In the Skaergaard Intrusion: Geologic Relations and the Origins of Rhythmic Modally Graded Layers

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. Geological Society of America Bulletin, November 1998 1399 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 IRVINE ET AL. 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

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