High-Temperature Ultramafic Complexes in the North Norwegian Caledonides: I - Regional Setting and Field Relationships

High-temperature ultramafic complexes in the North Norwegian Caledonides: I - Regional setting and field relationships

M.C. BENNETT, S,R. EMBLIN, B, ROBINS & W.J.A. YEO

Bennett, M.C., Emblin, S.R., Robins, B. & Yeo, W.J.A. 1986: High+emperature ultramafic complexes in the North Norwegian Caledonides: I - Regional setting and field relationships. Nor. geol. unders. Bull.405, r40.

Four major (25-100 km¡) ultramafìc complexes were developed in the Sørøy Nappe during the second phase of Finnmarkian deformation in the North Norwegian Caledonides. They were emplaced into layered mafic intrusions and show broadly similar emplacement histories and petrographic variations suggesting a related petrogenesis. The Nordre Bumannsford, Melkvann and Kvalford group of ulramafic complexes developed in part by emplacement ofboth replacive and dilational ne-normative ultramafic sheets and dykes which progressively fragmented the layered olivine gabbro envelope. The earliest dykes and sheets are from a few cm to l00m wide and show highly inegular, nondilational contacls. They are dominantly coarse-grained, xenolithic olivine clinopyroxenite varying locally to dunite, poikilitic wehrlite, feldspathic olivine clinopyroxenite and olivine melagabbro. Iater dykes are from a few cm to 20 m wide and also lack chilled margins but show regular, dilational contact relationships. They include poikilitic wehrlite (commonly spatially associated with veins and patches of secondary wehrlite and dunite), olivine clinopyroxenite, olivinehornblende clinopyroxenite, hornblende peridotite and homblende melagabbro. Some members of the dyke suite exhibit mineral lamination, modal layering and cyclic units, and others contain mosaic-porphyroclastic spinel lher¿olite nodules. Variably foliated and metamorphosed olivine gabbro, ankaramite and picrite dykes with chilled maryins were emplaced during the latest stages in the evolution ofthe complexes. The Reinfiord ultramafìc complex contains pronounced, subhorizontal modal layering and evidence of at least 3 major magmatic events. The earliest parts of the complex comprise hy-normative wehrlite and olivine clinopyroxenite (olivine and clinopyroxene-olivine cumulates) containing abundant blocks derived from the subalkaline layered gabbro envelope as well as blocks of crudely layered or massive olivine websterite and websterite derived from marginal zones. Plagioclase is present only in marginal zones and in cumulates in the vicinity ofgabbro xenoliths. The trânsgressive core ofthe complex is composed of dunite and poikilitic wehrlite (olivine cumulate) and is associated \Mith the development ofsecondary dunite and poikilitic wehrlite from earlier cumulates. The olivine-rich rocks are traYersed by dense swarms of veins and dilational dykes of dunite, wehrlite, lherzolite, olivine clinopyroxenite and olivine gabbro as well as amphibole-rich rock types. The intrusive æquences and petrographic variations represented in the complexes suggest the successive emplacement of mantle-derived magmas with a diminishing number of liquidus phases, accompanied by ln sltø liquid-crystal fractionation, subsidiary hybridization and metasomatism. M.C. Benneu, S.R. Emblin & W.J.A. Yeo, DepL of Geology, Ilniveßity of Bristol, l{ills Memoriøl Building, Queens Road, Bristol BS8 IRJ, England B. Robins, Geologisk instituu, ovd. A, Allëgt. 4 1, 5014 Universitetet i Bergen, Norway

Introduction The Seiland magmatic province of V/est Finn- fic complexes include those of Barth (1953, l96l) mark and North Troms (Roberts 1974, Robins and Oosterom (1963). & Gardner 1975) contains a variety of predomi- During the last two decades, research on the nantly basic plutonic rocks emplaced during the stratigraphic, structural and metamorphic evolu- Finnmarkian phase of the Caledonian orogeny tion of the province (Sturt et al. 1975, 1978, (Sturt et al. 1978). Although the province is Ramsay et al. 1985) and radiometric age deter- dominated by large syn-orogenic layered gab- minations (Pringle & Sturt 1969, Sturt et al. bros, it also contains several major ultramafic 1978) have provided a detailed temporal frame- complexes in addition to numerous small ultra- work of Finnmarkian events, a major feature mafic bodies. Early descriptions of the ultrama- of which involved the periodic injection of 2 M. C. Bennett, S. R. Emblin, B. Robins &W. J. A. Yeo NGU - BULL.4O5, I98ó

manlle-derived magmas into the dcveloping altcmpt to show that they share sufÏcient simi- orogenic zone (Robins & Gardner 1974, 1975). larities in rclativc ages, geological setting, petro- Although many ol the gabbro plutons ol thc logical variations and intrusive relations- province have becn studied in rccent years ships. lor thcm to bc regarded as a cogenetic (Hoopcr 1971. Robins & Gardner 1974. Gardne r group. Thcir characteristics are compared with 1980, Robins 1982), thc ultramafic complexes the Rcinljord complex where recent re-investi- have reccived littlc attention, except lbr thc gation suggcsts that whilc thc Rcinfjord intru- Reinfjord complex in thc southwest of the prov- sion rcnrains unique in its extensive develop- ince (Bennetl 1971. 1974) and more recently, nrcnt ol layering, its petrogenesis may well be the Nordre Bumannsfjord complex on western rclatcd to that ol the ullramafic complexes ol Seiland (Sturt et al. 1980). Sciland and Stjernøy. Ilcnnett (197 I, 1974) showcd the Reinford complcx to be a high-temperature syn-orogenic intrusion characterized by an unusual develop- Regional setting ofthe ment of ultramafic cumulates, and suggested that the crude concenlric zonation was produced ultramafic complexes by assimilation of the wall rocks.lJecause ol its T he re gional st ructural framework thick sequence olplagioclase-lree cumulates and The ultramafic intrusions of the West Finnmark its high temperature of emplacement Bennetl, and North Troms are confined to the Sørøy (1974) suggested that the complex may have Nappe, which is the uppermost tectonic unit crystallized from ultrabasic magmas. Sturt et within the Kalak Nappe Complex (Fig. i), a al. (1980) established that the Nordre Bumanns- group of thrust nappes constituting the Finn- fiord complex is also a high-temperature syn- markian allochthon of the northern segment of orogenic body characterized by extensive wall- the Scandinavian Caledonides (Ramsay et al. rock assimilation and argued that the pelrolog- 1985). Immediately to the east of the Seiland ical evolulion of this complex (and others in the magmatic province the Kalak Nappe Complex province, including that a1 Reinfiord) involved is thrust over Karelian metasediments and primary, relatively Fe-rich dunite magmas. metavolcanic rocks (the Raipas suite of Reitan Ultramafic complexes resembling those of the (1963) or Raipas Supergroup of Pharaoh et al. Seiland province do not appear to have been (1983)), with a thin autochthonous/parautoch- reported from elsewhere in the Scandinavian thonous cover of Vendian tillite, quartzite and Caledonides (Stephens et al. 1985). Discordant slate preserved in places beneath the lower ultramafic bodies involving rock types varying boundary of the nappe complex (Roberts & lrom dunite to clinopyroxenite are common Fareth 1974, Pharaoh et al. 1983). The thrust within the Lyngen gabbro but are an order of boundary represents a profound discontinuity, magnitude smaller (Randall 197 l), and the bringing the complexly-deformed, amphibolite- gabbro itself appears to be an ophiolite fragment facies rocks of the Sørøy Nappe above the (Furnes et al. 1980, 1985). According to Curry weakly metamorphosed and structurally simple ( 1975) the Honningsvåg ultrabasic complex, Karelian rocks and their Caledonian cover sequ- emplaced into Silurian metasediments within ence exposed in the Komagfiord and Altafiord the Magerøy Nappe of northernmost Norway, tectonic windows (Fig. l). Further to the east was intruded at a late stage in its development the Kalak Nappe Complex overrides the Gaissa, by xenolithic dykes and sheets varying from Jerta and I akseûord Nappes of low metamor- dunite to olivine melagabbro. Recent investiga- phic grades and traditionally regarded as par- tions by one of the authors (8.R.) show, how- autochthonous (but see discussion in Ramsay ever, that the ultramafìc rocks are olivine- et al. 1985). Southwestwards the Kalak Nappe chromite cumulates forming either [he basal Complex is progressively excised by higher parls of megacyclic units or the matrix to intrusi- nappes forming the late Silurian, Scandian on megabreccias which resulted from the dis- allochthon. ruption of pre-existing cumulates within a The Kalak Nappe Complex includes both magma conduit. far-travelled and less far-travelled nappes (the In this paper, we report results from recent distal and proximal allochthons of Ramsay et studies in the Melkvann, Nordre Bumannsfiord al. 1985) varying in metamorphic grade from and Kvalfiord ultramafic complexes in the lower greenschist facies in the lowest (proximal) northeastern part of the Seiland province and nappes to upper amphibolite facies in the hig- NGU - BULL. ¡105, 1986 High-temperatureultramaficcomplexes 3

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./ Base of Kalak Nappe Complex Major faults ./ Base of SØrØy Nappe 25km

Fig. l. Location and major tectonic features of the Seiland magmatic provice. Major ultramafic complexes (shaded black) are: L Melkvann; 2. Nordre Bumannsfiord; 3. Kvalf ord; 4. Reinfiord. Other ultramalic intrusions are: 5. Habuvarri (HappoÛell); 6. Kjerringford; 7. Hallarbukø (Havnbukt); 8. Nordbukt; 9. Tappe- luft; 10. Tverrfiôrddalen. 4 M. C. Bennett, S. R. Emblin, B. Robins &.W. J. A. Yeo NGU - BULL. ¡m5, 198ó

hest (distal) nappes. The nappes typically repear belong exclusively to the sillimanite zone, and stratigraphic couplets, in which the lower unit am phibolite-facies conditions persisted well into is either pre-Karelian gneiss or the Karelian D, (Robins l97l). Here, the local development rocks ofthe Raipas Supergroup, while the upper of granulite-facies parageneses in metasediments unit consists of either Karelian rocks or late (Barth 1953, 196 l. Krauskopf 1954, Oosterom Precambrian-Cambrian metasediments. Pri- 1963) is a consequence of the superimposition mary unconformities between the two units are of contact and regional melamorphism. preserved locally (Ramsay & Sturt 1977, Ram- With the exception of the late-stage alkaline say et al. 1979, 1985). The Sørøy Nappe, conrai- suite and associated intrusions, the igneous rocks ning the Seiland magmatic province, is the of the Seiland province generally show the uppermost distal allochthon and is characterized imprint of cooling al intermediate pressure by a couplet of pre-Karelian gneiss and Caledo- (6-10 Kb). This is expressed by the devel- nian metasedimenls which were clearly derived opmenl of pyroxene-spinel symplectites in oli- from an ensialic sedimentary basin and its vine-bearing gabbro (Oosterom 1963, Gardner Precambrian gneissic basement. & Robins 1974, Robins 1982) and the formation of garnet--quartz coronas between Ca-poor pyroxene plagioclase (Akselsen T he t ect o not hermal ev o lut io n and 1980, Gard- ofthe Sørøy Nappe ner I 980). The detailed emplacement hislory of the The lower stratigraphic unit of the Sørøy Nappe Sørøy Nappe is ambiguous. Pre-D. mylonites exposed along the southern and eastern part of present along the base of the nappe (Worthing present-day its outcrop, comprises Precambrian 1971, Akselsen 1980) and the folding of the paragneiss including feldspathic biotite schist, nappe boundary by major D, folds (Akselsen quartzite, pelitic gneiss and garnet gneiss (Aksel- 1980) suggest syn-D, translation (Ramsay et al. sen 1980). The overlying Caledonian cover 1985). The lower metamorphic grade of Caledo- sequence consists of a metasedimentary sequ- nian cover sequences within underlying nappes ence (Ramsay 1971, Roberts 1974), recording a points to final emplacement either during the transition from predominantly shallow-water D, phase of Finnmarkian deformation or the clastic deposition to turbidite-type sedimenta- subsequent Scandian orogenic phase (Ramsay & tion in a sequence has been dated as early Sturt 1977, Ramsay et al. 1985). Emplacement Middle Cambrian and younger on the basis of at a late stage in the orogenic evolulion is also archaeocyathids identified in limestone (Hol- suggested by the 40 km separation ofthe surface land & Sturt 1970, but see Debrenne 1984 for expression of the Seiland magmatic province a contrary view). from its associated + 100 mgal Bouguer gravily Caledonian deformation within the Sørøy anomaly (Brooks 1966, 1970). Reverse faults Nappe took place during two protracted phases and thrusts are also known to have affected the (D, and Dr), each initiated by episodes of strong alkaline suite and carbonatites which constitute folding and thrusting which were followed by the latest major intrusions in the Seiland provin- periods characterized by flattening strains (Ram- ce (Skogen 1980, Bruland 1980, Robins & Tysse- say & Sturt 1963, Ball er el. 1963, Roberrs 1968, land 1983). Ramsay 1971, Sturt et al. 1978), possibly related to gravitational spreading following nappe translation (Ramsay et al. 1985). The magmatic history The amphibolite-facies peak of the Barrovian ofthe regional metamorphism in the Caledonian cover Seiland province sequence was altained between the major defor- The emplacement of the high-temperature ultra- mational phases and metamorphic temperatures mafìc complexes occurred at a relatively late gradually waned throughout D, (Roberts 1968, stage in the protracted evolution of the Seiland Srurt & Taylor 1972, Sturt er al. 1978) (Table province .(Table l). The magmatism in the l). The D,-D, growth of garnet, staurolite, province, which gave rise to predominantly kyanite and sillimanite was accompanied locally basic, ultrabasic and alkaline rocks, commenced by the development of migmatites. In the Pre- during the first phases of Finnmarkian deforma- cambrian basement within the central portions tion (D,) and contiliued through the D,-D, peak of the Seiland magmatic province the D,-D, of regional metamorphism and the second phase regional metamorphic parageneses appear to of deformation (D,) (Robins & Gardner 1975). NGU-BULL.¡105, 198ó High-temperatureultramaficcomplexes 5

REGI ONA L RADIOMETRIC DEF MAJOR ORMATI ON METAMORPHISM INTRUSIONS ¡c¡s ¡F s.w N.E Locol ized folding ond Greenschist focies thrusli ng

Alkoline pyroxenite, Exlensiono I a+ao-¿çr olk,syenitef ¡¡o tectonics ? ne,syenite4 (K-Ar, .9 corbonotite, fenile nepheline) z è 3490 ! zl t't- o REINFJORD U.C N. BUMANNSFJORD, (Rb-Sr c w.r. F lotten i ng 0 Longstrond KVALFJ,, MELKVANN U.C isochron) strqi ns o gobbronori te Olivine gobbros o (grovi to tiono I E ú spreodi n9 ?) po Gobbronorite, perthosi te o o o Cylindricol folds, o E, vergence o z Thrusti ng t Diorite, monzonite, 1 Bqrrovion \¿ qtz. syenite omphibolite focies Sondlond - Middogsfi É lsza ! zz tto (gornet, stourolite, - Olderfjord Hosvik gobbronorite 2 's3o t ló Mo kyonite, sillimonite) gobbronori te (Rb-Sr w.r. ã E i sochrons) .9 z Jq Flottening o Bre ivi kbotn j u stroins o 1s¿o o Storelv - Husfiord, t tz ¡¡o = (grovi to tiono I E Hpnseby gobbrånorites spreodi ng ?) (Rb-Sr w.r. ! isoc hron) o Lorge-ompl i tude o o folds, vorioble L vergence Greenschist focies Thrusti nq * À8zn¡ = 1.42 x ro:ll o-l Table l, Summary of the tectonic, metamorphic and magmatic evolution of the Seiland province. Radiometric ages are quoted from the followed sources: l. Sturt et al. (1978).2. Pringle & Sturr (19ó9), Sturt et al. (1978). 3. Sturt et al. (1978). 4. Sturt et al. ( I 9ó7). For other references see Robins & Gardner (l 975) and text.

Pre-D, gabbroic intrusions are widespread in some olthe pre-D2 basic intrusions are spatially the province and typically form thick extensive associated with a suite of calc-alkaline iirtru- sheets, steep-sided dyke-like bodies or crudely sions including high-K diorite, monzonite and lens-shaped or lopolithic plutons emplaced in quartz syenite (Robins & Gardner 1975). both the Caledonian cover sequence and its Syn-D, activity in the southwestern part of Precambrian basement. These intrusions include the Seiland province continued with the em- the Storelv-Husfiord and Breivikbotn gabbro- placement of subalkaline basalt magmas (Robins norites on Sørøy (Stumpfl & Sturt 1965, Sturt & Gardner 1975). Both the small Marøen gabbro & Ramsay 1965, Speedyman 1983, Sturt & (Gronow 1967) and the major Langstrand laye- Taylor 1972) and the Hønseby gabbronorite on red gabbronorite (Bennett 1972, I9l4) þossess Seiland (Worthing 1971, Roberts 1974) all of tholeiitic characteristics, though the latter conta- syn-Dr age, as well as the Hasvik gabbronorite ins abundant xenoliths of highly-metamorpho- on Sørøy (Robins & Gardner 1974, 1975, Gard- sed paragneiss and its fractionation trend may ner 1980) and the Sandland-MiddagsÛell- possibly have been controlled by assimilation. -Olderfiord gabbronorite in the Kvænangen- The Langstrand gabbronorite is intruded by the (Hooper -Bergsfiord area l97l) of D,-D, age. Reinfiord ultramafic complex, described in the In several of the bodies cumulate structures and following text. textures are well preserved and some intrusions In the eastern part ofthe Seiland province the exhibit extensive cryptic variation. They are earliest major magmatic event during D, resulted uniformly of subalkaline aspect (Robins & in the emplacement of large, generally strongly Gardner 1975, Stephens et al. 1985). On Sørøy deformed gabbros associated with syenite (per- 6 M. C. Bennett, S. R. Emblin, B. Robins & W. J. A. Yeo NGU-BULL./m5, 1986 thosite) fractionates (Robins & Gardner 1974, a latc cxprcssion of thc D, phasc oldcformation 1975). These were followed by the emplacement (Sturt & Ramsay 1965. Skogcn 1980. Robins & of large plutons of olivine gabbro (the clino- Tysscland 1983) under condilions of lower pyroxene gabbros of Robins & Gardner (1974)) amphibolitc lacics to uppcr grcenschist lacics characterized by thick cumulate sequences con- mctamorphism (Ramsay & Sturt 1970, Skogcn taining aluminous diopside or salite, the absence 1980). Thc latcst mctamorphic cflects in thc of primary Ca-poor pyroxene and the early provincc wcre thc local dcvclopment of zcolite- appearance ol cumulus Fe-Ti oxides. The lacics paragencses in nephelinc syenite (Bruland mineralogy and chemistry of these cumulates r 980. are consistent with the fractional crystallization alkaline basalt parents (Robins & of olivine The absolute age of magmatic Gardner 1974, Robins 1982). The contacts of province the olivine gabbros are generally concordant events in the Seiland with the fabric of the host rocks. The latter, and Thc radiomctric ages listed in Table I suggesl the modal layering within the plutons them- that magmatic activity took placc in the Seiland selves, dip in most cases towards the centres provincc during middlc Cambrian to early of the plutons and defìne deep synforms. The Ordovician (Arcnig) time. On the basis ol the structures are suggestive of gravitational collapse rclativc age sequence, established on purely accompanied by viscoelastic deformation of the gcological criteria, the ultramahc complexes country rocks (Robins 1971, Petraske et al. would appear to have been emplaced between 1978). The olivine gabbro plutons are the loci 530 + 16Maand 490 + 27Ma. of emplacement of the Seiland-Stjernøy group of ultramafic complexes. The Langstrand gab- Plate-tectonic setting of the has a form to that of the olivi- bronorite similar province ne gabbros, and here also the southwestern part Seiland of the body is intruded by the Reinfiord ultrama- Until recently, palinspastic reconstructions of fic complex. the early-Caledonian evolution of the Seiland While thc Rcinljord ultramafic complex appe- province (Ramsay 1973, Robins & Gardner ars to rcpresent the last major intrusive event 1975, Sturt et el. 1978), although differing signi- in the southwestern part ofthe Seiland province, ficantly in detail, emphasized the transition from the Seiland-Stjernøy ultramafic complexes were a latc Precambrian-Cambrian rifled continental postdated by the Lille Kuliord intrusion, a small margin to a subduction-dominated regime. subalkaline layered body occupying an anomal- Finnmarkian deformation, metamorphism and ous position in the magmatic history (Robins & the magmatic activity represented within the Gardner 1974, 1975, Robins 1982). Members Seiland province were related in these models of an alkaline picrite-ankaramite-Mg-rich to persistent eastward subduction of oceanic alkaline olivine basalt dyke swarm cross-cut crust beneath a Baltic plate. The recognition of both the Lille Kufjord intrusion and the Sei- pre-Ordovician ophiolite fragments in the hig- lancl-Stjernøy ultramafites and are widely hest allochthonous units at several places along developed in the eastern and northern parts of the length of the Norwegian Caledonides (Furnes the province (Robins & Takla 1979). Some dykes et al. 1980, 1985) has, however, highlighted belonging to this widespread, and in places substantial deficiencies in these models and dense. swarm contain spinel lherzolite nodules Sturt et al. (1983) have suggested that initial of mantle derivation (Robins 1975). subduction in the middle to late Cambrian had The youngest magmatic events represented in a westward polarity, uplift of the Finnmarkian the northern and eastern parts of the province orogenic belt coinciding with, or succeeding, a led to the emplacement of a suite of alkaline changeover to eastward subduction. While a rocks and carbonatite in the form of isolated number of features of the Finnmarkian orogene- dykes and major complexes. The alkaline suite sis in the type area can be rather neatly explained includes alkaline pyroxenite, nepheline-free and within this plate-tectonic framework, including nepheline-bearing hypersolvus syenites and the internal deformation, P-T paths and displa- syenite and nepheline syenite pegmatites (Heier cement of tectonotratigraphic units, as well as 1961. Sturt & Ramsay 1965, Robins 1972,1974' ophiolite obduction (Sturt 1984), the interpreta- 1980). The alkaline suite and carbonatite intru- tion of the magmatism in the Seiland province sions were weakly to intensely deformed during remains largely enigmatic. Although the detailed NGU - BULL. /m5, 198ó High-temperatureultramaJiccomplexes 7 platc-tcctonic sctting of thc Sciland magmatic. fìc complexcs was aflected by similar magmatic provincc is as yct vcry poorly constrained, the and metamorphic events subsequent to their magmatism is still rcgardcd in a broad scnse dcvclopment and it is considered .reasonably as rclatcd to thc evolution ola destructivc platc ccrlain that thcy were emplaced contemporane- margin. Thc alkalinc igneous rocks and car- ously. From the close spatial and temporal bonatites do. however, tcnd to suggest a pcriod association of these ultramafrc complexes it ofcxtensional tcctonics during the latcst phases lollows that any major peirogenetic synthesis ol the evolution of the Finnmarkian orogen crccted on the basis of observations within one (Tablc l). complex should be equally applicable to the othcrs. Field relationships in the The Reinf ord complex (Bennett 197 1, 1974) is situated some 50 km to the southwest of the ultramafic complexes Seiland-Stjernøy ultramafìc plutons and crops Thc ultramafìc complexes within the Sciland out over an area of about 25 km' (Fig. l). The magmatic province lall naturally into two groups only other ultramafic bodies known in this part on thc basis of their spatial relationships' inter- olthe Seiland province are much smaller, dyke- nal structure, host rocks and petrography. The likc occurrcnces exposed in Tverrfiorddalen ultramafics exposed on the islands of Sciland betwccn the Svartfiell and Øksford ice caps and Stjcrnøy lorm a coherent group with indiví- (Hooper l97l). The Reinfiord complex, empla- dual complexes separated by no more than about ced into the Langstrand layered gabbronorite, 7 km. These includc the Nordre Bumannsfiord contains pronounced modal layering. Wehrlite (Sturt et al. 1980), Melkvann (Robins l97l) and and dunite are the principal rock types in the Kvalfjord complexes (Oosterom 1963), occupy- complex, and the$e are accompanied by olivine ing arcas ranging lrom 30 km' (Kvalf ord) to clinopyroxenite,lherzolite, feldpathic peridotite, 50 km' (N. Bumannsfiord) and possibly more pyroxene-rich ultramafites and olivine melagab- than 100 kmr (Melkvann), as well as substanti- bro. Ultramafìc dykes cutting the host gabbro ally smaller intrusions at Habuvarri (Happoljell) or gabbro enclaves are rare and the main petro- in the southeasternmost part of Seiland, Kjer- graphic variations within the complex are rela- ringford on the north coast ofStjernøy, Tudvik ted to megacyclic units, small-scale modal laye- between Lille Kvalfjord and Hallarbukta (Havn- ring, the vicinity of contacts with the enclosing bukt) on the east coast ofStjernøy and Nordbukt gabbro or garnet gneiss and a gross zonal structu- on the south coast ofStjernøy (Fig. l). Each one re. Due to the geographic separation of the of these occurrences is emplaced discordantly Reinfiord complex from the Seiland-Stjernøy in layered olivine gabbro plutons, although the group of intrusions, its emplacement into a ultramafic rocks in places cross-cut the margins petrographically different host gabbro and the of the gabbros and come into contact with absence in this area of the characteristic late- other, older host rocks. The main ultramahc stage magmatic events represented in the nort- rock type is coarse-grained olivine clinopyroxe- hern and eastern parts of the Seiland province, nite containing variable amounts of plagio- its contemporaneity with the other ultramafic clase and amphibole. This is accompanied by plutons cannot be demonstrated with any exact- smaller volumes ol wehrlite and dunite as well ness. All the ultramafic complexes were, howe- as pyroxene-hornblende peridotite and other ver, emplaced during the course of the D, defor- amphibole-rich ultramafic rocks, olivine mela- mational phase, and it would seem highly pro- gabbro and hornblende gabbro. Petrographic bable that they represent a unified magmatic variations within the bodies are generally highly event. irregular, and associated ultramafic dykes emp- The field relationships and petrography ofthe laced in gabbro âre common. Although the individual ultrama{ic complexes of the Seiland major ultramafìc complexes in the Seiland-- province are summarized in the following sec- Stjernøy region are emplaced into physically tions. The descriptions of the Kvalfiord and separate gabbroic intrusions, these belong to a Nordre Bumannsford complexes are based on single, distinctive petrographic suite believed to the earlier work of Oosterom (1963) and Sturt have resulted from the intrusion and fractional et al. (1980), supplemented where appropriate crystallization of critically-undersaturated ba- by the authors own observations in these intru- salt magma (Robins & Gardner 1974, Robins sions. The petrographic terminology employed 1982). As outlined earlier, this group of ultrama- is based on Streckeisen ( I 976). V%iZ til," Kufjord sabbro THE MELKVANN ! w"nrtite and dunite ULTRAMAFIC COMPLEX Olivine-pyroxenite, peridotite m SEILAND Layered olivine gabbro W Gabbronorite 4-_- E Metased¡ment r' Ultramafic nodule-bearing dykes Foliation FJORD ¡l nnyttmic layering / Layering in gabbro enclaves Ài.èå'nòt;iáópðð 0123km ..'...in detail.:.:':.:.:.:.:.:.:

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Fig. 2. Simplified geological map ofthe Melkvann ultramafic complex and its envelope. NGU-BULL.,l05, 1986 High-temperatureultramaJiccomplexes 9

Replocement of Loyered olivine host gobbro gobbro, to form olivine STEINFJELL olivine leuco- melogobbro & ROOF gobbro ultromofic rocks ZONE FoTg-70 Dykes postdoting lrregulor dykes the moin evolu of olivìne clino- of the complex pyroxeniÌe & wehrl Fo Br -7ó Regulor, dilotionol o) picrite- ullromofic dykes: onkorqmiìe ol ivine cl inopyroxenile, sui te Rheomorphism of wehrlite, omphibole- the gobbro envelope: beoring ultromofic b) olkoline olivine gobbro dyke suite, some pegmotites pegmotite dykes, with monÌle-derived intrusion brecciqs spinel lherzolite Fo 77-74 nodules, some Ioyered. to 78-69

Dykes & lorger bodies of non- CEN TRAL loyered olivine clinopyroxeni ZONE wehrlite & some corllondile Fo g2-76

lrregu dykes & replocive bodies of dunìte & poikilitic wehrl ite, Fo B2-go

T IMÊ

Fig. 3. Intrusive sequences in the central and roof zones of the Melkvann ultramafic complex (based largely on Yeo 1984). Olivine compositions by electron microprobe analysis.

The Melkvann ultramartc comp lex The outcrop of the complex lorms two lobes The Melkvann complex occupies an arca ol linked by a comparatively narrow ultramahc some 100 km' in the southern part of the island outcrop extending from Melkvann 1o the south- of Seiland and is emplaced into a lormcrly ern ice front of Seilandsjøkelen. Along parts of extensive layered olivine gabbro, the remnants its western and much of its eastern margins, of which are preserved in places around the ultramafic rocks extend beyond the confìnes of margins of the complex, and as enclaves within the olivine gabbro and intrude older gabbronor- it (Figs. | & 2). The body lies in the core ol a ite (the Seiland syenogabbro of Robins & Gard- large doubly-plunging synform, the southern ner (1974) and in Olderfiord, ultramafic rocks closure of which is locally overlurned. The are in conlact with gneissic melasediment (Fig. synform is defined by S, foliations in early-D, 2). One km west olSt. Bekkarfiord, rocks ofthe gabbronorite sheets interbanded with gneissic ultramafìc complex are strongly fenitized. This metasediments and by the modal layering in the alteration is associated with the emplacement syn-D, olivine gabbro, which locally contains of magnetite-apatite hornblende clinopyroxen- a recrystallised fabric subparallel to the layering. ites, one expression of the late-D. alkaline activ- The ultramafìc rocks lack a systematic penetra- ity, which elsewhere in the complex and its tive foliation, the most conspicuous structural envelope produced en échelon swarms of east- element within the complex being a prominent northeasterly trending dykes and small plugs set of steeply northerly-dipping joints. of syenite and nepheline syenite pegmatite. 10 M. C. Bennat,,S.R. Emblin, B. Robins &W. J.A.yeo NCU - BULL 405, 1986

Fig. 4. Boulder derived from a dyke of inhomogeneous, xenocrystic wehrlite grading in place inlo dunite and containing blocks of coarse-grained olivine clinopyroxenite. The alignment of the xenoliths and the crude banding suggest llowage of a viscous crystal mush. Melkvann u.c., southeast of Steinfell.

In general, contacts ol the complcx with ils gabbro in enclaves and along the margins of the country rocks are discordant. subvertical or complex, similar effects are not readily observed steeply inward dipping, but are generally poorly at contacts with older gabbronorite or metasedi- exposed. In a number of places the contacts arc ment, but this may simply be because these seen to be transitional, comprising subvcrtical contacts are generally not well exposed. At sheeted complexes some 300 m widc, in which Olderfiord, metasediment in contact with plagi- dyke frequencies increase gradually towards thc oclase-bearing hornblende clinopyroxenite con- complex. In several places l0-20 m widc dykc tains a low- to medium-pressure pyroxene gra- of hornblende-olivine melagabbro, in which nulite paragenesis, but does not display evidence amphibole and plagioclase form large oikocrysts, of extensive partial melting. The adjacent ultra- extend obliquely several hundreds of mctrcs mafic rocks do, however, contain small, rounded from the sheeted contacts. inclusions of quartz mantled by coronas of fi- At Olderford erosion has provided a vertical brous green amphibole. Elsewhere, metasedi- section through the complex o[ nearly 900 m ments show local D, superimposition of thermal but over much of the complex the topography metamorphism to pyroxene-granulite facies on lies between the 300 and 800 m contours. regional metamorphic assemblages of the al- Remnants of layered olivine gabbro occur on mandine - sillimanite - orthoclase subfacies. the high ground ofTverrfiell and are prescrvcd with the development of extensive rheomorphic as large enclaves invaded by ultramafic dykes breccias. It is not possible, however, to demon- and sheets. Similarly, layered olivine gabbro strate that these contact metamorphic effects occurs as dissected screens and irregular enclaves are due solely to the emplacement of the ultra- on Steinfiell, and on crests of ridges extending mafic complex, and they are proably due to the up to 3 km inland from lhe western contact combined thermal effects of the complex and south of Steintell (Fig. 2). Here, enclaves occur its olivine gabbro envelope. only at elevations above 400 m. The anitude The ultramafic rocks of the Melkvann com- of the modal layering within the screens and plex are medium- to coarse-grained and show enclaves is systematically related to the orienta- considerable modal variation involving olivine, tion ofthe layering in the olivine gabbro envel- clinopyroxene, amphibole, calcic plagioclase and ope. This consistent relationship suggests that spinel. The central parts of the complex contain the enclaves represent'in-situ' relics of a previ- an extremely complicated sequence of cross- ously more extensive gabbro body preserved at cutting ultramafic intrusions and modal layering a structural level close to the local roof of the is generally absent. The later intrusive events ultramafic complex. are represented by a sequence of ultramafic Although ihe high temperature of emplace- dykes, which in places constitute more than 500/o ment of the ultramafic complex can be inferred ofindividual outcrops and which have disrupted from field relationships implying partial melting, earlier ultramafic bodies, obscuring their origi- remobilization and assimilation of the olivine nal form. In the roof zones of the complex NGU-BULL.,m5, 1986 High-temperatureultamatìccomplexes ll

Fig. 5. lrregular patches and veins ofsecond' ary wehrlite and dunite developed from olivine clinopyroxenite close to discrete wehrlite dykes. Melkvann u.c., southeast ofSteinfiell.

clino- preserved on Sleinfjell and TverrÛell (Fig' 2)' to 200 m across emplaced in olivine ð*porut. is generally excellent and here it. is pyroxenite occur in a topographic depression (Fig. 2). In these apparent that the complex developed through a l-2 km southeast of Steinfell locally protracted process of dyke and sheet emplace- the clinopyroxene is oikocrystic and poikilitic wehrlite enclosing ment, accompanied by mechanical disruption' dunite grades into coarse olivine partial melting, remobilization and assimilation discrets subrounded blocks of (Fig. Small-scale layering, of the olivine gabbro envelope. The sequence clinopyroxenite 4). variations in the modes of olivine ofintrusive events in the central and roofzones . defìned by picotite been observed in one dunite is summarised in Fig. 3. and has indicate a magmatic Within the central parts of the complex, olivi- bodv. and would seem to cumülus processes, but the hap- ne clinopyroxenite is the main rock type and origin through distribution of small, irregular patches forms the host to later generations of ultramafìc haiard wehrlite and dunite in adjacent, relatively dykes. Locally it grades through an increase in of olivine clinopyroxenite (Fig' 5) modal olivine into wehrlite (and less commonly homogeneous wehrlite and dunite has increases in amphibole and suggests that some dunite), and through replacement of olivine plagioclase into a variably feldspathic olivine-- developed by hornblende clinopyroxenite. The latter rock type clinopyroxenite. roof zones on Tverrfiell and SteinÛell, is more characteristic of marginal zones ol the In ìhe clinopyroxenite is the most abundant ultramafic complex where it is usually xenol- olivine rock type, but plagioclase-bearing ithic, containing blocks derived from the olivine ultramafic melagabbro and gabbro occur gabbro envelope, and less frequently, olivine- wehrlite, olivine volumes, but never as large horno- ii.h ult.urnuhc xenoliths. Conspicuous dykes in signifìcant Small segregations and patches and irregular bodies of wehrlite and dunite up geneóus bodies. of wehrlite and dunite also occur. The olivine gabbro ofthe envelope and enclaves is characterised by an alternation of laminae and medium-thick layers of olivine leucogabbro and olivine gabbro (Fig. 6). Thick layers (l-5 m) of modally laminated olivine leucogabbro are, however, common. Primary structures such as modally-graded and grain-size graded layers are relatively rare in the gabbro, although slump structures and erosional discontinuities are preserved locallY. The gabbro enclaves are intruded by numerous generations of ultramafic and mafic dykes . and sheets, many of which contain irregularly distributed xenolithic and xenocrystic fragments Fig. 6. Modally-tayered olivine gabbro cut by narrow, dilatio' of the host rocks. The width of the individual wehrlite From an enclave within the Steinfell roof nal dykes. varies greatly. Some dykes are zone, Melkvann u.c, dykes and sheets 12 M. C. Bennet,,S..R. Emblin, B. Robins&.W. J.A.yeo NGU - BULL. ¡105, l9Eó

Fig. 7. Olivine melagabbro sheet containing slabs and schlieren of olivine leucogabbro aligned parallel to the fabric in the host olivine gabbro. Melkvann u.c., Steinfiell roof zone,

only a centimetre or so thick while others are involved the formation within the olivine gab- several tens of metres wide and can be traced bro of irregular, concordant and subconcordant for hundreds of metres. Towards the margins bodies of olivine melagabbro containing felds- of the larger gabbro enclaves, the proportion pathic schlieren and platy xenoliths ofleucogab- ultramafic of dykes and sheets increases, and bro orientated parallel to the fabric in the host these areas contain smaller, isolated blocks of (Fig. 7). The distribution of olivine melagabbro gabbro in which the modal layering has an is not obviously related to conduits and appears parallel orientation to that in the larger enclaves to be the result of replacement of layered olivine and in the olivine gabbro envelope. The concor- gabbro. Some bodies of olivine melagabbro can dance of the orientation of the layering from be traced laterally into xenolithic olivine clino- gabbro one enclave to the next, regardless of pyroxenite occurring as concordant sheets in the size and distance of separation over large areas layered gabbro. These commonly contain feld- requires lhat substantial parts of the complex spathic schlieren and tabular xenoliths ofleuco- developed through protracted emplacement of gabbro preferentially aligned parallel to sheet individual dykes and sheets which gradually margins, against which a gradational zone of fragmented the envelope. A bewildering variety feldspathic olivine clinopyroxenite is developed of intrusive phenomena are displayed within in places. and around enclaves. From cross-cutting relationships it has been possible, however, to deduce Irregular olivine clinopyroxenite dykes. A later the following general sequence of intrusive stage of emplacement is represented by discor- events (Fig. 3): dant ultramafic dykes varying from several centimetres to up to 100 m in width. Traced (l) The passive replacement of olivine gabbro laterally, such bodies locally display both discor- by more mafìc and ultramafic rocks to form dant and concordant relationships with the olivine melagabbro; modal layering in the hqsl olivine gabbro. Dis- (2) The development of irregular olivine clino- cordant, contacts typically have an irregular pyroxenite dykes associat€d with rheomorp- 'saw-tooth' form, and opposite side walls cannot hism of olivine gabbro and emplacement be matched (Fig. 8). The composition and struc- of gabbro pegmatite dykes and intrusion ture ofthe gabbro along the contacts frequently breccias; show considerable modification. Modal plagio- (3) Emplacement of regular (dilational) ultrama- clase in the olivine gabbro increases abruptly fic dykes; at the expense ofclinopyroxene, forming a troc- (4) Emplacement of dykes postdating the main tolitic halo along the contacts. The layering in evolution of the ultramafi c complex. the gabbro is destroyed during the formation of these haloes. Olivine is usually present in the Passive replacement of gabbro. The initial stages reaction zones and shows an increase in modal in the development of the ultramafic complex proportion toryards the dykes. Internally the NCU - BULL. 405. 1986 High-temperatureultramaÍiccomplexes 13

Fig. 8. Irregular olivine clinopyroxenite dyke cutting a modally layered, olivine gabbro enclave. Contacts are lined in places by dunite and show typical'saw-tooth' form, apparently controlled by compositional variations in the host. Melkvann u.c., Steinfell roofzone.

Fig. 9. Higly-inegular margin of an olivine clinopyroxenite dyke. Olivine gabbro enclaves within the dYke contain modal lamination parallel to and are form orientated in concord' ance with the layered fabric in the host olivine gabbro. Melkvann u,c.

dykes show irregular petrographic variations. in the xenoliths is virtually parallel to that in Dunite and wehrlite, generally associated with the wall iocks. (Fig. 9). Similarly, zones rich in troctolite, are commonly developed adjacent to plagioclase xenocrysts often extend inço the the irregular contacts, but elsewhere the dyke dykes from the serrated dyke walls and yet margins are composed of olivine mela¡jabbro. remain parallel to, and appear to represent a In contrast, the central parts of the dykes are continuation of, the layering in the adjacent more homogeneous and consist mainly of olivi- gabbro. Oblique sections through this type of ne clinopyroxenite. The dyke margins, and to a dyke margin show highly complex contact rela- lesser extent the cores, are characterised by tionships. Some of the irregular dykes pass into small gabbro xenoliths and plagioclase xeno- relatively homogeneous concordant sheets of crysts. These are irregularly distributed and a variably feldspathic olivine clinopyroxenite, perplexing feature in. some dykes is the orienta- which in general have sharp regular contacts tion ofelongate gabbro xenoliths at a high angle usually lacking reaction haloes. to the dyke margins such that the layered fabric The detailed relationships described above, yeo 74 M. C. Bennet,S.R. Emblin, B. Robins &W. J.A. NGU. BULL 405, 1986

xenoliths in various stages of disaggregation. Elongate xenoliths of anorthosite and leucogabbro, possibly representing restites, characteristically display disharmonic folding (Fig. l0).

Rheomorphism of olivine gabbro; gabbro pegmatite dykes and intrusion breccias Towards the margins of the larger olivine gabbro enclaves cut by irregular olivine clinopyroxenite dykes, modal layering in the gabbro exhibits various degrees of disruption and reconstitution. Leucogabbro neosomes Fig. 10. Anorthositic rc¡tites cncloscd in variably-fetdspathic are developed in the olivine clinopyroxenite that forms an irregular dyke cutting more mafic of the layers and cut across the olivine leucogabbro near Steinfiell, Melkvann u.c. Some rest- primary modal layering and ultramafic dykes ites display disharmonic folds. emplaced into the enclaves (Fig. I l). Discordant, irregular bodies of olivine gabbro pegmatite occur in places, sometimes grading locally into- in particular the non-matching, irregular dyke subconcordant anorthositic neosomes. Frequ- walls and the preservation of apparently undis- enlly the gabbro layering becomes highly irregu- turbed xenoliths within the dykes, indicate that lar, mesocratic parts having developed a swirling they developed al least in part by replacement appearance, while anorthositic layers largely ofgabbro, through a process involving dissoluti- retain their original form. Very locally, the on, mechanical disaggregation, transport and adjacent ultramafic dykes are cut by anorthositic assimilation by the invading magma as will be veinlets, while on a much larger scale, dykes discussed later. The structural relationships also of olivine gabbro pegmatite and coeval intrusive suggest that in such dykes, the magma flow breccias invade the ultramafic rocks, forming must have had a significant lateral component. sub-parallel dyke complexes extending over The wider dykes of this type (>3 m across) several kilometres (Fig. l2). Within these dyke exhibit the contact relationships described above complexes, gabbroic and ultramafic dykes dis- but show a composite internal structure. The play mutually cross-cutting relationships. The central portions of the dykes consist essentially olivine gabbro pegmatites typically have comb- of olivine clinbpyroxenite and/or olivine mela- textured margins and extremely coarse-grained gabbro containing narrow zones of plagioclase- cores, commonly containing large skeletal olivi- bearing clinopyroxenite and/or olivine melagab- nes up to l0 cm long. The intrusion breccias bro which define a crude fluxion banding. These occur as small plugs and consist of olivine leuco- bodies were emplaced by simple dilation within gabbro enclosing sub-rounded to angular blocks pre-existing, essentially non-dilational dykes. of various gabbroic and ultramafic rocks (Fig. The dilational cores are replete with gabbroic I 3).

Fig. ll. Maryin of an olivine gabbro cnclave in the Steinûell roof zone. Olivine clinopyroxenite dykes emplaced inlo rhe gabbro are back-veined or disrupted by a leucogabbro neosome. Melkvann u.c. NGU-DUIJ.¡I05, I98ó High-temperatureultramaJiccomplexes 15

Fig. 12. tnclined subparallel dykes ofolivine gabbro and comb-structur€d olivine gabbro pcgmatite emplaced in variably fcld- paihic olivine clinopyroxenite. The dykes form a NF-SW swarm subparallel to the margin of the Melkvann u.c. to the nolh ófsteinfell. Two sets ofnarrow, dilational amphibole-bearing ultramafic dykes post-date the olivine Sabbro dykes.

Dilational ultramalic dykes. A' further stage in the evolution of the complex is represented by a suite of coarse-grained (>5 mm), dilational ultramañc dykes. These include both simple and composite bodies, and have regular margins often with horns (Robins & Takla 1979), but lack chilled margins. Although represented in the enclave-rich roof zones (Fig. 6), they are most common in the central and eastern parts of the pluton. Cross-cutting relationships within the suite suggest a temporal variation in composition. The earliest dykes are of wehrlite. olivine clinopyroxenite and olivine-hornblende clinopyroxenite, while later dykes characteristically contain abundant primary amphibole with or without plagioclase (both usually occurring as oikocrysts enclosing olivine and clinopyroxene), and have compositional range from clino- Fig. 13. Intrusion breccia consisting of sub-rounded'blocks a of ullramafic rocks ând olivine gabbro enclosed in olivine pyroxene-hornblende peridotite to olivine-- leucogabbro. The breccia plug is intruded by an ultrabasic hornblende clinopyroxenite, olivine-clinopy- dyke (with chilled maryins) belonging to the late-ståge, picri- roxenite hornblendiie and hornblende melagab- teankaramite suite. Melkvann u.c., Steinfetl roof zone. bro. The dykes vary in width from a few centimetres to approximately 20 m, independently of dyke composition. In many areas, especially in that the dykes were emplaced along a number the central and eastern parts of the complex, of trends: one approximately ENE-WSW, sub- they have a generally east-west orientation, but parallel to the northwestern margin of the detailed observation in the Steinfiell area show complex, one approximately E-W and another 16 M. C. Bennet, S. R. Emblin, B. Robins &.W. J. A. Yeo NGU - BULL. ¡105, ¡986

*20 m

Fo7 4

n67 Ca48Mg39Fe 1 3 Fo7'l 70

Fo72 60 Ca4SMg4OFe 1 1 An65 Ca49Mg39Fe 1 2

Fo73 ^50E Ca49Mg42Feg E o)

Eoo Fo78 x o o (soo Fo77

Fig. 14. Olivine-hornblende clinopyroxenile dyke cutting 20 .olivine clinopyroxenite. The dyke is choked with angular xenoliths of olivine leucogabbro (foreground and background) and olivine clinopyroxenite (middle ground). Melkvann u.c., Steinfiell roofzone. 10 Fo78 Marginal approximately N-S, giving rise to a reticulate series w. dyke pattern. subvertical The latest in the suite are 0.2-1.0m-wide banding dilational olivine-hornblende clinopyroxenite dykes, commonly choked with angular xenoliths places of their wall rocks (Fig. la), and which in Fig. I 5. Simplified vertical section through a layered ulrama- occur in swarms providing evidence for local ficlmafic dyke cutting the margin of the Melkvann u.c. at linear extensions of up to 400/o (Yeo 1984). Steindal, SW ofSteinfiell, with mineral compositions as deter- (based Yeo 1984 Of particular interest within this suite are mined by electron microprobe analysis on and unpublished data). Cumulates within the narrow marginal dykes showing internal layering, and dykes that series are organized in cyclic units involving olivine cumula- contain mantle-derived ultramafic nodules. The tes (oC), diopside

Fig. ló. Spinel lherzolite and subordinate olivine-hornblende clinopyroxenite nodules packing an olivine clinopyroxenite dyke near Holmevann, Melkvann u.c. There is a distinct preferred orientation of xenoliths parallel to the margins of the dyke, Some nodules exhibit compositional banding and some are cut by narrow, plagioclase-bearing olivine-- hornblende clinopyroxenite dykes.

Within l-2 m of the margins there are olivine- The nodule suite consists predominantly of rich zones conlaining a banding parallel to the Cr-spinel lherzoliles (Carswell 1980), but with dyke walls. a subordinate population (<20o/o) of olivine-- The nodule-bearing dykes are generally hornblende clinopyroxenites. The lherzolites are 5-10 m wide and may be traced over distances mosaic-porphyroclastic varieties (Harte 1977), of up to I km. They are concentrated in a narrow probably derived from depths no greater than zone trending ENE-WSW from Holmevann to 60 km. Some of the lherzolite nodules contain Melkvann (Fig. 2), but similar dykes also occur veins and narrow dykes of plagioclase-bearing east and northeast of St. Bekkarfiord and at olivine-hornblende clinopyroxenite which may other localities outside the complex (see Robins well represent pre-inclusion products of partial 1975, Robins 1982). Those within the complex melting in the upper mantle beneath the ultra- are, however, contemporaneous with the suite mafic complex. The suite of olivine-hornblende of later ultramafìc dykes. clinopyroxenites have textures and mineral The ultramafic nodules characteristically have compositions that are similar to the ultramafic low to moderate degrees of roundness and rocks of the Melkvann complex and were pro- sphericity, many approximating to spindle or bably derived from lower, unexposed levels of plate shapes. The larger nodules measure the pluton. The dense packing of nodules within 40-50 cm across, with a maximum recorded the dykes suggests that the nodules accumulated diameter of 60 cm. A preferred orientation of within dyke fissures as the velocity of the entrai- nodules is apparent in some dykes and common- ning magmas decreased (cf. Robins 1975). ly they are so densely packed that they are in mutual contact, or separated by only thin septa Dykes post-dating the complex. Younger intrus- of matrix (Fig. I 6). Exposure is generally insuffr- ive rocks which post-date the formation of the cient for a detailed study of dyke walls, but in ultramafic complex have a regional distribution some dykes it is clear that the larger nodules and comprise a suite of alkaline olivine basalt are concentrated towards the centre, and nodule afiìnity dominated by picrite and ankaramite diameters decrease towards dyke margins into (Robins & Takla 1979) (Fig. l3) and later, more a zone of dispersed, more rounded nodules, localised pegmatite dykes forming part of the commonly with diffuse margins. The matrix in late-D, alkaline magmatic activity (Sturt & these dykes is variable in composition, and Ramsay 1965, Robins 1972,1974,1980). Within although locally rich in xenocrystic olivine deri- the Melkvann complex, the picrite and ankara- ved from the break-up of nodules, generally . mitic dykes represent a number of generations, consists of olivine clinopyroxenite with a variab- but all the dykes exhibit chilled margins ahd the le, but usually significant, amphibole content. majority have an ENE-WSW trend, following l8 M. C. Bennett,S.R. Emblin, B. Robins &,W. J.A.Yeo NGU - BULL ¡105, l9E6

FIT{ olivine pyroxenite/dyke lr{\ and peridotite /complex [fr.'ÏÏl ."r","0 orivine eabbro

IYYY rou","o sabbronorno Metasediment and dioritic gneiss

MANNSFJORD 71 rori^t,on

,( Rnytnri" tayering //: Layerißg in gabbro / enclaves

0123kmE=#

F", THE NORDRE BUMANNSFJORD ULTRAMAFIC COMPLEX,

BÀRDFJoRD SEILAND

Fig. 17. Simplified geological map of the Nordre Bumannsfiord u.c. and its envelope (based on Sturt et al. 1980, unpublished data from Dr. D.L. Speedyman and the authors' own observations). a conspicuous set of joints.. They must have and metamorphosed gabbronorite along its been emplaced after significant cooling of the southwestern margin, and the body terminates complex and were subsequently variably defor- southwards in a sheeted complex of ultramafic med and metamorphosed under lower amphib- dykes emplaced in strongly metamorphosed ole facies to upper greenschist facies metamor- metasediments and dioritic gneiss (Fig l7). To phic conditions (Ramsay & Sturt 1970, Robins the north and west the strongly discordant &Takla 1979, Skogen 1980). contact of the ultramafic complex dips steeply beneath olivine gabbros in which the modal The Nordre BumannsJj ord layering dips at low to moderate angles towards the complex. The gabbro appears to ultramaJìc complex envelope have been emplaced into the steep western limb The Nordre Bumannsford complex, occupies of the major synform developed around the an area of substantial relief in the western part Melkvann mafic-ultramafìc complex and has of the island of Seiland (Fig. l), its outcrop both concordant and highly discordant contacts being separated at the head ofSt. Kuford from with an earlier, strongly deformed and meta- that of the'Melkvann complex by a steeply morphosed (syn-D,?) subalkaline gabbro and dipping septum of strongly deformed country metasediments. The earlier gabbro contains rocks only 3 km wide. The ultramafic pluton is numerous rafts of metasediment (Barth l96l). emplaced mainly into layered olivine gabbro In the metasedimentary country rocks southwest but satellitic ultramafic dykes intrude deformed of the ultramafic complex and its host gabbro 19 NGU . BULL ¿O5, I98ó High-temperatureultramqlìccomplexes

Fig. 18. Modally-layered olivinc gabbro enclave intruded by crosr cutting, inhomogeneous ultramafrc dykes. N. Bumannsford u.c.

'.\\

a pyroxene-hornfels facies thermal aureole up present authors suggest, however, that the encla- related to to 3 km wide is developed in which rheomorphic ves have orientations systematically gabbro breccias are common (Sturt et al. 1980). Shear the orientation of the layering in the within appears to have accompanied contact mela- envelope, even when completely isolated morphism and anatexis ofthe metasediments. peridotite (Fig. l7). Ultramafic dykes within the marginal swarms and those cutting gabbro encla- Marginal dyke swarms' The margins of the ves lack chilled margins and are petro- ultramafic complex are marked by dense swarms graphically diverse and internally inhomogene- indica- of ultramafic dykes of variable orientation which ous (Fig. l8). Cross-cutting relationships according to Sturt et al. (1980) have led to the te that several generations of dyke emplacement detachment of roof pendants to form large rafts are represented, and although the dykes most and xenoliths within the complex. Their map- commonly consist of variably feldspathic olivine ping of the orientation of the modal layering clinopyroxenite, dunite is also represented as within gabbro enclaves and observations by the well as feldspathic lherzolite, wehrlite, pyrox-

Fig. 19. Margin of olivine clinopyroxenite dyke (rieht) cutting modålly-layered olivine gabbro enclave within the N. Bumannsford u.c., east of N. Bumannsfiord. The contact is highly irregular and apophys€s extend outwards along the mafic bases of modally- ¡¡aded layers in the host. Wehrlite is develo- pcd in the apophys€s and in patches elsewhere along the margins ofthe dyke. 20 M. C. Bennett, S. R. Emblin, B. Robins &W. J. A. yeo NGU-BULL.¿105, 1986

ene-hornblende peridotite and olivine gabbro, sing an ophitic texture and engulfing the layering petrographic the range being essentially similar and tectonic foliation of the host gabbro. Gabbro to within that the ultramafic pluton itself. The enclaves within the interior portions of the ultra- dykes range in width from several meters 10 mafic pluton show an increased degree ofstruc- veins thin and commonly display dilational lural compelxity. Modal layering, primary textu- contact relationships. Some of the dykes, how- res and tectonic foliation are rarely present. The ever, have irregular saw-tooth contacts which enclaves are strongly recrystallized and have a appear have to been controlled by the modal swirling banding. Olivine gabbro, gabbro and layering gabbros of the they cut or have such hornblende gabbro pegmatites are widely deve- highly irregular contacts that an origin by simple loped as patches within enclaves or along con- would dilation appear to be excluded. Such tacts with ultramafic rocks. Enclaves of mig- generally dykes contain narrow, irregular zones matitic aspect are closely associated with intrusi- of dunite and wehrlite along their walls grading on breccias of crudely banded leucogabbro inhomogeneous into olivine clinopyroxenite enclosing rounded to angular blocks ol ultrama- occupying the main part of the bodies (Fig. fic and mafìc rocks (Srurt et al. 1980, Figs. l3b, l9). Within haloes adjacent to some dykes, clino- l4b). Intrusion breccias are, however, also pyroxene gabbro in the is replaced by olivine developed beside wide satellitic peridotite dykes which form larger crystals and becomes more emplaced in gabbro to the west of the southern abundant towards the dyke contacts. Clino- part of the ultramafìc complex. pyroxenite or olivine clinopyroxenite reaction zones a few centimetres wide, grading into olivine gabbro are also present along certain dyke Petrographic variations. The coarse-grained ul- contacts. The dykes generally contain xenoliths tramafic rocks of the Nordre Bumannsfiord and xenocrysts derived from the wall rocks. The cornplex are rarely homogeneous and free from xenoliths have both rounded and irregular inclusions, and a crude interbanding of petrog- forms, cornmonly with ragged margins. In some raphic types is widely developed. The main dykes they are aligned within a crude banding rock type is a feldspar-bearing olivine clinopy- defined by the distribution ofxenocrysts derived roxenite which grades in places into wehrlite apparent from the disaggregation of xenoliths and dunite, and with an increase in the amount (srurt er al. 1980, Fig. l0a). some feldspathic of feldspar, into olivine melagabbro. pyroxene- pyroxene-hornblende peridotite or hornblen- hornblende peridotite containing poikilitic de-olivine melagabbro dykes, however, contain amphibole is present locally. Ultramafic rocks distinctly angular, randomly-oriented xenoliths carrying xenoliths of gabbronorite or metasedi- of modally-layered gabbro, olivine clinopyroxe- ment are generally feldspar-bearing lherzolite nite and wehrlite (Srurr er al. 1980, Fig. 8b). or spinel lherzolite, respectively. Around meta- One petrographically distinctive dyke, which can sedimentary xenoliths harzburgite is developed. be traced for about 2 km cuts obliquely across In a number of places, and in particular along the southweslern margin of the complex (Fig. the eastern margih of the complex, the ultramafi- l7). It contains oblate plagioclase and amphibole te possess a penetrative tectonic foliation and oikocrysts up to several centimetres across have been recrystallized at high temperatures. enclosing tabular olivine. The elongation of the Throughout most of the complex, deformation oikocrysts is parallel to the lamination defìned is restricted to widely spaced shear zones and by the olivine and both dip at a shallow angle the development of a fracture cleavage. between the steep dyke walls. This dyke would appear to be related to the layered dykes present Late ma/ìc and ultramatìc dykes.The ultramafic within the Melkvann ultramafic complex. complex was cut at a lage stage in its development by a variety of pegmatitic veins and dykes Remobilization of olivine gabbro. The modal of which only some of the youngest have chilled layering of the proximal olivine gabbro host margins. The veins, which commonly show rock and marginal enclaves has locally been comb structure, frequently form the margins of modified by the development of patches and composite dykes cored by either medium-grai- veins of leucogabbro within more melanocratic ned gabbroic rocks (Sturt et al. 1980, Fig. l3a) gabbro (Sturt et al. 1980, Fig. lla). In rhese or coarse-grained ultramahc rocks. Vein compo- places the gabbro may take on a migmatitic Sitions of olivine gabbro, olivine gabbronorite, appearance, the leucogabbro neosome posses- gabbronorite and hornblende gabbro have been NGU - BULL. ¡O5, I9E6 High-temperatureultramaficcomplexes 2l recorded. Pegmatites occurring as parallel-sided strongly delormed metagabbro, monzodiorite veins cutting ultramafics have been observed and perthosite which in turn is thrust over to pass laterally into zones where they become carbonatite and fenites (Robins & Tysseland flow-folded. Other veins are deformed and folia- 1983). The ultramafic rocks occurring around ted but are cut by later undeformed veins. The the head of Store Kjerringfiord appear also to late dykes in the complex are petrographically rcst on a thrust plane, and may.form a thrust diverse and include wehrlite, felspathic pyroxe- wedge sandwiched between slices of olivine ne-hornblende peridotite, gabbro, hornblende gabbro. Despite these local tectonic complicati- gabbro and blastoporphyritic amphibolite (picri- ons the gabbroic cumulates can be seen to form te and ankaramite). According to Sturt e1 a1.,, a layered sequence at least 4 km and possibly (1980), the dykes extend outwards into the as much as 8 km thick. Cumulate structures are envelope but are concentrated within the pluton. preserved only locally, having been overprinted The magmatic activity responsible for the dykes and highly modified by penetrative high-tempe- would appear to represent a late stage in the ralure delormation and recrystallization. Modal- evolution of the complex, since late phases ly-graded layers showing a consistênt way-up include wehrlite and other ultramafìc rocks, buf pattern have not been observed, and a systema- a comprehensive sequence of dyke emplacement tic cryptic layering has yet to be detected. has not been etablished. The dykes have a prefer- Oosterom (1963) reported large-scale alternati- red NW-SE orientation and in places occur as ons of modally-layered olivine gabbro with dense swarms associated with linear extensions leucogabbro and anorthosite, the latter locally of up ro 400/o (Sturt et al. 1980, Fig. l3 & Table lorming discrete units up to l0-20 m thick. In 2). Towards the central parts of the pluton, some areas the primary layering is highly distur- however, the dykes assume a more random bed and irregular in orientation, folded intensely orientation and some are included as blocks in by fìow folds or afnected by autobrecciation. intrusion breccias. Adjacent to uncomplicated contacts with the ultramafìc complex the olivine gabbro is grano- blastic and exhibits modal layering modified The Kvalfiord ultramafic rocks by recrystallization. To the southwest of the Ultramafic rocks on the eastern part of the island ultramafic complex the olivine gabbros are in- of Stjernøy (Fig. l) apparently occur in two truded by a considerable volume of coarse- steep-sided and virtually coalescent intrusive grained to pegmalitic rocks varying from horn- centres emplaced in tectonized, modally-layered blendite to hornblende gabbro. Contacts of the olivine gabbro (Fig. 20). On a large scale the amphibole-rich rocks are sharp and discordant, contacts of the ultramafìc body are clearly intru- and the surrounding gabbros are cut, and in sive, cutting discordantly through the layering places brecciated by networks of amphibole-rich of the host gabbro. Enclaves of gabbro up to dykes or veins. several hundreds of metres across are included within the ultramafic rocks (Fig. 2l) and sheets and dykes of wehrlite, peridotite and feldspathic Olivine clinopyroxenite and related rocl

c VZ Hornhtendite, hbl sabbro Wenrtit" drnit"o"bbroi. ! "nd r; A Olivine-ovroxen¡te/ ve¡ns and lÆsl and periobtite / intrusion ffi þreccias lïlÏll Layered oliv¡negabbro ,.€ ô fvYl Fol¡ated gabbronorite Ul-Yl and syen¡te -| - I Metased¡ment ,( Folialion ,l Rhythmic layering .1 Layering ¡n gabbro enclaves / Thruat O 1 2 3km

THE KVALFJORD ULTRAMAFIC COMPLEX SIJê.â,VSU,VO STJERNøY

Fig. 20. Simplilied geological map ofthe Kvalfiord ultramafìc complex and its envelope (based on Oosterom 1963, Robins & Tysseland I 963 and the authors' own observations).

lope is not apparent in the olivine clinopyroxeni- body of banded wehrlite (Fig. 20). The wehrlite te. In the vicinity ofcontacts with the host rocks contains small, elongate and parallel-orientated and olivine gabbro enclaves, however, the ultra- inclusions of olivine-rich wehrlite, olivine gab- mafic rocks generally have a streaky and imper- bro, olivine clinopyroxenite and pegmatitic cli- sistent banding defined by the distribution of nopyroxenite, all possessing highly irregular plagioclase xenocrysts and the crude alignment margins; the wehrlite itself grades into bands of concentrations of olivine and pyroxene. Olivi- of olivine clinopyroxenite and olivine melagab- ne clinopyroxenite dykes within gabbro enclaves bro. Where enclaves of olivine gabbro are asso- are commonly xenolithic, and the irregular ciated with the wehrlite these are cut by wehrlite contacts with olivine gabbro, olivine leucogab- dykes but the gabbro has a nebulous and irregu- bro or troctolite xenoliths and the wall rocks lar banding and appears in places to have back- may be marked in some cases by narrow zones veined the wehrlite dykes. The northern and of dunite or poikilitic wehrlite. Olivine-poor eastern margins of the wehrlite are marked by gabbro towards discordant contacts with olivine a zone of intrusion breccias (Fig. 23) in which clinopyroxenite commonly contains increasing a matrix of olivine gabbro or melagabbro carries amounts of large, , irregular olivine crystals. both rounded and angular blocks of olivine Concordant xenolithic olivine clinopyroxenite gabbro, some of which exhibit modal layering, intrusions in olivine gabbro can in some places as well as ultramahc rocks varying in compositi- be traced laterally into olivine melagabbro in on from wehrlite to olivine.clinopyroxenite. which much of the plagioclase present appears Poikilitic wehrlite (locally feldspathic and to be related to aligned olivine gabbro xenoliths often with patches ofdunite and coarse-grained and may be interpreted as xenocrystic. to pegmatitic olivine clinopyroxenite and clinopyroxenite) also forms tabular or irregular dykes Wehrlite. In the southern part of the ultramafic a few centimetres to several metres in width complex olivine clinopyroxenite and olivine traversing both olivine clinopyroxenite and oli- gabbro enclaves are invaded by a fairly extensive vine gabbro enclaves. Contacts of the wehrlite NCU-EULL¡O5, ¡986 High-temperatureultramaÍiccomplaces 23

Fig. 21. Maryin of large, olivine gabbro enclave cut by several generations of ultramafic dykes. Modal layering in the enclave is disturbed by fìow folds. Kvalfiord u.c., betv,een Lille and Store Kvalfiord.

dykes with olivine clinopyroxenite show no sign preclude emplacement by dilation, and they are of chilling and vary from sharp to gradational. believed to have developed by replacement. Narrow marginal zones of dunite are developed in some instances while other dykes grade into Late dykes and veins. The Kvallord complex marginal zones of olivine clinopyroxenite. Pods is intruded by several generations of coarse- and irregular patches of coarse-grained to peg- grained to pegmatitic olivine gabbro veins lack- matitic olivine clinopyroxenite and clinopyroxe- ing chilled margins and which in places form nite are commonly present along dyke margins dense swarms (Fig. 26). The veins occur both and are often crudely aligned with the contacts. as simple bodies from a few centimetres to some Wehrlite dykes emplaced in olivine gabbro metres wide, generally exhibiting a comb struc- enclaves typically contain irregular, slabby to ture and sometimes containing skeletal olivine, rounded xenoliths of olivine gabbro. Xenoliths and as the marginal zones of composite dykes, are frequently aligned parallel to the general the cores of which may be occupied by olivine-- trend of dyke margins but these may be highly hornblende clinopyroxenite or wehrlite. Oliv- irregular in detail. Coarse-grained dunite is ine-hornblende clinopyroxenite as well as oliv- locally found along contacts of the wehrlite dykes with the gabbro host as well as around the margins of xenoliths. Dunite, wehrlite and troctolite are developed in irregular patches and lenses extending along the relict modal layering of the host olivine gabbros adjacent to some wehrlite dykes (Fig. 24). Some steeply-dipping wehrlite dykes contain a more or less pronounced olivine lamination orientated at angles up to 90o to their contacts; strong mineral lamination has been noted in dykes as narrow as 20 cm. In the northeastern part of the ultramafic complex poikilitic wehrlite together with subsidiary olivine-rich olivine clinopyroxenite and dunite commonly form irregular veins and bodies a few centimetres to several metres across within olivine clinopyroxenite (Fig. 25). The Fig. 22. Rounded xenoliths ofcoarse-gra.ined wehrlite within complicated form of these bodies appears to olivine clinopyroxenite. Lille Kvalford valley, Kvalford u.c. 24 M. C. Bennet, S. R. Emblin, B. Robins & W. J. A. Yeo NGU - BULL.,m5, 198ó

The Reinfi ord ultramaJic comp lex The Reinfjord complex (Fie. l) lies some 50 km souhwest of Seiland and Stjernøy and allhough of comparable (syn-D,) age diflers from the other high-temperature ultramafic complexes in two important respects. Firstly, the Reinliord complex contains extensively developed layering together with a crudely concentric modal and cryptic variation. Secondly, it is emplaced into a layered gabbro of subalkaline character (the Langstrand gabbronorite), and to some extent this is shared by the ultramafic cumulates themselves. Recent re-investigation of the Reinfiord Complex has led to a revision of the nomencla- Fig. 23. Intrusion breccia conlaining angular blocks ofolivine ture originally proposed (Bennett 1974) lor the clinopyroxenite within a matrix ofolivine gabbro. Kvalfiord various parts of the complex. The subdivision u.c., Lille Kvalfiord valley. adopted here (Table 2) conforms more closely to the terminology for layered intrusions proposed by Irvine ( I 982). The exposed part of the Reinf ord ultramafic ine gabbro, hornblende gabbro and olivine leu- complex contains lwo early layered series, one cogabbro dykes of dilational form also occur in structurally above the other and separated by a the ultramafìc complex. These minor intrusions sub-concordant screen of gabbro that projects characteristically lack chilled margins and while from the envelope into the southwestern corner they cross-cut some ultramafìc rocks they are ol the complex (Figs. 2l &. 28). The Lower themselves cut by subsequent ultramafìc dykes. Layered Series, consisting mainly of lherzolite. A suite of dilational, variably foliated and wehrlite and olivine clinopyroxenite, is restric- amphibolitized olivine gabbro dykes preserving ted to a level below the gabbro screen and is chilled margins represents one of the latest exposed only in the southwestern corner of the magmatic events within the complex. These complex. The Upper Layered Series, consisting dykes are post-dated only by nepheline syenite of wehrlite and olivine clinopyroxenite, has a pegmatites which are widely developed within more extensive outcrop above the gabbro screen, the complex and its envelope. and forms a plateau 550-800 m a.s.l. Both laye-

Fig. 24. Relict modal layering within an olivine gabbro enclave partially replaced by troctolite, wehrlite and dunite. Kvaltord u.c. NGU.BULL.¡M5, 1986 High-ternperatureultramaÍiccomplexes 25

Fig. 25. Veins and Patches of secondary wehrlite and dunite developed within massive olivine clinopyroxenite. Kvalfi ord u.c.

Fig. 26. Swarm of pegmatitic olivine gabbro veins cutting peridotite and commonly forming the margihs of composite dykes. Kvalfiord u.c.

red series pass laterally through transitions of In the central part of the complex, an extens- variable width into marginal zones dominated ive outcrop of dunite and subsidiary poikilitic by websterite and olivine websterite but also wehrlite represents a later phase of intrusion. containing smaller volumes of olivine melagab- Towards the eastern and western margins of the bro and troctolite, spatially associated with complex, this later body is grossly discordant numerous gabbroic xenoliths in various stages and cuts both the Upper Layered Series and its of assimilation, and blocks of coarse-grained time-equivalent Upper Marginal Zone. Towards pyroxenite. the south, it extends as a relatively narrow 26 M. C. Bennet, S.R. Emblin, B. Robins&W. J.A. Yeo NGU.BULL¿I05, I98ó

VVVVVVV VVVVVVVVVVVVVVVVVVVVV :V Y.V.V .V ]/',V.V V V V VVV V V VV VVV V V V V V: iv YV jv v v.vv v \ lv.\Yy.-v-y""1v v.v.v.v.!.

Vr V': THE REINFJORD xþj I ULTRAMAFIC COMPLEX

V. v. V\i LANGFJORDJøKELEN Kr1..''

ü'v ,.. VVVVVVVVVVV VVVVVVVVVVV VVVVVVVVVVVVVV VVVVVVVVVVVVVV VVVVVVVVVVVV VVVVVVVVVVVV V,VVVVVVVVVVV vl-¿ovvvvvvvvv VIVVVvVVVVVVV VVVVVVVVVVVV VVVVVVVVVVVVVV a\/vVVVVVVVVVVVVVVV VVVVVVVVVVVVVVVVVVV

Central Ser¡es ffi Dunite, Wehrlite Upper Layered Series VVV Wehrl¡te, Olivine pyroxen¡te VSov ffi Lower Layered Series W Lhe( zolile, wehrlite Ol¡v pyrox. íf {r':, !,1å?'å1'",íï ñ" 0., þ,n Ø ",,, ñ Uooer Maroinal Zone " NNNNN WeUsterite: Oliv¡ne websterite qo Troctolite, Olivine melagabbro o f:- North East Marginal Zone Dunite, Wehrlite, Troctolite, ¡( :l----l I Olivine melagabliro qu cn"i". Y ffi å:iff:'"'J,1," ffi VVVVVVVVV VVVVVVV VVVVVVVV VVVVVVV VVVVVVV VVVVVVV ,< Foliation VVVVVVV Modol loyering l1ì w36;vvv VVVVVVV VVVVVVV O lkm VVVVVVV ^ VVVVVVV

Fig. 27. Simpliñed geological map ofthe Reinfiord ultramafic complex and its envelope. See Table 2 for the subdivision ofthe complex,

tongue to the southern margin of the complex, envelope are strongly discordant, being either forming a steep-sided, dyke-like body, near vertical or steeply inclined to the northeast 100-200 m wide. or east. Locally, however, contacts are irregular Extrapolation of outcrop data suggests that in detail and are to varying degrees influenced the Reinûord complex approximates to a cru- by the gently or moderately inclined modal dely elliptical cylindroid plunging very steeply layering in the gabbro envelope along which in a northeasterly direction. Contacts with its there are sill-like projections. This type ofcon- NGU - BULL. ¡105, 1986 High-temperatureultramaJìccomplexes 27

* srullig

ofthe envelope form the bluffs below the extensi- Fig. 2g. panorama ofthe southwestern corner ofthe Reinfiord u.c. Gneisscs cliff-and is separated from the upper layered Series ve screes. The l¡wer l:yered Series c¿n be seen in the loweipart ofthe and cumulates and large' lenso- by the light-coloured gabbro screen. Alternations ofsequences ofoliuint clinopyroxenenorite was earlier believed to represent the metasediments. oldest part of an Eocambrian (Vendian) cover M. C. Bennett, yeo 28 S. R. Emblin, B. Robins &, W. J. A. NGU-DULL¡I05, 1986

corrobortive evidence for subsolidus cooling at confìning pressures of 6-8 kbar (Kushiro & Yoder 1966, Ito & Kennedy l97l), corresponding to crustal depths of 22-28 km within the orogenic belt. Recently obtained analyses (Lord & Benneu, unpublished data) of coexisting cordierite (Fe/Fe+Mg = 0.39), almandine garnet (FelFe+Mg = 0.73) and plagioclase from a pelitic rafl within the Langslrand gabbronorite, close to the southern margin of the Reinûord complex, suggest that peak metamorphic conditions, corresponding with the emplacemenl of the gabbronorite, were c. 875.C and c. 7.5 kbar (Aranovich & Podlesski 1983, Ghent 1979) assuming PH2o = Prorur

The Layered and Central Ser¡'es. Field relations, modal and cryptic variations require that three major phases of intrusion are represented by the Lower and Upper Layered Series and the Cen- lral Series of the Reinûord complex. The Lower and Upper Layered Series contain sequences of olivine and modally-layered clinopyroxene-olivine cumulates, while the Central Series Fig. 29. The subhorizontal upper contact ofthe gabbro screen contains only olivine cumulates. with the Upper Marginal Zn¡e of the Reinfiõrd u.c. Thin The Lower Layered represents screens ofrecrystallized leucogabbro Series the are separated by modal_ earliest phase intrusion ly-layered variably felspathic olivine websterite and lhirzolite. of and comprises four cyclic units, each composed of olivine cumulates followed by clinopyroxene-olivine cumulates, in a 300 m-high section exposed in the southwestern part of the complex (Fig. 2S). The oliv- sequence (Hooper Gronow & 1969, Hooper ine cumulates are lherzolites containing large (1971), but more recent studies elsewhere in ihe bronzite oikocrysts, and poikilitic wehrlites. The province (Akselsen 1980, *r]1¿ Ramsay er al. clinopyroxene-olivine cumulates are lherzolites 1985) suggest that it is probably precambrian of with oikocrystic bronzite, wehrlites and olivine age, and forms the basal plinth of the Sørøy clinopyroxenites (Fig. 30). Nappe. The Upper Layered Series represents the Due west of Langfiordjøkelen, where the ultra_ second major phase of intrusion. Seven cyclic mafic complex is directly in contact with metase- units, each consisting of olivine diment, partial cumulates follo- melting has produced a wed by clinopyroxene-olivine cumulates, have 20-30 m-wide zone rheomorphic of breccias been recognised within the 250 m-thick sequ- with granitic and granodioritic neosomes. A ence preserved. The olivine çumulates are poiki- thermal aureole developed in the metasediments litic spinel wehrlites and the clinopyroxene-oli- extends up to 2 km from the western intrusive vine cumulates are wehrlites and olivine clino- contact and is characterised by sillimanite and pyroxenites. In the lower cyclic units olivine K-feldspar (Bennert 1974). Replacement of silli- cumulates dominate, whereas clinopyroxene- manite by kyanite in the outer parts of the olivine cumulates predominate in the higher aureole suggests that crystallisation and cooling units. of the ultramafic complex occurred under con- Vertical cryptic variation within the sequences fining pressures ofat least 6 kbar (Althaus 1967, of cyclic units is extremely limited. Recent work Richardson et al. 1969). Within rhe marginal has established a total vertical variation ofoliv- zones of the ultramafic complex, coronas along ine composition within the Lower Layered olivine-plagioclase grain boundaries provide Series of Fo¡¡_¡o and within the Upper Layered NGU-BULL/m5, 198ó High-temperatureultramaficcomplexes 29

Fig. 30. Cm- and dm-scale modal layering in clinopyroxene-olivine cumulates from the l.ower l:yered Series ofthe Reinford u.c.

Series ofFos5 - For, . The Upper Layered Series relationships indicate that the blocks are auto- appears also to show a slight lateral cryptic liths and were probably derived from the Upper variation amounting to c. 2 mole o/o forsterite Marginal Zone. over a distance of 500 m. This series contains The discordant Central Series represents a large blocks of massive websterite (Fig. 3l) and third major phase of intrusion and crystalliza- either massive or crudely layered olivine webste- tion, during which olivine was the only cumulus rite, sometimes including gabbro xenoliths. phase. The series consists mainly of dunite and Beneath such blocks, which appear to be concen- subordinate poikilitic wehrlite, and exhibits a trated at a particular stratigraphic horizon, the range in olivine composition from Fos6 to Foro modal layering is disrupted and folded, while However, within the sequence of olivine cumu- the overlying layers drape over the blocks. These lates there appears to be a regression in

I¿åle 2. Subdivision of the Reintord ultramafic complex. Olivine compositions by electron microprobe analysis (Emblin 1985)

ASSOCIATED SERIES ROCKTYPES MARGINALZONES ROCKTYPES

CENTRALSERIES OLIVINECUMULATES NORTHEASTMARGINAL Dunite, wehrlite, olivine Dunite, wehrlite ZONE websterite, websterite, For..-Fo,." lroctolite, olivine melagabbro Forr.,-Forr.,

UPPERLAYERED OLIVINEANDCLINO. UPPERMARGINAL Olivine websterile, websterite, SERIES PYROXENE-OLIVINE ZONE olivine gabbro CUMULATES Fo,..o-Fo.." wehrlite, olivine clinopyroxenite Fo,,.o-Fo,r,

LOWERLAYERED OLIVINEANDCLINO- LOWERMARGINAL Olivine websterite, websterite, SERIES PYROXENE.OLIVINE ZONE olivine melagabbro CUMULATES For,r-For,... lherzolite, wehrlite, olivine clinopyroxenite For.r-Fo.r," 30 M. C. Bennett, S. R. Emblin, B. Robins &. W. J. A. Yeo NGU-BULL4O5, 1986

Fig. 31. Iænsoid block of massive, coarse-grained websterite, probably derived from the Upper Maryinal Zone, enclosed within modallyJaye- red olivine

- {.- Mg/Mg+Fe ratio, suggesting that more than one The Mørginal Zones. Marginal zones, compri- pulse of magma was responsible for the Central sing pyroxene-rich ultramafic rocks characteri- Series. sed by variable contents oforthopyroxene and/- The contacts between the Central Series and or plagioclase and containing gabbroic wall-rock older ultramafìc cumulates, although discordant xenoliths, are lateral time equivalents of the and intrusive in a general sense, are generally various layered series. The Lower Marginal highly irregular and display evidence of metaso- Zone is developed adjacent to the Lower Layered matic activity, in which pyroxene was replaced Series, the Upper Marginal Zone adjacent to the by secondary olivine. Numerous examples exisl Upper Layered Series, and the Northeast Margi- of the partial replacement of clinopyroxene-rich nal Zone adjacent to the Central Series where layers within the Upper Layered Series by oliv- it intrudes the Upper Marginal Zone and the ine, without disruption of the layering, and gabbro envelope, to the southeast of Langford- where irregular bodies and veins of dunite cut jøkelen (Fie. 27 and Table 2). across but do not disrupt the modal layering Unlike the marginal (Fie. 32). Less commonly, non-dilational dykes . series of many layered intrusions, the Lower of dunite cut the cumulates of the Upper Laye- and Upper Marginal Zones within the Reinfiord red Series. These features are restricted to the complex lack any modal layering or banding orientated vicinity ofthe contact ofthe Central Series and roughly parallel to contacts. Instead, are clearly related to its emplacement. They give both these marginal zones contain various types evidence ofreaction between fluids derived from of macrofabrics that are laterally continuous the Central Series and pyroxenes of the Upper with the modal layering in the adjacent layered Layered Series. series. The narrow extension of the Central Series The Lower Marginhl Zone consists of a transi- southwestwards through the Upper Layered tional sequence of thick (> I m) gently-dipping Series (Fig. 27) consists mainly of poikilitic layers of olivine websterite containing large wehrlite. Although this body is essentially a bronzite oikocrysts and an outer part of appa- dyke, numerous thin sheets of poikilitic wehrlite rently massive websterite and olivine websterite. project laterally from the dyke into the Upper The massive rocks in places contain numerous Layered Series. Many of these sheets have irre- small (cm scale) corroded xenoliths of metasedi- gular contacts,and enclose partially disrupted mentary origin. Within the thickly-layered units, aggregates of pyroxene, presumably derived lensoid xenoliths of gabbro mantled by coarse- from the host rocks. The structures at the base grained websterite lie with their long axes within of one of the wehrlite sheets suggest that the the plane of the layering. Olivines within the dyke-like extension of the Central Series was Lower Marginal Zone are significantly more emplaced into variably consolidated earlier Fe-rich fhan in the Lower Layered Series, and cumulates. lie in the range Fo32 - Forr. NGU - BULL ¿I05, 1986 High-temperatureultramatìccomplexes 3l

Fig. 32. Replacement of clinopyroxene by olivine in the Upper layered Series of the Reinlord u.c. is common close to the discordant contact v/ith the Central series. Inegular bodies and fingers of wehrlite and dunite lransgress, but do not disrupt, the modal lamination in the earlier clinopyroxene- olivine cumulates.

The Upper Marginal Zone is developed be- Layered Series close to the marginal zones. The tween the Upper Layered Series and the envelo- xenoliths occur in various stages of assimilation, pe, which except in the extreme northwest, and vary in size from large recrystallized gabbro consists of the Langstrand gabbronorite. The slabs, up to 200 m long in section, to small lithologies developed are similar to those in the leucogabbro restites a few centimetres across. Lower MarginalZone, but also include abundant Most xenoliths are surrounded by a coarse olivine melagabbro and troctolite associated pyroxenite reaction rim, the outer margin of with gabbroic xenoliths. Although not shown in which sometimes contains olivine. Fig. 27 for reasons of clarity, the Upper Margi- nal Zone extends along the top of the gabbro Ultramafic and maJic dykes. Relatively narrow screen. Here the modal proportion of plagioclase (< l5 cm wide) ultramafìc and mafìc dykes are in the zone decreases upwards. Adjacent to the common within the layered series, but are par- rheomorphic breccias due west of Langfordjø- ticularly abundant within the main outcrop of kelen, the Upper Marginal Zone is dominated' the Central Series (Fig. 33). Dyke compositions by massive websterite, but with macropoikilitic vary from dunite, through wehrlite and lher- olivine melagabbro developed in places, partic- zolite to pyroxenite, olivine melagabbro and ularly within 5 metres of the contact. This part gabbro. Crosscutting relationships at individual ofthe zone contains higher modal pyroxene and stations indicate a complex sequence of empla- plagioclase and lower modal olivine than else- cement. Most dykes are dilational, but some are where. The range o[olivine compositions mea- replacive. Bennett (1974) suggested that the sured in the zone as a whole lies between Fo' earlier dykes were produced by remobilisation and Fo74. of intercumulus liquids, and the later gabbroic The Northeast Marginal Zone contains xenoliths dykes were derived from residual melts. This and hybrids resulting from intrusion of Central simple interpretation will undoubtedly be revi- Series into the Upper Layered Series, its associa- sed in the light offurther investigation. ted marginal rocks, and the gabbro envelope along the eastern margin of the complex (Fig. Deþrmation within the complex. Most rocks 27). Lithologies in this zone include dunite and within the complex show variable degrees of poikilitic wehrlite, in addition to olivine webste- high-temperature recrystallization. While this rite, olivine melagabbro and troctolite. Associa- tends to obscure primary cumulus textures at ted with these are gabbro xenoliths, some of the microscopic scale, mesoscopic and macro- which form large (> l0 m long) lensoid slabs. scopic features are extremely well preserved. A The presence ofgabbro xenoliths is a feature late-D, high-temperature foliation, sometimes common to all three marginal zones, but gabbro containing macropoikilitic schlieren of pyroxene xenoliths also occur within the Lower and Upper and/or plagioclase, is present in parts of the 32 M. C. Bennett, S. R. Emblin, B. Robins l*, W. J. A. Yeo NGU - BULL ¡I05, 1986

T he int rus iv e s equences Although the intrusive sequences represented within the complexes differ in detail there appears to be a general trend towards emplacement of more olivine-rich rocks with time. This is most clear within the Reinfiord complex where clinopyroxene-olivine cumulates forming the Lower and Upper Series are cut discordantly by the olivine cumulates of the Central Series. The regularity of the modal layering within the Lower and Upper Layered Series and the recognition of three marginal zones equivalent in time to the major cumulate subdivisions of the complex show that the cumulate sequences represented in the complex have resulted from fractional crystallization of essentially liquid magmas. The cyclic units which have been distingu- ished within each of the layered series give evidence of the repeated emplacement of pulses of magma. During the formation of the Lower and Upper Layered Series each emplacement of new magma resulted in the temporary precipitation of olivine on the floor of the magma Fig. 33. Narrow, cross-cutting, dilational, mafic and ultrama- chamber followed by the cotectic crystallization fic dykes emplaced into massive olivine cumulate of the Later magmas Central Series ofthe Reinford u.c. of clinopyroxene and olivine. must have been either appreciably more magnesian or more voluminous and capable of sustai- ning extended periods of olivine fractionation as witnessed by the volume of dunite and complex close to contacts with the gabbro enve- wehrlite represented in the Central Series. Thick lope, and dips at 25"-40" in a northeasterly sequences of olivine cumulate may, however, direction (Bennett 1974). Later, post-emplace- be present beneath the Lower or Upper Layered ment ductile deformation is mainly restricted Series and clinopyroxene-olivine cumulates to discrete shear zones whilst later brittle defor- could have been developed in connection with mation is represented by a number of NE-SW- the crystallization ofthe Central Series but have trending faults which are displaced by E-W fallen prey to erosion. trending fault zones. One of the latter cuts the A trend toward more olivine-rich rocks can complex east of Storvannet, and has provided also be discerned in the Seiland-Stjernøy group a pathway for the aqueous fìuids responsible for of ultramafic comp'lexes. The earliest major the high degree ofserpentinization present along magmatic event in the Kvalfiord complex resul- this lineament. ted in the emplacement of olivine clinopyroxenite. Wehrlite dykes clearly represent a subsequent episode of activity. The Melkvann and Summary Nordre Bumannsfiord complexes show complex The preceding descriptions show that while each emplacement histories and a wide variety of of the ultramaltc complexes of northern Norway magmatic phenomena but also here olivine cli- has a structural and petrographic individuality, nopyroxenite dominates the early magmatic there are a number of common features. The aclivity and appears to have been followed by latter must form the basis of any synthesis of dunite and wehrlite. Inhomogeneous, banded the genesis of the ultramafic rocks. In this sec- and xenolith-rich ultramafìc dykes may have tion the phenomena common to all of the com- resulted from the emplacement of viscous, crys- plexes and their distinctive features are sum- tal-charged magmas but the evidence in many marized and discussed. dykes for extensive wall-rock reaction and assi- 33 NGU. EULL.4O5. 1986 High-temperatureultramaJìc complexes

milation as well as their dilational form and 1 olten restricted widths suggests the emplacement olfluid magmas. Initial short-lived fractionation ol olivine resulted in the dunite and wehrlite which commonly lines contacts of olivine clinopyroxenite dykes (Fig. 34). The later wehrlite dykes were emplaced together with olivine clinopyroxenite and olivine-hornblende clinopyrox- cnite dykes and may be related to them by lraclional crystallization. The appearance at a late stage of numerous amphibole-rich ultramafic dykes and olivine clinopyroxenite dykes bcaring Cr-spinel lherzolite and olivine clinopyroxenite nodules appears to be restricted to the Melkvann complex. The waning stages of the magmatic activity represented in the Seiland-- Stjernøy complexes were characterized by the cmplacement of picritic and ankaramitic dykes which do not seem to be represented in the 2 Reinljord complex. Each olthe ultramafìc complexes is intimately associated with an earlier mafic pluton and it seems probable that the mafic and ultramafic rocks were emplaced vic common feeder sys- tems. Mineralogical evidence also suggests petrogenetic links. between the mafìc and subsequent ultramahc rocks. The mafic plutons may be interpreted as parts of major magmatic cyc- les. The mafic rocks were, however, separated lrom the emplacement of the ultramafic rocks by an indeterminate interval of time. The gabbros appear to have crystallized, suffered variable degrees of deformation and undergone recrystallization prior to the hnal emplacement of the ultramafic rocks. The recrystallization appears, however, to be a high-temperature phenomenon, and there is convincing evidence for remobilization of the host gabbros during 3 the emplacement of the Seiland-Stjernøy ultramafic complexes. The lack of chilled facies along contacts of the earliest ultramafrc dykes represented in the Seiland-Stjernøy complexes further suggests that the temperature of the host gabbros could not have decreased significantly

Fig. 34. The evolution of irregular olivine clinopyroxenite dykes involved the following stages: l. The emplacement of magnesian magma into dilational fissures in hot, modally- layered olivine gabbro. 2. Assimilation of the host by the through-flowing magma, the rate ofdissolution being controlled by the local composition ofthe gabbro. 3. Plating ofthe irregular walls of the enlarged fissure by olivine, forming maryinal dunite or wehrlite, followed by the coprecipitation ofclinopyroxene and olivine on the smoothed walls, resulting in the olivine clinopyroxenite core, 34 M. C. Bennett,,S.R. Emblin, B. Robins &W. J.A. yeo NGU - BULL ,105, ¡986

belore the cmplacement of the ultramafites. land magmatic province, we term A regressive Emplacement of the ultramafìc rocks into a intrusive sequence. A regressive sequence in high-temperature environment is also suggested which a magma which crystallized a thick series by the degree to which reaction, assimilation of clinopyroxene-olivine cumulates overlain and hybridizalion are in evidence within the by clinopyroxene-magnetite cumulates was complexes. Nor is there any evidence oldyking followed by the emplacement of a largely liquid of the host gabbros previous to the emplacement magma from which olivine and chromite were ol the ultramafic rocks. This contrasts sharply the first minerals to crystallize is represented in with the dense swarms of dykes cutting both the the Duke Island ultramafic complex (lrvine gabbros and the ultramafic rocks and the numer- 1974). Irvine (1974) suggests a number ofpossi- ous episodes of dyking which appear to have ble reasons for this regressive intrusive sequence: accompanied the earlier evolulion of the Seiland magmatic province (Robins & Takla 1979). l) Magmas rising aiong a feeder system heated These considerations suggest to the aulhors that, by the passage of earlier magmas may be the emplacement of the ultramafic rocks fiol- less subject to fractional crystallization as lowed rather rapidly on that of the mafic rocks they ascend. and may have taken place before the hosl gabbros were entirely consolidated. 2) Later magmas carried olivine in suspension, possibly The consecutive development of the mafic including some derived from earlier cumulates deposited plutons and the ultramafìc complexes and the within the feeder system. intrusive events represented within the complexes can be interpreted as the result of the succes- 3) Magmas may have been derived from pro- sive emplacement of magmas having fewer liqui- gressively greater depths in the mantle or dus phases. As noted above, however, early from a source region that became depleted olivine-rich cumulales may possibly be repre- in water. sented beneath the Reinfiord complex and clino- 4) pyroxenite-olivine cumulates may have crystal- The composition of the mantle source may have become lized later than the exposed portion of the enriched in olivine due to earlier Central Series. This reservation applies equally episodes of partial melting. to the gabbroic plutons hosting the ultramafìc complexes which could conceivably also be A rather diflerent hypothesis for the generation underlain by ultramafic cumulates, although of regressive intrusive sequences in complexes there is little field evidence to support such a in which the major units are cumulates is conjecture. Considering only the structural level upward migration of successive magma cham- represented by the present-day surface, the ini- bers such that later cumulate sequences at a tial, voluminous magmas may have had compo- particular structural level represent earlier frac- sitions close to the natural plagioclase-clino- tionation stages. Regressive intrusive sequences pyroxene-olivine cotectic. Subsequent magmas may also be envisaged as a result ofthe succes- were emplaced in diminishing relative volumes sive tapping of a deep-seated magma chamber and their compositions progressively changed which possessed a vertical chemical zonation in such a way that they were first close to the due to density differences. Evidence bearing on clinopyroxene-olivine cotectic surface and Iater the application ofthese hypotheses to the Finn- were within the olivine phase volume (Fig. 35). mark mafic-ultramafic complexes will be revie- Although detailed consideration of the nature wed in the second contribution in this series. and genetic relationships of the magmas involved in the genesis of the mafic-ultramafic complexes will be deferred to a later contribution, this sequence of intrusive events is worthy Assimilation of some further general comments. The succes- Evidence for local assimilation of the gabbroic sive emplacement of rocks whose compositions' hosts is widespread in all complexes and has two are related in such a way as to be approximately main expressions. the reverse of that due to fractional crystallization, such as can be deduced from the Lang- l) The form of intrusive contacts. strand gabbronorite/Reinfiord complex and the 2) The occurrence ofhybrids resulting from the other mafìc-ultramafic complexes in the Sei- crystallization of syntectic magmas. NGU.BULL¿I05, 1986 High-temperatureultamaÍìccomplexes 35

In the Melkvann complex the highly irregular, fìc rocks as hybrids. The present authors regard non-dilational contacts of the earliest ultramafic assimilation on this scale is unlikely and reject and olivine melagabbro bodies are interpreted a dunite liquid as the primary magma. The as due to local dissolution, the modal variations model does not adequately account for either in the host olivine gabbro having determined the composition ol the olivine clinopyroxenite the rate at which assimilation took place (Fig. within the northern group of ultramafic com- 34). The dykes and sheets represented in this plexes or its preponderance. complex show a clear tendency towards simp- The extreme view advanced by Sturt et al. ler contact relationships, suggesting that the (1980) of the Reinfiord olivine clinopyroxenites importance of assimilation diminished with and wehrlites as crystallization products of a time. This trend culminated in the dilational, syntectic magma is also not compatible with the chilled contacts of the late-stage picritic and evidence that they originated as cumulates from ankaramitic dykes. It is concluded that the inlru- an essentially liquid magma. They suggest that sive activity within the Melkvann complex took a primary dunite magma generated the olivine- place as temperatures waned. The early phases rich rocks in the core of the complex. That these of intrusion occurred while the envelope remai- are olivine cumulates with intercumulus clino- ned at such a high temperature that assimilation pyroxene suggests, however, that they crystalli- was promoted. Later, the temperatures within zed from a malma not far removed from the the complex had lallen and assimilation was natural clinopyroxene-olivine cotectic. less marked. The serrated dyke margins typical of the early intrusions in the Melkvann complex are repro- duced on a larger scale along parts of the con- Metasomatism tacts of the Reinfiord ultramafìc complex. The The results of metasomatic activity associated development of websterite and olivine websteri- with the emplacement of ultramafic rocks can te along these contacts and around gabbroic be observed in all of the ultramafic complexes. xenoliths buried within cumulates, appears to Both the gabbroic envelopes and earlier ultra- be the result of crystallization of hybrid magmas mafic rocks have been lransformed locally and which arose by the assimilation of either gabbro to varying degrees. or gneiss. Olivine melagabbro found in places The appearance of coarse-grained olivine in around the complex would appear to represent originally olivine-free gabbronorite within a few a higher degree of assimilation. metres of the margin of the Reintord complex Olivine melagabbro, common within the is a particularly stiking example of metasomat- earliest intrusions in the Melkvann complex and ism of envelope rocks. Similar phenomena are locally developed in the Kvalfiord complex,- known from the Seiland-Stjernøy complexes appears to be a hybrid which crystallized simul- where extreme local alteration of olivine gabbro taneously with the olivine clinopyroxenite into resulted in coarse-grained olivine-rich troctolitic which it grades. Minor amounts of olivine gabbro and troctolite varying to dunite. The melagabbro also occur along the margins of patchy emplacement of gabbro has resulted in some of the later, irregular olivine clinopyroxe- relationships resembling the haloes developed nite dykes in the Melkvann complex. Zones of around peridotite dykes within the Emigrant olivine clinopyroxenite and patches of coarse- Gap ultramafic complex, California (James grained clinopyroxenite along the margins of l97l). The olivine neoblastesis in the North wehrlite dykes in the Melkvann and Kvalfiord Norwegian complexes appears to be rèstricted complexes may also be interpreted as a result to zones a few decimetres to metres wide along of assimilation of the olivine clinopyroxenite contacts with ultramafìc rocks, the 'flooding' hosts. of gabbroic rocks by olivine having only taken place where the mahc rocks are surrounded on According to Sturt et al. (1980), assimilation all sides by ultramaftc rocks. The processes played the principal role in the origin of the leading to the transformation of gabbro to more . petrographic variations within the Nordre olivine-rich rock types thus appear to have Bumannsfiord ultramafìc complex. They appeal operated over a short range and possibly were to large-scale interaction of a primary dunite diffusion controlled. magma with gabbro and metasediment of the Although the metasomatic zones may differ envelope and interpret 60-700/o of the ultrama- signihcantly in detail from place to place the 36 M. C. Bennet,,S.R. Emblin, B. Robins&.W. J.A.yeo NGU - BULL. ¡105, 198ó most common alteration of olivine gabbro and Upper Layered Series, respectively. As appears to be a volume for volume replacement noted above, assimilation of the envelope took involving a reduction in the mode of clino- place along the contacts ol the complex and pyroxene and a corresponding increase in lhe must have resulted in enlargement of the magma amount of olivine, and in some cases plagio- chambers. Although the available evidence does clase, accompanied by an increase in grain size. not permit a detailed description of the evolu- The mode of olivine may subsequently increase tion of the Reinford complex with respect to at the exp€nse of plagioclase. A detailed ex- thc room problem, emplacemenl by dilational planation olthis zonation will not be attempted mechanisms accompanied by supplementary here but the main leatures ol the modal varia- stoping and assimilation is considered most tions can be regarded as an approach towards likely. equilibrium with nearby magmas which were In strong contrast to the Reinfiord complex, crystallizing only olivine. This is clearly demon- the Seiland-Stjernøy complexes give little evi- strated by the extensive constant-volume repla- dence of the existence of large magma cham- cement of clinopyroxene by olivine in modal- bers. Modal layering is restricted to certain ly-olivine cumulates around the Central Series small ultramafic bodies and picritic dykes. of the Reinf ord complex. The irregular develop- Furthermore, the margins of the compl€xes are ment of secondary wehrlite and dunite from commonly gradational with an inwardly increa- pre-existing olivine clinopyroxenite is also wide- sing frequency of ultramafic dykes and sheets. spread in parts of the Melkvann and Kvalfjord Much of these complexes appear to have evolved complexes, in association with discrete wehrlite by the recurrent emplacement of dykes and dykes. The metasomatic dunites and wehrlites sheets, gradually fragmenting the envelope and of the North Norwegian ultramafìc complexes resulting in the isolation of country-rock encla- resemble in many respects the replacement ves. Emplacement of the later intrusions was duniles and peridotites described from the Duke clearly accompanied by dilation while assimila- Island ultramafìc complex, Southeast Alaska tion of the envelope appears to have been (lrvine 1974). important during the earliest phases of development. Initiation of the early intrusions by dilation is considered probable, Intrusion mechanisms their subsequent enlargement and replacive structural relation- One of the principal distinctions between the ships following from the dissolution of the wall Seiland-Sdernøy ultramafic complexes and the rocks by actively flowing magma (Fig. 3a). Reinlord complex is the structural relationships of the ultramafic rocks with the gabbroic envelopes. The cumulates exposed in the Reinfiord F r act io nal c ry st al li z ati o n complex clearly must have accumulated within Evidence for fractional crystallization in the a magma chamber of substantial horizontal Reinford ultramafìc complex is both explicit cross-seclion and vertical extent. The observa- and dramatic. As noted above, the Lower and tions require, however, a more complex scenario Upper Layered Series resulted mainly from the embracing three successively developed cham- coprecipitation of clinopyroxene and olivine bers, the latest having evolved mainly within with the temporary fractionation ofolivine alone the fractionation products of two earlier magma following the ingress of new pulses of magma. chambers. The evenls which led to the formation The exposed part of the Central Series is compo- of these magma chambers are nevertheless by sed exclusively of olivine cumulates. The subal- no means clear. Structural evidence ofthe shoul- kaline affrnity of the parent magmas to the dering aside of the country rocks is lacking and Reinfiord cumulates is indicated by the presence the frequency of gabbroic xenoliths within both of intercumulus Ca-poor pyroxene and the the marginal zones and layered series suggests composition of the cumulus clinopyroxene (Ben- that emplacement was accompanied by upward nefi 1972,1974). or downward stoping. Dilation may also have Within the Seiland-Stjernøy complexes, field played an important role in the development evidence for the operation of fractional crystalli- of the chambers. This is suggested by the dyke- zation in the genesis of the ultramahc rocks is like southern extension of the Central Series as not obvious. In the Melkvann complex the well as the subconcordant screen of gabbro occurrence of modal layering in oge dunite body forming the local roof and floor of the Lower suggests fractionation of olivine and picotite. NGU - BULL. ¡105, 1986 H igh- temperature uhramaÍìc complexes 37

CaMgSizOo Ga-rich pyroxene

392 1 500 Olivine 1kb clinopyroxenite

127 4 7kb

1 325 1320 Olivine Forsterite gabbr \. Spinel 1 CaAl2Si206 Mg2SiO4 Ø Olivine

Dunite, wehrl¡te

Plagioclase Olivine Fig. 35. Preliminary, qualitative, petrogenetic model for the mafic-ultramafic complexes within the Seiland province based on the pseudo-ternary Fo-Di-An system at I Kb (Presnall et al. 1978). Initial voluminous magmas had compositions close to the natural plagioclase

Late picrite dykes, containing mineral lamina- ably constitr.rte the most persuasive evidence tion accompanied in some cases by modal laye- of an origin by fractional crystrallization. Oliv- ring and cyclic units, resulted from fractional ine clinopyroxenite, the dominant rock type, crystallization of magmas which probably were contains clinopyroxene and olivine in propor- emplaced laterally along these surprisingly nar- tions similar to those predicted by the iron-free row dykes. Mineral chemistry and the crystalli- Fo-Di-An pseudo-ternary system at moderate zation sequence suggests that the picrite dykes rpressure (Presnall et al. 1978) for the cotectic are cogenetic with the bulk of the ultramafìc crystallization of forsterile and diopside (Fig. rocks within these complexes. Early olivine 35). The wehrlite and dunite intrusions can also fractionation is proposed for the dunite and be satisfactorily explained by the accumulation wehrlite which common lines the walls ofolivine of olivine followed by varying degrees of intercu- clinopyroxenite dykes (Fig. 3a). Narrow wehrlite mulus crystallization of clinopyroxene. For these dykes containing laminated olivine are also principal reasons we suggest that the ultramafic believed to be due to the precipitation of olivine rocks forming the bulk of the Seiland-Stjernøy from magmas migrating laterally along dilatio- complexes are cumulates. In dykes the cumula- nal fractures. tes may have been generated by plating of the The modal compositions of the main rock walls or precipitation on the floors of conduits types in the Seiland-Stjernøy complexes prob' through which magma was moving for prolon- 38 M. C. Bennet,S.R. Emblin, B. Robins &.W. J. A.Yeo NGU - BULL. ¡105, 1986 ged periods. Crystal accumulation was assisted me magma compositions or temperatures. The by the low viscosities of the magnesian, ultraba- development olthe ultramalìc complexes within sic magmas and the high temperatures of the the Seiland magmatic province of North Norway mafic plutons into which they were emplaced. appears to the present authors to have involved The normative compositions of the Seiland-- a variety of mantle-derived magmas, the most Stjernøy ultramafic rocks and the Ca- and Al- magnesian of which were probably picritic, with contents of their clinopyroxene (Oosterom 1963, up to c. 20 wt. 0/o MgO and emplaced at tempera- Yeo 1984, unpublished data) suggest derivation tures of c. 1450'C (Yeo 1984, Emblin 1985). from critically undersaturated magmas. As poin- Some ol these magmas may be represented ted out by Irvine (1974), magmas with signif- among the picrite-ankaramite dykes which cut icantly different initial compositions may yield the Seiland-Stjernøy ultramafic complexes. A similar ultramafic rocks as early fractionates. detailed discussion of petrogenetic models based This appears to be borne out by the ultramafìc on rock and mineral chemistries will. however, rocks of the Reinfiord and Seiland-Stjernøy be presented in the second paper in this series. complexes. A notable feature of the Fo-Di-An 'system' Acknowledgements is the expansion of the stability fields for clino- D7,2The authors acknowledge with gratitude the financial pyroxene and spinel at the expense of forsterite support ofthe following bodies during the c,ourse ofthe research reported in this contribution: and anorthite as the pressure increases (Presnall The Norwegian Research Council for the Science and et al. 1978). The reduction of the plagioclase Humanities (8.R.); The Natural Environment Research field is enhanced by the presence of water Council (M.8., S.R.E., W.J.A.Y.); The Royal Society (M.8.). (Yoder 1965) as well as Na,O (Biggar 1983). The line drawings were consfructed by the staff of the carto- graphic office of the Geological Institute of the University These relationships suggest that at moderate of Bergen under the direction of E. Irgens. This work is pressure a far wider range of natural liquids Norwegian contribution no. 89 to project2T (The Caledonide will crystallize olivine followed by clinopyroxene Orogen) under the auspices of the International Geological than at lower pressures where olivine and then Correlation Programme. plagioclase is a common crystallization sequence in magmas of 'basaltic' composition (Fig. 35). Furthermore, for liquids of equivalent composition the experimental observations suggest that the coprecipitation of clinopyroxene and olivine References can be take place over a wide temperature inter- Akselsen, J. l98O: Tectonic and metamorphic development pressure. val at moderate pressure than at low ofthe Precambrian and Caledonian rocl

Brooks, M. l9óó: Regional gravity anomalies attributable to basalt-garnet granulie-eclogite transilion. In: Heacock, basic intrusions in orogenic bells. Geophys. J.R. astr. J.C. (ed.): The slructure and physical prop€rties of the Soc.12,29-31. earths crusl. Am. Geophys. Union Geophys. Mon. 14, Brooks, M. 1970: A gravity survey of coastal areas of West 303-3 r4. Finnmark, Northern Norway. Geol. Soc. London Quart. James, O.B. l97l: Origin and emplacement of ultramafìc Jour. 125, l7l-192. rocks ofthe Emigranl Gap area, California, J. Petrol. 12, Bruland, T. 1980: Lillebukt alkaline complex: Mafic dykes 512-560. and alteration zones in Nabberen nepheline syenite. Un- Krauskopf, K.B. 1954: Igneous and metamorphic rocks ofthe pub. Cand.real. thesis, Univ. ofBergen. Øksford area, Vest-Finnmark. Nor. geol. unders. 188, Carswell, D.A. 1980: Mantle derived lherzolite nodules asso- 29-50. ciated with kimberlite, carbonatite and basalt magmatism: Kushiro, L & Yoder, H.S., Jr. l9ó6: Anorthite-forsterite and

A review. L¡thos 1 3, l2l-138. anorthitelayered intrusion. ./. Pe¡- mental determination of kyanite-andalusite and andalu- rol.23,127-162. site-sillimanite equilibria; the aluminium silicate triple Ito, K. & Kennedy, G.C. l97l: An experimental study of the point. Am. J. Sci. 267,259-272. 40 M. C. Bennet, S. R. Emblin, B. Robins &, W. J. A. Yeo NOU - BULL. ¡105, l9Eó

Roberts, D. l9ó8: The structural and metamorphic history - Scandinavia and related areas. J. Wiley & Sons, Inc., oflhe langstrand-Finfiord area, Sørøy, N. Norway. y'Vor. New York. geol. unders. 253, 1 60 pp. Streckeisen, A. 1976:. To each plutonic rock its proper name. Roberts, D. l97l: A conspectus of Eocambrian-Lower Pa- Earth Sci. Rev. I 2, l-33. laeozoic sedimentation on Sørøy. 1{or. geol. unders. 269, Stumpfì, E. & Sturt, B.A. 1965: A preliminary account of the 242-245. geochemistry and ore mineral parageneses of some Cale- Roberts, D. 1974: Hammerfest. Description of the l:250000 donian basic igneous rocks from Sørøy, Northem Norway. geological map. ltor. geol. unders. 30 I, l-66. Nor. geol. unders. 234, I 96-230. Roberts, D. & Fareth, E. 1974: Conelation ofautochthonous Sturt, B.A. 1984: The accretion of ophiolitic terrains in the stratigraphical sequences in the Alta-Repparfi ord region, Scandinavian Caledonide$. In: Zwar1, H.J., Hartman, P. West Finnmark, Nor. Geol. Tidsskr. 54, 123-129. & Tobi, A.C. (eds.\: Ophiolites and ultramafic rocks - a Robins, B. l97l; The plutonic geology of the Caledonian tribute to Emile denTex.GeoL Mijnbouw ó3,201-212. complex of Southern Seiland, Northern Norway. Unpub. Sturt, B.A. & Ramsay, D.M. 1965: The alkaline complex of Ph. D. thesis, Univ. oflæeds. the Breivikbotn area, Sørøy, Northern Norway. Nor. geol. Robins, B. 1972: Syenite+arbonatite relationships in the unders. 23 l, 6-142. Seiland gabbro province, Finnmark, Northern Norway. Sturt, B.A. & Taylor, J. 1972: The timing and environment Nor. geol. unders. 272,4158. of emplacement of the Storelv gabbro, Sørøy. Nor. geol. Robins, B. 1974: Synorogenic alkaline pyroxenite dykes on unders.272, l-34. Seiland, northern Norway. Nor. Geol. Tidsskr. 54, Sturt,8.4., Miller, J.A. & Fitch, F.J. 1967: The age of alkali- 257-268. ne rocks from west Finnmark, nolhern Norway, and Robins, B. 1975: Ultramafic nodules from Seiland, Northem their bearing on the dating ofthe Caledonian orogeny. Nor. Norway.Iilúos& l5-27. Geol. Tidsskr. 47, 255-27 3. Robins, B. 1980: The evolution of the Lillebukt alkaline Sturt, 8.4., Pringle, LR. & Ramsay, D.M. 1978: The Finn- complex, Stjernøy, Norway (abs.). Lithos 13,219-220. markian phase of the Caledonian orogeny. Geol. Soc. Robins, B. 1982: The geology and petrology ofthe Rognsund London Quart. J our. I 3 5, 597 -610.' intrusion, West Finnmark, Nolhern Norway. Nor. geol. Sturt, 8.4., Pringle, I.R. & Roberts, D. 1975: Caledonian unders. 37 1: l-55. nappe sequence of Finnmark, Northern Norway, and the Robins, B. & Gardner, P.M. 1974: Synorogenic layered basic timing of orogenic deformation and metamorphism, Bull. intrusions in the Seiland Petrographic Province, Finn- Geol. Soc. Am.86. 710-718. mark. Nor. geol. unders. 3 1 2, 9 1- 1 30, Sturt, 8.4., Roberts, D. & Furnes, H. 1983: A conspectus of Robins, B. & Gardner, P.M. 1975: The magmatic evolution Scandinavian Caledonian ophiolites. /n. Gass, I.G., Lip- of the Seiland province, and Caledonian plate boundaries pard, S.J. & Shelton, A.W. (eds.): Ophiolites and Oceanic in northern Norway. Earth Planet. Sci. l¿tt. 26, 167-178. Lit hosphe re. Geol. Soc. Lond. Sp. Pub. No. I l, 38 l-39 l. Robins, B. & Takla, M.A. 1979: Geology and geochemistry Stufi, 8.4., Speedyman, D.L. & Griflin, W.L. 1980: The of a metamorphosed picrite--ankaramite dyke suite from Nordre Bumannsford ultramafic pluton, Seiland, North the Seiland Province, Northern Norway. Nol Geol. Norway. Part I: Field relations. Nor. geol. unders. 358, Tidsskr. 59,67-95. l-30. Robins, B. & Tysseland, M. 1983: The geology, geochemistry Worthing, M.A. l97l: Studies in Íhe slructure and melamorp- and origin ofultrabasic fenites associated wilh the Pollen hic petrology ofthe Eo-cambrian rocks of Eastern Seiland, carbonatite (Finnmark, Norway). Cheft.. Geol. 40, 65-95. North Norway. Unpub. Ph.D. thesis, Univ. of London. Skogen, J.H. 1980: Lillebukt alknline komplel