Included blocks (and blocks within blocks) in the Skaergaard intrusion: Geologic relations and the origins of rhythmic modally graded layers

T. Neil Irvine* Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015 Jens Christian Ø. Andersen† Department of Earth Sciences, Aarhus University, 8000 Aarhus C., Denmark C. Kent Brooks Geological Institute, Copenhagen University, 1350 Copenhagen K., Denmark

ABSTRACT broad stratigraphic zones. Their physical rela- the replacement process also occurred in the tionships to their host rocks—particularly the upper border environment. The early Eocene Skaergaard intrusion of way they indent older layers beneath and are Two mechanisms are described whereby Greenland includes enormous numbers of covered by younger layers above—provide graded cumulate layers can be sorted and de- rocks of both exotic and cognate origins. The abundant evidence that there was generally a posited by magmatic crystal-liquid suspension lower parts of the Marginal Border Series sharp, well-defined interface between the top currents. One, involving density surge cur- contain abundant fragments of feldspathic of the cumulate pile and the main body of rents, has been advocated previously; the other peridotite that are possibly autoliths, inter- magma in the intrusion while the Layered Se- is a new concept based on boundary flow sepa- mixed with occasional xenoliths of Precam- ries was forming. The distribution of the au- ration and reattachment vortex cells. The two brian gneiss and metasomatized Cretaceous– toliths between and through the well-known, mechanisms are used in complementary ways Paleocene sediments derived from adjoining rhythmic, thin, modally graded layers shows to illustrate the formation of (1) some of the country rocks. The Upper Border Series in- that these layers were spread by magmatic principal Skaergaard structures involving cludes one exceptionally large block of gneiss currents; and their relations to the more ex- blocks and layers; (2) modally graded layers in (several hundred meters across), and numer- tensive macrorhythmic layering suggest that it the Layered Series that rhythmically alternate ous smaller fragments, these originating from too was significantly shaped by currents. with uniform layers; and (3) modally sorted the intrusion’s footwalls, plus a few pieces of Many of the larger autoliths are crudely layers in the Upper Border Series featuring peridotite. The Layered Series contains layered internally, and in places it is evident “underside draping” beneath small included countless autoliths of troctolite, gabbroic that their stratification existed before they blocks. Explanations are provided for (1) why anorthosite, and oxide (magnetite-ilmenite) broke loose; therefore, it must have formed plagioclase did not float away from the tops of gabbro, broken from parts of the Upper Bor- in the Upper Border Series. One particularly graded layers even though it was less dense der Series that have otherwise been lost to large block of oxide gabbro exhibits extraor- than the liquid, and (2) how the liquid part of a erosion; at the upper midlevel of its western dinarily well-developed modal and textural current was fractionated away from the crys- half, it contains a few xenoliths of basalt, de- layering and includes small troctolitic au- talline materials. Modal and grain-size data rived probably from the now-eroded (Eocene) toliths of an earlier generation, and it provides from Skaergaard intrusion graded layers are roof of the intrusion. A distinctive postintru- evidence that currents also spread crystalline shown to be in excellent accord with charac- sion composite basaltic dike at one place con- materials across the top of the magma body. teristics predicted for layers sorted by cur- tains 40 or more xenoliths of troctolite, olivine Many of the very small autoliths in the Lay- rents; a synthesis diagram is presented illus- gabbro, and gabbroic anorthosite that may ered Series are highly anorthositic in composi- trating how all the above processes may have represent parts of the Layered Series still hid- tion, apparently because they were leached of functioned in concert in the intrusion. den at depth. mafic minerals, and some of the larger blocks The Layered Series autoliths range from show local patchy internal replacement by INTRODUCTION fragments a few centimeters on a side to anorthosite. Most large blocks show little sign blocks more than 400 m across, and they typi- of postaccumulation modification, and some The Skaergaard intrusion (Fig. 1) of Greenland cally are coarser grained than their host cu- have thin, fine-grained augite-rich rims or contains countless inclusions of rocks formed in mulates, being in this respect more like Upper rinds, demonstrating that even though they other places and environments. The lower (north- Border Series rocks. The autoliths are spread were out of thermal and chemical equilibrium ern) parts of its marginal border units contain stratigraphically through the lower 70% of with their host cumulates, they still were effec- heavy concentrations of peridotite fragments that the exposed 2500 m thickness of the Layered tively armored against extensive chemical are possibly cognate inclusions (autoliths), plus Series and are generally concentrated in three change. Also documented is a large block that occasional xenoliths of gneiss and metasedimen- was cut by several early basaltic dikes before tary rocks derived from adjoining host rocks. The *E-mail: [email protected]. it broke free from the top of the intrusion; well-layered interior contains enormous numbers †Present address: Camborne School of Mines, these early dikes transgress small anorthositic of autoliths of troctolitic, anorthositic, and gab- Redruth, Cornwall TR15 3SE, United Kingdom. replacement pipes in the block, showing that broic rocks that were apparently broken from the

GSA Bulletin; November 1998; v. 110; no. 11; p. 1398–1447; 29 figures; 1 table.

1398

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/11/1398/3382835/i0016-7606-110-11-1398.pdf by guest on 27 September 2021 Figure 1. Generalized geologic map of the Skaergaard intrusion, based on Wager and Deer (1939), Wager and Brown (1968), McBirney (1989b), and our observations. Abbreviations: PCgn—Precambrian gneiss; Ks—Cretaceous sedimentary rocks; Ebv—Eocene basaltic volcanic rocks; Egs—Eocene gabbro sills; Ebd—Eocene basaltic dike; Epd—Eocene peridotite; LZ(a,b,c)—MZ—UZ(a,b,c)—lower, middle, and upper zones (and subzones, respectively) of the Layered Series; MBS—Marginal Border Series; UBS—Upper Border Series; gr—granophyre; gn— granitic gneiss. Small arrows with numbers denote inclinations of contacts; strike and dip symbols pertain to foliation in the gneiss, bedding in the supracrustal rocks, and layering in Skaergaard. Uttental Plateau forms the peninsula north of the words Uttental Sound; WP is Wager Peak. Two main sections of macrorhythmic layering are indicated by series of subparallel dotted lines. For more detail, see Figure 3. The basalt dike shown extending north from Skaergaard Bay is termed the Campsite composite dike. Its continuity from the bay to Kraemer Island is reason- ably certain; the further extension to the Uttental Plateau is speculative. Maps of parts of the dike appear in Figures 5 and 16; and the dike ex- tends across the area of Figure 17A, but has been closed up there so as not to obscure host-rock features.

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upper border units while they were solidifying, pal contributions. Like Wager and his coworkers, GEOLOGY OF THE INTRUSION and includes a few xenoliths of basalt that proba- we see most Skaergaard intrusion layering as be- bly represent the roof of the magma chamber (in ing defined by “cumulus minerals” fractionated General Structure fact, one contains what might be part of the orig- directly from the main body of magma through inal roof contact). A distinctive, post-Skaergaard processes of both magmatic sedimentation and in The Skaergaard intrusion is a roughly oval- intrusion basaltic dike at one place contains sev- situ crystallization, and we support their view that shaped body, approximately 10 km long from eral dozen troctolitic and gabbroic xenoliths that the solidification process was highly dynamic; the north to south and 7–8 km wide (Fig. 1), located probably came from parts of the intrusion that are floor, walls, and roof of the magma body all being on the east coast of Greenland at the mouth of a still hidden at depth. swept and coated almost continuously by convec- large fjord called Kangerdlugssuaq. The intrusion The autoliths are abundant: they are spread tion and density currents. We also recognize that formed in early Eocene time, 55.7 ± 0.3 Ma through more than 70% of the 2500 m exposed compaction and filter pressing were probably (Hirschmann et al., 1997; see also Hirschmann, section of layered rocks, almost from its lowest widespread processes in the main interior part of 1992; Brooks and Gleadow, 1977), along with levels, and range in size from fragments a few the intrusion, and that the displaced liquid caused about a dozen other major plutons in association centimeters across to blocks more than 400 m significant infiltration autometasomatism. How- with large floods of basalt and at least three, and on a side. At the present erosion surface, au- ever, we find conspicuous effects of these possibly as many as five, extensive swarms of toliths as long as 1 m probably number in the processes to be quantitatively relatively minor, basaltic dikes (Nielsen, 1978; Brooks and Nielsen, thousands, and blocks tens to hundreds of me- and our observations of the autoliths indicate that 1982). All this magmatism accompanied the open- ters across must total in at least the hundreds. their compositional features are mostly original, ing of the North Atlantic and evidently stemmed The autoliths are important because (1) they and thus presumably developed in a source region from the mantle hotspot or plume that is now iden- constitute a sample of parts of the intrusion located just beneath the intrusion’s roof. tified with Iceland (Brooks, 1973). that have otherwise been lost through erosion; Principal among the newer unconventional The country rocks invaded by the Skaergaard (2) they yield valuable insights into the me- ideas that we oppose (e.g., as expressed especially intrusion consist of a basement complex of chanical processes that occurred during solidi- by McBirney and Noyes, 1979; McBirney and Archean granitic gneiss, amphibolite, and related fication of the Skaergaard magma; and (3) they Hunter, 1995; McBirney, 1995, 1996a; McBirney rocks, overlain unconformably by a thin unit of are useful in delimiting the physical and chem- and Nicolas, 1997; Boudreau and McBirney, Cretaceous to Paleocene sandstone and shale (the ical characteristics of the magma. 1997) are: (1) the concepts and terminology of ig- Kangerdlugssuaq Group) that is succeeded more In this paper we begin with a review synthesis neous cumulates are inappropriate and mislead- or less conformably by some 3–7 km of the of the Skaergaard intrusion geology and petrol- ing; (2) the prominent layering that characterizes Eocene basaltic volcanic formations (the Blosse- ogy; then we give a lithologic classification of the the main, interior part of the Skaergaard intrusion ville Group) and affiliated gabbroic sills. At the autoliths, describe their field relations, and explore was developed largely by mineralogical recrystal- present erosion surface, the intrusion is mostly in their implications with regard to the effects of lization and reorganization of an initially unstrati- contact with gneiss on the west and north, and postcumulus metasomatism and recrystallization fied, relatively stagnant crystal mush; (3) the crys- basalt and sills on the east and south. Its outer and the origins of Skaergaard intrusion layering. tal mush was fractionated and aggregated simply boundary on the west dips inward at about 80°, Up until the time of his death in 1965, by gravitational compaction; and (4) the autoliths and what can be seen of it on the north dips in- L. R. Wager closely coordinated investigations of have been extensively remade metasomatically in ward (southward) at 35°–40°. On the east, the the intrusion. Wager and Brown (1968) is a bal- composition (to anorthosite), texture, and internal contact in its upper reaches is inclined steeply out- anced, internally consistent account of the knowl- structure after they were emplaced at their ob- ward (eastward), but it turns inward at structurally edge of and ideas about the Skaergaard intrusion served sites. Because of the controversy, we in- lower levels. The southernmost Skaergaard out- that existed at that time. Since then, however, clude specific page, figure, or table numbers with crops retain a small portion of roof contact, dip- there have been many investigators, most working most of our reference citations to try to ensure that ping about 20° southward before turning down- either independently or in small groups, and usu- they are both identifiable and accurate. ward into the south wall contact, which is inclined ally only on specific features. Quality data and The detailed maps of block and layering struc- to the south at about 80°. important new insights have accrued, but with so tures presented here were controlled by triangu- The top of the Skaergaard intrusion is mostly many people trying to break new ground, numer- lated networks of rock cairns and, as much as eroded away, and a gravity model by Blank and ous questionable interpretations have arisen, possible, are oriented so that the layering appears Gettings (1973; see McBirney, 1975) suggests that some of them radical and unlikely, some overex- stratigraphically upright. Thus, north arrows its unexposed parts extend to depths of 3.5–4 km. tended, and a few even inconsistent with factual point to the lower left or downward in several The gravity model has large uncertainties, how- observations. The literature consequently has be- maps, to the right in one, and to the left in an- ever, because the intrusion’s field is heavily come both controversial and confusing. other. Down- (or up)-dip projections are pre- masked by a strong regional gradient associated Controversial matters arise repeatedly in this sented for some of the maps in order to suppress with the transition from continental granitic crust paper. The main problem we address concerns the irregularities due to topography and show fea- on the north and west to the oceanic basaltic crust origin of the Skaergaard intrusion modally graded tures at approximately the same horizontal and on the southeast. The model probably does not layers, but the broader issue is the way the intru- stratigraphic scales. The projections resemble take the intrusion deep enough (it does not seem to sion as a whole solidified and differentiated. In cross sections, but they entirely depict the surface accommodate adequately the lowest visible rock general, we agree with and follow the original relations of features that are three-dimensional in units), but we have found it satisfactory enough to themes of Wager and Deer (1939) and Wager and exposure as well as structure. It is best, therefore, use it in our cross sections in Figure 2 to portray Brown (1968)—as well as a few of our own to think of the diagrams in terms of what they the emplacement of the magma. In the emplace- (Irvine, 1980a, 1980b, 1987)—and we believe are—perspective views of outcrop surfaces in ment scheme, the room for the intrusion is made this paper logically evolves from these studies which features at the bottom are generally closer largely by displacements on faults now repre- without seriously discrediting any of their princi- to the observer than those at the top. sented by its east and south walls. Another fault

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Figure 2. Reconstructed west-to-east cross sections through the Skaergaard intrusion, illustrating the emplacement of the magma, modified from Irvine (1991). Diagram A shows the intrusion filling; B portrays it after it was solidified and intruded by the Campsite composite dike and the Basistoppen sheet. By the inferred relations, room for the intrusion was made largely by the displacement on the East Wall fault (and a South Wall fault, not visible here); then a block down-dropped on West Wall fault 2, forming a “stopper” in the feeder. Locality M is the island Mellemö, on the west side of the intrusion (see Fig. 1), where a small remnant of sediments and basalt rests unconformably on gneiss just at the intrusion con- tact. Epd—Eocene peridotite (see Figs. 1 and 7); Ebd—Campsite composite dike (Figs. 1 and 4). Note in Figure 1 that the West and East Wall faults converge to the north and meet near (if not at) the peridotite body on the south shore of Watkins Fjord.

block is then down-dropped on the west side, clos- dikes and sills that cut it, have unquestionably all lowing. The nature of the Hidden Zone is neces- ing off the feeder and isolating the magma body so been tilted some 15°–20° to the south-southeast. sarily inferred, but the other zones are defined ac- that it can undergo the closed-system fractional Thus, at the present erosion surface, the Layered cording to the cumulus arrivals and departures of crystallization that has been the hallmark of Skaer- Series dips to the south, its stratigraphically the main primary minerals, these reflecting the gaard intrusion petrology. higher (i.e., younger) units being exposed suc- liquidus path of the parental magma under condi- Internally, the Skaergaard intrusion divides cessively in that direction, and the Marginal Bor- tions of fractional crystallization (Wager and naturally into three major units: a Marginal Bor- der Series gradually increases in width from Deer, 1939). Thus, plagioclase and olivine are co- der Series, crystallized from the walls of the north to south with the progressive appearance of tectic precipitates in LZa; they are joined by cu- magma chamber inward; an Upper Border Se- its younger divisions (Figs. 1 and 3). The Layered mulus augitic clinopyroxene in LZb, and by ries, crystallized from the roof contact down- Series has a visible thickness of about 2500 m, abundant ilmenite and magnetite in LZc. Olivine ward; and the Layered Series, accumulated from and the Marginal Border Series widens from grades out through LZc and is largely absent an unexposed floor upward (Fig. 3). The sedi- about 100 m around its lower, northern reaches to through MZ, where cumulus pigeonite (now in- mentary rocks, basalt formations, and gabbro roughly 600 m at its southern limits (Hoover, verted to orthopyroxene and exsolved augite) lo- sills hosting the east side of the intrusion show 1978, 1982, 1989a, 1989b). The Upper Border cally occurs in its stead (see following). This suc- monoclinal flexuring to the south, their dips in- Series has a maximum preserved thickness of cession is then reversed at the base of UZa, where creasing (Fig. 1) from about 5° opposite its north about 950 m (Naslund, 1984). cumulus olivine reappears in abundance; then ap- end to roughly 40° at its south end (and to 50° atite becomes cumulus in UZb; and finally in slightly farther south). Wager and Deer (1939, Layered Series UZc, ferrobustamite, now mostly inverted to p. 51–55) believed that the intrusion’s internal hedenbergite, takes the place of clinopyroxene. structure was similarly warped, and attempted to As shown in Figure 3, the Layered Series is di- By this stage, the mafic silicates have all devel- reconstruct the original configuration of the Lay- vided into Hidden, Lower, Middle, and Upper oped very high Fe/Mg, and the plagioclase is rich ered Series on that basis. We have a slightly dif- Zones (with subzones), identified by the symbols in albite. Conventional names for the rocks, ferent view on this matter (as described under the HZ, LZ (a,b,c), MZ, and UZ (a,b,c) (Wager, based essentially on cumulus assemblages, are heading Associated Basaltic Intrusions), but the 1960). The main kinds of layering are illustrated given in Figure 3. intrusion and its structures, and all the many in Figure 4 and described more fully in the fol- Two problems relating to the above crystal-

Geological Society of America Bulletin, November 1998 1401

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/11/1398/3382835/i0016-7606-110-11-1398.pdf by guest on 27 September 2021 Figure 3. Lithostructural subdivisions of the Skaergaard intrusion. Symbols: HZ—LZ(a,b,c)—MZ, and UZ(a,b,c) denote the Hidden, Lower, Middle, and Upper Zones (and subzones, respectively) of the Layered Series; asterisks denote equivalent units for the Marginal Border Series; α(1,2,3), β, and γ(1,2,3) are correlative divisions of the Upper Border Series. Cumulus mineral arrivals and departures are indicated by plus and minus signs. Here and in Figure 1, the MZ-UZa contact is placed at the base of the melanocratic part of the uppermost Triple Group member, be- cause drill-hole intersections show that this is where olivine makes its most definite reappearance after a general absence through MZ. The Lay- ered Series is portrayed as being flat for convenience of illustration; in reality, it is relatively planar from north to south but distinctly synformal from west to east. The “stoped front” delimits the part of the Upper Border Series from which the included blocks in the Layered Series are evi- dently derived. An anorthositic facies change is suggested for the now-eroded source zone of the autoliths because many of them are distinctly more feldspathic than the preserved Upper Border rocks (Naslund, 1986, p. 366). If the granophyre existed, it has been lost to erosion. HZ rock types are inferred from the xenoliths in the Campsite composite dike (see Fig. 5). The larger autoliths are drawn roughly to scale, but the collo- form banding in the marginal units, and the bodies of pegmatite and secondary anorthosite are exaggerated for illustration. For other abbrevia- tions, see Figure 6.

1402 Geological Society of America Bulletin, November 1998

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C D

E F

Figure 4. Photographs of Skaergaard layering. (A) Thin, modally graded layers alternating with uniform (isomodal) layers in subzone UZa (cf. Figs. 1 and 3), near the area of Figure 5. (B) Colloform banding in the Marginal Border Series on the island Ivnarmiut. Antiforms point into the intrusion. A pod of mafic pegmatite replacement underlies the hammer, and more is to the right. Some anorthositic replacement oc- curs to the left. (C) Macrorhythmic layering in LZb, Uttental Plateau. Note the thin modally graded layer in the basal part of first dark macrolayer (just above the hammer). (D) Microrhythmic layering in MZ (Middle Zone), at the top of the section shown in Figure 15. (E) Modal layering in the cross-bedded belt, Uttental Plateau. (F) Layers selectively replaced by mafic pegmatite in UZa. A zone of 10–15 of these pegmatite units extends for some 400 m from the area of Figure 5 to the northeast.

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lization sequence are noted. The first concerns posited at the slope break between the intrusion ries, but the rocks are generally coarser grained, magnetite and ilmenite and arises partly be- walls and the accumulating Layered Series by commonly have pegmatitic and granophyric fa- cause their primary crystallization relations are convection currents descending from the walls cies, and also differ significantly in various as- extensively obscured by exsolution, recrystal- (Wager and Brown, 1968, p. 218; Irvine, 1980a, pects of their mineralogy and chemistry. Hoover lization, and coarsening associated with postcu- Fig. 13, 1987, Fig. 41). The cross-bedded belt is (1978, 1989a) assigned the Marginal Border sub- * * mulus cooling and oxidation (Vincent and also distinguished by numerous small normal zones the analogous symbols, LZa T, LZa B, Phillips, 1954; Buddington and Lindsley, 1964). faults dipping steeply into the intrusion, and some MZ*, UZa* and UZb* where the subscript T de- Wager and Deer (1939, Fig. 18) recognized that of them even break the contact with the Marginal notes the “Tranquil Division” of Wager and the two “iron ores” became distinctly more Border series (Wager and Brown, 1968, Fig. 70; Brown (1968), which is a fringe of olivine gabbro, abundant at the base of LZc, as would be ex- McBirney and Nicolas, 1997, Figs. 2 and 5). The 50–100 m wide, locally chilled on its outer edge pected if either or both had made their cumulus faults are generally healed magmatically, so they and relatively structureless except for local bands appearance there; but Wager and Brown (1968, evidently formed at the postcumulus stage. Some and splays of distinctive “perpendicular feldspar Fig. 14 and p. 48–49) inferred from textures that of them might reflect eastward or southward tec- rock” (see Wager and Deer, 1939, Pl. 3, Fig. 1; ilmenite is sporadically cumulus even in LZa tonic tilting of the intrusion, but our impression is McBirney and Noyes, 1979, Pl. 3; Irvine, 1987, and LZb, and inferred that its arrival preceded that they mostly developed because the compact- Fig. 25). The subscript B denotes the “Banded Di- that of magnetite. Our impression, however, is ing interior part of Layered Series repeatedly vision,” which is characterized by an irregular, that ilmenite is never abundant enough in LZa sheared off from the marginal parts as the latter corrugated to colloform modal stratification that and LZb to be a cumulus precipitate, so we sug- became frozen to the intrusion walls (Irvine, frequently is transgressed and partly replaced by gest that in the rare places where it is euhedral, it 1980a, Fig. 13). pods, veins, and patches of mafic pegmatite (Fig. was perhaps formed by postcumulus fractiona- The top boundary of the Layered Series where 4B; Wager and Brown, 1968, Fig. 85; McBirney tion from pore liquid filtering upward through it meets the Upper Border Series is called the and Noyes, 1979, Pl. 5, F; Irvine, 1982, Pl. 1, the cumulate pile. Sandwich Horizon, and Wager and Brown (1968, 1987, Figs. 24 and 25). However, banding is pres-

The second problem is that McBirney (1989a, p. 31) believed that it could be identified by a thin ent in all subzones from LZa*B to UZb*, so in p. 371) disputed the interpretation that pigeonite is rock layer compositionally at the extreme of the Figure 3 we have simply taken its presence as cumulus in MZ, claiming that it was never a liq- differentiation trends in the adjoining units. Geo- given and dropped the B subscript. Then, by using uidus phase but formed entirely by late replace- logically, this layer should crop out in a ring TZ* to denote the Tranquil Division, we were also ment of olivine and augite, even though excellent around the west, north, and east sides of a moun- able to drop the T subscript. The Marginal Border cumulus examples had been illustrated by Wager tain peak called Basistoppen. Its exposure is Series has no preserved equivalent of UZc, and and Deer (1939, Pl. 15, Fig. 2) and Wager and poor, however, and Wager and Brown (1968, according to Hoover (1978), apatite makes an Brown (1968, Fig. 22A). This view reflects a p. 95–96) could identify it at only one place on anomalously early appearance as a fractionated long-standing opposition to cumulate concepts the west side of the mountain, and on our visits to mineral in the highest reaches of LZb* along the and terminology (McBirney and Noyes, 1979, the area, our guide could only find it at one local- east side of the intrusion. p. 503–504; McBirney and Hunter, 1995), and ity on the east side. Wager and Brown (1968) The Upper Border Series is difficult to study be- was amplified by McBirney and Naslund (1990, viewed the rock as representing the last (lowest cause it is only exposed in rugged mountainous p. 237), who stated unequivocally that there is no temperature) residue of Skaergaard magma, but terrain. Wager and Deer (1939) recognized that it cumulus pigeonite in the Skaergaard intrusion. By Hunter and Sparks (1987, p. 452) have since sug- had differentiation zones comparable to those of our thin-section sampling, pigeonite is not cumu- gested that it is a cumulate, and that the last mag- the Layered Series (but formed from the top down lus throughout MZ (e.g., as indicated in matic residue was a granophyric liquid, the rather than from the bottom up) and, following a Fig. 14 of Wager and Brown, 1968), and nowhere crystallization products of which have been com- study of their samples by Douglas (1961), Wager is it abundant, but inverted cumulus crystals are pletely eroded away (see Figs. 2 and 3). The gra- and Brown (1968, p. 31) assigned these zones the common toward the top of this zone in the inter- nophyric residue idea was strongly criticized by symbols UBGα, UBGβ, and UBGγ (where the G val preceding the three prominent layering units McBirney and Naslund (1990), Morse (1990), denotes “group”). These authors also identified a called the Triple Group. Notably, pigeonite does and Brooks and Nielsen (1990), but by our calcu- major unconformity along which the lower not have to be abundant to be cumulus. The ratio lations, it appears almost certainly to be correct (younger) part of UBGα and a large part of UBGβ in which it coprecipitates with augite experimen- (even though the liquid earlier reaches FeO levels were evidently stoped away, and they correlated tally is only about 1:3 (Hoover and Irvine, 1978, of 18 wt% at the middle stages of its differentia- the stoping with the abundant large autolithic Fig. 87); thus its typical abundance of only tion, as argued by Brooks and Nielsen, 1990). blocks in Layered Series MZ. More recently, 6%–8% where we consider it to be cumulus (and Our impression of the Sandwich Horizon rock, Naslund (1976, 1980, 1984) climbed all the main where cumulus augite generally amounts to both from our sample and the one illustrated by mountain peaks and made a detailed sampling of 20%–25%) is entirely appropriate. Wager and Brown (1968, Fig. 67), is that it is a the rocks for a more comprehensive study. He used α β γ Around its edges where it adjoins the Marginal pegmatitic cumulate with plagioclase, olivine, the symbols UBZT, UBZ , UBZ , and UBZ for Border Series, the Layered Series has a spectacu- hedenbergite, apatite, and magnetite being re- the main lithologic zones, the subscript T referring lar marginal facies called the cross-bedded belt, crystallized cumulus minerals, and granophyre to a Tranquil Zone comparable to that of the Mar- measuring from a few tens of meters to about 0.5 forming a postcumulus mesostasis. ginal Border Series, and he introduced numbered km in width, featuring complex arrays of small subscripts for two subzones in UBZα and three in angular unconformities developed in thinly lay- The Marginal and Upper Border Series UBZγ, thereby matching most of the Layered Se- ered cumulates (Fig. 3; Wager and Brown, 1968, ries subzones. However, from his petrographic ob- Pl. VIII, Fig. 68). The unconformities are readily The Marginal and Upper Border Series were servations, Naslund concluded that (1) apatite and ascribed to repeated erosion or slumping of over- recognized by Wager and Deer (1939) as having the Fe-Ti oxides all appear as primocrysts (frac- steepened stratified piles of crystals rapidly de- divisions analogous to those of the Layered Se- tionated minerals) in UBZα, earlier than their re-

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spective UZb and LZc stages of cumulus arrival in phyre(?) unit indicated in the diagram can be ob- and mafic minerals to shift compositionally to- the Layered Series; (2) Ca-rich pyroxene does not served, however, because even if they once ex- ward plagioclase. This effect is exemplified by the appear until UBZβ, later than its LZb arrival; and isted, they are now eroded away. The layering in cotectic of anorthite and diopside on the join

(3) Ca-poor pyroxene (pigeonite) is present only upper border rocks is briefly described later. CaAl2Si2O8-CaMgSi2O6-H2O (Yoder, 1965), and as a rare interstitial phase, never as primocrysts. it has now also been demonstrated for anorthite

Naslund also emphasized that the Upper Border Secondary (Metasomatic) Anorthosite and forsterite on the join CaAl2Si2O8-MgSiO3- Series is generally richer in K2O and SiO2 than the and Olivine Clinopyroxenite H2O, which is more pertinent here (Yoder and Layered Series, and he attributed all these charac- Irvine, unpublished data). If H2O were being lost teristics to formation of the rocks from a magmatic At several places in the Layered Series, there from a hydrous cotectic liquid filtering through a

liquid layer richer in SiO2, K2O, P2O5, and H2O are clusters of small (generally 1–5 m) irregularly Skaergaard gabbroic cumulate, the tendency of the than the parental liquid of the Layered and Mar- shaped bodies of anorthosite, evidently formed liquid to stay in equilibrium with its environment ginal Border Series. by (approximately) volume-for-volume replace- should cause it to reabsorb the cumulus mafic min- In preparing Figure 3, we initially tried to fit the ment of their host cumulates. These bodies are erals and precipitate plagioclase in their place, Upper Border zonal sequence to the Naslund’s pertinent here because of the claims that the au- thereby producing the secondary, feldspar-en- crystallization order, but we encountered diffi- toliths have undergone extensive anorthosite riched rock. (We suspect that selective vapor trans- culty with his placing of the Fe-Ti oxides and ap- metasomatism (e.g., McBirney, 1995, p. 431). fer was also involved, but an appropriate experi- atite in that his whole-rock chemical data seemed The replacement anorthosite bodies are largest mental model is not yet available.) The presence of in better accord with the paragenesis of the Lay- and most numerous in LZa and LZb on the Utten- the mafic rims on the undersides of some of the ered Series. Thus, in the zone-composition aver- tal Plateau, and a few small patches are scattered anorthosite bodies suggests that the replacement ages given in his Table 1A (Naslund, 1980), based among the trough layering structures in UZa and advanced downward as the reacting hydrous liquid β on 99 whole-rock analyses, TiO2 peaks in UBZ , UZb. Some of the thinner, tabular occurrences seeped upward around their lower edges. Irvine matching the abundant cumulus presence of Fe-Ti were mistaken by Wager and Brown (1968, Figs. ascribed the associated mafic pegmatite bodies to

oxides in LZc and MZ, and P2O5 increases 31 and 71) as deformed layers, but the replace- partial melting of neighboring (hot) cumulates be- γ sharply in UBZ 2, matching the abrupt cumulus ment origin was demonstrated by Irvine (1980b, cause of fluxing by the H2O, the hydrous melt then appearance of apatite in UZb. We therefore un- Fig. 21; 1987, Figs. 42 and 44), and further exam- resolidifying with pegmatite grain sizes. dertook an extensive reexamination of all the pet- ples were illustrated by McBirney and Sonnenthal Naslund (1986, Figs. 2–5; see also Naslund rographic, chemical, and mineralogical informa- (1990, Fig. 2; McBirney and Hunter (1995, Fig. and McBirney, 1996, Fig. 19) also documented a tion in Naslund (1980), and our finding is that 3), McBirney (1996a, Fig. 10B), and Naslund and cluster of irregular anorthositic bodies in the his suggested differences of paragenesis appear McBirney (1996, Fig. 8). The evidence of re- olivine ferrobustadiorite of Upper Border subzone γ largely to be a matter of rock unit identification. placement is that some of the bodies transgress 3 at a locality called Nunatak I, just below a large Thus some of the sections that he has assigned layering without displacing or deforming it, and basaltic intrusion known as the Basistoppen sheet, to UBZα (in Naslund, 1980, Pl. 4) seem better in places, remnants of their layered protolith can which cut into the Skaergaard intrusion shortly af- placed in UBZγ, because when we reassigned be traced through them. Some of the bodies are ter it had solidified (see section on Associated them, their petrographic, compositional, and highlighted by dark brown fringes rich in olivine Basaltic Intrusions). Most of these anorthositic stratigraphic relations all become much more and subordinate augite, especially on their under- bodies are 1–5 m in size, but one is more than consistent. The distribution of autoliths in the sides; strands and irregular bands and patches of 60 m long. They are of interest here because they Layered Series shows that stoping of the Upper similar dark, mafic to ultramafic rock are com- embody the only anorthosite that has been re- Border Series was well under way even at the mon nearby, and apparently represent part of the ported in the Upper Border Series, even though it LZa crystallization stage—earlier perhaps than displaced mafic minerals. is the apparent source of the abundant anorthositic Naslund realized in 1980 and 1984—and it is also Irvine (1983a, Fig. 17, 1987, Figs. 42–44) also autoliths in the Layered Series. Although the out- evident, by way of the basalt xenoliths in the Lay- noted an affiliation of the replacement anor- crop relations of the Nunatak I bodies are not suf- ered Series, that the stoping had extended com- thosite with small pods, patches, and branching ficient to prove that they formed by replacement, pletely through the Upper Border Series into the veins of mafic pegmatite, and this association we believe this origin is indicated because the intrusion roof by the end of MZ stage. On these was further documented by Sonnenthal (1992, smaller bodies are physically identical to the re- bases, then, it is fully possible for oxide- and ap- Fig. 2), and by Larsen and Brooks (1994, Figs. placement anorthosite bodies in LZa and LZb, es- atite-rich UBZγ rocks to have formed in juxtapo- 4 and 13). Like the anorthosite, the pegmatite pecially in the respects that they have similar sition with the early subzones of UZα. bodies commonly have mafic fringes on their discontinuous melanocratic fringes and are ac- In Figure 3, we show the Upper Border Series lower edges; some have diffuse feldspathic cap- companied by bands and strands of mafic to ultra- as having the same zone sequence and crystalliza- pings and small cores of melanogranophyre. mafic rock (here composed mostly of fayalite and γ tion order as the Layered Series (except pigeonite The pegmatite ranges from gabbroic to ferrodi- ferrobustamite, as would be expected for the 3 is excluded), and we have simplified the zone oritic, depending on its host rocks, and an am- environment; see Naslund, 1986, Fig. 7). symbols by omitting the UBZ and by using τ to phibole-bearing zone commonly adjoins the However, there is another view, and there are denote the Tranquil Zone. The diagrammatic granophyre. other complications. Naslund (1986) interpreted γ stoped front delimits the structural interval from Irvine (1980a, 1980b, 1987) ascribed the re- the 3 anorthositic rock as restite from rheomor- which the Layered Series autoliths and basalt placement anorthosite to infiltration metasoma- phic melting of the ferrobustadiorite by the Basi-

xenoliths were evidently derived, and an anor- tism by migrating, H2O-enriched intercumulus stoppen sheet. His reasons were (1) the bodies α thositic facies is suggested for zone because liquid when the H2O was escaping from the liquid. are obviously within the thermal halo of the many of the autoliths are much richer in plagio- This interpretation stemmed from the observation sheet, and (2) the sheet is cut by some of the

clase than any of the preserved upper border rocks. that increased H2O in mafic silicate liquids causes melanocratic veins, which he regarded as the Neither the anorthositic facies nor the grano- cotectic liquidus boundaries between plagioclase rheomorphic melt. However, Wager and Deer

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(1939) observed that the base of the sheet on that they represent processes that controlled devel- Layered Series where there are no gneiss xeno- Nunatak I is also veined (replaced?) by an irreg- opment of some of the Skaergaard intrusion layer- liths, and in part because unequivocal gneiss ular network of feldspathic material, and this was ing, and McBirney (1995, p. 421) claimed them to xenoliths in the Marginal Border Series character- illustrated by Douglas (1964, Fig. 4) and attrib- be evidence that buoyant liquid rich in volatiles istically have sharp contacts with little or no asso- uted by him to “back-veining” by what he and incompatible elements rose through the Lay- ciated pegmatite (e.g., Kays et al., 1981, Fig. 4, A thought was a salic rheomorphic melt produced ered Series, permeating and extensively modifying and B). An alternative origin, suggested by Taylor by the sheet in its host rock. On the basis of phase the upper half of the intrusion. and Forester (1979, p. 400–401) and advocated by equilibria considerations, our view is that neither Larsen and Brooks (1994, Fig. 2) identified Irvine (1987, p. 227–232), is that the marginal

the melanocratic nor the feldspathic veins could 35–40 mafic pegmatite occurrences through the zone pegmatite bodies mark places where H2O have been melts, because both would have had to western half of the Layered Series (in detail there from outside the intrusion infiltrated the relatively lose substantial flux; however, replacement is an are many more), and a similar distribution proba- anhydrous original Skaergaard rocks while they alternative. Further problems in the field relations bly exists in the less accessible eastern half, were still hot enough to be fluxed and remelted. are: (1) the anorthositic bodies are only seen at which was not examined. Three of the localities The pegmatite and the associated granophyre two places beneath the sheet, not everywhere that feature stratiform pegmatite units, typically 5–30 (and their constituent amphibole, biotite, and ap- its base is exposed (Naslund, 1986, p. 360); and cm thick (Fig. 4F), some of them following cu- atite) then differentiated from the anatectic hy- (2) some of the bodies are reported to be as mulate layers for tens to hundreds of meters, but drous melt as it resolidified. We hereafter refer to much as 50 m below the sheet (Naslund, 1986, even in the area where these are most abundant this process of replacement by a flux melting, fol- p. 361), which is distant for rheomorphism. We (partly shown in Fig. 5), their aggregate thickness lowed by coarsening during resolidification, as suggest, therefore, as a way of rationalizing these is only about 3 m. Most of the other pegmatite “pegmatitization.” observations, that the two anorthosite clusters occurrences (especially in LZa–c) feature one to In the UZa area illustrated in Figure 5, where mark localized channels of postcumulus replace- several small pipes, branching veins, or lenses the stratiform pegmatite units extend for tens to ment that had perhaps been in operation even and pods, typically 1–3 m in size; some of the hundreds of meters along thin modally graded lay- while the Skaergaard intrusion was undergoing pods are shaped like toadstools spreading on ers, these units occasionally branch or offset into primary crystallization, and which were still host-rock layers (Irvine, 1983a, Fig. 17, 1987, other layers (Fig. 4F; McBirney and Noyes, 1979, functioning weakly—or were reactivated—when Fig. 43; Larsen and Brooks, 1994, Figs. 2, 6, 11, Pl. 5E), and in a few places they contain disori- the Basistoppen sheet was emplaced. Thus the re- 13, and 15). There are a few near-vertical peg- ented inclusions of layered rock (Norton et al., placement process could precede the sheet and be matite dikes, generally less than 0.5 m wide, ex- 1984, Fig. 8). These features show clearly that the essentially independent of it, but still affect it. tending for as much as 50 m (McBirney and pegmatite-forming process was imposed on exist- McBirney and Sonnenthal (1990, p. 248– Noyes, 1979, Fig. 4), and small pegmatite ing strata (it did not produce them), and that mag- 257; McBirney, 1996a, Fig. 10A) described patches and veins can be found in some of the au- matic liquid was involved. As first noted by Irvine two occurrences of metasomatic olivine clino- toliths. In total, however, as assessed in part from (1987, p. 232), the pegmatite units often preferen- pyroxenite along the west side of the Skaer- detailed maps like those presented herein, these tially follow the plagioclase-rich tops of graded gaard intrusion, one near the south end of Krae- various occurrences probably do not amount to layers (Wager and Brown, 1968, Fig. 51; Larsen mer Island, the other on the island Ivnarmiut. even as much as a 0.1% of the Layered Series, and Brooks, 1994, Fig. 5c), a feature that can read- The pyroxenite occurs as ovoid and amoeboid and replacement anorthosite is proportionately ily be attributed to the same effect that is invoked bodies to a few meters in length, roughly less abundant by perhaps a fifth. in our explanation of replacement anorthosite: i.e.,

aligned with the intrusion wall in feldspathic Mafic pegmatite is abundant, though (to per- that addition of H2O shifts cotectic mafic melt gabbro that features local comb texture and haps 10% locally), in the Banded Division of the compositions toward plagioclase, in this case mak- weak layering. On Ivnarmiut, the pyroxenite Marginal Border Series, where it occurs as small ing the plagioclase-rich layer tops selectively sus- bodies form as much as 35% of an area at least pods, lenses, and veins, both concordant and ceptible to flux melting. However, the prime evi- 500 m long and 300 m wide, and they are un- transgressive, and as diffuse irregular patches. In dence that the pegmatite parental liquid was like anything that has been described elsewhere some outcrops it permeates host rocks, obscuring formed by flux melting is the delicate way in in the intrusion. From reported petrographic and and obliterating their primary structures (Fig. 3B; which pegmatite units follow particular layers for compositional characteristics, there seems no Wager and Deer, 1939, Pls. 4 and 14; Wager and distances that are enormous in comparison to their doubt that the pyroxenite is metasomatic, but its Brown, 1968, Figs. 85–87; McBirney and Noyes, thickness. There are bands of fine pegmatite, uni- occurrence is so extraordinary as to suggest that 1979, Pl. 1A; Irvine, 1983a, Fig. 17). The larger formly 1–2 cm thick, that follow layer tops for it replaced some exogenous mafic or ultramafic pods and lenses commonly have comb texture, 10–20 m, and coarser units, 5–10 cm thick, that material; e.g., as might have been affiliated and are often differentiated from mafic to felsic similarly extend for 50–100 m (Fig. 5; Wager and with peridotite and metasomatic hercynite in- away from the intrusion contact. (An excellent ex- Brown, 1968, Fig. 51). Given that the host cumu- clusions present farther north along the same ample is shown in McBirney and Noyes, 1979, Pl. lates were almost certainly still at red heat (and contact (see below). 5F, but is incorrectly identified as crescumulate therefore probably somewhat plastic) when the layering.) Some of these bodies also have distinc- pegmatite formed, it is highly unlikely that liquid Mafic Pegmatite and Melanogranophyre tive cores of fine-grained melanogranophyre and could have been intruded along the layers with associated miarolitic cavities (Wager and Deer, such precision. Such conditions would, however,

Besides their association with replacement 1939, Pl. 14; Wager and Brown, 1968, Fig. 87). be ideal for selective melting by H2O fluxing. anorthosite, mafic pegmatite bodies like those de- Wager and his colleagues—and McBirney (1979, The way the pegmatitization has spread on scribed here have a much broader distribution Fig. 10-8) and Kays et al. (1981, Fig. 4C)—con- layers is beautifully displayed in an arrested state through the Skaergaard intrusion, but they do not sidered the granophyre cores to be relics of gneiss on two layering plane exposures discovered by comprise a major rock unit. They are relevant here xenoliths, but this interpretation is unlikely, in part McBirney and Sonnenthal (1990, Fig. 3) in UZa because Sonnenthal (1992, p. 229) has suggested because there are similar pegmatite bodies in the near the area of Figure 5. In these exposures, den-

1406 Geological Society of America Bulletin, November 1998

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/11/1398/3382835/i0016-7606-110-11-1398.pdf by guest on 27 September 2021 Figure 5. Map of a complex array of basaltic and granophyric dikes and pegmatite bodies in UZa (Upper Zone a) along the northwest edge of the trough layer zone shown in Figure 1 (approximately lat 68°10.2′N, long 31°43.6′W). The pegmatite bodies are oldest, the granophyre dikes are next in age, and three ages of basaltic dikes are evident. What is probably the oldest basaltic dike is numbered 1. It consists of a plagioclase por- phyry, and it shows a transform offset from E to F (invaded by a younger dike) at the top right corner of the map. From F, it extends beyond the map area to the south-southeast (upper right) for several hundred meters. Next in age is the dike numbered 2, called the Campsite composite dike. It has been traced for some 500 m to the south (right) of D, and in Figure 1 we suggest that it extends to the north (left) for about 5 km. Its intersection with dike 1 must be near B but is not exposed, so their relative ages are not conclusively established. However, at locality A, dike 2 con- tains troctolitic, anorthositic, and gabbroic rock xenoliths, and the troctolite appears to represent a cumulate facies of dike 1 (see text). Dikes 1 and 2 are both intruded by numerous dikes of the younger swarm numbered 3.

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dritic fronds of embryonic pegmatite fan out lat- phibole, and associated fluid inclusions are all ing this possibility experimentally, and results erally (up-dip in one case, down-dip in the other) rich in chlorine (Sonnenthal, 1992; Larsen et al., thus far (Irvine, unpublished data) show that, in a along the feldspathic upper parts of graded lay- 1992; Larsen and Brooks, 1994). The fluid inclu- haplobasaltic liquid saturated with forsterite, ers, and the spreading stems from transgressive sions, for example, typically contain 17–23 wt% diopside, anorthite, and apatite, fluorine is parti- fractures that presumably were access channels NaCl. Our contention is that the high NaCl indi- tioned in favor of apatite over liquid by a factor of

for H2O. At another place, partly illustrated at the cates that the infiltrating H2O was originally sea- 4–5. Thus apatite crystals fractionated out of the bottom edge of Figure 5, a vertical pegmatite water brine, probably in the marine Kangerd- liquid would be expected to have appreciably dike, 20–50 cm wide, connects with several peg- lugssuaq Cretaceous–Paleocene sediments, but higher fluorine contents than intercumulus crys- matitized layers and appears to represent an ac- perhaps also in the volcanogenic sediments that tals that grew and equilibrated in the liquid until it cess channel that was subject to flux melting. We occur with the neighboring Eocene basalts. Some was completely solidified. also favor flux melting as an origin for the verti- of this brine would necessarily have to enter the Our interpretation of the NaCl-rich fluids cal pipes, branching veins, and toadstools of peg- intrusion by way of fractures through the gneiss also differs from ideas favored by the authors matite that occur in LZa–c. that underlies the sediments, but by the structural who analyzed the pegmatite minerals and fluids. A second of the three areas featuring strati- relations suggested in Figure 2, much of it could Sonnenthal (1992, p. 209) argued that the NaCl- bound pegmatite units is near the top of MZ on have flowed in directly from the sediments where rich fluids must have been derived from the Kraemer Island, and the third is in the lower part of they extend beneath the intrusion from the east. Skaergaard magma, as opposed to being intro- UZc, just north of Basistoppen peak. That all three We suppose that the NaCl content of the brine duced hydrothermally from an external source, localities are in the upper half of the Layered Se- initially was about 3.5 wt%, as is typical of sea- because the hydrous minerals in nearby hy- ries led Larsen and Brooks (1994, p. 1674–1675) water, but in time, because of fractionation ef- drothermal veins are low in chlorine, and be- to believe that the stratiform pegmatite units fects variously associated with boiling, selective cause the pegmatite he had studied is far distant formed because of a stratigraphic control. Larsen channeling, and preferential dissolution during from the intrusion’s margins (the apparent im-

and Brooks (1994, p. 1671) cited several studies the flux melting, with removal of H2O by amphi- plication here being that externally derived flu- reporting that the residual porosity (the erstwhile bole and biotite during resolidification of the ana- ids could not have penetrated that far). He did content of trapped intercumulus liquid) in the Lay- tectic magma, and with preferential early exsolu- not present any data on the hydrothermal vein ered Series decreased upward, and they argued tion of salt-poor aqueous fluids, the much higher minerals, however, and although Sonnenthal that, above the MZ level where this porosity be- concentrations observed in the fluid inclusions (1992, p. 224) cited Bird et al. (1986) as saying came less than 20 vol%, the cumulates were no were eventually established. that biotite in hydrothermal veins crosscutting longer permeable vertically, so the pegmatite par- We would exclude a magmatic source for the Skaergaard layering is poor in chlorine, they did ent liquid had to spread laterally. However, the chlorine on the grounds that the Layered Series not present any data on chlorine in biotite. Bird three areas of stratibound pegmatite units are postcumulus biotite and amphibole are almost et al. (1988, p. 449) claimed that fluid inclusions widely separated, and the view here is that the fac- everywhere poor in chlorine (only 0.04–0.06 wt% in vein quartz within Skaergaard and throughout tors controlling their formation were fundamen- and 0.05–0.12 wt%, respectively; Nash, 1976, Ta- its contact aureole contain less than 3 wt% tally more site specific than stratigraphic. Two es- bles 3 and 4), and its apatite is relatively poor (only NaCl, and cited supporting data in Bird et al. sential requirements for layer pegmatitization are 0.2–0.8 wt%) where the mineral is postcumulus in (1986); but the latter data pertain to fluids, not to (1) an availability of appropriate layers to replace, LZa to UZa, and becomes extremely poor (only minerals, and they all came from four samples

and (2) an availability of H2O, including access 0.04–0.16 wt%) where it is cumulus in UZb and collected in the aureole. channels for its influx. Thus the exceptional devel- UZc (Nash, 1976, Tables 1 and 2; Brown and The analyses of Layered Series apatite that opment of stratibound pegmatite in the area of Fig- Peckett, 1977, Table 1). Among all the apatites that Sonnenthal obtained (see McBirney, 1995, ure 5 could arise because, per (1), the area features have now been analyzed from layered mafic cu- Fig. 14) exhibit the same chlorine and fluorine a plethora of modally graded layers with plagio- mulates, those from the Skaergaard intrusion rank variations that were found by Nash (1976) and clase-rich tops; and per (2), dikes of granophyre with the lowest in chlorine (e.g., Boudreau et al., Brown and Peckett (1977), whereas Sonnenthal’s and basalt are exceptionally numerous and com- 1995, Fig. 3). data (1992, Table 3 and Fig. 9) from the dendritic plex in the vicinity, suggesting that this part of the The slight decrease of chlorine in the Layered pegmatite show that its biotite is as much as 10 intrusion was especially susceptible to invasion by Series apatite where it becomes cumulus is paral- times richer in chlorine than the Layered Series magmas and fluids from outside sources—the leled by a small decrease in its hydroxyl content, postcumulus biotites. Sonnenthal (1992, Figs. likely reason being that the area overlies the heart and these changes are compensated by a substan- 6–8) also showed that the pegmatite has large ap- of the intrusion’s feeder system (see Fig. 2). Two tial increase in fluorine (from approximately atite crystals that are step zoned both toward their stratigraphic features that might have imposed 2.4–2.6 wt% to 3.1–3.4 wt%; Nash, 1976, Table edges and toward NaCl-rich fluid inclusions, from some control, though, are (1) the modally graded 1). McBirney (1995, p. 433) attributed the high fluorine (3.0–3.3 wt%) and very low chlorine layers in the upper half of the Layered Series tend changes to removal of chlorine from the Skaer- (0.2–0.3 wt%) to moderate fluorine (2.1%– to have better defined plagioclase-rich tops, and gaard magma by way of vapor exsolution, but this 2.8%) and exceptionally high chlorine (0.9–1.2 (2) their plagioclase is more sodic (lower tempera- interpretation neglects a fundamental geological wt%). The first of these zone compositions is ap- ture) and therefore was probably more susceptible relationship. The cumulus arrival of apatite is the proximately matched in the Layered Series cumu- to melting. most sharply delimited fractional crystallization lus apatite (which appears only shortly above the The notion that the pegmatitization was in- event in the Layered Series: the mineral appears dendrite locality), and the low fluorine of the sec-

duced by H2O from outside the intrusion is plau- abruptly, in about a 10 cm stratigraphic interval, ond is matched in the postcumulus apatite, but its sible for the Marginal Border Series simply by often in amounts of 7–9 wt%. Thus a more likely high chlorine seems appropriate to be the indicator virtue of its location, but the concept appears also explanation of its composition change is simply of an influx of exogenous Cl-rich fluid. Our exper- to be justified for pegmatite occurrences in the that, when it began to precipitate, it preferentially imental study of apatite crystallization is relevant Layered Series in that their apatite, biotite, am- concentrated fluorine. One of us is currently test- here in that it shows that the apatite/liquid parti-

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tionings of fluorine and chlorine are very similar. ferentiation yielded a substantial proportion of intrusion, generally 200–300 m wide (White If this relationship is general, then it implies that granophyre that is now eroded away. et al., 1989), and the Upper Border Series is in- fluorine-rich magmatic crystals should reflect liq- truded by the Basistoppen sheet, which is a dif- uid with high F/Cl, and chlorine-rich crystals Transgressive Granophyre Intrusions ferentiated sill-like layered ultramafic-mafic should reflect liquid with high Cl/F, and these re- intrusion about 600 m thick (Hughes, 1956; lations accord with the notion that the late Cl-rich Next in age after the mafic pegmatite and Douglas, 1964; Naslund, 1989). The Skaer- apatite found by Sonnenthal (1992) in the peg- melanogranophyre are some well-defined grano- gaard intrusion is also cut by basaltic dikes of at matite formed because exogenous NaCl had been phyre bodies that are clearly magmatic injections. least three ages, these typically measuring from added to the system. As noted above, our Figure 2 The largest is a sheet in the Upper Border Series, a few centimeters to 20 m in width (see Fig. 5). indicates a direct way for brine to enter the intru- roughly 3 km long and 30 m thick, having a light The Basistoppen sheet has an apparent feeder- sion from the Kangerdlugssuaq sediments, and pink upper part and a darker basal section (Wager dike appendage, about 80 m wide, that extends through the map relations in Figure 5, it is appar- and Brown, 1968, Pl. IX). Wager and Brown for a short distance toward the macrodike (see ent that brine could have reached the pegmatite (1968, p. 137) were ambivalent about this body, in- Fig. 1), so they appear to be coeval, the macro- area by essentially the same pathways that only terpreting it first as one intrusion, then as two, and dike probably being the ultimate feeder. Wager shortly after were invaded by transgressive grano- although they saw no clear contact between the and Brown (1968, Fig. 105) observed that the phyre intrusions (see the following). two parts, they called the upper one the Tinden sheet is cut by a number of granophyre “veins,” Larsen and Brooks (1994; also see Larsen acid granophyre sill, and the lower one the Syd- so if they are transgressive acid granophyre, et al., 1992) also advocated a magmatic source toppen transitional granophyre sill. Also present then the sheet and the macrodike represent the for the NaCl-rich fluid inclusions. Their argu- within the Skaergaard intrusion are numerous first magma to intrude the Skaergaard intrusion ments were: (1) the pegmatite must have crys- small, sharply defined dikes and sheets of what is after it had solidified. However, Wager and tallized from segregated intercumulus liquid called transgressive acid granophyre (Wager and Brown (1968) referred to the granophyre veins because its plagioclase and pyroxenes are com- Deer, 1939, Fig. 39; Hirschmann, 1992, Fig. 3). as being contemporaneous with the sheet, positionally similar to the intercumulus plagio- These bodies are clustered in three areas along the which could mean that their relationship has no clase and pyroxenes in neighboring layered shores of Uttental Sound, spanning MZ, UZa, and broader age significance. Like the Skaergaard gabbros (Larsen and Brooks, 1994, p. 1651); UZb (Wager and Deer, 1939, Fig. 39; Wager and Layered Series, the sheet is tilted about 20° to and (2) fluid inclusions similarly rich in NaCl Brown, 1968, Fig. 101; Bird et al., 1986, Fig. 7). the southeast (Fig. 1), and the macrodike and had been found in intercumulus quartz and Wager and Deer believed that the transgressive many of the basaltic dikes are similarly tipped feldspars from MZ and from the Sandwich granophyre represented residual siliceous liquid in that direction (see White et al., 1989, Fig. 2). Horizon (Larsen and Brooks, 1994, p. 1670). that had been filter pressed from the more evolved Regionally, the older dike swarms are usually However, our contentions are: per (1), pegmati- Skaergaard mafic units, but Sr and Pb isotopic inclined more than the younger ones, showing tization provides a better explanation of the studies have shown that its parent liquid was that their tilting was progressive through the field relationships of the pegmatite and would melted from Archean gneiss (Hamilton, 1963; episodes of their emplacement (Nielsen, 1978; probably also yield its mineralogical character- Leeman and Dasch, 1978; Hirschmann, 1992). Brooks and Nielsen, 1982, Fig. 21). istics; and per (2), the apatite, biotite, and am- The contributing investigators all agreed that the Since the basaltic dike swarms are all younger phibole data gathered by Nash (1976) and heat for the melting came from Skaergaard, but than the Skaergaard intrusion, much of the tilting Brown and Peckett (1977) constitute a far more they expressed differing views about the extent to had to occur well after its solidification. How- complete sampling of the Layered Series, and which the siliceous melts might have mixed with ever, the relatively early emplacement of the Ba- are inherently more reliable indicators of chlo- mafic liquids (see Hirschmann, 1992). sistoppen sheet and Vandfaldsdalen macrodike rine and fluorine in the Skaergaard magma be- Our only additional comment about the gran- appear to be specifically tied to the southward cause the process we advocate could also lo- ophyre concerns the Tinden and Sydtoppen sills. monoclinal flexuring that is so prominent in the cally have left high-NaCl fluids in MZ and the On the basis of admittedly limited observations, sediment and basalt formations east and south of Sandwich Horizon—and in virtually any other we suggest that, rather than being two discrete the Skaergaard intrusion (see Fig. 1). A key ob- part of the intrusion. (separate) intrusions, they more likely represent servation here, due to Wager and Deer (1939, We do not see any compelling reason to think density-stratified layers of contrasting magmas p. 51–55), is that, in contrast to the sediments and that such fluids were critical to the general differ- emplaced as a single intrusion after derivation basalt, the Layered Series appears to be almost entiation of the intrusion. It is well established, for from different sources or by different degrees of unwarped from north to south. They assumed, example, that the Upper Border Series is richer in melting. The vagueness of the boundary be- however, that the intrusion had been similarly af- T SiO2,K2O, and BaO, and poorer in FeO than the tween the two granophyre types suggests this in- fected, and they attempted to reconstruct an orig- Layered Series (see Naslund, 1984; McBirney, terpretation, and the upward succession from a inal form for the Layered Series by graphically 1995, Fig. 4), and McBirney (1995, p. 421) as- denser, warmer, relatively mafic variant to a unbending it. Our impression, though, is that the cribed this to upward migration of volatile-rich more buoyant but cooler acidic one is appropri- Skaergaard intrusion did not flex much, but siliceous liquid into the top part of the intrusion. A ate for crystallization below and above a diffu- cracked open instead, thereby making room for simpler explanation, however, is that the Layered sive liquid-liquid interface (see Irvine et al., the sheet and macrodike. In support of this no- Series compacted under gravity as it accumulated, 1983, Fig. 2). tion, we note that Nielsen (1975) and Brooks and thereby eliminating intercumulus liquid enriched Nielsen (1982) showed that the flexuring was ac- in granophyric constituents and poor in FeOT, Associated Basaltic Intrusions tually effected by the southward rotations of nu- whereas the Upper Border Series could not com- merous north-south fault blocks on north-dipping pact and therefore retained most of its pore liquid. The Skaergaard intrusion (see Fig. 1) is antithetic fault planes. Three such faults have This possibility is consistent with the Hunter and breached on its east side by the Vandfaldsdalen been mapped within the Skaergaard intrusion Sparks (1987) view that Skaergaard intrusion dif- macrodike, which is a steep-walled layered (McBirney 1989b), and exploration drilling has

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shown that others underlie Forbindelses Glacier, propriate to the emplacement scheme in Figure 2. entire Skaergaard intrusion (and far beyond), be- which spans the eastern half of the intrusion These possibilities hinge strongly, however, on coming generally denser to the south. Most of (T. Nielsen, 1995, personal commun.). the relative ages of the two older dikes. As seen in them dip 80°–85° to the north, so they were prob- The three ages of basaltic dikes can be distin- Figure 5, the two converge and obviously must ably tipped to the south along with their host guished in the map in Figure 5 by their trends and cross, but unfortunately their intersection is not rocks. The swarm actually comprises dikes of intersections, and their compositions. Each of the exposed, so a direct age comparison is precluded. more than one age, because some of them cut two older ages are represented by only a single However, the troctolitic xenoliths in the Campsite others (see Fig. 5), but they are all broadly similar dike, but the youngest encompasses an extensive composite dike are characterized by large (1–2 in appearance, trend, and disposition, and our swarm. One of the two older dikes, numbered 1, cm) tabular plagioclase crystals heavily laden data are not good enough to separate them here. measuring 12–15 m in width, consists of a dis- with what once were obviously inclusions of con- Some of the young dikes cut the Basistoppen tinctive porphyry with prominently abundant tab- temporary liquid (these inclusions are now either sheet (Naslund, 1980, Fig. I-8), and several in- ular plagioclase phenocrysts trachytically aligned altered to a chloritic matte, or crystallized to py- truded the Vandfaldsdalen macrodike (White in a fine, dark gray, slightly alkaline basaltic ma- roxene and oxide minerals with reaction fringes et al., 1989, Fig. 3). trix (cf. Nicolas, 1992, Fig. 11). So far as we of relatively sodic plagioclase, but in melting ex- know, no other dike of its kind has been found in periments, the latter type yielded basaltic liquid as Hydrothermal and Isotopic Alteration the Skaergaard area. the fringes dissolved), and the large plagioclase The other early dike, labeled 2 in Figure 5, is a phenocrysts in the porphyry dike are virtually Although Skaergaard intrusion rocks are rela- north-trending, brown-weathering, composite in- identical, with similar abundances of erstwhile tively fresh by common petrographic standards, trusion of transitional tholeiite, 12–15 m wide, liquid inclusions. We therefore favor the less ap- they become increasingly altered to the east and extending for about 1200 m from Skaergaard pealing interpretation that the troctolitic xenoliths south; Taylor and Forester (1979) found that their Bay through UZb and UZa to Uttental Sound. probably represent a cumulate facies of the pla- oxygen isotopic compositions are increasingly Each of its sides is lined by a partly chilled mar- gioclase porphyry dike, rather than the Hidden modified in these directions owing to exchange ginal unit, 10–15 cm wide, and they are separated Zone. This interpretation is additionally reason- with hydrothermal fluids. Norton and Taylor by a sharply delimited, coarser central core. The able because the liquid inclusions would be more (1979) simulated the alteration effects by a com- dike is well exposed near a popular campsite in likely to survive in the hypabyssal dike environ- puter model that coupled the isotopic exchange UZa (Fig. 5, loc. A), so we call it the “Campsite ment than in the plutonic Hidden Zone. It implies, and mineral-fluid reactions with the fluid dynam- composite dike,” and it is especially interesting at however, that the Campsite composite dike is yet ics of the hydrothermal system and the thermal this locality because it visibly includes 35–40 another dike generation younger than the Skaer- history of the intrusion. In subsequent studies, xenoliths of gabbroic and anorthositic cumulates gaard intrusion (it is definitely younger than the Norton et al. (1984), Bird et al. (1986, 1988), that might have come from the Hidden Zone. transgressive acid granophyre, and probably Manning and Bird (1986, 1991), and Rogers and Again, so far as we know, it is the only dike in the younger also than the Vandfaldsdahlen macrodike Bird (1987) documented in detail the structural area carrying such inclusions. About 2 km farther and the Basistoppen sheet, although that is not and mineralogical characteristics of hydrother- north, across Uttental Sound on the shore of proven), and that it formed after the major mal veins and alteration as found along fractures Kraemer Island, there is a virtually identical, changes of stress direction recorded by the vari- in the intrusion and its host rocks. early brown-weathering, north-trending compos- ous strike directions of the dikes. The hydrothermal system evidently had a long ite basaltic dike, and because the two occurrences Some of the xenoliths in the Campsite compos- life, beginning while the Skaergaard intrusion are so similarly distinctive and approximately in ite dike other than the anorthosite and troctolite was still solidifying and extending through the line, we have joined them as a single dike in Fig- could still come from Skaergaard units, but by the times of emplacement of the youngest basaltic ure 1, and we suggest as well (although much reasoning just described, the possibility of a direct dikes. In the earliest stages, the rocks now seen as more tentatively) that it continues still farther genetic connection between the dike and Skaer- autoliths in the Layered Series were altered in north, back beneath Uttental Sound, to connect gaard magmatism seems remote. Indeed, two bet- oxygen isotopic composition before they broke with an early, brownish basaltic dike mapped in ter possibilities, in timing at least, are (1) the pla- loose from the Upper Border Series (Taylor and LZb on Uttental Plateau. Brooks and Nielsen gioclase porphyry dike arose from the Skaergaard Forester, 1979, p. 403); there was extensive peg- (1978) believed the Campsite segment belonged feeder system, or (2) it is kindred to the Vandfalds- matite formation in the Marginal Border Series to a regional north-trending post-Skaergaard dike dahlen macrodike and the Basistoppen Sheet. Per (and some in the Upper Border Series) and more swarm that they thought might be composition- (1), however, the porphyry appears slightly too al- local replacement of the Layered Series by peg- ally representative of the Skaergaard liquid, and kaline to represent Skaergaard liquid, and it is matite and secondary anorthosite. At later stages,

indeed, this segment is coincident with the map probably too poor in TiO2 to represent the parent fracture-controlled hydrothermal veining and al- trace inferred here for the intrusion feeder system liquid of the abundant Fe-Ti oxides in the upper teration occurred throughout the intrusion, but es- (cf. Fig. 2). Intriguing speculations, therefore, are half of the Layered Series; and per (2), the pecially along the dike swarms (Bird et al., that the dike is a late manifestation of the feeder macrodike and sheet respectively commence their 1986). As a population, the autoliths were un- fracture system, and that continued crustal exten- differentiation series with gabbroic and ultramafic doubtedly affected at all stages, but details cannot sion on this system, through the time when the rocks (see White et al, 1989; Douglas, 1964; always be resolved. Some autoliths underwent Layered Series was accumulating, served to dis- Naslund, 1989), and not with anorthosite or troc- minor replacement by anorthosite before they lodge many of the upper border autoliths that are tolite, as one might expect from the porphyry par- separated from their source zone; some were lo- now embedded in the Layered Series. The au- ent magma. cally replaced by small dikes and veins of peg- toliths are notably largest and most numerous The youngest dikes (numbered 3 in Fig. 5) matite, probably at various times; and some were along the exposure line of the dike, and the dike typically are greenish gray and commonly have cut and replaced by veins of Fe-Ti oxides and/or generally dips steeply to the west, as though it slightly alkaline compositions. They form a re- pyroxene after they were buried in the Layered had been tilted to the east, as would also be ap- gional swarm that trends east-northeast across the Series. A few examples of these effects are de-

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scribed below in the section Reaction and Re- presumably downward away from the contact, cordierite (Fig. 8). The geometry and massive- placement Effects. rather than from the bottom up as in the modally ness of the hercynite bands suggest that metaso- graded layers of the Layered Series. Apart from matism might have occurred before they became ROCK INCLUSIONS IN THE INTRUSION being slightly more delicate, the layers do not inclusions, perhaps through their association with seem appreciably different from their Layered the peridotite, given that it is compositionally Basalt Xenoliths in the Layered Series Series counterparts; we later show how such lay- rather extreme and probably formed at a consid- ers might have been deposited by crystal-sus- erably higher temperature than the Skaergaard Some 10–15 basalt xenoliths are exposed in pension currents flowing laterally across the top gabbro. We could not, however, establish any the Layered Series in the central part of MZ on of the magma body. strong evidence in support of this possibility. the south side of Kraemer Island, approximately If the block containing the basalt-gabbro con- Wager and his coworkers (e.g., Wager and 1.5 km in from the west side of the intrusion (see tact does indeed represent the original Skaer- Brown, 1968, p. 115) thought that the peridotite Fig. 1). Figure 6 is a map that shows two princi- gaard intrusion roof contact, then it is rather sig- blocks represented an early differentiate of the pal examples (the largest about 20 m across) plus nificant. By virtue of occurring well within the Skaergaard magma that had accumulated on several small fragments. Most of the xenoliths intrusion, intermixed with probable Upper Bor- the more gently dipping parts of the intrusion’s occur with large autoliths below the prominent der Series autoliths, it constitutes evidence (al- wall and then slumped inward, thus breaking macrorhythmic layering that underlies the Triple beit limited) that even the middle part of the roof into blocks. However, M. A. Kays discovered a Group, but a few are scattered within this layer- was basalt (as shown in Fig. 2) and not, for ex- small outcrop of petrographically similar perid- ing along the peninsula at the southeast corner of ample, a capping of brecciated gneiss as envis- otite isolated in the Precambrian gneiss on the Kraemer Island. Very few sizable autoliths are aged by Wager and Deer (1939). shore of Watkins Fjord immediately north of present above the basalt xenoliths; thus, the re- the Skaergaard intrusion (see Fig. 1), and Kays lease of the basalt appears to have marked the Inclusions in the Marginal and and McBirney (1982) correlated the Skaer- beginning of the end of the autolith stoping event Upper Border Series gaard peridotite inclusions with it. The correla- that is so prominent through LZ and MZ. tion is geologically and petrographically rea- The basalt xenoliths characteristically are an- In the Marginal Border Series along the north sonable and is generally accepted, but Kays and gular and have sharp contacts and distinctive red- side of the Skaergaard intrusion there is a layer or McBirney (1982, p. 29–30) claimed that the dish brown weathered surfaces, although a few zone, roughly 50–70 m thick, in which countless peridotite represents a source unrelated to the are more gray. The basalt is strongly hornfelsed subangular to angular blocks and fragments of Skaergaard intrusion, and that is less certain. and locally features numerous irregular, quasipeg- brown-weathering ultramafic rock are set in a ma- Their reasons were (1) the Watkins Fjord perid- matitic veins of recrystallization, but in places (as trix of olivine gabbro. The blocks are generally 10 otite is structurally below the Skaergaard intru- was first noted by Wager and Deer, 1939, p. 199), cm to 3 m in size and typically make up 35%–50% sion and appears to be a separate body, and it is still conspicuously amygdaloidal. In thin sec- of the layer (e.g., Wager and Brown, 1968, Fig. 82; (2) the olivine in both it and the Skaergaard in- tions, the least modified basalt consists of meta- McBirney, 1996a, Fig. 9A). The ultramafic rock trusion peridotite blocks is much more magne- morphic assemblages of plagioclase, augite, and was originally identified as gabbro picrite or pic- sian than the earliest cumulus olivine in LZa

minor Fe-Ti oxide, plus either olivine or hyper- rite, but where it is least altered, it is essentially a (Fo78–81 vs. Fo68), and bridging of this gap would sthene; some samples have patchy aggregates of cumulate of olivine plus minor chromite, with sub- require fractionation of a large volume of dense fine plagioclase grains formed as pseudomorphs ordinate augite and plagioclase and only minor rocks that are not evident in the intrusion’s grav- after large phenocrysts. McBirney (1979, Fig. 10- postcumulus bronzite, and biotite, and a more ap- ity model. Although these arguments are reason- 7) obtained chemical and oxygen isotopic analy- propriate name is gabbroic peridotite (or gabbroic able, they are not incontrovertible. The apparent ses for a series of samples across the edge of one wehrlite). A subordinate variant has a small pro- HZ xenoliths in the Campsite composite dike of the xenoliths, and the data suggest some zonal portion of what appears to be fine-grained cumu- contain olivine that partly fills the composition interaction with the adjoining cumulates, but ex- lus plagioclase, and it might be called troctolitic gap (Table 1); as we noted above, the gravity tending inward for only about 0.5 m. peridotite. Some of the larger blocks have feld- model has large uncertainties. In the area shown in Figure 6, the basalt xeno- spathic marginal zones, apparently because of re- The age of the Watkins Fjord peridotite is also liths are clustered with several relatively large action with the gabbro, and many of the smaller crucial in this problem. Kays and McBirney blocks of coarse oxide gabbro, plus some small- fragments are similarly altered. (1982) did not discuss it, but their Introduction to medium-size blocks of a finer microgabbro; A few gneiss xenoliths are present below the suggests that they viewed it as being Precambrian, one block contains a contact between gabbro and peridotite block layer in the north edge of the in- and Kays et al. (1989, Fig. 1) subsequently basalt that may originally have been part of the trusion, and occasional xenoliths of hornfelsed grouped the peridotite with Archean ultramafic Skaergaard roof contact (Fig. 6, location 9). The sediments, featuring prominent cordierite and bodies in a geological map of the gneiss terrain. contact is partly obscured by a small mafic dike, minor hercynite, are scattered within it. Most of The Watkins Fjord peridotite is, however, extraor- but what can be seen is sharp to within a few the sediment xenoliths are only 3–5 m in length, dinarily fresh and undeformed, whereas the centimeters, and although the gabbro is not truly but one is more that 30 m long (Wager and Deer, Archean bodies have rinds and haloes of meta- chilled, it is relatively fine grained. Starting from 1939, p. 31), and it locally features prominent morphic hornblende, and are highly deformed about 1 m below the contact and proceeding metasomatic banding. A similar population of and foliated with their olivine extensively altered downward, the gabbro shows several modally peridotite and sediment inclusions occupies a to serpentine (see Kays et al., 1989, p. 337–341, graded layers, each 4–8 cm thick, spaced at in- small alcove in the west margin on the intrusion 350). Such petrographic differences are not proof tervals increasing from 20 to 40 cm (Fig. 7). The on Kraemer Island, and here the sediments are of an age difference, but they cast doubt on a Pre- layer grading is weak, but appears to be normal largely metasomatized to dense, black magnetic cambrian assignment for the Watkins Fjord body. from mafic to felsic upward, even though the di- oxide layers, 5–15 cm thick, composed largely of By contrast, there are several small pluglike Ter- rection of crystallization or accumulation was hercynite, subordinate magnetite and minor tiary ultramafic bodies along Kangerdlugssuaq

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/11/1398/3382835/i0016-7606-110-11-1398.pdf by guest on 27 September 2021 1, 3, 4, “microgab- those near 2 and at 8 are composed of coarse oxide gabbro; and 5 are bro.” and basalt is contact between microgabbro What might be part of an original roof contained in the block at 9, the contact, and just below well-de- shows the microgabbro 7). (see Fig. layering rhythmic fined N, ′ W). The outcrop surface is generally inclined to the south,The outcrop W). in the same direction ′ Figure 6. Map of basalt xenoliths (at locations 6,Figure 7, and 9) among a cluster of gabbroic as the layering, at Autoliths topographic irregularities. reflect traces strongly hence layer autoliths in MZ (Middle Zone) at the south end of Kraemer Island (~68°10.7 31°45.3

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crystallized sufficiently rapidly for them to be frozen to the roof contact before they could set- tle away. There is, however, an alternative for both xenolith types. In the emplacement scheme of Figure 2, the sediment-gneiss unconformity with the peridotite body was initially situated di- rectly opposite the top of the intrusion along the now eroded west side of the intrusion; thus, rather than being carried upward for more than 500 m by the magma, the blocks of both rock types would need only have been moved a short distance laterally.

Autoliths in the Layered Series

The autoliths in the Skaergaard Layered Series (see Figs. 9–23) were mentioned only briefly in the original description of the intrusion by Wager and Deer (1939, p. 42 and 200, and Pl. 10, Fig. 1), but their presence was emphasized by Wager and Brown (1968), who illustrated the general distri- bution of the larger blocks and recognized that Figure 7. Photograph of thin, modally graded layers in microgabbro, just below its contact they probably came from the Upper Border Series. with basalt, in the block at locality 9 in Figure 6. Taylor and Forester (1979, p. 403) determined oxygen isotopic compositions of the plagioclase and pyroxene in some of the blocks, and Irvine with which the peridotite might be correlated— began to open, and then disrupted as more liquid (1980d, 1983a, 1987) further illustrated their field most notably the East Kaerven complex, dated as was added. We cannot make a definite choice characteristics. Some of the observations from 55 ± 2 Ma (see Brooks and Nielsen, 1982, p. 12), here, but it seems significant that the opening of these works are necessarily repeated here, but gen- and the Kartografvig and Kaelvegletscher com- the Skaergaard chamber was almost certainly a erally with more documentation. To facilitate our plexes (Kempe et al., 1970). According to P. M. lengthy event. By the kinematics illustrated in descriptions, we categorize the autoliths by their Holm (1993, personal commun.) and S. Bernstein Figure 2, the opening would entail some 3 km of maximum size dimension: “fragments” are small- (1995, personal commun.), the peridotite shares displacement on the East and South Wall faults, er than about 0.5 m; “small blocks” are 0.5–3 m; some of their isotopic and geochemical traits. an adjustment that could have spanned the forma- “medium-size blocks” are 3–30 m; “large blocks” The best available indication of the age of the tion of an early cumulate unit as well as cause its are 30–200 m; and “exceptionally large blocks” peridotite blocks may be their intermixing with breakup. In magma calculations, we have found are 200–500 m. Cretaceous–Paleocene sediment xenoliths (e.g., it necessary to assume the intrusion originally The autoliths are spread stratigraphically from Fig. 8). This association suggests strongly that embodied peridotite like that in the included the top part of LZa to the middle of UZb, and they their source regions were spatially close, and a blocks in order to balance its bulk composition are especially abundant in the middle of LZb on plausible scenario is that the peridotite came against those of its chilled margins in terms of Cr the Uttental Plateau, and in LZc and the lower half from a cumulate unit situated on or near the un- and Ni. This is only a numerical exercise, but it of MZ on Kraemer Island and within Wager Peak conformity separating the sediment formation shows that the idea of an early peridotite differ- (Figs. 1 and 3). As seen in Figure 1, they tend to be from the gneiss beneath it. Such an arrangement entiate may be justified on chemical grounds. concentrated in rudely stratigraphic zones, typi- is illustrated schematically in our emplacement The rock inclusions in the Upper Border Se- cally 50–100 m thick and up to 1 km in length. model in Figure 2; in the model, the Watkins ries are also pertinent to this problem. It contains Three exceptionally large blocks have been identi- Fjord peridotite is practically at the intersection a major gneiss xenolith, about 500 m across, fied, and we suspect the presence of one or two of the East and West Wall faults, which are the plus numerous smaller fragments (Wager and more, but they have not been mapped sufficiently breaks on which the intrusion chamber is be- Deer, 1939), and according to Naslund (1984, well to be confirmed. The stratigraphically upper- lieved to have opened (see Fig. 1). Within this p. 191), its Tranquil Zone, τ, also includes a few most large block is an anorthositic slab about 50 m framework, the peridotite could have either of fragments of peridotite. The large gneiss xeno- long in UZa, but fragments can be seen for another two origins: (1) it could have differentiated from lith is prominently exposed in a steep mountain 200–300 m into UZb. The cross-bedded belt rarely a Tertiary magma slightly older than the Skaer- face, lodged next to a gabbro sill in the intru- contains inclusions of any kind, and the few we gaard intrusion that had risen to the unconform- sion’s roof, and it would appear to have risen to have seen within it are all small and situated to- ity along essentially the same feeder system (in this location in the Skaergaard magma from ward its inner side (e.g., Irvine, 1987, Fig. 36). which case, the hercynite recorded in Fig. 8 more than 500 m below (Wager and Deer, 1939 Some of the cross-bed truncations are very sharp, might represent metasomatism associated with p. 34). Whereas the gneiss is sufficiently buoyant but we know of no evidence that they were ever the early magma); or (2) it might represent a first to have floated up, the peridotite fragments are stoping surfaces from which autoliths originated. Skaergaard cumulate differentiate, as postulated not, so for them to rise from a source beneath The autoliths have various shapes—many are by Wager and his coworkers, precipitated next to the basalt would require flow transport by the angular, a few are vaguely rounded, others are ir- the sediment formation as the magma chamber magma, which presumably then cooled and regular and have peculiar reentrants (see Figs.

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Figure 8. Map of a part of the Skaergaard Marginal Border Series (~68°10.9′N, 31°46.6′W), showing blocks and fragments of peridotite, and one block of basalt, together with xenoliths of Kangerdlugssuaq sediments that are now extensively metasomatized to rock rich in hercynite. The locality is about 30 m in from the west edge of the intrusion, near the south side of Kraemer Island. The hercynite is largely contained in three dis- continuous massive layers, up to 15 cm thick, intercalated with peridotite fragments. The gabbro around the blocks is more leucocratic and finer grained than the neighboring pegmatitic marginal gabbro, presumably because of reaction effects, and it contains local zones of perpendicular feldspar rock (see Wager and Brown, 1968, Fig. 77).

9–23). Some are equant, but many more are slab- appears to have been “mushy.” Inclusions smaller against the surrounding gabbroic cumulates, and like. The angular blocks were well solidified when than about 30 cm on a side are often leucocratic in the field we generally typed them informally they broke loose, and their edges are usually sharp and diffuse, and in the extreme, there can be un- as gabbroic anorthosite, anorthositic gabbro, and to within a few grain diameters. A notable excep- certainty as to whether they are actually fragments, leucogabbro. Subsequent petrographic work has tion is the large anorthositic slab in UZa. Its upper or whether the anorthosite formed by some other shown, however, that their mafic mineral con- side is a cleanly broken surface with angular cor- process (such as replacement; see the following tents are more variable in both abundance and as- ners, but its lower side has irregular protuberances discussion). semblage than we had realized, and are important of mixed anorthositic and gabbroic material and The autoliths commonly appear leucocratic to their classification. We still occasionally use

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“anorthositic” and “leucogabbroic” as descrip- equigranular rock—perhaps a cumulate—com- would seem to be an Upper Border equivalent of tors because our thin-section sampling covers posed mostly of plagioclase, augite, and olivine; HZ; (2) the gabbroic anorthosite matches best α only a small fraction of all the many blocks that and (3) gabbroic anorthosite, an aggregate of with LZb*, 1, and LZb in terms of plagioclase are exposed, but we now formally recognize four fine- to medium-grained plagioclase crystals set compositions, but it is poorer in mafic minerals; main rock types (plus minor variants), specifi- in large poikilitic grains of augite. Also present and (3) the oxide gabbro correlates best—al- cally, gabbroic troctolite, gabbroic anorthosite, are a few small, brown-weathering slabs that though by no means perfectly—with LZc* and α oxide gabbro, and microgabbro. The troctolite could not be identified with certainty in the field MZ*, or with 3 and LZc. These correlations are typically is a cumulate of plagioclase and subor- but that appeared to be pieces of either hornfelsed reasonable in that, in terms of stratigraphic distri- dinate olivine. It can be distinctly anorthositic in sediment or fine-grained basaltic rock. Those ex- bution, no autolith rock type appears before its appearance, and locally the olivine is poikilitic, amined in thin section appear to be fine-grained Layered Series counterpart. but more often there is a significant proportion of dike rock, and some show rosettes of plagioclase subpoikilitic postcumulus augite, hence the “gab- and comblike growths of olivine such as occa- BLOCK-AND-LAYERING broic” prefix. The anorthosite most commonly sionally occur in chilled margins, so they are RELATIONSHIPS appears as a plagioclase cumulate, with sub- probably cognate pieces of the early marginal poikilitic augite again imparting a gabbroic tenor, zones of the composite dike. but some blocks resemble plagioclase adcumu- Types of Skaergaard Intrusion Layering lates. There is always a possibility that the Compositional Correlations with the Main anorthosite formed by replacement, but the Intrusion Divisions Wager and Deer (1939) and Wager and Brown blocks are not usually large enough to apply the (1968) called the visible layering at the Skaer- field criteria by which this origin can be identi- Possible correlations of the Skaergaard intru- gaard intrusion “rhythmic layering” because of its fied. The replacement probably occurred in the sion lithostructural divisions with the xenoliths in prominently repetitive nature; we now also distin- Upper Border Series, because as Taylor and the Campsite composite dike and the autoliths in guish “macrorhythmic” and “microrhythmic” Forester (1979, p. 403) found in about six exam- the Layered Series can be examined in a prelimi- layering, depending on the kinds and thicknesses ples, the plagioclase in the blocks is lighter in nary way through Table 1. The left side of Table 1 of the defining strata (see Fig. 4; Irvine, 1987, oxygen isotopic composition than that in the ad- lists the common compositions of olivine, Ca-rich Figs. 21–23). The traditional rhythmic variety is joining cumulates; they claimed that this shows pyroxene, and plagioclase in the main lithostruc- characterized by thin modally graded layers, 5–30 that the blocks were hydrothermally altered be- tural divisions; the right side gives comparative cm thick, featuring “normal” grading by density, fore they became inclusions. The oxide gabbro data for the xenoliths and autoliths at places where mafic minerals being concentrated at the base and qualifies as a cumulate of plagioclase, augite, and correlations are indicated or expected. Correla- plagioclase at the top. These layers can occur suc- Fe-Ti oxide + minor olivine, but it is coarser than tions presumably are stronger if mineral assem- cessively, but more commonly they alternate with the cumulate oxide gabbros of LZc and MZ. The blages and modal proportions also match, but lim- compositionally “uniform” (isomodal) strata (Fig. microgabbro is a relatively fine, plagioclase- its are not clearly defined, and because mineral 4A). The latter may be anywhere from a few cen- augite rock that has a weakly cumulate texture. compositions often reflect complicated reactions timeters to several meters in thickness at any Stratigraphically, the gabbroic troctolite blocks with liquid and other minerals, all matching is given place, but broadly speaking, they are rela- appear in LZa and are found at all levels through necessarily imprecise. tively less prominent in LZ, and they are thinner to UZb. Gabbroic anorthosite blocks are present The plagioclase and olivine of the gabbroic in MZ than in UZa. Many of the modally graded in LZb and are most common at midlevels in MZ, troctolite xenoliths have higher temperature layers can be traced for tens of meters, but de- but they are a minority. Oxide gabbro blocks do compositions (respectively more calcic and tailed mapping of selected occurrences suggests not appear until the base of MZ and occur only in more magnesian) than any of the lowest exposed that they nowhere extend for more than about 300 its lower two-thirds. They had not been recog- cumulates of the Layered Series or the earliest m, even where they characterize sections of layer- nized prior to this study, probably because they units of the Marginal and Upper Border Series. ing that are continuous and regular for as least have only limited stratigraphic distribution, but Thus, because plagioclase-olivine cumulate is twice that far. The uniform layers are not easy to perhaps also because their coarseness makes them the modal precipitate of choice for HZ (on the follow for long distances, but by virtue of being seem more feldspathic than they are. The only oc- basis both of the natural crystallization charac- host layers, they are inherently more extensive. currences of microgabbro that we know are with teristics of the intrusion and of melting experi- The macrorhythmic layering, which Wager the basalt xenoliths illustrated in Figure 6. ments on its chilled margin samples), the trocto- and Brown (1968, p. 74) called “zebra banding,” lite could represent its main rock type. The is defined by relatively thick (1–5 m), broadly Hidden Zone Xenoliths in the Campsite fine-grained olivine gabbro xenoliths have dis- uniform layers that are alternately slightly leuco- Composite Basaltic Dike tinctly higher temperature mineralogy than the cratic and slightly melanocratic (Fig. 4C; Irvine, olivine gabbro of LZb, but they are mineralogi- 1980d, 1982, 1987; Naslund et al., 1991). These The xenoliths in the Campsite composite cally similar to TZ* olivine microgabbro, so they strata, which we refer to as “macrolayers,” typi- basaltic dike range in size from a few centimeters may be HZ counterparts to it. The few data avail- cally appear prominent from a distance, and in to 3 m on a side, and they are congregated in the able for the gabbroic anorthosite xenoliths indi- some sections they can be traced for several kilo- younger central zone of the dike, near the end of cate that they have LZa and LZb mineral com- meters. However, the light and dark layers gener- one of its offset segments. Thin-section study has positions. ally have the same mineral assemblages and their revealed three main plutonic types: (1) gabbroic Notable mineralogical features among the au- contacts tend to be diffuse, so where their modal troctolite, a well-laminated apparent cumulate of toliths are as follows: (1) although the gabbroic differences are subtle (or the outcrops dirty), they coarse plagioclase laths and subhedral olivine troctolite partly overlaps the gabbroic troctolites can be difficult to distinguish. We have consid- α grains cemented by subpoikilitic postcumulus of LZa*, 1, and LZa, it generally matches better ered the possibility of grouping the light and dark augite; (2) olivine gabbro, a moderately fine, with TZ*, τ, and the inferred HZ range, so it layers in pairs, but their bottom and top contacts

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TABLE 1. POSSIBLE CORRELATIONS OF THE XENOLITHS IN THE CAMPSITE COMPOSITE BASALTIC DIKE AND THE AUTOLITHS IN THE LAYERED SERIES WITH THE MAIN SKAERGAARD LITHOSTRUCTURAL DIVISIONS Intrusion unit, rock type Mineral compositions† Inclusion, rock type Mineral compositions† Olivine Augite Plagioclase§ Olivine Augite Plagioclase§ 100Mg/(Mg+Fe) 100An/(An+Ab) 100Mg/(Mg+Fe) 100An/(An+Ab) Marginal Border Series Suggested correlations TZ* Olivine microgabbro 78–57 74–60 76–65 Xenolith Olivine gabbro 75 78–74 74–65 LZa* Gabbroic troctolite 62–55 74–68 66–60 Autolith Gabbroic troctolite 60–41 78–66 74–62 LZb* Olivine gabbro 56–50 70–65 63–55 Autolith Gabbroic anorthosite 74–42 62–58 LZc* Oxide-olivine gabbro 55–42 65–60 57–54 Autolith Oxide gabbro 49 69–60 59–50 MZ* Oxide ferrogabbro 60–52 56–53 Autolith Oxide gabbro 69–60 59–50 UZa* Oxide olivine ferrodiorite 38–30 57–48 54–50 UZb* Ap-ox-ol ferrodiorite 30–3 49–16 50–38 Upper Border Series τ Olivine microgabbro 67–60 77–65 72–62 Autolith Gabbroic troctolite 60–41 78–66 74–62 α1 Gabbroic troctolite 60–37 70–60 62–59 Autolith Gabbroic troctolite 60–41 78–66 74–62 α1 Gabbroic troctolite 60–37 70–60 62–59 Autolith Gabbroic anorthosite 74–42 62–58 α3 Oxide-olivine gabbro 60–37 60–54 59–56 Autolith Oxide gabbro 49 69–60 59–50 β Oxide ferrogabbro 40–25 56–40 γ2 Ap-ox-ol ferrodiorite 25–0.2 25–12 44–36 γ3 Ap-ox ferrobustadiorite 25–0.2 5–0.2 36–28 Layered Series UZc Ap-ox-ol-fbus ferrodiorite 2–0.5 5–1 34–32 UZb Ap-ox-ol ferrodiorite 32–5 50–8 41–34 UZa Ox-olivine ferrodiorite 40–31 57–50 46–40 MZ Ox-pigeonite ferrogabbro 64–55 52–46 LZc Oxide-olivine gabbro 55–45 70–64 52–52 Autolith Oxide gabbro 49 69–60 59–50 LZb Olivine gabbro 59–55 76–65 62–55 Xenolith Gabbroic anorthosite 71 72 LZa Gabbroic troctolite 65–55 80–76 67–59 Autolith Gabbroic troctolite 60–41 78–66 74–62 HZ Olivine gabbro(?) >65 >80 >67 Xenolith Olivine gabbro 75 78–74 74–65 HZ Troctolite >65 >80 >67 Autolith Gabbroic troctolite 60–41 78–66 74–62 HZ Troctolite >65 >80 >67 Xenolith Gabbroic troctolite 79–78 82–80 82–79 Notes: Abbreviations: ol—olivine; ox—oxide (magnetite ± ilmenite); ap—apatite; fbus—ferrobustamite; An—anorthite; Ab—albite. Series data are compiled from Wager and Brown (1968), Naslund (1976), Hoover (1978, 1989), McBirney(1989), and C. Tegner (1995, personal commun.). Autolith and xenolith data are from this study. †Atom or mol percent ratios. §Cumulus crystal compositions for the Layered Series; crystal core compositions for the other units and the inclusions.

do not differ in any regular way, and their internal abundant, one at the top of LZb, the other in the graded layers, generally 10–20 m wide and grading is rarely strong or systematic, so there is upper half of MZ. The MZ interval is just above 30–100 m high. They mostly occur in a 150 m not a clear basis for choosing lower and upper the largest autoliths in the intrusion and features a interval at the top of UZa (a few extend slightly members for such pairing. The layers commonly magnificent sequence of about 70 combinations of into UZb), where about 24 are spread laterally show internal stratification, however, and many light and dark layers, totaling about 200 m in over a distance of 3 km, alternating with ridge- (especially the darker ones) contain thin, modally thickness, topped off by three especially promi- like units of relatively massive rock (see Wager graded layers of more limited extent (see Fig. nent units known as the Triple Group (Wager and and Deer, 1939, Fig. 11; Wager and Brown, 4C). The “uniform” layers that rhythmically al- Brown, 1968, Pl. IV). Each Triple Group unit has 1968, Fig. 89; Irvine and Stoeser, 1978; Irvine, ternate with the thin, modally graded layers prob- a lower leucocratic layer and an upper melano- 1980d, 1983b; 1987, Figs. 38–40.). Such promi- ably also qualify as macrolayers, or at the least, cratic one, the pair totaling about 15 m in thick- nent troughs are not found elsewhere in the Lay- parts of them. ness. The two lower units are successive, but the ered Series, but more subtle occurrences are The microrhythmic layering has only minor upper one is separate, 40–50 m higher above a sec- widespread; and we have come gradually to re- occurrence. It is defined by layers or laminae a tion of somewhat vague macrorhythmic layering. alize that most modally graded layers occupy (or few centimeters thick (Fig. 4D), and it bears Mining exploration work has shown that the two define) trough or basinform structures, a feature some resemblance to the classic “inch-scale lay- lower units are accompanied by several thin (typi- that is not immediately obvious because the ering” of the Stillwater Complex (Hess, 1960, cally 1–3 m) zones rich in palladium and gold that structures are shallow. The clearest sign of a Pl. 8). The examples we have seen are scattered are scientifically important as well as economi- “low-amplitude” trough is that the modally de- from LZc to UZa, and there is no obvious con- cally intriguing (Bird et al., 1991). The exploration fined layers tend to end in bunches at its edges, trol on their occurrence. Some are located in drilling also revealed that the Triple Group is con- occasionally with slight upturning of their tips. troughs, some on antiforms, and some where tinuous at depth through much of the southern half Graded layers are confined to troughs and the layering is broadly planar. of the intrusion. A few autoliths are present within basins, whereas macrolayers are not. Some The autoliths can occur with either modally the Triple Group interval (one is prominently macrolayers even contain small trough struc- graded layers or macrolayers, but neither affilia- lodged between the first and second units along a tures, and a leucocratic macrolayer extends tion is universal. We have not seen them in contact west spur of Wager Peak), but they are much less through several of the main troughs. None of the with microrhythmic layering. The association with frequent here than in the lower two-thirds of MZ. main troughs is obviously related genetically to the graded layers is usually the closer, but A distinctive, much photographed Skaergaard the autoliths, but we know of three places where macrorhythmic layering is best developed in two structure is trough layering. The classic Skaer- they are closely associated, and all three show stratigraphic intervals that immediately succeed gaard trough structures (Wager and Deer, 1939, hints of ties (see section Block Relations to the zones in which autoliths are especially large and p. 45–50) are synformal stacks of modally Trough Layering Structures).

1416 Geological Society of America Bulletin, November 1998

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Concepts and Controversy of ing roller convection cells, whereby the ridges erals in response to physical flow of partially crys- Layer Formation between the troughs grew by continuous in situ tallized magma (McBirney and Nicolas, 1997, crystallization from the descending, supercooled p. 569–571). The nondynamic variants are inter- Wager and Deer (1939, p. 262–277) believed liquid in the rollers, and the modally defined preted as follows (cf. Boudreau and McBirney, the Skaergaard Layered Series to be the product trough layers were concurrently deposited by re- 1997, p. 1004). (1) The macrorhythmic layering of gravitational sorting of crystals settling from peated density currents flowing down the gullies and most of the layering in the Marginal and Up- their parental magma under the influence of con- between them. per Border Series are attributed to transitory com- vection currents that descended from the walls of The fundamental contention behind these con- positional excursions of the magmatic liquid the intrusion and then swept inward across the cu- siderations was that the field evidence of currents across cotectic boundaries, these excursions vari- mulate floor. Wager (1963) and Wager and Brown among the Skaergaard intrusion layers is so com- ously occurring in response to convection, inva- (1968, p. 210–220) further postulated that the pelling, and currents are so appropriate to the sions of new magma, country-rock contamination, thin, modally graded layers were deposited by pe- overall geology of the intrusion, that the plagio- gains and losses of volatiles, or any other such fac- riodic rapid, crystal suspension (surge-type den- clase flotation problem has to be resolved on tors as might affect the liquid’s composition, tem- sity) currents similar to sediment turbidity cur- those terms. Possible rationalizations of the flota- perature, or oxygen fugacity. (2) The microrhyth- rents in aqueous environments, and that the tion problem were the following. (1) Because the mic layering is ascribed to accentuation of “small intervening uniform layers were laid down by high-density liquid compositions derive from cal- initial differences” (weak primary layering) by a broader, more gentle, continuous convection cur- culations and experiments that are rife with uncer- process of solution, reprecipitation, and coars- rents. Wager and Deer also suggested that the tainties, they may not be completely realistic, so ening similar to Ostwald ripening. (3) The thin, Triple Group units each recorded a half overturn through at least some stages of the intrusion’s modally graded layers are said to have similarly of almost the entire magma body. solidification, the feldspar may well have been developed by solution and reprecipitation of min- Despite the splendid array of field observa- denser than the liquid. (2) While being transported erals in partially crystalline material in response to tions on which these concepts were based, in the magmatic currents, the plagioclase crystals the stresses of compaction and the associated flow McBirney and Noyes (1979) rejected them on were so entangled with denser mafic minerals that of the displaced interstitial liquid. Boudreau and the grounds that the magma compositions esti- their buoyancy was not a dominant factor. (3) The McBirney (1997, p. 1008–1010) also reexamined mated for the intrusion required liquids so dense mechanics of density currents are such that the Coats (1936) observation that crude layering that the plagioclase crystals in the Layered Se- graded layers are formed by crystal deposition, can be produced by the settling of crystals of ries should have floated (Bottinga and Weill, not just crystal settling, and plagioclase buoyancy markedly different densities in an otherwise stag- 1970; Murase and McBirney, 1973). As alterna- only became a factor in the final sorting of the nant dense suspension, and they advocated this tives, they postulated that the rock layers formed graded layers. And (4) once deposited, the plagio- process as an origin for the weak layering that in situ, either by crystallization from a density- clase crystals could not float away because (a) the could then be enhanced by recrystallization during stratified liquid undergoing double-diffusive intercumulus liquid began to crystallize as soon as compaction. (They did not, however, consider the convection—an idea subsequently abandoned the minerals were deposited (e.g., in response to possibility that the sorting might be sufficiently en- by McBirney (1985)—or by a process of miner- diffusive loss of water, if not to actual precooling), hanced by current flow to produce the observed alogical and textural differentiation and reorgan- and this cemented them in place; (b) the yield layering directly.) ization in a stagnant zone composing the top- strength of the pore liquid trapped between the We would not reject all these possibilities— most few meters of the Layered Series while it minerals increased rapidly as it stagnated (as because they encompass almost every conceiv- was accumulating (McBirney and Noyes, 1979, claimed by Murase and McBirney, 1973, from ex- able origin for layering except magmatic sedi- Fig. 7). Their model for the latter process was the perimental measurements) and prevented buoyant mentation—but we find the following. growth of Liesegang bands (McBirney and crystals from rising; or (c) the plagioclase-en- 1. The two-fold classification is too limited Noyes, 1979, Fig. 10, and Pl. 11B), which they riched layer tops were immediately buried be- and subjective to be of value. thought were due to oscillatory nucleation and neath layers of denser cumulates. 2. The discussion of magmatic deformation crystallization, but Boudreau and McBirney McBirney (1995, 1996a) continued to oppose (McBirney and Nicolas, 1997, p. 570–573) is (1997, p. 1004) have also abandoned this model. magmatic sedimentation, however, and McBirney based on mineral lineations (Nicolas, 1992) of Irvine (1978, 1980a, 1980d; 1983a, 1983b; and Nicolas (1997, p. 570) and Boudreau and uncertain origin (see Layer Streamlining and 1987), by contrast, reemphasized that the Skaer- McBirney (1997) rejected the layering terminol- Other Evidence of Current Flow), and it gives no gaard intrusion modally graded layers have nu- ogy reviewed here and proposed instead a two- insight into how deformation might produce merous structures indicative of current sedimen- fold genetic classification whereby Skaergaard in- cross-bedded layering. tation, and on that basis, he investigated the flow trusion layers are categorized as either “dynamic” 3. The descriptions given for alleged nondy- structure of experimental surge-type density cur- or “non-dynamic.” It is difficult to be sure where namic layering tend to be misleading, and some rents, scaled them to magmatic conditions, and our layering types fall in these categories, or how are incorrect. For examples, Boudreau and showed how they might deposit modally graded each is thought to have formed, but broadly speak- McBirney (1997, p. 1004) said that the modally cumulate layers. He suggested that such currents ing, it seems that almost the only layering that is graded layers bear only “superficial resemblance” might have been generated in the Skaergaard viewed as dynamic is that in the cross-bedded belt. to sedimentary deposits (by contrast, see our doc- chamber by the slumping of unstable crystal piles The layering of the Marginal and Upper Border umentation below); they claimed (p. 1006) that the from places now recorded by unconformities, es- Series, the macrorhythmic layering (including the trough structures do not show cross-bedding, pecially in the cross-bedded belt, but also at nu- Triple Group), the microrhythmic layering, and scour-and-fill, or lateral channel migration (by merous other places in the Layered Series (but the thin, modally graded layers appear to be typed contrast, see Irvine, 1978, Fig. 53, 1982, Figs. 18 not in the Marginal Border Series, as others have as nondynamic (Boudreau and McBirney, 1997, and 19); and they state (p. 1004) that modally intimated). Irvine proposed that the trough layer- p. 1003–1004). The cross-bedded belt is attributed graded layers suddenly disappear above the trough ing structures were formed by a process involv- to mechanical separation and reorientation of min- structures near the base of UZb (whereas such lay-

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ers occur in significant numbers for about another structures that we consider to be primary, but generalization is that equant and irregular blocks 200 m, up to the middle of UZb; Wager and many more can be seen at virtually all levels of have usually caused more deformation than slab- Brown, 1968, p. 225; and our own observations). the Layered Series, from LZa to the lower part of like ones. Fluid dynamic experiments show that 4. The one-dimensional mathematical model UZb. We also illustrate some examples of layer nonequant blocks should tend to settle in hori- devised by Boudreau to simulate layer formation replacement, but such occurrences are rare by zontal orientations (especially if the Reynolds by recrystallization and coarsening is theoretically comparison. number of the flow regime is beyond the Stokes elegant, but it is unsupported by any independent In the most common structures related to range, as it probably was for the larger Skaer- evidence. If all the Skaergaard modally graded lay- blocks, layers are deformed beneath them, and gaard autoliths; see Irvine, 1974, p. 150–152), ers were formed by recrystallization, as the model draped and smoothed above them. Some blocks and it seems likely that a slab landing flat would assumes, then some of them should have yielded appear simply to have dropped or flopped into cause less disturbance than a more irregular clear field support—but none is described. place, because the underlying strata are only block with an angular bottom configuration. The descriptions of block-and-layering to fol- slightly depressed (Figs. 9, A and B; 10A; 12A). low provide further evidence that the Skaergaard Others have knifed into layers and cut them off Layer Streamlining thin, modally graded layers were deposited by (Fig. 11B), and many blocks appear almost to have and Other Evidence of Current Flow magmatic currents, and they give some additional been driven into the floor, because they have crum- insight on how such currents might have origi- pled, overturned, and torn up layers as they evi- If the layers adjoining a block are relatively nated. This is not to say, however, that we think all dently skidded to a stop (Figs. 12, B and C; 13B; thin compared to its height, they usually lap up Skaergaard layers formed the same way. Macro- 14, C and D; Wager and Brown, 1968, Fig. 46). on its sides and drape across it, thinning above it layers show some signs of current deposition Some of the very large slabs shattered on landing, and thickening off its flanks, with the effect that (Irvine, 1983a, 1987, Fig. 23), but on the whole and their fractures were both intruded from be- the layering plane irregularities associated with they appear much too thick and extensive to be neath by dikes derived from the underlying cumu- the block are quickly smoothed over (Figs. 9, sorted and deposited by surge-type currents. It is lates, and filled from above by later cumulates A–F; 10A; 11B; 13B). Such “streamlining” of doubtful too that the macrolayer compositional (Wager and Brown, 1968, Fig. 47). Indeed, the layering is strongly suggestive of current sed- relations could be produced simply by mineral through the Layered Series as a whole, deforma- imentation, and the phenomenon is similarly sorting, especially as they are developed in the tion structures of these kinds are so numerous that, common around the blocks in the Duke Island Triple Group. Even though we have been dis- in our view, they constitute virtually incontrovert- Ultramafic Complex in southeastern Alaska, posed since the early 1980s to believe that the ible evidence that: (1) there was almost always a where there is a variety of other evidence of layer macrolayers formed by some process of in situ sharp, clean interface between the top of the cu- deposition by magmatic currents (Irvine, 1974, crystallization (cf. Irvine, 1983a, p. 289), our ob- mulate pile and the main body of overlying liquid; 1987). At Skaergaard, the uplapping and draping servation is that structures, fabrics, and textures (2) when blocks landed on this interface, the cu- are most frequently shown by thin, modally that might be indicative of this origin—such as mulates beneath were usually coherent (perhaps graded layers, but they are also prominent in crescumulate growths of crystals oriented normal 50% crystalline), and any layering within them macrolayers where they adjoin and cover large to the layering planes, or colloform layering struc- was generally fully developed right up to the inter- blocks (Figs. 14B; 15). This suggests that even tures—are notable only by their absence. face; and (3) the interface environment was highly the macrorhythmic layering was broadly shaped Finally, we might emphasize that, despite all dynamic. On these bases, the interpretation ad- by currents, which is an important observation, the attention that Skaergaard layering has re- vanced by McBirney and Noyes (1979, Fig. 7), given that this layering has yielded almost no ceived, there are still large gaps in the available in- whereby Skaergaard layers form mainly by reor- other clues to its origin. formation on its field relations. Detailed maps ganization and recrystallization within the cumu- Layer draping does not occur around all blocks, have been made of most of the prominent trough late pile, appears largely excluded, and the sce- however, and tentative impressions are that (1) it is structures (Irvine and Stoeser, 1978; Irvine, nario advocated by McBirney (1995, Fig. 1a; most pronounced where it is equally developed on 1983b, and unpublished data); and we present 1996a, Fig. 3) whereby the Layered Series gradu- both sides of the block (e.g., Fig. 10A), and (2) if it here similar maps of some of the block-and-layer- ally evolves from a crystal mush compacting is not appreciable, then the layers ending against ing structures. There are, however, many more beneath ponded (stagnant?) magma, seems highly the block change in thickness from one side to the structures that should be studied and documented; inappropriate. The Skaergaard microrhythmic other (e.g., Fig. 9E). What these relations mean is the only reliable mapping of the extent of the layers do resemble considerably the Stillwater not entirely clear, but the contrast might just be a cross-bedded belt is that shown diagrammatically inch-scale layering that Boudreau (1987) has matter of two-dimensional exposure. Thus, the by Wager and Brown (1968); Figures 1 and 3 rep- simulated by theoretical models of postcumulus symmetric draping perhaps occurs on block edges resent a first attempt to portray the distribution of differentiation, but in two principal occurrences that were parallel to the current flow, while the ab- macrorhythmic layering; a comprehensive, de- they seem clearly to have formed by primary sence of draping might reflect “shadow zones” at tailed stratigraphic-structural study has not been crystallization rather than by recrystallization the lee and stoss ends of the block, where deposi- attempted; and there is no map or stratigraphic (see Irvine, 1987, Fig. 20). tion was too limited for it to develop. record of the distribution and kinds of layers pres- The deformation associated with autoliths Another feature of the layers that cover blocks ent in the Marginal and Upper Border Series. tends to increase with their size and color index in LZc and MZ is that they are sometimes ex- These gaps of knowledge are undoubtedly a ma- (density), but exceptions are common. Some ceedingly rich in Fe-Ti oxides. There are two such jor factor behind the controversy described here. medium-size, relatively leucocratic blocks have zones in the lower part of MZ, each about 1 m caused marked disturbance of the layers beneath thick, that have been traced laterally for several Layer Relations to Individual Blocks them (Figs. 12, B and C; 13, location 3; 14, C and hundred meters (part of one appears at location 4 D), while other, much larger and slightly darker in Fig. 13), and by virtue of their extent, they seem The photographs and maps in Figures 9–23 il- ones produced relatively little (Figs. 15, below lo- clearly to be primary precipitation effects (e.g., an lustrate many Skaergaard layering and block cation 7; and 16, locations 7, 8, and 9). A better oxide-mineral overproduction induced because

1418 Geological Society of America Bulletin, November 1998

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C D

E F

Figure 9. Photographs of small autoliths in the Skaergaard Layered Series (all views to the east-southeast). (A) Angular anorthositic block in- denting modally graded layers beneath and covered by planar layers above, in LZc (Lower Zone c) on Kraemer Island. (B) Subrounded block in MZ (Middle Zone) on Kraemer Island, with modally graded layers draped and smoothed across it. Note the feldspathic layer that tapers out as it turns up near the block, and that the block has a reaction zone, 5–8 cm thick, around its upper reaches. (C) Imbrication of three slabs of anorthositic rock in a graded layer in LZc on Kraemer Island. (D) Layers containing anorthositic fragments in MZ on Kraemer Island. All layers have oxide- rich basal zones, and a thin one toward the top of the sequence has a plagioclase-rich top. The fragments are poorly sorted by size and are almost randomly distributed and oriented in the thicker layers, but two flat-lying slabs above the hammer are lodged at the top of the layer in which they occur, as is a thin slab just to the left in the succeeding layer. (E and F) Successions of graded fragmental layers in LZb on Uttental Plateau (Fig. 16, location 5), some fragments are highlighted for clarity. These layers have heavy concentrations of small (5–10 cm) feldspathic fragments through most of their thickness, but grade sharply at their tops into a more feldspathic cumulate containing larger (10–50 cm) fragments. Note in E the ab- sence of uplapping against the larger block at the top, and the change in the thickness of the dark layer that adjoins it.

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the influx of the blocks mixed liquid layers at slightly different stages of differentiation). How- ever, in other cases, the oxide-rich layers are only developed immediately adjacent to the blocks and are probably better attributed to mineral sorting by eddy currents flowing over, around, or be- tween them (e.g., Fig. 12F). We have found two places, both on Kraemer Island, where the layers torn up by autoliths have been redeposited in approximately reverse strati- graphic sequence (see Fig. 14, C and D; Irvine, 1987, Figs. 28–30). These structures are particu- larly important because they constitute virtually unequivocal evidence of magmatic sedimenta- tion. The reasons are (1) the source region of the redeposited material is identified (it was where the block landed and caused the layer erosion); and (2) unconsolidated crystalline material was clearly transported to another place (albeit close by). In the example in Figure 14, the transport distance was possibly as much as 20 m, and in the other occurrence, it was more definitely about 15 m; but even though the redeposited layer se- quences are only about 10 cm thick, they are still well defined—and their plagioclase did not all float away. As noted by Irvine (1987, p. 198, 216–218), almost identical redeposition struc- tures are present among the ultramafic grain-size graded layers of the Duke Island Complex. The redeposition structures are also of interest as indicators of current flow direction. They are not very conclusive in this respect because of their limited number, and their two-dimensional expo- sures reveal only a component of the flow direc- tion. In both cases, though, the discernible com- ponent is directed east and accords with directions indicated by convoluted layering structures in the same vicinity (see Irvine, 1987, Figs. 31 and 32). The convolution structures closely resemble con- voluted bedding in water-laid turbidity current de- posits and appear similarly to have been caused by current drag. Nicolas (1992) measured the strike directions (but not the inclinations) of mineral lineations through most of the exposed Layered Series and found that they generally converge from the northwest and northeast edges of the intrusion to- ward the south. (His data can also be seen on the revised Skaergaard intrusion geological map by McBirney, 1996b.) Nicolas attributed the lin- Figure 10. Photograph tracings illustrating layer relations to small autoliths. (A) MZ (Middle eation pattern to flow organization of crystals by Zone) block with layers draped across it. The characteristics of layer 1, which is a fragmental magmatic gravity currents “sliding” down the unit about 2 m thick with a thin feldspathic top, are illustrated over a 250 m stretch in Figure 33 walls of the intrusion and over the cumulate floor of Irvine (1987). Layers 2, 4, 6, and 8 are graded with feldspathic tops; 3, 5, 7 and 9 are more toward a basinal low in the southeast. This inter- uniform. More fragmental layers are present in the top part of the outcrop. The block was evi- pretation follows the inferences of Wager and dently emplaced during deposition of layer 3. Layer 2 is almost planar and does not thin ap- Deer (1939), which were mainly based on the preciably beneath the block; layers 4 and 5 lap up and pinch out against it; and 6 and 7 thin alignment of the trough layering structures, and it markedly above it, with the effect that strata from 8 up are again virtually planar. These rela- is compatible with the flow directions that we in- tions obviously are not due to compaction. (B) A 2 m modally graded layer with anorthositic au- fer. However, Nicolas’s data encompass a mixed toliths concentrated in its upper part, at locality 5 in Figure 16. population of lineations, and considered individ-

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ually, most of them would probably be indefinite in origin. Those recorded along the west side of the intrusion are described as being developed on the normal faults in the cross-bedded belt, in A which case they are younger than the trans- gressed layering and, thus, probably had nothing to do with its formation. There are some strong lineations aligned with UZa trough layers that do seem to reflect magmatic flow (Wager and Brown, 1968, p. 102; and our own observations), but they constitute only a small part of Nicolas’s sample. Most of his other measurements repre- sent only very weak fabrics, and while they might be due to magma flow, they could simply reflect slight shifting of the cumulate pile: e.g., in re- sponse to compaction or tilting. Nicolas made a valuable contribution by measuring the lineation pattern, and its regularity is impressive, but de- spite signs that it might reflect primary flow, this interpretation requires independent support. The block-and-layering structures are certainly the best places to look for this support, and they war- rant more systematic study in this respect.

Fragmental Layers

In places where small autoliths are abundant, they commonly are spread through layers like clasts in water-laid fluxoturbidite and debris- flow beds (Figs. 9, C–F; 10B, 13, locations 8 and 9; 16, location 5; Wager and Brown, 1968, B Fig. 39). As in the Duke Island Ultramafic Com- plex (Irvine, 1974, 1987), such “fragmental lay- ers” probably constitute the strongest evidence of magmatic current transport and sedimenta- tion, because the fragments had to be spread lat- erally. The Skaergaard intrusion fragmental lay- ers range in thickness from a few centimeters to about 1 m, and although they invariably are less extensive than ordinary, nonfragmental layers in the same section (e.g., at location 5 in Fig. 16), a few have been traced for as much as 200 m (Irvine, 1987, Fig. 33), so transport distances at times were appreciable. The blocks and fragments in fragmental layers most commonly are aligned parallel to the layer- ing, but many are randomly oriented, and some slabs are at high angles to the stratification (Fig. 9, D and F), suggesting that they were deposited by relatively dense suspensions of crystals and frag- ments. Fragment imbrication can also be seen, but it is rare and seems only to have local signifi- cance. The outstanding example in Figure 9C Figure 11. Photographs of layering structures adjoining small- to medium-size autoliths. probably reflects current deposition, but another (A) A medium-size anorthositic block, mapped at locality 2 in Figure 15. The block was appar- illustrated by Irvine (1987, Fig. 33) appeared to be ently emplaced concurrently with a layer that tops out just above the hammer. Ensuing layers due to layer compaction beneath a large autolith. first lap up against it, then some drape across it. Note the angular corner of the block and its Fragmental layers tend to be somewhat darker sharp edges, virtually free of reaction effects. (B) A small block (or two) in UZa; relations are than other kinds of layers in the same section. highlighted for clarity. Layer 1 was apparently broken by the block’s impact (probably as it was They occasionally are slightly enriched in mafic moving to the left); layer 2 appears to have been deposited along with the block; and layer 3 was minerals at their base, and some have a thin zone then draped across it.

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C

E F

Figure 12. Photographs of layering structures associated with medium- to large-size autoliths. (A) Anorthositic block with layers depressed be- neath and draped against it, in LZc on Kraemer Island. The layer depression might be due to compaction, but our impression was that it more probably reflects the block’s impact. (B) Deformation along the lower edge of a leucogabbroic block, LZc on Kraemer Island. The layering in the left foreground was depressed by the block, but a segment near its edge was pushed upward, apparently because the block skidded into place while moving in the direction of the arrow. (C) Layering overturned and smeared (small arrows) along the lower edge of the MZ troctolitic block mapped in Figure 22 (location 8), again probably because the block slued into place. (D) Layering lapping up against the end of an anorthositic block in LZc, Kraemer Island (Fig. 13, location 2). (E) A small, low-amplitude trough structure adjacent to the large block illustrated in Figure 15 (location 5). An unconformity truncating the top of the structure is slightly highlighted. Several small dark spots are photographic flaws, not features of the rocks. (F) Thin layers rich in Fe-Ti oxides synformally filling a gap between blocks, in LZc on Kraemer Island (Fig. 23, location 2). A 10 cm zone of rind rock in front of the hammer, spotted with dark olivine grains, fringes a block in the foreground.

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of plagioclase enrichment at the top; but many tilt away from it. This is the sort of effect we associated layering relations can be chaotic. The occur between thin, modally graded layers in the would expect from compaction, but in this case, area mapped in Figure 16 is exemplary, but simi- intervals ordinarily occupied by “uniform” layers the amount of the tilting is probably within the larly complex relations can be seen at many of (Fig. 10A). The usual first impression of frag- uncertainty of the projection, so we cannot claim the places in LZb, LZc, and MZ. The details of mental layers is that they are poorly sorted, con- it to be real. Figure 16 are described in the caption; the fol- taining inclusions of various sizes scattered al- The general lesson on compaction that we lowing considerations are more general. most randomly through an otherwise relatively would take from the block-and-layering struc- The complex association of numerous blocks, uniform cumulus aggregate of mafic minerals tures is that it is difficult to identify at this scale— deformed layering, unconformities, and local and plagioclase (Fig. 9, A, D, and F). However, probably because its effects are subordinate to, fragmental layers in the stratigraphically lower, an important new observation about them is that, and heavily masked by, those of impact and southwest part of the map area suggests that the in numerous occurrences, the fragments are con- streamlining. blocks and affiliated debris detached en masse centrated in their upper parts, sometimes yielding from the Upper Border Series and, together with a crude inverse size grading (see Figs. 9, D–F; Block Relations to the Trough Layering abundant unconsolidated crystalline material 10B; and two excellent examples in Pl. 6B of Structures from the upper boundary environment, cascaded McBirney and Noyes, 1979, incorrectly de- in rapid succession to the cumulate floor, proba- scribed as showing only density grading with lit- One of the three localities where trough struc- bly as a plumelike current. As they impacted the tle or no size sorting). In some places, the larger tures and autoliths are closely associated is illus- floor and spread laterally, the blocks variously fragments protrude above their host layer to be trated in Figure 13. At this LZc site, the two uprooted existing strata (both old and new) and covered finally by the succeeding layer. From largest blocks (at locations 1 and 5) are within the accumulated with new ones, thereby producing these relations it is evident that the layers were span of a small shallow trough that stems from a their observed association with discontinuous, sorted so that the fragments accumulated in a local erosional unconformity (at location 6). The unconformable successions of modally graded feldspar-enriched matrix of roughly equivalent association might be coincidental, but a consider- and fragmental layers. average density or buoyancy. The apparent im- able possibility is that the blocks were guided Up-section to the east, however, the stratigraphy portant implication is that the process of current into the trough by currents flowing through it. changes rather abruptly and becomes dominated transport and deposition was controlled more by A second example, located in UZa and by extensive, almost undeformed macrorhythmic bulk density relations than by individual mineral roughly illustrated in Figure 3, features two ad- layering and one exceptionally large block. The densities (particularly that of plagioclase). jacent major troughs. After being initially promi- reason for the change is not clear, but it is not just nent, these troughs almost died out (upward), but a local feature. The blocks pictured in Figure 14A, Layer Compaction Around Blocks were abruptly rejuvenated when, first, a large for example, are several hundred meters to the east block landed in the one, and then, a medium-size at about the same stratigraphic level as the jumbled One of our reviewers suggested that the de- block landed in the other. Formation of promi- blocks, and the macrorhythmic layering extends pression of layers beneath blocks, and the drap- nent layering then resumed in both troughs, and above them and far beyond. In Figures 1 and 3 we ing and smoothing above them, are simply effects they continued to grow side by side for another show two levels in the Layered Series where zones of differential compaction. We cannot agree, 130 m. No other blocks are exposed in the vicin- of abundant blocks are overlain by thick sections however, even though we advocate Layered Se- ity, so the impression is that the two in question of macrorhythmic layering, one in the upper part ries compaction on other grounds (as in the were guided into the troughs, presumably by of LZb, the other about two-thirds the way up in formation of the normal faults in the cross-bed- currents. MZ (Fig. 3). The area of Figure 16 straddles the ded belt). On some blocks, the layer draping is The third example of an association of a block first, and in view of the unconformities at localities pronounced, yet the layers beneath them are vir- with a trough is illustrated in Figure 15 (locations 1, 6, and 15, it might record a time when the tually planar with no thickness change attribut- 1 and 5). At this MZ locality, a small, low-ampli- magma chamber was being tilted or reshaped tec- able to compaction (e.g., Fig. 10A). On other tude trough defined by about 20 thin, modally tonically. Thus, the blocks might have detached blocks, the underlying layers are bent or broken graded layers (Fig. 12E) is present near the edge when the cumulate floor was being downdropped in ways that clearly indicate impact (Fig. 11B). of the largest block. The timing of events is not on the east (or uplifted to the west) along the Many layers succeeding blocks pinch out as they clear (see Fig. 12 caption), but the preferred inter- feeder fracture system that we speculatively linked lap up on their sides, and those that drape across pretation is that the trough layers could be de- to the basalt dike extending north-northeast across almost characteristically thin down far more than posited by currents flowing around the block, the area (see Associated Basaltic Intrusions). can reasonably be accounted for by compaction. through a moat-like depression attributable to its The emplacement of the block can often be iden- weight or impact. Whatever its history, the overall Layers and Blocks within Blocks tified with the formation (deposition) of a partic- structure portrayed in Figure 12 is extraordinary ular layer from the way this layer thickens and in the way it interrelates troughs, unconformities, It has long been known that some Skaergaard thins around it. and layer streamlining through blocks of different autoliths are internally layered, but occurrences These comments pertain mainly to the thin, sizes and layers of different kinds. The mere sys- are more numerous than has generally been real- modally graded layers, but they also apply to tematics of the relations seem to constitute evi- ized. A splendid example in MZ, involving an ex- macrorhythmic layering. As illustrated in Figure dence that the structures are primary, and that the ceptionally large anorthositic block prominently 15, macrolayers frequently lap up and pinch out layers are properly ascribed to current deposition. exposed in the cliff face of Wager Peak, has been against large blocks (as at locations 1 and 2; see controversial for many years. We have found an- also Fig. 11A), and those that extend over them Layer Relations among Clusters of Blocks other on the east side of Kraemer Island, com- characteristically show pronounced thinning. posed of coarse oxide gabbro, exhibiting extraor- Also notable, though, is the apparent tendency of At places where large blocks are numerous, dinarily well-developed layering and numerous the trough layers at the sides of the large block to they frequently occur in jumbled piles, and the small autoliths of an earlier generation. In all

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Figure 13. Map (A) and section (B) of several small anorthositic gabbro blocks in LZc on Kraemer Island (~68°11.4′N, 31°43.7′W). In the section, the map relations have been projected on lines inclined approximately at the average layer dip of 27° onto a perpendicular plane through EW (east-west), and they are viewed in the direction of the arrows. The largest block, a slab 23 m long at locality 1, has crude layer- ing parallel to its length. The adjoining host layering at 2 shows an unconformity marking the level at which the slab was emplaced. Layers above the unconformity lap up against the slab (see Fig. 12D); those below are depressed beneath it. On its opposite side at 3, the underlying host layers are crumpled in a way that suggests a component of impact motion to the left (east). At 4, layers rich in Fe-Ti oxides cover the slab; at 5, the next largest block shows internal replacement by Fe-Ti oxides; at 6, underlying host layers show an unconformity beneath a trough; and 7 marks the end of a trough layer. Fragmental layers are at 8 and 9. That the largest blocks are situated in a trough suggests that they were guided there by magmatic currents flowing into it.

A B

C D

Figure 14. Photographs of medium to large cognate inclusions. (A) Medium-sized, probably troctolitic blocks in LZb above Uttental Plateau. (B) Large anorthositic block with macrorhythmic layering draped across it, in MZ in a western spur of Wager Peak. The cliff is about 200 m high. (C) Medium-sized anorthositic block in MZ on Kraemer Island. The block evidently skidded into place while moving to the left, and some of the layers beneath it were torn up and redeposited farther to the left in approximately reverse order. (For a map of this area, see Irvine, 1987, Fig. 30). D. Closer view of the redeposited layers.

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Figure 15. Projected section of a medium-size anorthositic autolith, plus smaller blocks and fragments, in MZ near the east shore of Uttental Sound (~68°11.2′N, 31°41.9′W); constructed from a map in Irvine (1987, Fig. 26). Compare with Figures 14B and 20B. Only the top of the large block (location 1) is visible, but its overall relations are probably much like those of the smaller block at 2, which is partly photographed in Fig- ure 11A. The layer deformation at 3 may be due to emplacement of the large block, but the overlying layers lapping up against it are clearly younger. In this scenario, the unconformity at 4 would stratigraphically mark the block’s emplacement; the trough structure at 5 would be older; and the troughs at 6 would be younger. An alternative possibility, however, is that emplacement is marked by the small autoliths to the right of 7. The trough at 5 would then be younger, and the deformation at 3 and unconformity at 4 might be affiliated only with the introduction of the block at 2. The latter interpretation appears geometrically more likely, but it was not realized during mapping when it might have been confirmed. Ei- ther way, though, the host ferrogabbro layering around 6 laps against and over both blocks 1 and 2 and clearly did not “propagate” into them. The origin of the narrow strip of anorthositic gabbro in the edge of the large block above 3 is uncertain, but it gave the impression of being an orig- inal feature. (It is not a concentric reaction rim.) It might be layering from the block’s source region, but a perhaps better possibility may be that it represents an alteration zone along the fracture from which the block detached.

other occurrences that we are aware of the block tends to be concordant with the layering and sistent as that in the modally graded layers in the layering is relatively inconspicuous; e.g., the lamination of the host cumulates (see Fig. 13, Layered Series, and nowhere is it as sharply de- medium-size block at locality 1 in Figure 13 ex- locations 1 and 4), even though it is older and limited, although the feldspathic zones occasion- hibits only weak textural layering, and the excep- was formed in a different place. ally show faint pegmatitization. Collectively, the tionally large block underlying localities 10, 11, The Kraemer Island oxide gabbro block that block layers form a stratigraphic stack about 80 m and 12 in Figure 16 has only rather vague modal contains layering and small earlier autoliths is il- thick, part of which extends laterally for 170 m, stratification (but note too that it contains a small lustrated in maps and a section in Figures 17 and and part for about 80 m; at the stack edges, the anorthositic block of an earlier generation at lo- 18, and in photographs in Figure 19. It is about layers end in bunches against massive rock in cation 13). The important point is that some fab- 250 m by 150 m in plan and at least 90 m thick. much the same way that the Layered Series ric of this type is often present. The early autoliths are from a few centimeters to trough layers terminate collectively. As can be In some cases, the layers within blocks are 5 m across and are usually finer grained and more seen in Figure 17B, the layers are roughly parallel sharply truncated at their edges, and occasion- leucocratic than their host rock. The larger ones to those in the cumulates outside the block, but at ally they are adjoined or covered discordantly are troctolitic, and two of them are crudely strati- locality 6 along its upper (west) side, they are cov- by younger layers in the host cumulates (Figs. fied (Fig. 17, locations 3 and 5). The layers within ered discordantly; it is clear that two distinctly dif- 13, location 2; 15, location 6; Irvine, 1987, Figs. the oxide gabbro typically are 5–50 cm thick and ferent ages of stratification are present. The early 26 and 36). However, many blocks are slabs are well defined by both modal and grain-size autoliths were evidently captured while the oxide broken parallel to their layering, so when they variations (Fig, 19, B–D). The grading tends to be gabbro layers were forming, because the layers al- lie flat in the Layered Series, their stratification from mafic to felsic upward, but it is not as con- ternate with and warp around them in much the

1426 Geological Society of America Bulletin, November 1998

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/11/1398/3382835/i0016-7606-110-11-1398.pdf by guest on 27 September 2021 ties of these layers mostly reflect topography, mostly reflect ties of these layers in the sides in the peculiar reentrants even between 9 and 10, block is crudely layered The large of the block at 9 and 14. and it in- re- patchy cludes a small anorthositic block of an earlier generation at 13. It also shows placement by secondary anorthosite at 11, “rind rock” and it is cut by brown-weathering anorthosite,A body of secondary (replacement) between 13 and 14. dikelets 20 by 25 m in size, 24C), at 2 (see Fig. truncates layering sharply ob- and patches of similar replacement The timing of the re- small blocks between 3 and 4. and partly obliterate several scure placement is uncertain, at 6, the unconformity just preceded it may have but because This possi- by the local deformation. smaller patches beneath it (near 3) appear smeared anorthosite of outstanding diapirism a replacement bility accords with the presence meters to the north just a few hundred the same stratigraphic level body at approximately (see Larsen and Brooks, 1994, 4b). Fig. W). In the lower part of the illustrated section, W). In the lower ′ N, 31°42.5 ′ Figure 16. Map of a complex swarm of autoliths and related layering structures in LZb structures layering 16. Map of a complex swarm autoliths and related Figure in the vicinity of localities 1–4, thin, deformed medium-size blocks have modally graded layers,Then the influx near 1.) (Note the unconformity by more. covered and then were locality 5 proba- sizes to the south and west around blocks of similar and larger of more at 6. During this angular unconformity in a prominent represented bly caused the erosion episode, limited lateral extent at of very in fragmental layers spread small autoliths were 5 (see Figs. 9, appears in the vicinity of 6 and layering E and F; 10B). Macrorhythmic to 7,dominates through 8, than more block extending for An exceptionally large and 9. 10 and 12 was then emplaced,300 m from at 15, layers and in the covering is an- there In contrast to its predecessors,other unconformity. block caused the exceptionally large beneath it, of the layers almost no visible deformation The map irregulari- as at 7 and 8. on Uttental Plateau (~68°12.5

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same ways that Layered Series layers enclose au- common perception of the Upper Border Series combination, these three blocks clearly represent toliths. An important difference is that warping is is that, in freezing to the intrusion roof, it crystal- an enormous stoping event. It is intriguing that mainly a downward bulging or “underside drap- lized in situ, but our observations suggest that it they are succeeded by the most spectacular ing.” This seems strongly to imply that the layers actually consists largely of transported crystals. macrorhythmic layering in the intrusion (Wager accumulated from the top down, as is appropriate In later discussion we propose a mechanism and Deer, 1939, Pl. 6). This strongly suggests that for the Upper Border Series, and it suggests, too, whereby currents convecting and crystallizing their release in some way initiated the layer- that the block did not turn over as it settled to the along the upper border solidification front might forming process. cumulate floor. have deposited modally sorted layers and small The problem presented by the layered block is The similarity of the layers and small autoliths autoliths. that its stratification aligns well with the macro- in the oxide gabbro block to those in the Layered A related question is whether the small early rhythmic layering. McBirney and Noyes (1979, Series does raise the possibility that the block autoliths might have been floating in the Skaer- p. 519–520) interpreted this alignment as indicat- might have come from there, rather than from the gaard magma at the time they were trapped as ing that the host-rock stratification “propagated” Upper Border Series. This seems unlikely, how- inclusions. This is significant to the problem of into the block after it was emplaced. They then ever, for two reasons (in addition to the underside cumulus plagioclase buoyancy during Layered claimed this interpretation as support for their draping). One is that a major disruption of the Lay- Series accumulation, because the small autoliths view that Skaergaard intrusion layering formed ered Series would be required to dislodge such a contain mafic constituents and thus are denser by processes of recrystallization and reorganiza- large block, but none has been recognized; the than plagioclase. If they floated, then plagioclase tion, and McBirney (1987, 1995, 1996a) also second is that the oxide gabbro is much coarser should have floated too. Specific gravity measure- cited it as evidence that autoliths in the Layered than any Layered Series cumulate, whereas it ments show, however, that the small early au- Series have undergone wholesale mineralogical matches well in this respect with the upper border toliths are just as dense as the large troctolitic au- and compositional changes by way of chemical rocks. We have not seen upper border layering toliths that are embedded in the Layered Series, exchange and reequilibration with the cumulates close at hand, but the descriptions of Wager and and the latter are so numerous, it is inconceivable around them. Deer (1939) and Naslund (1980, p. 48–59) suggest that they could have floated. It is likely, therefore, Several geologists, including McBirney, have that it can also be matched in the autoliths. Some that the small autoliths are simply minor fine de- climbed onto the Wager Peak block, but the lay- layers are thick and broad like those in the Wager bris that broke loose with the abundant larger ering involved is mostly of the type that is diffi- Peak layered block; others are thin and closely re- blocks, and were selectively recaptured and cult to see at close range, and all published infor- peated in local stacks like those in the layered ox- frozen into the oxide gabbro before they could be mation and inferences about the block (including ide gabbro block (see Bird et al., 1986, Fig. 10A), swept away. Within the framework of Figure 2 our own) are based only on distant observations. and they are similarly defined by both modal and this interpretation can also be applied in principle It is clear, however, that despite their alignment, grain-size variations, with some pegmatite devel- to the small (dense) peridotite fragments observed the layering units inside and outside the block opment. Photographs by Naslund (1980, Figs. by Naslund (1984) in the Upper Border Series. differ markedly in frequency and color index. As I-10–I-13; also McBirney and Noyes, 1979, Pl. 1, The layering and autoliths in the oxide gabbro noted by Irvine (1987, p. 233), the host layers B) suggest (to us) that, if there is a dominant sense block constitute rather profound evidence that that are alleged to have propagated through the of grading in the layers, it is “gravitationally nor- Skaergaard layering is a primary feature in both block actually turn up (lap up?) against its side mal” from mafic to felsic upward. the Upper Border and Layered Series. Although (see Fig. 20B); furthermore, when they are traced The large oxide gabbro block, by virtue of its mineralogical recrystallization and reorganiza- in the opposite direction (to the left), they are accessibility, exposure, and exceptional internal tion may produce layering in some circum- seen to be part of a succession that drapes over structure, may be the best place in the Skaergaard stances (Boudreau, 1987), it seems impossible the neighboring block. Thus the notion that the intrusion to study upper border processes. The that they could ever yield the intricate relations of layers “propagated” through the block appears to small early autoliths, in particular, lead to impor- fragments surrounded by layers inside a block be both unwarranted and unrealistic. tant inferences. In being troctolitic, they appar- surrounded by layers that we have documented Our interpretation of the Wager Peak layered ently represent an upper border differentiate ear- here. By contrast, these relations are fully in ac- block is the same as we have indicated for lier than the oxide gabbro in a physicochemical cord with traditional Skaergaard concepts of blocks with similar relationships that we have sense as well as in actual age, so their source unit magmatic crystallization and sedimentation. mapped in more accessible parts of the Layered was probably situated stratigraphically higher. Series; i.e, (1) the layers in the block derive We may infer, therefore, that in becoming inclu- Wager Peak Layered Block from its source region, presumably in the Upper sions, they first had to be broken from a place Border Series, and probably underwent little or where the troctolite unit had been exposed to the The controversial layered block in the cliff no subsequent modification, and (2) the layers magma through the oxide gabbro (presumably by face of Wager Peak is illustrated in Figure 20A, in the host cumulates are distinctly younger, stoping); then they had to be transported laterally and its principal features are highlighted in Fig- having been deposited around and over the to the sites where they accumulated among the ure 20B. Its lower side is covered at the base of block after it was emplaced. These inferences oxide gabbro layers that now surround them. On the cliff, but there almost certainly is a second seem fully justified by a comparison of Figure these bases, then, the formative conditions of the block on the left, and if that is the only neighbor, 20 with Figures 13, 15, 16, and 17, wherein they Upper Border Series seem to have been like those then the layered block is at least 400 m wide and are well supported. indicated for the Layered Series; i.e., there was 250 m high. Although its three-dimensional form We note also that the layering in the oxide gab- generally a sharp interface between the magmatic is not discernible, the block could be an enor- bro block in Figures 17–19 is so similar to that of liquid and the solidifying rock units, and this in- mous slab. It is situated at roughly the same its host rocks in disposition as well as appearance terface was highly dynamic, some parts being stratigraphic level as both the 400-m-wide slab that, until this study, the block itself had not been stoped while others were swept by currents and on the east shore of Kraemer Island (see Fig. 1) recognized. Although the alignment of layers in a coated with additional crystalline materials. A and the large block shown in Figure 14B, and in block with the younger layers of its host cumu-

1428 Geological Society of America Bulletin, November 1998

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/11/1398/3382835/i0016-7606-110-11-1398.pdf by guest on 27 September 2021 Figure 17. Map (A) and projected section (B) of two large autoliths in MZ on the east shore of Kraemer Island (~68°11.25′N, 31°43.73′W). The smaller (left) block consists of anorthosite; the larger one is oxide gabbro. The map is oriented with north to the right to facilitate comparison with the section, and it has been simplified by removing (closing) more than a dozen dikes of basalt and granophyre. The section shows the block rela- tions approximately as they would appear from an aircraft flying low and just to the east over Uttental Sound. It clarifies relations by eliminat- ing most topographic distortions of the layer trends. Note that both blocks are fringed on top by “rind rock” (see text). The oxide gabbro block has well-developed internal layering, especially between localities 2 and 4, and it includes numerous small troctolitic blocks and fragments of an earlier generation (Figs. 18 and 19). Note the close association of the fragments and layering inside the block, the somewhat troughlike distribu- tion of the layers from 3 to 5, and the way this layering is covered discordantly by the host gabbro layers at 6. Exceptionally coarse-grained olivine- bearing oxide gabbro occurs in an unlayered part of the block at 7.

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Figure 18. Detailed maps of the early troctolitic autoliths at localities 1–5 in the large oxide gabbro block mapped in Figure 17. Those in areas 1 and 3 are weakly layered.

lates might seem an improbable coincidence, at thus they probably broke from a horizontally lay- 12F, 17, 21A, 23, location 1). These generally the Skaergaard intrusion it appears to be coinci- ered source and then settled in horizontal orienta- have relatively sharp contacts against the block, dental only in a physical sense. In terms of fre- tion to the horizontally layered floor. Almost in- but are gradational into the adjoining gabbroic quency of occurrence, it is more the rule than the evitably, therefore, they now rest with their cumulates. We refer to them as “rind rock,” but exception, and for two good reasons: (1) when the stratification broadly parallel to that around them. similar material of about the same age occurs as Layered and Upper Border Series were forming, small dikes in some of the larger blocks (Figs. 16, both were stratified more or less horizontally (see REACTION AND REPLACEMENT 21, B and C, 22, location 4), so we use the name Wager and Brown, 1968, p. 131); and (2) fluid ASSOCIATED WITH BLOCKS for them as well. The light brown weathering es- mechanics experiments show that large blocks sentially reflects an abundance of augite, but should tend strongly to settle with their largest Rind-Rock Rims and Dikes some rims are additionally spotted with darker cross-sectional area horizontal, which implies brown, ovoid olivine grains up to about 1 cm in also that they should not rotate much (Irvine, A few autoliths, especially in the medium to length, and minor Fe-Ti oxides are generally 1974, p. 150–152). Many of the blocks in the Lay- large size range, are bordered by light brown present. Plagioclase only occurs interstitially. ered Series are slablike bodies with subparallel weathering, fine-grained, mafic to ultramafic We observed rind-rock rims on all three of the layering or lamination (albeit weakly developed), rims or rinds, typically 5–15 cm thick (e.g., Figs. principal autolith rock types (gabbroic troctolite,

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Figure 19. Photographs of blocks and layers within the oxide gabbro block mapped in Figures 17 and 18. (A) Early generation troctolite block, locality 1. The edge of the host oxide gabbro block is slightly highlighted (on the left and in the foreground); layers in the host cumulates are visi- ble in the left background. (B) Layers near locality 2, some with weak “normal” modal grading (mafic at the base, feldspathic at the top) above an early troctolitic autolith in the foreground. (C) Well-developed layers between localities 2 and 4, the lowest one showing slight downward bulging or “underside draping” beneath a very small leucocratic fragment, possibly signifying accumulation from the top down. (D) Layer show- ing pronounced underside draping beneath a small fragment; relations are highlighted for clarity.

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gabbroic anorthosite, and oxide gabbro), and al- small blocks in MZ have faint concentrations of with the host cumulates (McBirney, 1987, p. 439). though they are most common along the tops of Fe-Ti oxides, only 1–2 mm thick, along their top Little evidence has been presented in support of medium to large blocks, they also are on edges and surfaces that might be reaction effects, although these claims, however, and we can only recom- undersides, and in rare cases, they fringe small they could be cumulus deposits sorted from mag- mend them in minimal ways. Our experience is blocks. The main occurrences are in the lower half matic currents flowing over the blocks. Rare dis- that most autoliths show no clear sign of having of MZ on Kraemer Island, and the most prominent continuous fringes of truly massive Fe-Ti oxides, lost mafic minerals (see following for some ex- examples we have mapped are along the tops of up to about 1 m in length and 6 cm in width, lo- ceptions), and the anorthositic blocks do not ap- the anorthosite and oxide gabbro blocks illustrated cally border a few of the larger anorthositic blocks pear to be any more altered than the oxide gabbro in Figure 17. Only dikelets of rind rock were found in LZc and MZ (see McBirney, 1989a, Fig. 7 a), blocks, which have compositional counterparts in in the autoliths in LZb (Fig. 16), and no rinds were and small patches of similar material locally re- the Upper Border Series. We do not know of any recognized on the blocks in the upper half of MZ place some of them internally (Fig. 13, location medium-size or large block that is prominently or and in UZa. 5). These occurrences appear to be hydrothermal, extensively zoned concentrically because of pro- The rind-rock dikes are more feldspathic than but whether the oxides came from the block or gressive anorthosite replacement, and our specific the rims, and some are partly pegmatitized. Our from its host rock (or the intercumulus liquid in gravity measurements show that, although the best examples, illustrated in Figure 21, B and C the host) is debatable. gabbroic anorthosite blocks would probably have and at locality 4 in Figure 22, clearly predate the Even though the rind-rock rims are the result of floated in relatively iron-rich liquid, the more many basaltic dikes that intruded the Skaergaard chemical interaction between blocks and the ma- abundant gabbroic troctolite autoliths are dense intrusion after it solidified, and in being trun- terials around them (be it only intercumulus liq- enough to have settled. We do not find the inter- cated at the edges of their host block, it is evident uid), they also constitute armoring, and in that role pretation that layers “propagated” into the Wager that they formed in the source area of the block they have clearly limited the interaction. The oxide Peak block to be credible; and in the single docu- before it detached. They may represent injec- gabbro block mapped in Figure 17 serves ad- mented case of continuous lamination between a tions of Skaergaard magma into fractures that mirably as a case in point. Its extensive, but thin block and adjoining cumulates (McBirney and developed as the block was breaking loose. The rind-rock armoring implies that it did not react Hunter, 1995, Fig. 4), the block is one of a rubbly pegmatitization of the dikes seems to stem from much, and the fact that it has beautifully preserved, cluster of flat-lying slabs that could well have ac- a pegmatite pod outside the block, so it probably primary internal block-and-layering structures quired their shapes and inherited their fabric developed after the block was buried. proves the point. If the Wager Peak layered block through being broken from a laminated source. The tendency for the rind-rock rims to be more has a rind-rock rim, then the interpretation offered Our data on mineral compositions (see summary common on the larger blocks suggests that their for it above would seem to be well justified. in Table 1) show that the autoliths generally have thermal mass was an important factor, and the plagioclase crystal core compositions markedly impression is that the typical rim is a cross be- Replacement Anorthosite in Autoliths different from the cumulus plagioclase composi- tween a chilled margin and a reaction rim, tions of their host cumulates, and although the formed when its block detached from a relatively The small replacement anorthosite bodies in mafic minerals in blocks show some evidence of cool part of the Upper Border Series and settled the Layered Series (Fig. 24A) are pertinent here compositional modification, these changes often into hotter liquid where it was no longer in ther- because of claims that the anorthositic autoliths are such that they could represent postcumulus re- mal and chemical equilibrium. The rim then de- were initially gabbroic bodies, but were metaso- actions that occurred in the Upper Border Series. veloped by a process coupling rapid primary matically leached of mafic minerals after they The small thicknesses of the described rind-rock crystallization of the liquid, with bimetasomatic were buried. This interpretation both was based rims show that chemical exchange between the chemical exchange between it and the block. The on, and has led to arguments that: (1) the autoliths autoliths and their host cumulates was limited. exchange probably continued into the postcumu- are more feldspathic than any existing Upper Bor- Irvine (1987, p. 232) said that he had not seen lus stage, but the rims invariably are thin, so the der Series units, and therefore must have lost any blocks typical of the preserved parts of reactions were limited. In the dike rock, the mag- mafic minerals (McBirney, 1987, p. 438–439); the Upper Border Series in a state of partial matic liquid presumably entered fractures in (2) some autoliths are compositionally zoned transformation to anorthosite. This statement cooled blocks and then reacted with their walls as from mafic cores to feldspathic margins because was not correct, however. Anorthositic replace- it crystallized. of chemical exchange with surrounding cumu- ment is not conspicuous in most autoliths, but it It would be of interest to know where blocks lates and their interstitial liquid (McBirney, 1979, has considerably affected many of the small and were when their rims formed. One possibility is p. 325; McBirney and Noyes, 1979, p. 510); thin fragments (see following), and a significant that they were still settling to the magma floor, (3) many autoliths are so feldspathic that they larger example was recognized by Tacinelli and and in cases where the rim is only on the top and should have floated in the Skaergaard magma, Naslund (1990) in a medium-size block that had upper edges of the block, the block may have rather than settling to its floor, particularly dur- been mapped by Irvine (1983a, Fig. 14). The lat- been significantly cooler on top by virtue of in- ing the LZb to MZ stages of differentiation ter replacement is illustrated in Figure 24B, and heriting a strong thermal gradient through the (McBirney, 1987, p. 438; also Naslund, 1986, a broader view of the block appears in Figure Upper Border Series. Alternatively, such a rim p. 366–367); (4) layering “propagated” into 14C. The block was anorthositic initially, but the might have formed after its block landed, but be- the Wager Peak block from its the host cumu- replacement produced a more feldspathic zone, fore its top was covered by younger cumulates. It lates (McBirney and Noyes, 1979, p. 519–520; about 2 m long and 15–30 cm wide, with diffuse is difficult to make specific choices, however, and McBirney, 1987, p. 439); (5) plagioclase lamina- boundaries and a discontinuous axial veinlet of Figure 23 illustrates block-rim relations that we tion is continuous between some blocks and their pegmatitic pyroxene and Fe-Ti oxides. Several cannot explain. host cumulates (McBirney and Hunter, 1995, similar mafic veins with narrower anorthositic A few of the smaller autoliths have diffuse, p. 117); and (6) the minerals in most autoliths fringes occur nearby, and collectively they ap- paired light and dark marginal zones that are un- have been thoroughly reequilibrated, both com- pear to represent an arrested residue of some of doubtedly reaction rims (Fig. 9B), and many positionally and texturally, by chemical exchange the replaced mafic minerals (Fig. 24B). Similar

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B

Figure 20. (A) Photograph of blocks and macrorhythmic layering in the cliff face of Wager Peak. (B) The same photo with layering units and the block outlines highlighted. Although the macrorhythmic host layers to the left of block 1 are broadly aligned with the layers inside it, in de- tail, they are seen to lap up against its side, and some drape over its top. The uplapping shows in photographs taken from various angles, and un- doubtedly is real. It may be compared with the uplapping of layers against the blocks in Figures 10A, 11A, 12D, 13 (location 2), and 15 (above lo- cation 3). In many photographs of this cliff, the layers within block 1 appear also to turn up at its left edge (see McBirney, 1987, Fig. 2b), but this is not clear in all views, so it might represent topographic distortion. Layers L1 and L2 are the leucocratic parts of the two lower Triple Group units. (They appear distorted because of topography.) At this location, the gold enrichment of the Platinova reef is just below L1.

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B

A

D C

Figure 21. Photographs of reaction effects and early dikes in anorthositic autoliths. (A) Rim of rind rock along the north edge of the large anorthosite autolith mapped at the left end of Figure 17. (B) A quasipegmatitic rind-rock dike in the troctolite block mapped in Figure 22. The pegmatitic facies is mainly at the dike margins, but some occurs along its axis. Replacement anorthosite pipes are faintly visible in the left fore- ground and to the right. (C) Broader view of the rind-rock dikes in the same block. Aligned pipes of replacement anorthosite are on both sides of the two dikes on the left, and the dikes are seen to be slightly transgressive to them. (D) Closer view of some of the anorthosite pipes. With their mafic fringes, they are reminiscent of larger replacement bodies in the Layered Series (see Fig. 24, A and C).

patchy anorthosite replacement can be seen in while the block was still part of the Upper Border the block when it broke from its upper border the exceptionally large block mapped in Figure Series. In this case, the replacement bodies are source. An intriguing possibility is that the pipes 16 (location 11). small aligned feldspathic pipes, and as explained and dikes originally were both oriented more or Figures 21D and 22 illustrate anorthosite re- in the caption of Figure 22, they are transgressed less vertically (approximately normal to the roof placement of a large block that evidently occurred by mafic dikes that were truncated at the edges of contact), the pipes marking the escape channels of

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Figure 22. Map of a leucocratic troctolite block, approximately 70 m by 30 m in plan, in MZ on Kraemer Island. When the block landed on the cumulate floor, it overturned thin, modally graded layers along its lower side at locality 8 (see Fig. 12C), then it was overlain by younger layers at 2 (a topographically higher place). The block troctolite is partly sieved with small aligned pipes of replacement anorthosite at 4 and 5 (Fig. 21, B–D), and between these localities, it is cut by three principal (and several smaller) early, quasipegmatitic mafic dikes (see Fig. 21, B and C). These dikes end at the edges of the block, so almost certainly were emplaced before it broke from the Upper Border Series; and they are transgressive to the anorthosite pipes (Fig. 21, B and C), which implies that the pipes also formed in the Upper Border Series. A likely possibility is that the pipes and dikes initially were both approximately normal to the roof contact, the pipes defining channels along which water-rich fluids escaped from the in- trusion, and the dikes occupying fractures that opened as the blocks were breaking free. By contrast, the pegmatite facies of the dikes appears to stem from a relatively large pod of mafic pegmatite in the layered cumulates that underlie the block at locality 10, so it probably developed after the block came to rest on these layers. All the above rocks are cut by late basalt dikes trending north-south (at 6), N40°–50°E (at 3 and 7), and N50°W (at 1 and 9), with the last set probably being the youngest.

magmatic fluids that left the intrusion before the lar size are numerous in the area, it might be that about 10 cm across or 10 cm thick commonly ap- block was fractured, and the dikes occupying a block was selectively replaced. pear only as feldspathic patches with diffuse fractures that opened while it was breaking free. The place where anorthositic replacement is boundaries (Fig. 24D), and although it can be dif- A fourth example of possible anorthosite re- most conspicuous in autoliths, though, is in very ficult to be sure they are fragments, it is clear that placement in a large block is shown in Figure 24C small and thin fragments. Medium to large au- small size greatly facilitated the replacement. Ex- (from location 2 in Fig. 16). The anorthosite trans- toliths occasionally have anorthositic fringes that amples can be seen at many places in the Layered gresses layering and has a thin mafic fringe typi- are a few centimeters thick, some of them enclos- Series, but perhaps the most striking occur among cal of replacement, and if that was the only ing mafic reaction zones of similar width (e.g., the uppermost autoliths in the intrusion, those in process involved, then this is the largest replace- Fig. 9A), but more frequently they have moderate the middle of UZb. ment body of its kind we have seen in the Skaer- contents of mafic minerals and sharp, clean con- We would emphasize that replacement anor- gaard intrusion. Its map-shape, however, is also tacts (Figs. 9A and C–F; 11, A and B; 12, A–D; thosite bodies have to be identified by their field that of a block; thus, given that autoliths of simi- 14C). By contrast, fragments that are less than relations. Most examples that have been described

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Figure 23. Map of a problematical unit of rind rock in a cluster of several small- to medium-size anorthositic blocks in MZ (Middle Zone) on Kraemer Island (symbols as in Fig. 22). The block at the right has a highly irregular bottom contact at 4 but is subangular on top, and magnetite- rich layers drape off its flanks at 2 (Fig. 12F) and 6. It shows weak textural layering at 5, and there is similar layering in the neighboring block below 3. Rind rock forms a brown-weathering band at 1. On the left, it rims the block that is stratigraphically above it, but to the right it switches without apparent break and fringes the top of the block beneath; the reason for this is not clear.

previously, both by us and by other authors, illus- uids that had been estimated for the intrusion by sions that would displace crystal-poor liquid trate the essential criteria, whereas our present ex- Wager (1960) and Wager and Brown (1968) and from the floor of the magma chamber, no matter amples serve more to illustrate the problems and found that the cumulus plagioclase should have how Fe rich that liquid might be. Almost the uncertainties of their application. In thin section, floated. This disparity with field observations has only bulk cumulate units in the Layered Series replacement anorthosite typically has all the ap- not been satisfactorily resolved, and McBirney that might have floated are the most anorthositic pearances of a plagioclase adcumulate, so there (1995, 1996a, Fig. 3 and p. 176) recently claimed autoliths (e.g., Fig. 17), and their typical occur- simply are no microscopic criteria for identifying that the dense liquids ponded (stagnated?) on the rences suggest they were dragged down to the it. Most who study layered intrusions understand cumulate floor of the magma body, with the ap- cumulate floor amid swarms of denser blocks. this (cumulate recrystallization and replacement parent implication that the ponding would have This argument assumes, however, that the have been recognized for years), but McBirney prevented current sedimentation. mineral and liquid densities only became indi- and Hunter (1995) found it reason to criticize cu- Our own calculations of the liquid compositions vidually significant during the actual sorting of mulate concepts and terminology. Our view on and densities are still provisional, but they too in- the modally graded layers. The challenge, then, is this matter is that, if replacement anorthosite is dicate that plagioclase should have floated through to identify a mechanism whereby the minerals in mistaken as an adcumulate, or vice versa, it prob- much of the crystallization history of the magma. a suspension current might be separated from one ably is because the interpreter did not properly They require, however, that the differentiation another while the liquid is fractionated away. evaluate the field relations. Even if the field crite- process yield a substantial portion of granophyre, ria cannot be applied, or if the distinction is hazy and as Hunter and Sparks (1987) showed, when Boundary Flow Separation and Reattachment for particular occurrences, that does not mean that this is incorporated into the estimated mafic liq- cumulate concepts and terminology are funda- uids, their densities decrease markedly, and pla- In this section and the two that follow, we il- mentally invalid or misleading. gioclase buoyancy becomes very slight. lustrate two processes of graded cumulate layer More important, though, we believe, is our deposition from magmatic currents and show DEPOSITION OF MODALLY SORTED observation that the sorting and deposition of the how they might have functioned in the Skaer- LAYERS AND AUTOLITHS fragmental layers were essentially controlled by gaard intrusion. The concepts derive from flume bulk density relationships. On this basis, even experiments by Irvine (1978, 1980a), but we now Dense Liquids and Buoyant Plagioclase the plagioclase-rich tops of the modally graded place greater importance on boundary flow sepa- layers (with their typical mafic mineral contents ration and reattachment as a controlling mecha- The contention that the Skaergaard Layered of 15%–25%) are dense enough to have been de- nism in the mineral sorting, and we apply the sec- Series could not have accumulated by magmatic posited by the most iron-rich liquids that anyone ond process to the Upper Border Series as well as sedimentation arose when Bottinga and Weill has proposed, and virtually all cumulate units in the Layered Series. (1970) calculated densities for the iron-rich liq- the Layered Series could have involved suspen- Flow separation is a phenomenon that occurs

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A B

C D

Figure 24. Photographs of anorthositic bodies showing replacement and reaction effects. (A) Bodies of secondary anorthosite in LZa on Ut- tental Plateau. The complex, partly transgressive shapes are typical, as is the dark (olivine-rich) fringe visible on right (stratigraphically lower) side of the largest body. (B) Zones of anorthositic replacement with medial mafic segregation veins, in the MZ (Middle Zone) block shown in Fig- ure 14C. The replacement appears to have been arrested while mafic constituents were being removed via the dark central pyroxene vein. (C) Corner of an anorthositic body approximately 25 m by 20 m in size, in LZb on Uttental Plateau (Fig. 16, location 2). The sharp truncation of the adjoining layers and the mafic fringe are typical of replacement anorthosite, and if the entire body formed that way, then it is the largest of its kind we have seen in the Layered Series. However, the overall form and relations of the body suggest that it might be a partly replaced autolith. (D) Small autoliths in MZ on Kraemer Island. The upper one does not appear leached of mafic minerals, but has a mafic fringe on top that might represent a reaction effect; the thin lower slab is anorthositic, but has only a faint upper mafic fringe.

in the boundary parts of currents advancing refer to the vortices of separation and reattach- ers formed by current deposition, but our diagrams against an adverse pressure gradient (Schlichting, ment as “S-R cells.” show that it is a fine line (essentially defined by a 1968, p. 121–123; Tritton, 1977, p. 106–109). In sedimentary processes, flow separation and threshold velocity gradient) that distinguishes the The general effect, portrayed schematically in reattachment have a wide variety of effects (Allen, two processes. Prograde sedimentation tends, Figure 25A, is that the fluid next to the boundary 1984). Their roles in prograde deposition and moreover, to produce lateral facies variations in can advance more rapidly by breaking away from headward erosion are illustrated schematically in sediments, and such variations are common in the it and reattaching downstream, a vortex cell de- Figure 25, B and C, and through these diagrams it cross-bedded belt (e.g., Irvine, 1987, Fig. 34). veloping on the wall in between. On separation, is easy to envisage them controlling the alternate the flow often, but not necessarily, becomes tur- episodes of layer formation and erosion that are so Layers Formation via Density Surge Currents bulent, and it may reattach in either laminar or prominent in the Skaergaard cross-bedded belt (cf. turbulent form. We assume laminar flow here Fig. 4E). Some authors have argued that, just be- Our concept of the deposition of a modally because it is may be more likely at magmatic vis- cause the unconformities of the cross-bedded belt graded layer by a surge-type density current is il- cosities, and it is easier to illustrate. Hereafter, we imply current erosion does not mean that the lay- lustrated in Figure 25, C–D. The current structure

Geological Society of America Bulletin, November 1998 1437

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/11/1398/3382835/i0016-7606-110-11-1398.pdf by guest on 27 September 2021 Figure 25. Schematic diagrams illustrating boundary-layer flow separation and reattachment and the deposition of a modally graded layer by a density surge current. The streamlines ostensibly portray instantaneous flow relations that are persistent but not truly steady. They are drawn relative to flow points R, S, and V, which may move as indicated by the large arrows at points V. (A) Steady-state vortex S-R cell of flow separa- tion and reattachment (cf. Schlichting, 1968, Figs. 16–14). (B) Prograde sedimentation by way of flow reattachment. In real situations, the vor- tex cell can also fill or empty in the third dimension. (C) Headward erosion caused by flow separation. The vortex cell may empty as indicated, or expand, and in real situations, it could be fed via the third dimension. (D) Structure of a density surge current, idealized from experiment re- sults by Irvine (1978, 1980a). The main body of the current effectively rolls forward, in part because of gravitational acceleration when the floor slopes as shown here, but more essentially because of its own inertia, as is evident when the floor is flat. The head vortex at the front and the zone of vortices along the back facilitate the advance (and they may variously form and be shed in this capacity); the tail at the rear is left because of joint drag effects of the no-slip floor and the overlying host liquid. The rapid flow zone (hachured) encloses materials that are gaining on the front tip of the current at R (so its motion determines the current flow). Host liquid caught beneath the current at R usually escapes upward to the vor- tex zone via cleftlike channels associated with roller circulation parallel to the flow direction (cf. Allen, 1984, v. I, Figs. 1–6 and 1–28). For further description see text. (E) Inferred sorting of plagioclase, mafic minerals, and liquids in a magmatic density current. The current is being deceler- ated by the drag of the floor and the resistance of the host liquid; thus the dense mafic minerals are shifting forward as well as settling, with the effect that they are selectively caught by the no-slip floor drag to form the bottom of the graded layer, and plagioclase lags behind and is stripped off by the host-fluid drag to form the layer top. The bulk tendency of the crystals to shift forward and down also expels current liquid into the vor- tex zone, where experiments indicate that it mixes with the host fluid.

1438 Geological Society of America Bulletin, November 1998

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/11/1398/3382835/i0016-7606-110-11-1398.pdf by guest on 27 September 2021 Figure 26. Diagrams depicting current deposition of some Skaergaard layering structures involving autoliths. Small solid circles represent crys- tals of mafic minerals; open circles, plagioclase crystals. In A, B, and C, streamlines portray instantaneous flow relative to the moving reference point R at the front tip of the current; in D, motion is relative to the floor. Pathlines similarly depict trajectories of specific fragments through a period of flow. (A) Formation of a Layered Series graded fragmental layer with the fragments concentrated in its upper part (as in Fig. 9, D and E, and 10B). (B) Deposition of slablike autoliths with reverse imbrication (as in Fig. 9C). Implied is that this process can also deposit slabs at steep inclinations (as in Fig. 9, D and F). (C) Draping of a layer over a medium-size block (perhaps as in Figs. 9B, 10A, and 15, location 2). The current separates and reattaches on both lee and stoss sides of the block as it transfers across it, with the effect that its deposits lap up on the block’s sides

and thin over its top. (D) Uprooting and inverted redeposition of layers (from S1 to R1, and S2 to R2) due to low-angle impact by a medium-size block moving from 1 to 2 (as in Fig. 14, C and D). Note: the displaced layers are always in front of the block as it moves from 1 to 2.

Geological Society of America Bulletin, November 1998 1439

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is based on experimental models by Irvine (1978, 1980a), and his scaling of the models showed that analogous currents in basaltic magmas would flow at rates of 2 to 6 km per hour (depending mainly on their size and relative density and the slope of the floor). Thus, given that currents large enough to travel several kilometers across the Skaergaard cumulate floor could be generated by slumping in the cross-bedded belt at the edge of the magma chamber, the crystals could be rede- posited as layers far into its interior in less than an hour. Density currents on the ocean floor flow for hundreds of kilometers in hours. The capacity of such currents to acquire and maintain momentum is extraordinary. The currents that deposited the Skaergaard intrusion fragmental layers were lo- cal, however, so they probably stemmed from cas- cades of crystals and debris that plummeted di- rectly to the floor from the Upper Border Series. The flow structure of the surge current por- trayed in Figure 25D is drawn relative to the moving point R at its front tip in order to reveal its internal circulation. Most streamlines, there- fore, are directed to the rear, including those within the floor, but material in the indicated “rapid flow zone” is gaining on the front tip, so this zone defines how the current advances. The current as a whole divides naturally into (1) a main body; (2) a head vortex plus one or more back vortices; and (3) a “tail.” The main body ef- fectively rolls forward along the floor and is an S-R cell. The vortices function like roller bear- ings to facilitate advance against the resistance of the overlying host liquid, and in this capacity, they may also split and be shed (effects not illus- trated). In experimental models of aqueous den- sity (turbidity) currents, the vortex zone typically appears turbulent, but the main body usually is not, and scaling of the Irvine (1978, 1980a) ex- periments suggested that this relationship would probably also obtain in large magmatic currents. The tail of the current comprises materials that are left behind because of the joint drag effects of the floor and the host liquid, and we contend that it represents the cumulate layer that would be de- Figure 27. Diagrams depicting the formation of modally differentiated layers from convective posited by a magmatic current. boundary-layer currents exhibiting separation-reattachment vortex cells (S-R cells). (A) Con- As a density surge current advances, it pro- current formation of thin, modally graded, isomodal (uniform), and fragmental layers in the duces a wavelike motion of the host liquid, and Layered Series. The mineral sorting is effected within the S-R cells; fragments can end up in iso- in Figure 25, D and E, point R denotes where modal as well as graded layers (in contrast to Fig. 26, A and B); and the plagioclase concentrated this liquid parts from the floor, as well as where at the top of the graded layers is retained at the floor, even though it might be slightly buoyant, the main current body attaches. Point S is because it is immediately covered by dense unsorted cumulate. Note, too, that because the cells where current materials of the rapid flow zone advance more slowly than the general convective flow, the graded layers are effectively being

separate from the floor to move forward, and T1 deposited by slow currents rather than fast ones, as in the case of density surge currents in Fig- at the rear ostensibly marks where the host liq- ure 26. (B) Three-dimensional depiction of the same process portraying the elimination of frac- uid reattaches to the top of the new floor tionated liquid from the S-R cells (cf. Allen, 1984, v. II, p. 101–132). (C) Formation of Upper Bor-

formed by the tail. The forward motion from T1 der Series layers with autoliths. A key factor is the heat loss to the roof, because it causes dense to S represents the tendency of currents to drag materials to be captured and frozen in place. The mineral sorting in the S-R cells is reversed their tails behind them, and we suggest that the from that in A, so here too the layer grading tends to be gravitationally “normal” (from mafic to differential motion involved here may be sig- feldspathic upward, as in Figs. 5 and 19B), even though accumulation is from the top down. Note nificant in the production of mineral lamination also the “underside draping” beneath autoliths (as in Fig, 19, C and D).

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host-liquid drag to form the leucocratic upper part.

The reattachment of the host liquid at T1 finally brings the plagioclase crystals close to the floor, even though they might be slightly buoyant. We listed several reasons earlier (Concepts and Prob- lems of Layer Formation) why buoyant plagio- clase crystals might stay on the floor after being deposited there, and the one we favor here is the simplest; i.e., that they were soon covered by lay- ers of denser cumulate. Thus, if the host fluid were moving as a slower current depositing an unsorted cumulate of mafic minerals and plagioclase, then as soon as the surge current passed, the resumed deposition of this cumulate would cover the pla- gioclase-rich top of the graded layer and thereby prevent it from floating away. Figure 26 illustrates the formation of four dif- ferent block-and-layer structures observed at the Skaergaard intrusion. Diagram A depicts the dep- osition of a graded fragmental layer (like those in Fig. 10B). The mafic minerals in the current shift forward and accumulate at the bottom of the layer, while the fragments concentrate where they are approximately at neutral buoyancy in the feldspathic suspension that lags behind, and thus are deposited in the top part of the layer. Diagram B shows slablike autoliths being deposited with reverse imbrication (like those in Fig. 9C). In wa- ter-laid sediments, imbricated particles (such as shale fragments) characteristically are shingled in the upstream direction, but we suggest that, with viscous magma and the larger autolithic slabs, the conveyor-belt circulation of the current will turn the slabs over as they approach the floor and shingle them downstream. This kind of particle rotation was observed in the experimental mod- els (Irvine, 1978, Fig. 49A), and it could also yield steeply inclined slabs like those visible in Figure 9, D and F. Figure 26D portrays layers being uprooted by Figure 28. Modal and (average) grain-size variations in three thin, modally graded layers in the low-angle impact of a medium-size block; they UZa (data redrawn from Conrad and Naslund, 1989, Figs. 2, 6, and 9, after correction of the are redeposited in approximately reverse order, as grain-size dimensions in their Figs. 6 and 9 from square centimeters to square millimeters and in the structure in Figure 14, C and D. We suggest

conversion of their area measures to linear values by taking square roots). Also shown are min- that the maximum transport distances, S1R1 and eral (m) sorting velocities (v) relative to pyroxene based on equation 1 in the text. Abbreviations: S2R2, might approximate the length of an S-R cell. pl—plagioclase; ol—olivine; px—pyroxene (ferroaugite); ox—Fe-Ti oxide. For the velocity cal- ρ δρ δρ δρ δρ culations, we let liquid = 2.76; hence ol = 1.1; px = 0.8, pl = –0.08; and ox= 2.3, all in Formation of Layering via Boundary Flow g/cm3. Values for r were set to half grain size. Separation and Reattachment

Although surge-type density current deposition in graded cumulate layers. (In real currents, ing forces, so together with gravitational accelera- seems an appropriate origin for some of the block-

T1S may be proportionately much longer than tion they should cause the denser mafic minerals to and-layering structures, we conclude that it is not indicated here.) migrate continuously forward and down, and the a satisfactory explanation for most Skaergaard in- Figure 25D shows our inferences concerning lighter plagioclase crystals to lag behind (see the trusion graded layers, even some of the fragmental mineral sorting by the current. To advance, the cur- small arrows on selected particles). The predicted layers. One problem is the enormous number of rent must overcome floor drag and displace host result is that the mafic minerals will be continu- modally graded layers in the intrusion. It is trou- liquid. Although these are both decelerating effects ously stripped from the front bottom of the current bling to think that a separate density current has to in a fixed reference system, in terms of the moving by the floor drag to form the melanocratic lower be invoked to account for each occurrence. An- current framework they would influence individ- part of the graded cumulate layer, while the pla- other problem concerns the characteristics of the ual suspended mineral grains as relative accelerat- gioclase crystals are stripped from the rear by the intervening “uniform” (isomodal) layers (see Fig.

Geological Society of America Bulletin, November 1998 1441

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/11/1398/3382835/i0016-7606-110-11-1398.pdf by guest on 27 September 2021 Figure 29. Diagrammatic synthesis of processes occurring in the western half of the Skaergaard magma chamber early in its MZ stage of dif- ferentiation (cf. Fig. 3). The streamline pattern is based on convection experiments by Irvine (1980c, Fig. 3). The main rock units and the larger autoliths are drawn roughly to scale; the inclusions and banding in the Marginal and Upper Border Series, and probably the separation- reattachment (S-R) flow cells, are all exaggerated for illustration. Layered upper-border oxide gabbro enclosing small autoliths is shown being formed and stoped at the same time, which is possible, but not in the way it is illustrated. The diagram implies that the peridotite blocks and the sediment xenoliths in the Marginal Border Series are both derived from source units between the Archean gneiss and the Eocene basalt. Cooling along the top and side of the magma body causes crystallization, and the “two-phase” (crystals + liquid) boundary flow along the wall drives the overall convection. Some crystals are frozen to the roof and walls, while others are carried down and spread across the cumulate floor. The S-R cells deposit modally sorted layers in all three environments and release fractionated liquid (dotted streamlines) as in Figure 27B. The flow along the wall is relatively rapid, as indicated by the close spacing of the streamlines (many are omitted), but it slows abruptly with the slope change at the cumulate floor, and the resulting hydraulic jump and associated complex circulation patterns (such as Görtler vortices) cause al- ternate episodes of erosion and deposition, producing the cross-bedded belt. Blocks are stoped from the Upper Border Series in response to fault extension of the roof, and some of the small fragments are transported laterally, and thus are occasionally captured in upper border layers. Larger blocks descend more directly to the floor, and they occasionally contain small early autoliths as inclusions (as in the block in Figs. 17–19). The larger blocks settle in horizontal orientation, and at times they detach en masse, producing “autolith plumes.” The plumes are deflected, first one way, then another, by the broader convection; and as their contents are strewn over the floor, their finer debris occasionally breaks out into den- sity surge currents that deposit graded fragmental layers. The return convective flow is essentially a slow lifting of the magma, as implied by the wider spacing of streamlines, and it is shown transporting a residue of buoyant plagioclase back to the roof to produce the now-eroded anorthositic Upper Border Series facies (which is evidenced by the more leucocratic autoliths). Meanwhile, compaction causes the interior of the Layered Series repeatedly to shear off at its solidus front from the parts that have frozen to the wall, thereby producing the numerous normal faults observed in the cross-bedded belt. Water from fractures in the gneiss infiltrates the Marginal Border Series and the Layered Series subzones LZa and LZb, producing local pods of anatectic melt that eventually resolidify as mafic pegmatite. Implied also is that, during formation of LZa and LZb, some of this water dissolved in the crystallizing part of the intercumulus liquid, causing its cotectic composition to shift toward plagio- clase. Then as compaction filter-pressed this hydrous liquid upward toward the main magma body, the water began to exsolve, and the cotectic shift reversed, causing the liquid to reabsorb cumulus mafic minerals and precipitate plagioclase in their place to produce the illustrated bodies of secondary (replacement) anorthosite.

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4A). Although the thicknesses of these layers can S-R cells advance on the cumulate floor at about that all minerals have identically the same grain- vary greatly in any given area, they clearly increase 0.3 times the velocity of the convective flow, and size distributions, the densest mineral should in a general way from around 5–30 cm in LZc on under the roof at about 0.5 times. These crude es- concentrate in the bottom part of a graded layer, Kraemer Island to about 1–3 m among the trough timates derive from considerations of the amount and the least-dense one should be enriched at its layers in UZa. In their alternations with thin, of liquid that might have been fractionated out of top—the distribution that characterizes “normal modally graded layers, the uniform layers typi- the crystal-liquid suspension during deposition of modal grading.” cally begin and end sharply (even where the the sorted cumulate layer. Thus, if the flow sus- 2. Under the same conditions, minerals of in- graded layers are fragmental), and there is little or pension crossing the floor carried 15% crystals termediate density should also show normal no sorting or grading at their bottom and top con- and deposited a cumulate layer containing 50% modal grading, but because modal proportions tacts. This last characteristic is very distinctive and crystals, then the layer would represent 30% of necessarily sum to 100%, abundance patterns requires explanation. its contents. If the suspension flowing under the may be controlled in complicated ways by the Figure 27A depicts the modally graded and uni- roof carried 20% crystals and deposited a cumu- grading of the most and least dense minerals. In form layers (and even fragmental layers) that are late layer containing 40% crystals, this layer principle, though, the intermediate minerals deposited concurrently. The mineral sorting for the would represent 50% of its contents. These vol- should show successive modal maxima in the graded layers is effected by the circulation of the ume proportions are then converted to relative reverse order of their sorting velocities pro- S-R cells; their feldspathic tops can be retained flow rates on the further assumption that each cu- gressing upward through a graded layer. even if the plagioclase is slightly buoyant, because mulate unit derives from a flow layer of approxi- 3. A mineral having the same density as the their burial begins as soon as they are formed; mately the same thickness. liquid should not show any size grading. there is no occasion for the tops and bottoms of the 4. In the (unlikely) end-member circumstance intervening uniform layers to be sorted because PARTICLE SORTING AND GRADING that all minerals have the same density, and it is the crystals required for their formation are always DURING CURRENT DEPOSITION different from that of the liquid, the mineral rang- present immediately above the floor. The S-R cells ing to the coarsest grain sizes should show the advance more slowly than the general convective It was not possible in the Irvine (1978, 1980a) most pronounced size grading, whereas those flow, so the graded layers are deposited by a rela- experiments to study particle sorting by currents with fine upper limits should show very little. tively slow component of the flow system, rather (that would have required an apparatus much 5. If a mineral was denser than the liquid of the than by a fast one, as inferred in the case of a surge larger than could be accommodated in the labo- depositing current, it should show “normal,” current. The cells presumably form at places ratory), but the more important effects can evi- coarse-to-fine-upward size grading; if a mineral where the boundary flow is becoming clogged dently be inferred through the equation: could be deposited despite being slightly less with crystals, so a graded layer should be de- dense than the liquid, then it should show “re- posited much more rapidly than an equivalent = 2 ∆∆ρ ρ verse,” fine-to-coarse upward grading. vv12()( rr 12 ) (1) thickness of uniform layer, and although the thick- 12 6. A mineral with no grain-size variation cannot nesses of the uniform layers necessarily reflect the show any size sorting, but it might show modal

rates of their deposition, they are more an inverse where: v1 and v2 denote the sorting (relative) ve- grading because of its density characteristics. measure of the frequency of the S-R cells. Figure locities of two particles of differing size and den- The concepts of crystal sorting portrayed in ∆ρ 27B also depicts a way that liquid might be frac- sity; r1 and r2 are their nominal radii; and 1 and Figures 25–27 can be tested by comparing these ∆ρ tionated away from graded layers. By virtue of its 2 are the differences between their densities predictions with modal and grain size data from fluidity, the liquid selectively segregates within the and that of the host fluid (which for the Skaer- Skaergaard intrusion graded layers. The data cell and then is expelled laterally from its ends. gaard intrusion minerals was presumably the liq- we use, shown in Figure 28, were obtained by The process suggested here can also be applied uid in the current, but for the fragments and Conrad and Naslund (1989, Figs. 2, 6, and 9) to the modal layering in the Upper Border Series blocks was more probably the enclosing crystal- from three modally graded layers in UZa (their (particularly as represented in the oxide gabbro liquid suspension). The equation is based on L-1, L-2, and L-3). To make the comparisons block in Figs. 17–19). The key factor in this case, Stokes Law, and a derivation that gives it rela- more quantitative, we also show sorting veloci- besides the physical inversion of the system, is the tively broad applicability is presented in Appen- ties relative to pyroxene as calculated by equa- heat loss to the intrusion’s roof. (Losses through dix 1. The general rules of application are that the tion 1. We neglect the possibility of postcumulus the cumulate floor are probably not comparable, mineral grains with the largest positive velocities coarsening or recrystallization of the minerals, because the thicker Layered Series would have should be concentrated in the bottom part of a because these effects are not evaluated in the been a more effective insulator; cf. Irvine, 1970.) graded layer, and those with the smallest (or neg- data collecting. Our impression from examining With the upward heat loss, there can be a “capture ative) velocities should be enriched at the top. similar layers in the same area is that olivine and front” above which crystallization is sufficiently However, both the equation and these rules are de- the oxide minerals are usually coarsened more rapid to freeze crystals and rock fragments in ceptive in their simplicity. When several minerals than the plagioclase and pyroxene, but not enough place even though they are dense enough to settle. of various grain sizes and densities are involved, to undo their primary relations. The grading of the layers should tend to be gravi- the manifestations can be diverse. Unless other- Even though olivine is much less dense than tationally normal (from mafic to feldspathic up- wise specified, we assume that the minerals all magnetite and ilmenite (see Fig. 28, caption), it ward, as in Figs. 7 and 19C), even though accu- have similar grain-size populations, ranging from is as much as twice as coarse. Thus, in L-3 it has mulation is from the top down; but by contrast, very small sizes to individual maxima that are sig- about the same maximum sorting velocity as the the layers should show “under-side draping” be- nificantly reflected in averaged measurements. oxide minerals, whereas its values in L-2 and neath autoliths (as in Fig. 19, C and D). Implied, The rationale is pragmatic: the only data available L-1 are 1.5 to 2 times larger; and appropriately of course, is that even the Upper Border Series is for Skaergaard layers are averaged values. The (per the first of the general rules enunciated largely composed of transported solids. predictions are as follows. above), it shows about the same (normal) modal In Figure 27, we tentatively suggest that the 1. In the end-member circumstance (idealized) and grain-size grading as the oxides in L-3, and

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it is substantially more graded (especially in the caption, so we emphasize that the diagram is rock, showing that the replacing material was a size) in L-2 and L-1. meant to give a general impression, not to be rig- magmatic liquid, which then resolidified as peg- Plagioclase, by contrast, apropos to having orous in detail, and that no attempt is made to ex- matite. The presence of NaCl-rich fluid inclu- probably been slightly buoyant, exhibits strong plain macrorhythmic layering. sions in the pegmatite minerals suggests that the

upward modal enrichment in all three layers (per H2O originally was seawater brine, presumably prediction 1 above), despite showing almost con- SUMMARY OF OBSERVATIONS AND derived from neighboring marine sediments. stant grain size in L-1 and L-2 (per 3 perhaps) CONCLUSIONS 4. Small irregular bodies of replacement and a slight, but fairly definite upward coarsening anorthosite in the Layered Series probably also in L-3 (per 5). 1. If the source of the peridotitic blocks in the formed because of the infiltration of exogenous

The pyroxene (ferroaugite) is only slightly less Skaergaard Marginal Border Series is represented H2O. We suggest that the H2O first dissolved in dense than the olivine, but it has a much finer, al- by the Watkins Fjord peridotite (as is generally crystallizing intercumulus liquid, inducing a co- most constant grain size; and appropriately, it believed), then the blocks are almost certainly tectic shift toward plagioclase; then, as this liquid

shows no significant size sorting in any of the Tertiary, and not Archean in age. The Watkins filtered upward through the cumulate pile, the H2O layers (per 5, or perhaps 6) and (per 4) almost no Fjord peridotite is extraordinarily fresh, unlike the began to disperse, reversing the shift and causing modal grading (except perhaps for faint midlevel local Precambrian ultramafic rocks, whereas it is resorption of mafic minerals and precipitation of modal maxima in L-2 and L-1, per 2). comparable to several small Tertiary peridotite in- plagioclase in their place. Vapor transfer effects The oxide minerals exhibit, per 1, substantial trusions present along Kangerdlugssuaq. The in- may also have been effective. Associated fringes, normal size grading in L-3 where they reach termixing of Cretaceous-Paleocene sediment strands, and veins of ultramafic rock apparently their coarsest sizes, but per 4, they show only xenoliths with the Skaergaard intrusion peridotite represent part of the displaced mafic minerals. slight coarsening at the base of L-2 and none in blocks indicates that their sources were spatially 5. The Skaergaard intrusion is cut by basaltic L-1, layers in which they are relatively fine. associated, which also suggests that the peridotite dikes of at least three ages, and what is probably Their relative sorting velocities are also appro- is Tertiary in age, and a possible structural recon- the oldest dike includes an apparently unique col- priate to these relations. The oxides show dis- struction is demonstrated in an emplacement lection of xenoliths of troctolitic, anorthositic, tinct normal modal grading in L-3 and L-2 (per model for the intrusion. The block peridotite and gabbroic rocks, some of which appear appro- the general rule above), and they possibly show a might represent magma slightly older than the priate to represent the HZ. faint midlevel modal maximum in L-1 (per 2, Skaergaard magma, and the sediment xenoliths 6. Basalt xenoliths in the upper interior of perhaps). might have been metasomatized in it; but the MZ probably represent the intrusion’s roof, and The fragmental layers might also be consid- proposition that the peridotite is an early Skaer- one even contains what may have been part of ered. As described earlier, their fragments seldom gaard cumulate is not ruled out, because it is the the original roof contact. These xenoliths con- show any systematic size grading, but they tend only associated rock known to contain enough Cr stitute limited evidence that the middle of the in- to be concentrated in the leucocratic upper parts and Ni to balance the intrusion compositionally trusion’s roof was basalt, and not a capping of of the layers as though they had been sorted to against its chilled margins. gneiss breccia as once believed. positions of approximate neutral buoyancy as in 2. The Upper Border Series was probably dif- 7. The autoliths of troctolite, anorthosite, and Figure 26A. If the fluid medium “seen” by the ferentiated according to the same crystallization oxide gabbro that abound in the Layered Series fragments was the bulk crystal suspension, and order as the Marginal Border and Layered Series. were apparently stoped from the Upper Border Se-

not just the liquid fraction, then this distribution The reason this series is richer in SiO2,K2O, and ries along fronts recorded by at least one major un- accords with prediction 3. BaO than the Layered Series, and poorer in FeOT, conformity (Wager and Brown, 1968; Naslund, Two observations can also be made about the might be that it was infiltrated by fluids or magma 1984). However, the troctolite and anorthosite are apparent sorting velocities of the mineral grains from below. However, that requires petrographic both much more feldspathic than any the pre- in graded layers. They should show variations demonstration, and a simpler alternative is that the served upper border rocks, so there probably once appropriate to the sorting and grading, but as il- Layered Series compacted as it accumulated, was an anorthositic facies in the top of the intru- lustrated in Figure 25E: (1) there should rarely, thereby eliminating liquid with granophyric con- sion where it is now eroded away. The troctolite if ever, be “hydraulic equilibrium” (similar ve- stituents, whereas the Upper Border Series could and anorthosite blocks first appear in LZa, apropos locities) among different minerals, even at spe- not compact. The alternative is consistent with the to this stage when only plagioclase and minor cific levels in a layer; but (2) there should prob- proposal that the Skaergaard magma yielded a olivine were precipitating; the oxide gabbro blocks ably be gradual upward trends toward such substantial proportion of granophyre. appear near the top of LZc, shortly after the cumu- equilibrium. Both these conditions are met in 3. The mafic pegmatite that occurs as abundant lus arrivals of ilmenite and magnetite. All three au- the data in Figure 28. pods, veins, and patches in the Marginal Border tolith types are abundant in MZ, and three huge In summary, then, the main modal and grain- Series, and as minor but locally prominent lenses, anorthositic blocks toward the top of this zone cul- size characteristics of Skaergaard intrusion veins, dikes, and stratiform bodies in the Layered minate the stoping event; the basalt xenoliths mark modally graded layers and the particle sorting Series, was formed because its host rocks were the beginning of its end. Only a few autoliths are

concepts appear to be in excellent accord. infiltrated by H2O from outside the intrusion present in UZa and UZb, and none are in UZc. The while they were still hot enough to be fluxed and areal distribution of the autoliths suggests that they SYNTHESIS OF THE MAGMATIC remelted. The hydrous melt then resolidified with broke from the Upper Border Series because it was EVENTS pegmatite grain sizes, and in most bodies it also repeatedly destabilized by extensional movements yielded minor granophyre. Stratiform pegmatite in the intrusion’s floor. We attempt to illustrate in Figure 29 how all units locally follow modally graded layers with a 8. The physical relationships of autoliths to the the various processes discussed here might have precision that indicates formation by replace- Skaergaard layering show that, while the Skaer- functioned in concert, ostensibly at a stage early ment, but they also branch or offset, and in places gaard intrusion was crystallizing: (1) there was in MZ differentiation. The events are explained in the units contain disoriented inclusions of layered almost invariably a sharp interface between the

1444 Geological Society of America Bulletin, November 1998

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magma and the top of the accumulating Layered “propagate” into it, as has been claimed. The 19. The proposed concepts of layer sorting and Series; (2) the observed layering structures were broad alignment of the block layers with the host grading by currents are shown, on the basis of generally fully developed right up to this inter- strata is rationalized on fluid dynamic evidence Stokes Law considerations, to be consistent with face; and (3) the interface was highly dynamic, that slablike autoliths should settle in horizontal published data on the modal and grain-size char- being repeatedly struck by autoliths and locally orientation, and not rotate. acteristics of Skaergaard intrusion modally eroded while it was continuously swept by cur- 15. Some autoliths have fine-grained augite- graded layers. rents and coated with new cumulates. rich mafic to ultramafic rinds, generally 10–20 20. From the indications that the graded frag- 9. The ways that the smaller autoliths are cm thick, and some were cut by small dikes of mental layers were sorted according to bulk spread in the rhythmic modally graded layers and similar rock before (or when) they broke from the density relations in the transporting solid-liquid affect their structure show that these layers were Upper Border Series. It appears that these blocks suspension currents, the importance of the al- deposited by crystal-liquid suspension currents. were cooler than the Skaergaard magma and not leged high densities of the Skaergaard intrusion Fragments are concentrated in the upper parts of in equilibrium with it chemically; hence the rims liquids and the plagioclase flotation problem ap- some of the layers, indicating sorting in accord- formed by a process combining moderately rapid pear to have been greatly overemphasized. In ance with the bulk density of the suspension, and crystallization of the liquid with limited bimeta- terms of bulk density, even the plagioclase-rich some fragments are at high angles, suggesting somatic exchange between it and the blocks. tops of the rhythmic modally graded layers con- that the suspension was rich in solids. 16. Some autoliths underwent minor replace- tain enough cumulus mafic mineral grains to 10. The thicker, more extensive macrorhyth- ment by anorthosite before they detached from make them denser than any liquids that have mic layers often lap up against and drape over their source region, and some were similarly re- been proposed; and the only rocks in the Lay- large autoliths, indicating that they too were placed after they were embedded in the Layered ered Series that might have floated are some of significantly shaped by magmatic currents, but Series. These changes cannot always be identi- the truly anorthositic autoliths, and these could the overall process of their formation remains fied with certainty, but they are most pronounced have been carried down to the magma body unresolved. in small fragments, and comparatively rare in floor amid cascades (plumes) of denser blocks. 11. A few rock units in the Layered Series medium to large blocks. The larger blocks rarely Suspension currents appropriate to deposit the (e.g., the ridges between trough layers) may show any appreciable modification by chemical Layered Series cumulates would also have been have formed by primary in situ crystallization, exchange and reequilibration with the surround- dense enough to displace crystal-poor liquid but virtually no textural or structural evidence ing cumulates and their pore fluids, as has been from the magma floor, so ponding of dense liq- has been found of layering being formed by re- claimed. uid would not be expected, and the autolith field crystallization and mineralogical reorganiza- 17. Replacement anorthosite necessarily has to relations indicate strongly that it did not occur. tion, even around the autoliths where one might be identified by its field relations, because it can- be expect it to be most conspicuous. not be distinguished texturally (or, probably, APPENDIX 1 12. The frequent normal faults in the cross- chemically) from adcumulate anorthosite. Thus, bedded belt around the edges of the Layered Se- if replacement anorthosite is mistaken as a cumu- Derivation of the Relative Sorting Velocity ries indicate that compaction of the interior cu- late, or vice versa, it is because the observer did Equation mulates caused them repeatedly to shear off from not properly evaluate the field relations; it does those that had frozen to the intrusion walls. Com- not mean that cumulate concepts and terminol- The equation used in the text to compare the sorting paction effects are not usually detectable in the ogy are invalid or misleading, as some authors velocities of different minerals has traditionally been de- block and layering structures, apparently because have argued. rived simply as the ratio of the Stokes terminal (steady state) settling velocities of two spherical particles (e.g., of masking by the strong primary effects of block 18. Two mechanisms of graded layer deposi- Jackson, 1961, Table 3). Irvine (1987, p. 239–240) gave impact and layer draping. tion by magmatic crystal-liquid suspension cur- a derivation whereby the accelerating force could be in- 13. A substantial number of the larger au- rents are described. One, entailing density surge ertial rather than gravitational, but it too assumed toliths are internally layered, and some include currents, is used to illustrate the formation of steady-state velocities. By the present treatment, the ve- locities can vary. small autoliths of an earlier generation. Detailed (1) graded fragmental layers, (2) fragment imbri- There are two fundamental equations. One defines maps of several occurrences demonstrate that cation within layers, and (3) layers lapping up the accelerating force (Fa) on a particle: most layering within the blocks existed before against and draping over blocks. The other, based they broke from their source region, so it pre- on boundary flow separation-reattachment vor- F = 43πρr3 ⋅⋅∆ a (A1) sumably developed in the Upper Border Series. tices, is applied to the formation of alternating a One unusually large oxide gabbro block features graded and uniform layers in the Layered and Up- where r is radius, ∆ρ is the difference between its den- exceptionally well-developed layering enclosing per Border Series (and is probably pertinent to the sity and that of the liquid, and a is acceleration. The numerous small autoliths, and its relations sug- Marginal Border Series). By the second mecha- other (which is the true Stokes Law) gives the drag force due to the viscous resistance of the liquid: gest that, like the Layered Series, the Upper Bor- nism, the graded layers are deposited by a rela- der Series consists largely of transported materi- tively slow-moving part of the flow system, rather = π µ FD 6 r v , (A2) als. Evidently, heat loss to the roof was rapid than by a fast one as with the surge currents, and enough for crystals and autoliths with negative the process also fractionates liquid away from the where u is the liquid’s viscosity, and v is the particle’s buoyancy to be captured and frozen in place be- sorted crystals. Plagioclase in the tops of modally velocity. If these two forces are equated, a steady-state velocity can be defined. fore they could settle away. graded layers is prevented from floating away be- Assume instead, though, that the ratio of the two 14. The relations of the controversial layered cause it is immediately covered by dense “uni- forces is the same for each of two particles, 1 and 2: autolith in the cliff face of Wager Peak are shown form” cumulate. Inverted redeposition of layer 6πrvµ 6πrvµ to be generally identical to those of blocks sequences after uprooting by block impact “under- 11 = 22 (A3) π 3∆∆ρ π 3 ρ mapped elsewhere. The uplapping and draping of side draping” of layers beneath autoliths is also il- 43ra1 1 43ra2 2 the adjoining host layers show that they did not lustrated, as is in the Upper Border Series.

Geological Society of America Bulletin, November 1998 1445

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hence, (Transactions, American Geophysical Union), v. 54, jecting the lavas: Meddelelser on Grønland, v. 137, p. 507. p. 1–27. = 2 ∆∆ρ ρ Bottinga,Y., and Weill, D. F., 1970, Densities of liquid silicate Hunter, R. H., and Sparks, R. S. J., 1987, The differentiation of vv12()( rr 12 12 ). (A4) systems calculated from partial molar volumes of oxide the Skaergaard intrusion: Contributions to Mineralogy components: American Journal of Science, v. 269, and Petrology, v. 95, p. 451–461. By this derivation, the particle velocities are not nec- p. 169–182. Irvine, T. N., 1970, Heat transfer during solidification of lay- Boudreau, A. E., 1987, Pattern formation during crystallization ered intrusions. I. Sheets and sills: Canadian Journal of essarily at steady state; they could be increasing or de- and the formation of fine-scale layering, in Parsons, I., Earth Sciences, v. 7, p. 1031–1061. creasing. The tenet is that any change of acceleration ed., Origins of igneous layering: North Atlantic Treaty Irvine, T. N., 1974, Petrology of the Duke Island ultramafic will similarly affect both velocities, so their ratio will Organization ASI series C, v. 196, p. 453–471. complex, southeastern Alaska: Geological Society of remain the same. This seems appropriate for the cir- Boudreau, A. E., and McBirney, A. R., 1997, The Skaergaard America Memoir 138, 240 p. cumstance that the particles are being transported side- Layered Series. Part III. Non-dynamic layering: Journal Irvine, T. N., 1978, Density current structure and magmatic sed- by-side in the same current. of Petrology, v. 38, p. 1003–1020. imentation: Carnegie Institution of Washington Year Boudreau, A. E., Love, C., and Prendergast, M. D., 1995, Halo- Book, v. 77, p. 717–725. gen geochemistry of the Great Dyke, Zimbabwe: Contri- Irvine, T. N., 1980a, Magmatic density currents and cumulus butions to Mineralogy and Petrology, v. 122, p. 289–300. processes: American Journal of Science, v. 280A, p. 1–58. ACKNOWLEDGMENTS Brooks, C. K., 1973, Rifting and doming in southern East Irvine, T. N., 1980b, Magmatic infiltration metasomatism, dou- Greenland: Nature Physical Science, v. 244, p. 23–25. ble-diffusive fractional crystallization, and adcumulus This study stems from many field seasons at the Brooks, C. K., and Gleadow, A. J. W., 1977, A fission-track age growth in the Muskox Intrusion and other layered intru- for the Skaergaard intrusion and the age of the East sions, in Hargraves, R. B., ed., Physics of magmatic Skaergaard intrusion, and we are indebted to nu- Greenland basalts: Geology, v. 5, p. 539–540. processes: Princeton, New Jersey, Princeton University merous people for support and assistance. Princi- Brooks, C. K., and Nielsen, T. F. D., 1978, Early stages in the Press, p. 325–383. differentiation of the Skaergaard magma as revealed by a Irvine, T. N., 1980c, Experimental modeling of convection in pal contributors (in roughly chronological order) closely related suite of dike rocks: Lithos, v. 11, p. 1–14. layered intrusions: Carnegie Institution of Washington are Hatten Yoder, Alexander McBirney, Leslie Brooks, C. K., and Nielsen, T. F. D., 1982, The Phanerozoic de- Year Book, v. 79, p. 247–256. Coleman, Douglas Stoeser, Alan Kays, Richard velopment of the Kangerdlugssuaq area, East Greenland: Irvine, T. N., 1980d, Observations on layering in the Skaer- Meddelelser om Grønland, Geoscience, v. 9, p. 3–30. gaard intrusion: Carnegie Institution of Washington Year Naslund, Jim Hoover, Tony Chen, Denis Norton, Brooks, C. K., and Nielsen, T. F. D., 1990, The differentiation Book, v. 79, p. 257–262. Dennis Bird, Ella Hoch, Eric Schou Jensen, Rune of the Skaergaard intrusion: Comment: Contributions to Irvine, T. N., 1982, Terminology for layered intrusions: Journal Larsen, and Karen Bollingberg. In 1989 and 1990, Mineralogy and Petrology, v. 104, p. 235–240. of Petrology, v. 23, p. 127–162. Brown, G. M., and Peckett, A., 1977, Fluorapatites from the Irvine, T. N., 1983a, Observations on the origins of Skaergaard logistical support was made available by Platinova Skaergaard intrusion, East Greenland: Mineralogical layering: Carnegie Institution of Washington Year Book, Resources, Ltd., and we thank Robert Gannicott, Magazine, v. 41, p. 227–232. v. 82, p. 284–289. Buddington, A. F., and Lindsley, D. H., 1964, Iron-titanium ox- Irvine, T. N., 1983b, Skaergaard trough-layering structures: William Mosher, and Patricia Turner for this and ide minerals and synthetic equivalents: Journal of Petrol- Carnegie Institution of Washington Year Book, v. 82, other courtesies. Nabil Boctor assisted with some ogy, v. 5, p. 310–357. p. 289–295. of the electron microprobe analyses. Irvine’s work Coats, R. R., 1936, Primary banding in basic plutonic rocks: Irvine, T. N., 1987, Layering and related structures in the Duke Journal of Geology, v. 44, p. 407–419. Island and Skaergaard intrusions: Similarities, differences was funded partly by the Carnegie Institution of Conrad, M. E., and Naslund, H. R., 1989, Modally graded and origins, in Parsons, I., ed., Origins of igneous layer- Washington and partly by U.S. National Science rhythmic layering in the Skaergaard intrusion: Journal of ing: North American Treaty Organization ASI series C, Foundation grants EAR-79-11188 and EAR-83- Petrology, v. 30, p. 251–269. v. 196, p. 185–245. Douglas, J. A. V., 1961, A further petrological and chemical in- Irvine, T. N., 1991, Emplacement of the Skaergaard intrusion: 08933. Brooks was supported by the Danish Sci- vestigation of the upper part of the Skaergaard intrusion, Carnegie Institution of Washington Year Book, v. 91, ence Foundation, and Andersen’s contributions East Greenland. Part I, The Basistoppen Sheet. Part II, p. 91–96. The Upper Border Group [Ph.D. dissert.]: Oxford, United Irvine, T. N., and Stoeser, D. B., 1978, Structure of the Skaer- stem from a Master’s thesis study supported in the Kingdom, University of Oxford, 200 p. gaard trough bands: Carnegie Institution of Washington field by Brooks and supervised at Aarhus Univer- Douglas, J. A. V., 1964, Geologic investigations in East Green- Year Book, v. 77, p. 725–732. sity by Richard Wilson. The manuscript was re- land, Part VII. The Basistoppen sheet: A differentiated ba- Irvine, T. N., Keith, D. W., and Todd, S. G., 1983, The J-M plat- sic sill enclosed in the Skaergaard intrusion, East Green- inum-palladium reef of the Stillwater Complex, Montana: viewed at various stages by Alan Boudreau, David land: Meddelelser om Grønland, v. 164, p. 1–66. II. Origin by double-diffusive convective magma mixing Mogk (twice), Tony Morse, Richard Naslund, and Hamilton, E. I., 1963, The isotopic composition of strontium in and implications for the Bushveld Complex: Economic Richard Wilson. the Skaergaard intrusion, East Greenland: Journal of Geology, v. 78, p. 1287–1334. Petrology, v. 4, p. 383–391. Jackson, E. D., 1961, Primary textures and mineral associations We acknowledge also the contributions of Hess, H. H., 1960, Stillwater igneous complex, Montana: A in the Ultramafic Zone of the Stillwater Complex, Mon- Lawrence R. Wager, W. Alexander Deer, and Sir G. quantitative mineralogical study: Geological Society of tana: U. S. Geological Survey Professional Paper 358, America Memoir 80, 230 p. 106 p. Malcom Brown (who died while the paper was in Hirschmann, M., 1992, Origin of the transgressive granophyres Kays, M. A., and McBirney, A. R., 1982, Origin of picrite its final stages of preparation). From our perspec- from the Layered Series of the Skaergaard intrusion, East blocks in the Marginal Border Group of the Skaergaard tive, Wager ranks as the most remarkable geologist Greenland: Journal of Volcanology and Geothermal Re- intrusion, East Greenland: Geochimica et Cosmochimica search, v. 52, p. 185–207. Acta, v. 46, p. 23–30. of the twentieth century, and we have been privi- Hirschmann, M., Renne, P. R., and McBirney, A. R., 1997, Kays, M. A., McBirney, A. R., and Goles, G. G., 1981, Xeno- leged in being able to build on the work that he and 40Ar/39Ar dating of the Skaergaard intrusion: Earth and liths of gneisses and the conformable, clot-like gra- his coworkers did on the Skaergaard intrusion. Planetary Science Letters, v. 146, p. 645–658. nophyres in the Marginal Border Group, Skaergaard in- Hoover, J. 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K., 1984, The geom- placement in the Skaergaard intrusion, East Greenland: etry and high temperature brittle deformation of the MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 28, 1996 Preliminary observations: Chemical Geology, v. 88, Skaergaard intrusion: Journal of Geophysical Research, REVISED MANUSCRIPT RECEIVED MAY 19, 1997 p. 245–260. v. 89, p. 10178–10192. MANUSCRIPT ACCEPTED SEPTEMBER 29, 1997

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