JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 86, NO. Bll, PAGES 10617-10632, NOVEMBER 10, 1981

THE LATE ARCHAEAN Q6RQUT GRANITE COMPLEX OF SOUTHERN WEST GREENIAND

Michael Brown and C. R. L. Friend

Department of Geology and Physical Sciences, Oxford Polytechnic: Headington, Oxford OX30BP, United Kingdom

V. R. McGregor

Atangmik, 3912 Sukkertoppen, Greenland, Denmark Geological Survey of Greenland, Copenhagen, Denmark

W. T. Perkins

Department of Geology and Physical Sciences, Oxford Polytechnic.• Headington, Oxford OX30BP, United Kingdom

Abstract. Granites and granite pegmatites composing the • 2550-Ma QSrqut granite complex occur in a SSW-NNE trending linear belt >150 km long extending through the Buksefjorden-Ameralik-

O , o Godthabsf3ord region of southern West Greenland. The main body of the complex crops out over a distance of 50 km from Ameralik to Kapisigdlit kangerdluat and reaches a maximum outcrop width of 18 km between Stor• and Q6rqut. Around Q6rqut the complex comprises three •ain groups of granites: early leucocratic granites, various grey biotite granites, and late aplogranite- granite pegmatites. Within the 1500-m vertical section available in this area the complex has a tripartite structure comprising a lower zone dominantly of polyphase granite, an intermediate zone where country rock occurs as rafts in polyphase granite with a complex sheeted structure, and an upper zone dominantly of country rock sheeted by granite. Fifty-two specimens of granite have been analyzed for major, minor, and some trace elements. Geo- chemical variation within the complex is consistent with either fractional crystalli- zation or partial melting, hut in both cases, feldspar + biotite must have been involved either as fractionating phases or as residual phases during melting to account for the trace element chemistry. Two possible models for the generation of the complex are either anatexis of granulite facies rocks in the lower crust following an influx of volatiles and heat from the mantle or melting at intermediate depths of amphibolite facies rocks with vola- tiles supplied by breakdown of hydrous phases.

Introduction

By far the most voluminous rocks in the Archaean of southern West Greenland are grey gneisses with tonalitic-trondhjemitic- granodioritic compositions. In the Godth•bsfjord-Ameralik-Buksefj orden region (Figure 1), two generations of grey gneisses have been recognized: the first comprising a major part of the • 3700-Ma-old Am•tsoq gneisses; and the second comprising the • 3000- Ma-old N•k gneisses [McGregor, 1973, 1979]. A plausible model for the origin of these grey

Copyright 1981 by the American Geophysical Union.

gneisses involves, first, partial melting of metabasaltic rocks followed by intrusion and differentiation of the resultant magmas under syntectonic conditions [O'Nions and Pankhurst, 1978; Compton, 1978b; McGregor, 1979]. It has been argued that the continental crust in this region was built up mainly of grey gneisses derived from the mantle by this two- stage process during two major episodes (termed accretion-differentiation superevents) approximately 3800-3600 Ma and 31OO-2800 Ma ago [Moorbath, 1977, 1978; McGregor 1979]. During both of these crust-forming episodes, remelting of older sialic rocks is considered to have been, at most, only a minor factor in the production of the grey gneisses.

In contras't, potassic granites are much less abundant in the Archaean of southern West Green-

land. Quartz-monzonitic augen gneiss occurs as a late phase in the Am•tsoq gneisses between Ameralik and Buksefjord [Bridgwater et al., 1976; Chadwick and Nutman, 1979]. This may be the product of partial melting of Am•tsoq grey gneisses under granulite facies conditions during the later part of the first accretion- differentiation superevent [McGregor, 1979; Griffin et al., 1980]. To the south of the Godth•bsfjord-Ameralik-Buksefjorden region, in the Sermilik-Fiskenaesset region, the •, 2800-Ma- old Ilivertalik granite [Kalsbeek and Myers, 1973; Myers, 1976; Pidgeon et al., 1976; Kalsbeek, 1976] has similar chemical character- istics to the Am•tsoq augen gneiss, for example high Fe/Mg ratios (see data of Hall [1977] and Compton [1978a] and compare those of McGregor [1979]). It, too, may be the product of part- {al melting of $ranulite facies õrey gneisses [Compton, 1978bj. In the Godth•bsfjord- Ameralik-Buksefjorden region, the • 2550-Ma- old QSrqut granite complex represents the last major plutonic event in the Archaean history of southern West Greenland.

The occurrence of a discordant body of potassic granite at Q6rqut, a small branch of

o

Godth•bsfjord (Figure 1), was first recorded by Noe-Nygaard and Ramberg [1961, p. 7], but, presumably as the result of a drafting error, the granite-,,as designated as homogeneous quartz diøritic to granodioritic gneiss on their map. McGregor made a reconnaissance of this granite in the area between Ameralik and Kapisigdlit kangerdluat (Figure 1) in 1965 and

Paper number 1B0461. 0148-0227/81/00lB- 0461 $01.00

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Brown et al.' QSrqut Granite Complex of Southern West Greenland 10619

1966, completed in 1978 and 1979, and named it the QSrqut granite [McGregor, 1973].

Detailed mapping in the type area of the QSrqut granite, around Q•rqut and as far north as Sulugssugutip kangerdlua, was undertaken during 1978 [Burwell and Friend, 1979] and 1979 [Brown and Friend, 1980a, b], and it is from this area that most of the information on the

internal structure of the granite has been obtained. Much of the mapped Q•rqut granite between Ameralik and Kapisigdlit kangerdluat has in fact a highly complex internal structure with considerable amounts of country rock cut by sheets of many different phases of granite. It is misleading to think of the granitic rocks as forming a clearly defined pluton even in the type area around Q•rqut where granite is the dominant lithology. In order to avoid giving this impression and also to emphasize the polyphase nature of the granite, Brown and Friend [1980b] have suggested that the name be changed to the Q•rqut granite complex. Mapping by m•ers of Exeter University team between 1972 and 1977 [Sharpe, 1975; Gibbs, 1976; Compton, 1978a; Coe et al., 1976; Chadwick, et al., 1978; Chadwick and Nutman 1979 ] has documented the existence of micro- granite and banded aplite-pegmatite sheets along the extension of the axis of the main body of the Q•rqut granite complex to the south-southwest between Ameralik and

Buksefjorden. Walton mapped the north- northeastern extension of the Q•rqut granite complex btween Kapisigdlit kangerdluat and Kangersuneq [Walton, 1976], but he included with the granite many rocks considered by us to be late-stage, relatively homogeneous phases of the N•k gneisses. The extent of the Q•rqut granite complex in this area is, there- fore, uncertain. Further to the northeast on Ivis•rtoq, Friend and Hall mapped garnet- bearing granite and granite pegmatite sheets [Friend and Hall, 1977] that subsequent iso- topic work (S. Moorbath, verbal communication, 1980) confirmed as being of the same age as the QSrqut granite complex in the type area around Q•rqut.

Thus g•anites and associated granite pegma- tites composing the Q•rqut granite complex are found in a linear belt, greater than 150 km long, that extends from the margin of the inland ice on Ivis•rtoq, south-southwest at least as far as the outer coast at Faeringehavn (in Figure 1, only areas dominantly of granite are ornamented as Q•rqut granite complex).

This paper presents the current state of knowledge on the rather unusual late Archaean Q•rqut granite complex of southern West Green- land. Much of this material, including fifty- two chemical anlyses, is previously unpublished. We aim to give field and geochemical evidence Which we interpret as showing that the Q•rqut granite complex represents a substantial example of reworking of sialic crustal rocks by anatexis to produce granite magmas.

Review of Isotope Geology

Rb-Sr, U-Pb, and Pb-Pb methods have been used in determining the age of the QSrqut granite complex. Pankhurst et al. [1973]

obtained a Rb-Sr mineral isochron age of 2520+30 Ma (recalculated using k = 1.42-• 10 -11 y_l) on a late pegmatite from the south coast of Ameralik 5 km south-east of the contact of the main body of the Q•r•qut granite complex. Moorbath and Pankhurst [1976] reported a Rb-Sr whole rock isochron age of 2460+_90 Ma from seven granite samples from the north coast of Ameralik. Reanlysis of these samples and a•nalys. is of a further sixteen samples from Umanap suvdlua and Sulugssugutip kangerdlua has revised the whole rock Rb-Sr isochron age to 2530+30 Ma, the initial 87Sr/86Sr ratio being 0.7081 +0.0007 [Moorbath et al., 1981]. The age agrees well with a

Pb-Pb whole rock isochron ale of 2580+80 Ma obtained by Moorbath et al 1981] on •wenty- eight samples of Q6rcut granite and three pegmatitic K-feldspars, and with a U-Pb zircon age of 2530+30 Ma obtained by Baadsgaard [1976].-- The high initial 87Sr/ 86Sr of the granite and the relatively unrad- iogenic nature of the contained Pb suggest that crustal anatexis was a major contributory factor in the genesis of the granites [Moorbath et al., 1981].

Regional Setting of the Q6rqut Granite Complex

The QSrqut granite complex, whose age is taken to be 2550 Ma [Moorhath et al., 1981], was em- placed toward the end of a late Archaean period of plutonic activity 2700-2500 Ma ago. This activity affected not only parts of the Archaean crAton but also areas to the north which

subsequently became part of the early Proterozoic Nagssugtoqidian mobile belt. Large areas, especially in the northern part of the Archaean craton, escaped or were only weakly affected by this plutonic activity. In these areas, granulite facies mineral assemb/lages dating from the • 2800-Ma metamorphic culmination •Black et al., 1973; Pidgeon and Kalsbeek, 1978J are widely preserved. Elsewhere, extensive retro- gression of granulite facies mineral assemblages was caused by an influx of water and other mobile components such as C1, K, Rb, and Si [Bridgwater, 1979].

Deformation in the period i•nediate!y pre- ceding the formation of the Q•rqut granite complex appears to have been concentrated in SSW- NNE trending linear belts of intense ductile strain. Several linear belts of this type have been recognized in the Buksefjorden-Ameralik- Godth•bsfjord region [Sharpe, 1975; Chadwick and Nutman, 1979]. One such linear deformation belt is cut by the eastern contact zone of the Q•rqut granite complex on the north side of Ameralik. Another important linear deformation belt extends through southwestern Bj•rne•en, Godth•b town, and the islands to the south-south west, some 10 km to the west of the Q•rqut granite complex. In this belt the foliation is generally steeply inclined, and both earlier structures and the lithological units are strongly attenuated. A strong extension lineation was produced that plunges regularly at 20o-30 ø to the SSW.

Widespread, but relatively minor, intrusion of granitic rocks occurred before and during the formation of these linear deformation belts.

10620 Brown et al.: Q•rqut Granite Complex of Southern West Greenland

On Bj6rne6en and Sadel6, for example, a suite of thick granite pegmatites and thinner granite aplite dykes postdates the main fabric-forming deformation in the N•k gneisses. The aplites are strongly deformed in the linear deform- ation belt exposed on south-western Bj6rne6en. Zircons from an aPlite belonging to this suite from south-eastern Bj6rne6n have yielded a U/Pb concordia/discordia age of 2670 Mm [Baadsgaard and McGregor, 1981] which thus constrains the deformation within the linear belt which crosses southwestern Bj•rne•en to the period immediately before the formation of the Q6rqut granite complex.

It appears, then, that shortly before the formation of the Q6rqut granite complex, the region was affected by activity that included the development of SSW-NNE trending linear belts of ductile deformation, the intrusion at the present level of erosion of small volumes of granitic magma, and the influx of aqueous fluids over large areas. Since the deeper parts of the crust in the Godth•bsfjord- Ameralik-Buksefjorden region can reasonably be assumed to have been depleted in H20 during granulite facies regional metamorphism around the 2800-Ma metamorphic peak, the fluids must have come from below the crust.

One of the most interesting features of the Q6rqut granite complex is its location within a SSW-NNE trending linear belt at least 150 km long but no more than 18 km wide at the present level of erosion. The belt within which the QSrqut granite complex was formed differs, however, from the linear deformation belts formed earlier in the same general period of activity in that a much larger volume of granite has been emplaced in it, at least at the present level of erosion, and that it does not appear to have been the site of significant ductile strain.

Field Relations

Country Rocks

Along much of its length the main body of the Q6rqut granite complex lies subparallel to

fragments, but these amphibolites do not cut the granites themselves. The striking simil- arity between the amphibolites in the rafts and enclaves and those found in the country rock Am•tsoq gneisses indicates that many of the rafts and enclaves within the complex are, in fact, derived from Am•tsoq gneisses.

N•k gneisses, with enclaves of anorthositic rocks, crop out deep within the complex on the south side of the mouth of Q6rqut. Further north, between Sagdlia and Sulugssugu- tip kangerdlua (Figure 2), the lower levels Of the complex contain rafts and enclaves of N•k gneisses together with anorthositic rocks and amphibolites, the latter comparable in litho- logy to components of the Malene supracrusta!s [McGregor, 1973]. Within the area between QSrqut and Sulugssugutip kangerdlua, the original configuration of the country rocks prior to granite emplacement is preserved as shown by careful mapping of the rafts and enclaves of country rock gneisses found with- in the granite complex (Figure 2). North of Kapisigdlit kangerdluat, the granites cut mainly N•k gneisses, many of which contain enclaves and thin, semicontinuous units of anorthositic rocks.

Form and Internal Structure of the

Q6rqut Granite Complex

Granite (as defined by Streckeisen [1976]) is the dominant lithology in an elongated area that extends for some 50 km from

Ameralik to Kapisigdlit kangerdluat and which reaches a maximum width of 18 km from

southern Stor• to Q•rqut (Figure 1). The form of the complex in this area is diffi- cult to establish because the margin of the complex in most places consists of a broad zone in which country rocks are cut by varying amounts of granite and granite peg- matite as sheets. There is generally no simple intrusive contact. Only on the south coast of Stor6 has a relatively sharp contact been observed. This contact is steeply inclined to the northwest and slightly discordant to the lithological

the regional structure, although in detail at its boundaries and gneissose foliation in the margins the complex is usually.sharply discordant. country rocks (Figure 1). Compared with Around the northern part of Umanap suvdlua, how- the margins of the complex elsewhere, there ever, the structure of the country rocks is more complicated than in the area to the south, and in its northern part the main body of the complex is strongly discordant to the regional structure.

Between Buksefjorden and the northern part of 0manap suvdlua the complex lies within a major

are relatively few angular, unmodified rafts and enclaves of country rock within the granite close to the contact and relatively few sheets of granite within the country rock outside the contact.

The granite complex appears to lie in the unit of Am•tsoq gneisses (Figure 1). The core of a broad antiformal structure that country rocks around the main body of the complex, •lunges 20o-30 ø to the south-southwest between Ameralik and Kapisigdlit kangerdluat, [McGregor, 1973; Bridgwater et al., 1976], are almost entirely composed of Am•tsoq gneisses. Amitsoq gneisses are recognized in the field by the presence of amphibolite layers. These layers are sometimes discordant to the gneiss- ose banding but occur more commonly as strongly disrupted remnants. They have been derived from an abundant swarm of basic dykes m the Ameralik dykes {McGregor, 1973]. Cornonly the rafts and enclaves of country rock gneisses which are found within the Q6rqut granite com- plex are seen to contain amphibolite layers or

parallel to the strike of the linear deform- ation belts. Four sets of joints are established from a stereographic analysis of joint orientations within the granites in the area north of Q6rqut. Two of these sets,

o o oriented NO50 E and N160 E, are symmetrically disposed about the SSW trending antiformal axis of the complex; one set, oriented NllOøE, is perpendicular to this axis, and a fourth set, which is gently dipping and apparently randomly oriented, appears to be related to

Brown et al.: Q•rqut Granite Complex of Southern West Greenland 10621

the margins of randomly oriented shallowly inclined sheets of granite or composite aplogranite-$ranite pegmatite [Burwell and Friend, 1979J. This symmetrical arrangement of three sets of joints about the axial direction of the antiformal structure suggests some tectonic influence during the crystal- lization stage of the complex. The anti- formal structure is probably the result of the diapiric rise of the granitic magmas which built up the complex.

In the area between QGrqut and Sulugssugutip kangerdlua, the QGrqut granite complex comprises three main groups of granites [Brown and Friend, 1980b]: early leucocratic granites which are often characterized by the presence of biotite schlieren and lamellae (group 1 granites); essentially homogeneous, grey biotite granites (group 2 granites); and composite aplogranite- granite pegmatite sheets (group 3 granites).

The early leucocratic granites contain discrete to diffuse enclaves of gneiss which exhibit a range of states of modification of their original textures and structures by segregation and dis- ruption during anatexis. These modified enclaves are regarded by Brown and Friend [1980a] as fragments brought in by the leucocratic granites from a melting zone, rather than representing locally derived fragments of country rock gneisses, since clear examples of the latter are generally little modified after incorporation in the granite. The grey biotite granites, which are free of modified gneiss enclaves of the type found in the group 1 granites, occur as sheets from <1 m to >10 m in thickness which cut country rock rafts, the early leucocratic granites, and each other. At many localities, up to four penecontemporaneous phases of grey biotite granite occur, but although a consistent intrusive sequence occurs

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OOrqut Granite Complex I•1 granites undifferentiated

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Am•tsoq • agmatite enclaves

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Fig. 2. Geological map (post QSrqut granite complex dykes omitted) of the area east of •m•nap suvdlua between QGrqut and Sulugssugutip kangerdlua showing in detail part of the QGrqut granite complex (the geology on Stor6 is not shown). Note that Talorssuit is 1490 m high; spot heights have been omitted for clarity but are shown in Figure 3.

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in the area east of 0m•nap suvdlua between QSrqut and Sulugssugutip kangerdlua. Spot heights are used to illustrate the strongly dissected nature of the terrain which exposes the complex through approximately 1500 m of relief in this area.

Fig. 3b. (Opposite) Composite section, about 2000 m thick, through the Q•rqut granite complex compiled from field data, field sketches, photo- graphs, and photographic mosaics of steep to vertical fjord walls and mountain faces to show the internal structure of the complex and the three zones referred to in the text. Country rocks are shown in black, early leucocratic granites (group 1 granites) are shown in jack- straw ornament, grey biotite granites (group 2 granites) are shown in various dotted ornaments, and late aplogranite-granite pegmatite sheets (group 3 granites) are shown unornamented.

Brown et al.' QSrqut Granite Complex of Southern West Greenland 10625

at some localities, it has not proved possible to construct an intrusive sequence of grey biotite granites applicable to the whole outcrop of the QSrqut granite complex. The aplogranite- granite pegmatite sheets often show a distinct layering between biotite-bearing and biotite free types and between aplitic texture and pegmatitic texture. They cut through all of the earlier granites and become volumetrically the most important group at higher levels in the complex.

Within the 1500 m vertical section through the Q6rqut granite complex provided by the relief in this area, the complex exhibits a distinct tripartite structure [Brown and Friend, 1980b] (Figures 2 and 3) comprising (1) a lower zone dominantly of polyphase granite with group 1 granites comnonly containing enclaves of modi- fied gneiss but only rare enclaves of unmodified country rock cut by various group 2 granites and occasional group 3 granites, (2) an intermediate zone with a substantial proportion of country rock as rafts and enclaves in polyphase granite and with a complex sheeted structure dominantly of group 2 granites (shown semidiagramuatically in Figure 3), and (3) an upper zone dominantly of country rock cut by sheets of group 2 granite and of group 3 composite aplogranite-granite pegmatite (Figure 3, upper zone drawn from a number of photographic mosaics). The sheets in the upper zone fill what appear to be extension fractures rather than shear fractures, which suggests that on a regional scale the minimum principal compressive stress was acting vertically. On the steep sections through the complex provided by the mountain peaks, the upper zone is seen to comprise lozenge-shaped areas which consist of country rock cut by group

common in a belt that extends to the south-

southwest (see, for example, Chadwick and Nutman [1979]), suggesting that the complex extends at depth in this direction. More continuous areas of granite occur around Qara. jat [Compto•n, 1978a], on the Agpanguit peninsula '[Gibbs, 1976J, and on Skinderhvalen, north of Faeringehavn, where the

ranite is thought ta'form an overthrust structure Sharpe, 1975] (Figure 1).

North of Sulugssugutip kangerdlua the complex is poorly known. Polyphase granite is the dominant lithology on the peninsula between Sulugssugutip kangerdlua and Kapisigdlit kangerdluat. Along the north coast of Kapisigdlit kangerluat, around Kfnaussaq, older rocks, mainly NuM gneisses, are cut by only a few widely spaced granite sheets, although gently dipping granite sheets become progressively more abundant upwards on the mountain wall. To the northwest, rocks believed to be part of the Q6rqut granite complex crop out extensively on the plateau northeast of Ivnarssuit (A. Steenfelt, verbal communication, 1979) and reach the coast on the northwestern part of Ivnarssuit as a number of sheets several tens of meters thick that dip at moderate angles to the north-west. Structurally below these sheets, to the southeast, there is little Q6rqut granite in the gently dipping Nfik gneisses. Further northeast, on Ivis•rtoq, bodies of aplitic and pegmatitic granite, which are part of the QSrqut granite complex, have been eraplaced into the core and along the limbs of an earlier antiformal structure [Friend and Hall, 1977].

Petrography

2 granites with the lozenge-shaped areas sep- Although it is clear that the QSrqut granite arated by anastomosing sheets of group 3 granites. complex comprises a number of different types of

Between Q6rqut and Ameralik the internal granite throughout much of its outcrop, detailed structure of complex is broadly similar to that north of Q6rqut (described above), although it has not been studied in such detail. An upper zone, similar to that established north of Q6rqut, crops out on the tops of the mountains and on the slopes facing Ameralik. Country rocks, mainly Amitsoq gneisses, are cut by sheets of grey granite and recut by a network of composite aplogranite-granite pegmatite sheets which generally dip broadly east or west at low to moderate angles. Granite sheets become thicker and more abundant downward, and an intermediate zone, some hundreds of meters thick, is dominated by. more massive granite with sub- ordinate rafts and enclaves of country rock.

The deepest ex•po. sed part of the complex, along the coast of Ureanap suvdlua north and south of the mouth of Q6rqut and on the southeast corner of Stor•, is made up of polyphase granites including early leucocratic granites with abundant strongly modified rafts and enclaves of Am•tsoq gneisses. N•k gneisses, virtually continuous and unmodified, crop out along the coast on the south side of the mouth of Q6rqut.

To the south the main body of the Q6rqut granite complex disappears down the plunge of the antiform as it crosses Ameralik. However, gently dipping sheets of microgranite and more variab!y inclined sheets of banded ap!ite- pegmatite, similar to those in the upper zone of the complex further north (described above), are

mapping and subdivision of the granite has been attempted only in the area between QBrqut and Sulugssugutip kangerdlua. For the purpose of this review, we offer some general petrographic remarks which apply to members of the complex as a whole. Table 1 gives a selection of repre- sentative modes of QSrqut granite complex rocks.

Most of the rocks that make up the Q•rqut granite complex are medium grained (in the range 1-5 am), although they vary from coarse-medium to fine-medium, with a texture which varies from hypidiomorphic-granular (common) to allotriomorphic-granular (rare). A porphyritic texture, with slightly larger plagioclase crystals, is occasionally developed. With the exception of a few thin, late tonalire dykes, the rocks examined in thin section are mostly granites bu.t with some granodiorites (see Figure 4 and Table 1) and comprise quartz, sodic plagioclase, microcline, and biotite as the major phases. The plagioclase, which has a composition generally within the middle part of the oligoclase range, often has an albitic rim and occasionally is zoned, sometimes exhibiting patchy zoning. The microcline, which is often perthitic, is generally anhedral and slightly replacive toward quartz and plagioclase. Biotite (• = light brown, C = Y = very dark green-brown) is the only mafic phase; it contains zircons and needles of rutile and may be fringed by opaques. Biotite is often closely associated

10624 Brown et al.: QSrqut Granite Complex of Southern West Greenland

TABLE 1. Modes of Granites From the QSrqut Granite Complex, Southern West Greenland

Specimen Quartz Plagio- Micro- Biotite Others* clase cline

86619 24.0 39.2 20.4 12.6 3.8

79798 32.0 30.7 27.3 6.5 3.4

195322(1) 32.1 29.1 35.5 2.6 0.8 195337(1) 28.9 34.5 32.1 1.9 2.7 195352(1) 32.2 30.5 34.3 2.3 0.6 195360(1) 33.8 32.7 29.0 4.1 0.3 195361(1) 27.3 45.9 23.4 1.3 1.2

195304(2) 26.5 42.9 20.8 9.3 0.5 195309(2) 30.9 32.3 30.4 4.9 1.6 195317(2) 20.5 49.0 18.2 12.0 0.4 195325(2) 34.0 36.0 24.1 4.7 1.2 195329(2) 30.0 29.5 30.4 6.4 3.7 195338(2) 31.5 40.4 19.5 6.8 1.8 195339(2) 29.5 31.9 31.5 5.7 1.4 195344(2) 26.1 35.2 32.0 4.3 2.6 195350(2) 28.3 35.4 28.5 6.8 0.9 195359(2) 32.5 30.6 34.0 1.1 1.8 195363(2) 26.7 37.4 26.8 6.3 2.9 195390(2) 29.5 35.0 23.1 7.4 5.0

195377 (3) 32.0 26.0 39.1 1.5 1.3 195397(3) 33.9 34.5 27.5 1.9 2.3

79635 24.6 35.0 35.6 1.9 3.0 79700 32.3 33.1 30.4 1.4 2.8 79701 28.5 38.6 24.9 3.3 4.7 79711 31.5 34.6 29.1 3.4 1.3 79712 27.7 35.8 27.7 5.5 3.3 79714 26.3 41.9 24.8 6.4 0.) 79717 36.4 31.4 26.6 4.0 1.7 79722 25.9 30.9 38.2 2.7 2.4

Specimen 86619 is a granite from along the north coast of Sulugssugutip kangerdlua. Specimens 79798 and 195304-195397 are granites from the area between Sulugssugutip kangerdlua and QSrqut. The number in parenthesis refers to the groups established by Brown & Friend [1980b] in this area. Specimens 79635-79722 are granites which •rop out in the mountains and along the coast on the north side of Ameralik. *Others includes epidote, muscovite, allanite, magnetite, zircon, apatite, and sphene.

with muscovite and epidote, both of which appear to be secondary and associated with the alteration of plagioclase. Accessory minerals, which occur in virtually all rocks examined in thin section, are magnetite, allanite (co•,,only metamict), zircon, apatite, and sphene. Three main factors in the petrography of the granites account for much of the variation observed in the

field between different phases. These factors are grain size variations; the appearance of the felsic minerals, which varies from translucent through cream to grey; and the modal amount of biotite, which, in homogeneous varieties of granite, varies from >12% down to <2%. There is

some tendency for rocks with higher modal biotite to have a higher proportion of plagioclase feld- spar relative to microcline; also, these rocks tend to be finer grained.

Textural relations in granitic rocks tend to be ambiguous and therefore difficult to interpret. Two general points may be stated, however, about those specimens of Q6rqut granite which we have examined in thin section. First, the biotite, especially in those rocks with higher modal biotite, often shows some evidence of instability and replacement. Second, there is some textural evidence of subsolidus reequilibriation between the felsic phases ( for example, the occasional development of myrmekite). Bearing these two points in mind, the generalized crystallization sequence in many of the Q6rqut granite complex specimens, especially those with a hypidio- morphic-granular texture, is biotite with contained zircons and rutile needles, slightly before plagioclase followed closely by quartz; or, biotite with contained zircons and rutile needles slightly before quartz followed closely by plagioclase. In both cases, anhedral, slightly replacive microcline is clearly later. In some specimens, particularly those with an allotrimorphic-granular texture, quartz and microcline appear to have crystallized essentially simultaneously, but shortly after the appearance of plagioclase. No interstitial micrographic intergrowth of the felsic phases is found, and no miarolitic cavities have been found.

Geoch emis try

A detailed geochemical study of the QSrqut granite complex in the area between Q6rqut and Sulugssugutip kangerdlua is in progress at Oxford Polytechnic by W. T. Perkins. Compton [1978a, b] has analysed specimens of QSrqut granite from the Buksefjorden area, but most of the data remain unpublished. At present, therefore, our knowledge of the chemical variation within the complex is based upon eighteen chemical analyses of specimens

60• ! 60

3, _.

A 35 65 90 p Fig. 4. Plot of Q•rqut granite complex speci-

mens in the •9 part of the QAPF diagram of Streckeisen 76]. Squares represent undiffer- entiated granites (collected before 1978), diamonds represent early leucocratic granites (group 1 granites), circles represent grey biotite granites (group 2 granites), and stars represent late aplogranite-granite pegmatite sheets (group 3 granites).

Brown et al.: O•rqut Granite Complex of Southern West Greenland 10625

collected from the north coast of Sulugssugutip kangerdlua, from the sheets of QSrqut granite which crop out along Ivnarssuit, and from the mountains and north coast of Ameralik (all collected before 1978, referred to as 'undiff- erentiated granites') and on thirty-four chemical analyses of specimens of group 1, group 2, and group 3 granites collected from the area between QSrqut and Sulugssusutip kangerdlua which form part of a postgraduate study by one of us (W. T. Perkins).

.Ana. lytical Methods

Specimens 79701-79798 were analyzed by X-ray fluorescence (XRF), major and minor elements were determined at the Geological Survey of Canada, and trace elements at the University of Leeds, United Kingdom. Specimens 86606-86622 and 171933 -171955 were analyzed at the Greenland Geological Survey, Denmark, MgO and Na20 were determined by atomic absorption spectrophotometry (AAS), and other elements by XRF. Specimens 195304-195399 were analyzed for major and minor elements at Oxford Polytechnic, United Kingdon, P205 was determined by colorimetry, and other elements by AAS; trace element determinations on these specimens were made by XRF analysis at the University of Nottingham, United Kingdom.

Discussion of the Geochemical Data

Major, minor and selected trace element cont- ents of the fifty-two specimens analyzed are given in Table 2. Also tabulated for each spec- imen are a differentiation index (DI) value (where DI is the sum of the mesonormative comp- onents Q+Or+Ab) and the proportions of Qz:Ab:Or: An (the granite system of Winkler [1979]) based upon the mesonormative proportions for each specimen. On plots of oxide/element against the differentiation index, TiO2, A1203, total FeO, MgO, CaO, Na20, and Sr all decrease with increasing values of DI, while Ba first increas- es with increasing values of DI to about DI 86 and then decreases, Rb shows little change, and K20 shows a general increase with increasing values of DI (see Figure 5). This variation is consistent either with differentiation of a

liquid of appropriate composition by fractional crystallization of a suitable phase or phases or with progressive fractional (batch) melting of an appropriate,parent with a suitable residuum. The specimens have high contents of K20, Rb, Sr, and Ba, low K/Rb ratios, and high Rb/Sr and Ba/Sr ratios, features which mead that a basaltic parent (with typical contents K = 21OO, Rb = 5.9, and Sr = 175, average values for Archaean basalts from Hart et al. [1970]), either as the parent liquid to be modified by •ractional crystallization or as the source rock or progressive partial melting, may b•

considered unlikely [Hanson, 1978]. With increasing values of DI, K/Rb and Rb/Sr show a general increase, Ba/Sr increases to about DI 86 and then decreases, and Ba/Rb shows a general

likely participating phases. It is clear that whether the process involved is fractional cry- stallization or partial melting, biotite and the feldspars must be important phases either as fractionating phases in crystallization or as residual phases in melting.

Plots of Ba against Rb and of Rb against Sr are given in Figure 6 together with vectors' which indicate (1) the change in composition of a melt as a result of fractional crystallization (FC) of the named phenocryst phases in acid rocks and (2) the change in composition of a melt as a result of equilibrium partial melting (PM) of the named residual phases in acidic rocks. The vectors were constructed using the data on mineral/melt distribution coefficients given in Table 3 and the method summarized by Hanson [1975]. If fractional crystallization is the dominant process giving rise to the observed geochemical variation, then fractionation of biotite and/or K-feldspar is necessary to produce the observed trend of Ba against Rb, but fractionation of plagioclase and/or K-feldspar is necessary to produce the observed trend of Rb against Sr. Alternatively, if partial melting is the dominant process giving rise to the observed geochemical variation, then residual K-feldspar and/or biotite is required to produce the observed trend of Ba against Rb, and residual K- feldspar and/or biotite is required to produce the observed trend of Rb against Sr. The data appear to be more compatible with partial melting as the dominant process giving rise to the observed geochemical variation. The effect of fractional crystallization superimposed on different partial melts cannot be assessed by this qualitative approach but will be assessed by modeling of the trace element distributions as part of our planned future work on the Q6rqut granite complex.

Plots of oxide/element against height above sea level (not figured) exhibit no rational variation whatsoever, so that granite at a higher level in the complex need not be more fractionated than granite at lower levels.

Plots of mesonormative compositions for the Q•rqut granite specimens within the Qz-Ab-Or-An tetrahedron (the granite system) for conditions where PH20 = P total = 5 kbar [Winkler et al., 1975] (note that this is the lowest value of PH20 for which comprehensive experimental data are available) show that the granite composit- ions fall on a smooth curve close to the quartz saturation surface (see Figure 7, top diagram). Individual specimens plot either in the p lagioclase primary phase volume or in the quartz primary phase volume or just below the two feldspar cotectic surface in the alkali feldspar primary phase volume. Since quartz or plagioclase was the first felsic phase to have crystallized in all specimens from the QSrqut granite complex Which have been examined in thin section, all of the specimens would be expected to plot above the two-feldspar corectic surface in the Qz-Ab-Or-An tetrahedron. The two-feldspar cotectic surface is located

decrease (see Figure 5). The observed variations closer to the base (tnc Qz-Ab-Or face) of the in K/Rb, Rb/Sr, Ba/Sr, and Ba/Rb combined with a tetrahedron at lower values of PH20; thus it consideration of the mineral/melt distribution seems likely that PH20 must have been less than coefficients for these elements in granitic rocks 5 kbar at the time of crystallization of the (Table 3) allow some assessment to be made of Q6rqut granite complex magmas. The smooth curve

10626 Brown et al.' QSrqut Granite Complex of Southern West Greenland

TABLE 2. Major(Percent), Minor(Percent), and Trace(parts per million) Element Compositions, Differentiation Index (Where DI= Mesonormative Q+Ab+Or) and Quaternary Qz, Or, Ab, and An

Proportions of Granite Samples From the Q6rqut Granite Complex, Southern West Greenland

171933 171937 171940 171949 171950 171951 171952 171955 86606 86615 86619 86622 79798

Si02 TiO 2 A1203 Fe203 FeO

MnO

CaO

Na20 K20 P20S H20 Total

73.1 73.3 75.2 72.30 73.7 74.3 72.8 71.5 76.4 72.5 70.7 73.5 0.20 0.21 0.04 0.31 0.15 0.12 0.19 0.28 0.01 0.16 0.38 0.16

14.6 14.6 14.3 14.8 14.5 14.3 15.1 15.O 13.8 15.2 15.4 14.4 0.3 0.1 - 0.1 - 0.2 - 0.7 0.1 - - -

1.3 1.2 0.5* 1.6 1.3' 0.9 1.6' 1.3 0.4 1.6' 2.4* 1.53' 0.01 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.00 0.01 0.03 0.01

0.56 0.45 0.27 0.67 0.52 0.37 0.55 0.72 0.16 0.48 0.84 0.43

1.63 1.31 1.40 1.63 1.30 1.19 1.32 1.64 1.01 1.34 2.00 1.29 3.6 3.2 3.6 3.4 3.3 3.4 3.3 3.4 3.0 3.4 3.6 3.3 4.94 5.50 5.02 4.78 5.22 5.12 5.47 5.48 5.40 5.55 4.43 5.40

0.10 0.08 0.03 0.12 0.08 0.05 0.11 0.10 0.05 0.08 0.15 0.08

0.5 0.6 0.4 0.6 0.6 0.7 0.4 0.6 0.5 0.6 0.6 0.4 100.84 100.57 100.77 100.33 100.68 100.66 100.85 100.73 100.83 100.92 100.53 100.50

68.12

0.38

16.09 0.07

2.27

0.05

0.89

2.14 4.94

3.95

0.13

0.61

99.64

Rb 250 240 200 250 170 170 170 200 220 200 260 250 207 Sr 200 190 250 250 80 160 170 210 220 220 230 140 275

Ba 820 1000 780 1200 750 620 840 740 n.d. 890 990 n.d. 1245

DI=Q+Ab+Or 86.86 88.27 90.61 85.00 87.85 89.75 86.81 86.O1 92.30 87.11 80.98 88.09 81.72

Qz) 29.96 31.17 30.70 32.10 32.08 32.08 30.58 27.89 34.92 29.01 30.73 31.22 20.96 Or) 28.24 32.25 29.22 27.27 30.15 30.07 31.54 31.93 32.28 31.99 23.93 31.13 20.37 Ab) =100 34.59 30.94 33.27 33.75 31.95 32.38 32.25 33.11 27.97 32.98 36.68 31.88 49.28 Ah) 7.21 5.65 6.81 6.88 5.83 5.47 5.63 7.06 4.83 6.02 8.65 5.78 9.38

195332 195337 195352 195360 195361 195362 195380 195385 195391 195304 195307 195309 195317

SiO 2 74.72 73.95 75.80 74.10 72.72 74.99 75.10 74.24 74.44 69.93 66.55 74.27 Ti02 0.11 0.10 0.15 0.24 0.19 0.18 0.11 0.16 0.06 0.29 0.40 0.31 A120 • 13.76 14.17 13.42 13.35 14.65 14.13 12.88 13.92 13.04 16.76 17.62 13.38 Fe20 • 0.13 0.48 0.02 0.17 0.12 0.02 0.55 0.56 0.13 0.29 0.56 0.32 FeO 0.74 0.80 0.57 1.20 1.11 0.38 0.79 1.19 0.46 2.07 1.93 1.21 M_nO O. O1 O. 02 O. O1 O. 02 O. 02 O. 02 O. 02 O. 03 O. O1 O. 03 O. 03 O. 03

MgO 0.12 0.10 0.12 0.34 0.28 0.09 0.24 0.37 0.04 0.83 0.97 0.31 CaO 1.07 1.16 0.97 1.42 1.61 1.58 1.13 1.29 1.24 2.12 2.62 1.21

Na20 3.31 3.66 3.34 3.10 3.49 3.53 2.96 2.97 3.13 4.11 5.04 3.32 K20 5.43 4.96 5.38 4.59 4.74 4.80 5.50 5.62 5.13 4.30 2.87 4.80 P205 0.01 0.01 0.02 0.05 0.03 0.02 0.08 0.04 0.03 0.13 0.14 0.05 H20 n.d. n.d n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. Total 99.41 99.41 99.80 98.58 98.96 99.74 99.30 100.39 79.71 100.86 98.73 99.21

67.92

0.46 16.33

0.44

2.19

O. 04

1.02

2.69

4.12

3.74

0.17

n.d.

99.12

Rb 205 217 178 169 163 144 205 186 149 218 169 211 198 Sr 104 95 111 186 184 188 116 264 159 261 367 123 375

Ba 197 283 300 710 707 756 287 942 475 1161 987 480 1892

DI=Q+Ab+Or 92.05 91.30 93.14 88.18 87.55 90.67 91.64 88.98 92.43 80.84 77.88 89.58 77.69

Qz) 31.50 30.36 32.59 36.14 30.87 31.45 33.73 32.50 33.19 26.29 22.40 34.43 26.12 Or) 32.33 29.63 31.95 27.10 27.86 28.53 32.89 33.14 32.21 23.22 14.19 28.36 19.54 Ab) =100 31.09 34.41 31.14 30.35 33.62 32.71 28.01 28.41 29.55 40.90 51.29 32.24 42.21 An) 5.08 5.59 4.32 6.41 7.65 7.31 5.37 5.95 6.04 9.60 12.12 4.97 12.13

Specimens 171933-171955 are undifferentiated granites from the QSrqut granite complex sheets cutting NQk gneisses •along the coast of Ivnarssuit, north of the mouth of Kapisigdlit kangerdluat. Specimens 86606-86622 are undifferentiated granites from the Qbrqut granite complex which crops out along the north coast of Sulugssugutip kangerdlua. Specimen 79798 is an un•dif, ferentiated granite from the Q5rqut granite complex which crops out along the east coast of Umanap suvdlua at Serfarssuit. *Total Fe as FeO.

Brown et al.: QSrqut Granite Complex of Southern West Greenland 10627

TABLE 2. (continued)

195325 195329 195338 195339 195344 195350 195363 195364 195365 1953•0 195372 195374 195381

SiO 2 TiO 2 A1203 Fe203 FeO

MnO

•go CaO

Na20 K20 P205 H20 Total

74.88 73.08 72.81 72.38 71.70 71.35 72.04 72.99 71.38 ?3.88 74.45 67.25 68.16 0.22 0.32 O.16 0.17 0.29 0.32 O.51 0.40 0.32 0.16 O. 21 O.51 0.26

13.37 13.91 13.40 14.11 13.93 14.65 14.20 13.95 14.59 13.88 13.79 15.74 15.O0 0.22 0.60 0.75 O.21 0.49 O.41 0.66 O.61 0.28 0.40 0.49 0.54 0.94 1.O7 1.62 1.49 1.33 1. O3 1.67 1.47 1.60 1.45 1.19 1.16 2.60 1.57 O. 04 O. 04 O. 03 O. 03 O. 03 O. 03 O. 02 O. 03 O. 02 O. 04 O. 03 O. 05 O. 03

0.29 0.55 O.61 0.50 0.44 0.63 0.63 0.47 0.56 0.32 0.24 0.80 0.67 1.29 1.61 1.71 1.75 1.31 1.68 1.75 1.39 1.69 1.26 1.45 2.34 1.77

3.66 3.14 3.54 3.98 3.20 3.39 3.42 3.38 3.47 3.24 3.30 3.67 3.65 4.39 4.67 4.20 5.19 4.79 4.80 4.84 5.08 4.82 5.21 4.89 4.57 5.25 0.05 O.11 O.10 0.08 O.10 0.09 O.12 0.08 O.11 0.06 0.04 O.14 O.12

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 99.48 99.65 98.80 99.73 97.31 99.02 99.66 99.98 99.69 99.6.• 100. O5 98.21 98.42

Rb 303 223 200 168 176 196 174 190 179 220 190 182 235 Sr 129 260 251 220 223 205 286 260 237 181 177 262 195 Ba 639 1782 1632 1161 1099 789 1338 954 1269 606 588 918 631

DI=Q+Ab+Or 89.89 85.63 86.O1 88.54 88.19 84.79 85.72 87.56 85.76 89.27 88.89 79.15 85.02

Qz) 33.85 34.88 33.33 26.24 33.46 31.o6 31.22 31.78 30.33 32.43 33.17 25.95 24.36 or) 25.51 27.13 23.94 29.84 29.17 27.74 28.20 29.66 28.07 30.73 28.77 25.54 31.18 Ab) =1OO 34.98 31.18 34.77 38.25 31.99 33.84 33.83 33.12 34.37 31.16 31.47 38.18 36.55 An) 5.65 6.81 7.96 5.68 5.38 7.37 6.74 5.44 7.23 5.68 6.59 10.33 7.91

195382 195390 195396 195398 195316 195377 195397 79701 79711 79713 79714 79722 195399

SiO2 TiO 2 A1203 Fe20$ FeO

MnO

CaO

Na20 K20 P205 H20 Total

71.50 71.51 72.32 75.34 77.31 75.20 76.70 73.64 73.63 73.43 71.63 71.69 71.88 O. 19 O. 40 O. 20 O. 09 O. 10 O. 15 O. 10 O. 15 O. 15 O. 09 O. 31 O. 20 O.3 2

14.O5 14.63 14.06 13.50 13.92 13.49 13.46 14.O8 13.81 13.66 14.45 14.42 13.78

0.45 0.49 0.62 0.23 0.O0 0.68 0.09 0.28 0.29 O.10 0.42 0.57 0.42

1.35 1.54 1.27 0,84 0.67 1.O2 0.59 0.89 1.O3 0.77 1.54 1.61 1.66 0.02 0.03 0.03 0.02 O.01 0.03 O.O1 0.05 0.04 0.02 0.05 0.05 0.03

0.40 0.68 0.44 O.16 0.05 O.14 O.15 0.27 0.34 0.16 0.80 0.43 0.36

1.43 1.84 1.47 1.16 !.32 1.19 1.33 1.12 1.32 1.06 1.51 1.16 1.35 3.19 3.46 3.36 3.12 4.06 2.87 3.35 3.89 3.82 3.81 %.32 3.87 3.36

5.27 4.47 5.10 5.06 3.51 5.62 4.59 4.87 4.68 5.17 4.77 5.14 5.12 O.10 O.11 0.07 0.02 O.01 0.04 O.O1 0.07 0.06 0.02 0.09 O. 12 0.05

n.d. n.d. n.d. n.d, n.d. n.d. n.d. 0.44 0.23 0.17 n.d. 0.78 n.d. 97.95 99.16 98.94 99.54 101.16 1OO.43 1OO.38 99.75 99.40 98.46 99.87 1OO. O4 98.34

Rb 216 168 214 159 118 158 170 263 292 203 242 204 245 Sr 198 229 156 116 89 178 156 112 159 86 194 156 143

Ba 690 1495 639 318 186 665 592 565 515 235 840 450 820

DI=Q+Ab+Or 87.83 84.19 87.85 90.89 90.75 90.74 90.94 91.O9 89.86 92.53 87.00 88.84 87.95

Qz) 30.66 31.91 30.56 34.68 35.77 33.84 35.43 29.66 30.44 28.50 25.49 27.70 30.76 Or)=lOO 31.53 25.71 3O.ll 3o.14 20.43 33.62 26.99 28.62 27.21 3o.61 26.44 29.93 30.28 Ab) 31.46 34.58 32.68 29.57 37.50 27.15 31.17 36.88 36.37 35.84 41.80 37.72 33.18 An) 6.35 7.81 6.65 5.61 6.31 5.39 6.41 4.83 5.98 5.05 6.28 4.64 5.78

Specimens 195332, 195337, 195352, 195360, 195361, 195362, 195380, 195385, and 195391 are group 1 granites; specimens 195304, 195307, 195309, 195317, 195325, 195329, 195338, 195339, 195344, 195350, 195363, 195364, 195365, 195370, 195372, 195374, 195381, 195382, 195390, 195396, and 195398 are group 2 granites; and specimens 195316, 195377 and 195397 are group 3 granites, all from the area between Sulugssugutip kangerdlua and Q•rqut. Specimens 79701-79722 are undifferentiated granites from the Qdrqut granite complex which crops out in the mountains and along the coast on the north side of Ameralik. Specimen 195399 is an undifferentiated granite from the Q6rqut granite complex which •rops out on the southwest side of Ameralik.

10628 Brown et al.' QOrqut Granite Complex of Southern West Greenland

0.6

0.4

0.2

0.5

MgO

ß

CaO =- top,

K20 , ß,Pro% * 4m, ß " '

- Rb f

ß Io ßus ß .. %. .'t ,

. Inqeq•n-&. ß '"1%,.

- 30O

- 200

- 300

200

100

1500

1000

500

300

200

-8

-6

4

2

- lO

-8

4 . ß B ß - b

75 80 85 90 95 75 81O 85 90 95

Fig. 5. Plots of selected oxides (in weight

The Q6rqut granite compositions seem likely to have been cotectic melts. Figure 7 also shows the mesonormative compositions of the QSrqut granite complex specimens projected from An onto the Qz-Ab-Or face and projected from Qz onto the Ab-OrrAn face of the Qz-Ab-Or-An tetrahedron. The QSrqut rocks plot inside the projection of the 685øC isotherm determined by Winklet et al. [1975], and it is quite clear that they could represent low temperature granitic melts [Winkler and Breitbart, 1978] whether or not such melts are roetastable

[Johannes, 1980]. Plagioclase and/or quartz could be residual phases during melting of an appropriate source material, and plagioclase and/or quartz will be the first felsic phases expected to crystallize from liquids with compo- sitions corresponding to the Q6rqut granite complex specimens, followed by alkali feldspar, either closely or after some crystallization of plagioclase and quartz; this accounts for the range in textures observed in these specimens from allotriomorphic-granular to hypidiomorphic- granular. If PH20 approached P total during the production of the Q6rqut granites by crustal anatexis but was lower than 5 kbar, the maximum depth at which anatexis could have occurred is of the order of 15-20 km.

General Discussion

In the area between Ameralik and Kapisigdlit kangerdluat, the country rock geology is dominated by Amftsoq gneisses at the present level of erosion. Around Q•rqut, some of the early leucocratic granites (group 1) carry schlieren of biotite and contain enclaves of

modified gneiss thought originally to have been mostly amphibolite facies Amltsoq gneisses.

percent) trace elements (in parts per million) These enclaves are interpreted by Brown and ' ' Friend [198Oa] as having been carried from the and element ratios against a differentiation upper margin (?) of a zone of melting at a deeper index (where DI is the sum of the mesonormative components Q+Ab+Or) for the fifty-two analyzed specimens from the QSrqut granite complex (symbols as in Figure 4).

defined by the QSrqut granite complex specimens thus trends along the quartz saturation surface and terminates in the cotectic curve formed by the intersection of the quartz saturation surface and the two-feldspar cotectic surface.

level in the crust. Pegmatite increases in abundance upward through the exposed section of the Q6rqut granite complex, and composite aplogranite-granite pegmatite sheets (group 3

ß

granites) are the most important granite component in the upper zone of the complex. Thus the granite magma or magmas from which the various granites composing the complex have been derived probably had a moderate content of water, although they may not have been water saturated.

TABLE 3. Mineral/Melt Distribution Coefficients for Common Rock-Forming Minerals From Dacitic and Rhyolitic Rocks

Element Garnet Ortho- Clino-

pyroxene pyroxene Hornblende B i oti te K-f e lds par P lag i oc lase

(•) (2) (3) (•) (2) (3) <•) (2) (3)

Rb 0.0085 0.0027 0.032 O.014 0.0077 3.26 2.24 3.0 0.659 0.34 0.8 0.O41 0.048 0.04 Sr O.O15 0.0085 O.516 0.22 0.094 O.12 - 0.4 3.87 3.87 3.6 4.4 2.84 3.35 Ba 0.017 0.0029 O.131 0.044 0.054 6.36 9.7 10.O 6.12 6.12 6.0 0.308 0.36 0.4

Data for garnet, orthopyroxene, clinopyroxene, hornblende, biotite (1), K-feldspar (1) and plagio- clase (1) are from the compilation by Hanson [1978]; data for biotite (2), K-feldspar (2) and plagioclase (2) are from the compilation by Arth [1976]; and data for biotite (3), K-feldspar (3) plagioclase (3) are from the compilation by McCarthy [1976].

Brown et al.: QOrqut Granite Complex of Southern West Greenland 10629

Ba 100

1000

99,,

3O

,3O

kh

6O

0

PM FC Fig. 6. Plots of Ba against Rb and Rb against Sr for the fifty-two analyzed specimens from the Q0rqut granite complex (symbols as in Figure 4). Also shown are vectors which indicate the change in composition of a melt as a result of fractional crystallization (FC) of the named phases in acid rocks and the change in composition of a melt as a result of equilibrium partial melting (PM) of the named•phases in acid rocks. The percentage of fractional crystallization of the original melt or the percentage of partial melting of the original solid to produce corresponding changes in Ba and Rb and Rb and Sr is marked along the appropriate vectors. The vectors were calcfilated using the data in Table 3 and the method summarized by Hanson [1978]. Key: g = garnet, o = orthopyroxene, c -- clinopyroxene, a -- hornblende, bh = biotite [Hanson, 1978•, ba = biotite [Arth, 1976], bm = biotite [McCarthy, 1976], kh-- K-feldspar IHanson, 1978], ka = K-feldspar [Arth, 1976I, km = K-feldspar [McCarthy, 1976I, ph =_plagioclase [Hanson, 1978], pa = plagioclase [Arth, 1976], and pm--plagioclase [McCarthy, 1976]. Exceptions are Ba against Rb, for fractional crystallization (FC), a = hornblende, orthopyroxene, and garnet, since all three produce closely similar vectors, and for both fractional crystallization (FC) and partial melting (PM), pm-- plagioclase for the data of Hanson [1978] and McCarthy [1976] which produce indistinguishable vectors; Rb against Sr, for fractional crystallization (FC), o = orthopyroxene and garnet, since both produce closely similar vectors, and pm= plagioclase for the data of Hanson [1978] and McCarthy [1976] which produce indistinguishable vectors, and for partial melting (PM), p = plagioclase for the data of Hanson [1978], Arth [1976], and McCarthy [1976] which produce closely similar vectors.

A high water content in such magmas places severe restrictions on their ability to move up through the crust [Tuttle and •-oweh, 1958; Cann, 1970]. The form and internal structure of the Q0rqut granite complex, shown schematically in Figure 3, suggest emplacement at a relatively high level in the crust in a regime of brittle/semibrittle failure rather than one'of ductile flow in response to deforming stresses. However, the absence of interstitial micrographic intergrowths between the felsic phases and miarolitic cavities places a restriction on the minimum thickness of overburden which must have been present at the time of granite eraplacement. The geoc•hemistry (this paper) and the isotope geology [Moorbath et al., 1981] of rocks from the QSrqut granite complex strongly suggest that the granites were produced by partial melting of sialic continental crust. The position of mesonormative composit-

ions of the analyzed Q•rqut granites in the Qz-Ab-Or-An tetrahedron and the inferred order of crystallization of the main phases established petrographically suggest a depth of generation of the parent magmas of less than 20 km A• temperatures of up to 685øC if PH20 approached P total.

Rocks with hornblende granulite facies mineral assemblages, produced during the metamorphic peak about 2800-Ma ago [Black et al., 1973], crop out extensively in the Archaean of West Greenland [Bridgwater et al., 1976; Allaart et al., 1977, 19781 . Rocks retrogressed from granulite facies mineral assemblages to amphibolite facies mineral assemblages crop out on the south coast of Ameralik just to the east of the Q6rqut granite complex, and it is likely that granulite facies conditions were reached at depth beneath the belt into which the complex was later eraplaced.

10630 Brown et al.: QSrqut Granite Complex of Southern West Greenland

An

Alkali-feldspar Ab Or

Ab Or

However, enclaves with preserved granulite facies mineral assemblages have not been found within the exposed part of the QSrqut granite complex, and locally derived enclaves and rafts of country rocks within the complex and the country rocks immediately adjacent to the complex all have amphibolite facies mineral assemblages.

We do not at present have sufficient inform- ation to determine conclusively whether the source rocks in the crust from which the QSrqut granite magmas have been derived by partial anatexis had amphibolite facies or granulite facies mineral assemblages. One of us (W. T. Perkins) is currently determining the rare earth element contents of a representative sample of 16 specimens from the Q•rqut granite cornpie. x, and we are hopeful that trace element modeling. of these data will constrain further the likely mineralogy of the source rocks.

One possibility, given the regional setting of the QSrqut granite complex, is that partial melting of granulite facies rocks deep in the crust could have occurred following an influx of volatiles. The rocks may have been sufficiently hot to melt in the presence of a volatile phase, or there may have been an enhanced heat flow accompanying the influx of volatiles. If this

Fig. 7. (Opposite) The top diagram shows the granite system of Winklet [1979] for conditions of PH20 = 5 kbar. The quartz saturation surface separating the liquidus phase volume of quartz from that of either plagioclase or alkali feldspar is shown, and the two feldspar surface separating the liquidus phase volume of plagio- clase from that of alkali feldspar is shown. The general trend of the QSrqut granite complex specimens, plotted as mesonormative proportions, is shown as a solid curve above the two feldspar surface and is dotted below the two feldspar surface (see text for discussion). The project- ion of this general trend from Qz onto the Ab-Or-An face of the tetrahedron is also shown as a dashed curve. The middle and bottom

diagrams show a Qz-Ab-Or plot of the QOrqut rocks projected from An and an Ab-Or-An plot of the Q•rqut rocks projected from Qz in the Qz-Ab-Or-An tetrahedron. The cotectic lines shown are for the condition PH20 = Ptotal = 5 kbar. The projection from An of the 670øC, 655øC, and 700øC isotherms on the quartz- plagioclase and plagioclase-alkali feldspar cotectic surfaces onto the Qz-Ab-Or face are shown, together with the An contents of liquid compositions lying on the cotectic surfaces or along the cotectic curve formed by the inter- section of the two cotectic surfaces for condit-

ions where PH20 = Ptotal = 5 kbar [after Winkler et al., 1975]. On the Ab-Or-An face, the projection from Qz of the 670øC, 685øC, and 700øC isotherms on the quartz-plagioclase and quartz-alkali feldspar cotectic surfaces are shown, together with Qz contents of liquid compositions lying on the corectic surfaces or along the cotectic curve formed by the inter- section of the two cotectic surfaces for condit-

ions where PH20 = Ptotal = 5 kbar [after Winkler et al., 1975].

Brown et al.' Q6rqut Granite Complex of Southern West Greenland 10651

model is correct, the liquids produced by partial melting will need to have fractionated a substantial amount of feldspar (both alkali feldspar and plagioclase) and biotite to account for the trace element distributions, since phases such as biotite cannot normally be expected to have been present in a source with a granulite facies mineralogy. In such a model it is difficult to envisage a mechanism of emplacement of the granite sheets which make up the complex which will allow the apparent random observed sequence of intrusion of the fractionated liquids, which themselves define rational geochemical trends on variation diagrams, without postulating the presence of a cumulate granite (probably of tonalitic composition) at deeper levels in the crust than presently exposed. The proximity of the analyzed Q•rqut granite cooplex specimens to the quartz saturation surface in the granite system makes substantial fractional crystallization of plagioclase unlikely.

An alternative model is one in which the

source material comprised amphibolite facies rocks at intermediate crustal depths. Suff- icient water to permit melting to occur in the volumes necessary must have come from hydrates such as mica (biotite) and possibly amphibole (hornblende) once temperatures exceeded that on an appropriate equilibrium curve for melting. In this model, in rocks with a suitable mineral assemblage, alkali feldspar might be expected to melt out first, with mica (biotite) + plag- ioclase + quartz + amphibole (hornblende) contributing to the melt fraction. Hornblende in the residue has little effect on K20, P,b, Sr, or

Nottingham University; and Anne Lloyd for typing the manuscript. Work in progress at Oxford Polytechnic on the QSrqut granite complex is supported by research grants from the Oxford Polytechnic Research and Advanced Study Committee.

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(Received November 18, 1980; revised March 2, 1981;

accepted March 13, 1981.)