Archaean crustal evolution in southern West Greenland: A review based on observations in the Buksefjorden region

Tectonoph.ysrcs, 105 (1984) 121-130

121

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

ARCHAEZANCRUSTAL EVOLUTiON IN SOUTHERN WEST GREENLAND: A REVIEW BASED ON OBSERVATIONS IN THE BUKSEFJORDEN REGION

BRIAN CHADWICK and KENNETH COE Department of Geoloa,

University, Exeter EX4, 4QE (United Kingdom)

(Submitted to the Editors February 6, 1982; received by Publisher October 30. 1983)

ABSTRACT

Chadwick, B. and Cue, K., 1984. Archaean crustal evolution in southern West Greenland: a review based on observations in the Bukse~orden region. In: SM. Naqvi, H.K. Gupta and S. Baiakrishna (Editors). Lithosphere: Structure, Dynamics and Evolution, Srectonoph~ysies.105: 121-130. Thermal, metamorphic and tectonic processes associated with the two major crust-forming events expressed as voluminous generation of Amitsoq and Nhk tonalitic-granitic gneisses, ca. 3750 Ma and ca. 2800 Ma respectively, in southern West Greenland are reviewed briefly in the light of recent studies in the Bukse~orden region.

INTRODUCTION

The geology of the Archaean block in southern Greenland (Fig. I) is now well known, notably through the writings of McGregor (1973) who demonstrated the existence of two major suites of gneisses, the older Amitsoq and the younger Nuk, separated in time by ca. 800 Ma. Other significant contributions have been made by Windley (1969a, b) who drew attention to the extensive sheets of stratiform anorthosite, Bridgwater et al. (1974) who speculated on a horizontal thrust tectonic regime and Black et al. (197I), Moorbath et al. (1972) and Pankhurst et al. (1973) whose isotopic age data confirmed the early deductions based on field evidence. An excellent review is provided by Bridgwater et al. (1976). The general interpretations given in the works cited are still valid, but recent studies have necessitated modifications of detail and some probtems have been aggravated rather than solved. In this brief review we draw attention to some of the recent work, much of which is based on Buksefjorden and adjacent regions (Figs. 1 and 2), and comment on its significance. The terminology and chronology are summarised in Table I, OO40-1951./84/%03.00

a 1984 Elsevier Science Publishers B.V.

i:

123

TABLE

1

Summary

of the Archaean

Principal

chronology

and terminology

in the Buksefjord

Approximate

events

age

region and adjacent Basic dykes

(Ma) Injection

of Qorqut

microgranites

in the

Buksefjorden

region

High-grade

and

ca. 2800

metamorphism

(granulite

facies in

southeast

Buksefjorden)

outlasting

deformation

Upright

ca. 2600-2400

granite,

mainly as pegmatites

Injection

folds and high-grade

Intra-Nbk

metamo~hism

of dykes

(Qaqatsiaq Injection

of major granitic

complex

in southeast

(llivertalik

granite

dykes)

ca. 3000

Bukse~orde~

and related

rocks) in late stage of recumbent folding Recumbent injection

folds and prolific

ca. 3000

of Ntik tonalitic-granitic

gneisses Thrusting

and tectonic

Depositional

dykes)

Some Ameralik dykes(?)

of magmas

the stratiform

dykes

(Neriunaq

rocks and

gneisses in northwest

Buksefjorden injection

parental

to

anorthosites and volcanic episodes

of the Malene supracrustal

ca. 3100?

rocks

Malene dykes and some Ameralik

Uplift and erosion of the Amitsoq sialic microcontinent Deformation (granulite

and high-grade associated gneisses

and volcanic episodes

of the Akilia association rocks

with the

of the Amitsoq

tonalitiogranitic Depositional

metamorphism

facies in northwest

Buksefjorden) generation

of

interleaving

of Maiene supracrustal Amitsoq

injection Irma-Nhk

supracrustal

ca. 3750-3600

dykes

areas

Anorthosite metagabbroic @j

Maiene

m

Amitsoa

augen

Amttsoq

multiphase

i’

fig.

2. Maps showing the position of the Archaean hiock of southern

and ntber localities

mentioned in the text.

and rocks

Greenland.

SUDracruStal

the

rocks

gneisses gneissss

Bukscfjnrdcn

region

125

our conclusion that the Ameralik dykes post-date all stages of metamorphism and deformation related to the Amitsoq event ca. 3750-3600 Ma. Griffin et al. (1980) calculate T and approximate P vaIues for equilibrium assemblages of this early metamorphism and suggest that by 3600 Ma the crust had accumulated to a thickness of 25-35 km comparable with the mean of modern continents. Retrogression from granulite facies involved an influx of water and it is assumed that this was related to NDk magmatism and the second period of crustal growth ca. 3000-2800 Ma (Table I). In view of this it is remarkable that the Amitsoq gneisses in question yield an Rb-Sr isochron which matches the Pb-Pb ages. It has been alleged that acid gneisses from further east in Buksefjorden and in eastern parts of Ameralik (Fig. 2) are also of Amitsoq origin, extremely depleted in U and Rb but sllbsequently refurbished in these elements. These gneisses give an Rb--Sr isochron of 2800 Ma and attempts to extract older ages using various minerals and isotope systems have failed. It is thus apparent that contradictions arise from isotopic age studies and we accept that only a combination of Rb-Sr and Pb-Pb systematics, together with Sm-Nd, can give reliable results. AMPHIBOLITE

DYKES

AS CHRONOLOGICAL

MARKERS

Recognition of the greater age of the Amitsoq gneisses was based on the presence of the Ameralik amphibolite dykes (McGregor, 1973) which were believed to be related to events taking place ca. 3700 Ma (McGregor, 1979). These dykes constitute an essential field criterion in the recognition of Amitsoq gneisses. However, it has been shown that similar amphibolite dykes in the Malene supracrustal association (Table I) cannot be separated from Ameralik dykes in time (Chadwick and Coe, 1976; Chadwick, 1981). These relations are highly significant since they indicate that the Amerahk dykes may be the early phases of a long period of dyke emplacement related to events ca. 30~-2800 Ma (Niik superevent; Coe, 1980) rather than marking the termination of the event in which the Amitsoq gneiss formation was the central part. At least two phases of Intra-Nuk amphibolite and related dykes are known, the subdivisions being erected on structural and chemical bases (Table I; Coe et al., 1976; Chadwick, 1981). However, there are many amphibolite sheets especially in the eastern part of Buksefjorden which cannot be placed with confidence in either category. Some of these sheets are inte~reted as Malene supracrustal amphibolites, but others may be contemporaneous with injection of NOk gneiss parents. The presence of Intra-Nuk dykes and other amphibolite sheets of Nuk affinity show that basaltic magma was generated and raised to crustal levels during a large part of the evolution of the Niik gneisses. Combined with the Ameralik and Malene dykes we conclude that there was basaltic injection during much of the NGk superevent which we interpret as the result of a major, deep-seated mantle disturbance. The simultaneous generation and existence of acid and basic magma compels comparison with

126

Phanerozoic

Cordilleran

necessarily Greenland. extensive

rocks (Windley

to imply a Phanerozoic Equally

compelling

basic igneous

activity

and Smith,

plate tectonic

is the contrast

1976). although model

we do not wish

for the Archaean

with the Amitsoq

of West

event

in which

has not been recorded.

‘I‘EC‘TONIC PROCESSES ca. 3000-2X00 Ma

In other respects Amitsoq petrographical

similarities

and Nbk gneisses

on the one hand

are similar. and

scales from fabrics to isoclinal folds of nappe considerable difficulties in field identification. structures although

Indeed

the structural

the chemical similarities

and

on all

dimensions on the other have led to Most of the fabrics and large-scale

in the Amitsoq gneisses were generated during the events of ca. 2800 Ma. possible pre-Malene isoclines may exist on the islands of northwest

Buksefjorden. Regional structural similarities the work in Buksefjorden and were assumed

were recognised in the early stages of to owe their character to comparable

and contemporary evolution. Recently. however, significant differences in the nappes have been described which must reflect differences in their generation. For instance, in the northern part of Buksefjorden immediately south of Ameralik (Fig. 2). the lnugsugssuaq rise of Nbk magma fold nappe

generated

nappe (Chadwick

into a distorting substantially

et al.. 1982) was generated

membrane by magma

of anorthosite. movement

by buoyant

The structure

which was overtaken

is a by

tectonic processes at a later stage. The magmatic parent now appears as a xenolithic gneiss in the core of the fold. The present outcrop pattern (Fig. 2) is related to superimposition of younger jorden are in sharp contrast. of comparable

upright folds. Nappes from two other parts of BuksefFirst. in the islands of northwest Buksefjorden a nappe

age and dimensions

to the lnugsugssuaq

nappe is outlined

by Malene

supracrustal rocks forming an envelope to a core of Amitsoq gneisses (Chadwick and Nutman. 1979). Thrusting of a cover-basement (MaleneeAmitsoq) association (Beech and Chadwick. 1980) took place before nappe development. Secondly, in southeast Buksefjorden.

large-scale

recumbent

isochnes

are cored by granitic

gneisses (Iliverta-

with lik granite and related rocks: Table I. Fig. 2) emplaced contemporaneously membrane can be deformation. Unlike the lnugsugssuaq nappe. no enclosing defined unambiguously. While the thrusts and nappes in northwest Buksefjorden may be interpreted as the products of crustal shortening by tectonic slicing, they may also have originated as the result of buoyancy phenomena which gave rise to uplift and consequent gravitational gliding and lateral spreading according to principles outlined. for example, by Ramberg (1981a. b). The gravitational instability which led to the nappes in north and southeast Buksefjorden was directly related to magma injection. diapirism and lateral spreading, whilst the instability which initiated the thrusts and nappes in northwest Buksefjorden may be regarded as the result of regional heating and buoyant uplift of pre-existing gneissses and supracrustal rocks. Myers (1976) suggested that Himalayan-type continental collision may have

127

been significant in the evolution of structures of similar age and geometry in the Fiskenaesset region to the south (Fig. 1). Whilst Archaean crustal shortening may have taken place on a very large scale as a result of subduction in this kind of tectonic regime, the nappes emplaced during the Niik superevent in West Greenland appear to have had a common parentage in buoyancy phenomena rather than shortening produced by tectonic slicing on a more localised scale. The nappes in north and southeast Buksefjorden were clearly developed at a deep level in the crust at a time approaching the climax of metamorphism in the N2k superevent (Table I). In parts of southeast Buksefjorden, acid magmas like those which generated the nappes crystallised directly to granulite facies assemblages. In these circumstances it follows that crustal growth to continental thickness was entirely due to magmatic injection with tectonic processes making only a negligible contribution. Thus the NClk superevent can he regarded as a continental accretion-differentiation superevent type as described by Moorbath and Taylor (1981). THERMAL.

PROC’ESSES ca. 3000-28(K) Ma

At present erosion level granulite facies rocks are best displayed in the south and southeast of the Boksefjorden region but in a zone which is known to extend southeast to Fiskenaesset (Fig. 1). Further south still, whilst amphibolite facies dominate there are areas, notably around Ravns Storer (Fig. 1). where supracrustal rocks contain pillow and other primary structures. The northwestern limit to the granulite facies area in Buksefjorden is complicated by a zone which may be regarded as a transition in which patches of granulite facies rocks are isolated from each other by zones of amphibolite facies. The boundaries of the granulite areas are remarkably sharp and whilst presumably marking limits of zones of dehydration they are not normally related to local tectonic structures, neither are they generally steep. Wells (1976) has shown them to be pro-grade boundaries but there are areas where retrogression, i.e. rehydration and breakdown of orthopyroxene, has taken place as part of the prograde metamorphic process. The cause of the late stage metamorphism is controlled by the complex relationship between fall in t,.nperature* ~,iUll,and Pi,>&,and reaction activities. There is at present no explanation for the distribution of granulite facies rocks. The thermal evolution of the high-grade Ntk area has been deduced by Wells (1981) who favours an over-accretion model. That is. successive sheets of granitic magma were emplaced subhorizontally at progressively higher levels in the crust. the lowest and earliest expanding the Malene supracrustal rocks by wedge injection. In contrast, under-accretion may have been more likely in the formation of the Amitsoq gneisses because an under-accretion model can more easily account for the inferred PT conditions ca. 3700 Ma and the thickness of siaiic crust which is preserved below present erosion level in the areas of Amitsoq gneisses.

L.ATE ARCHAEAN-EARLY

The last plutonic generation

GRANlTb

event in the Buksefjorden-

and emplacement,

complex. (Brown

PRO-I-EROZOIC‘

arguably

probably

rocks

tract (Fig. 1) w;~s the

Godthabsfjord

to a high crustal

the first alkaline

et al.. 1981). Moorbath

DEVEL.OPMEN~I

level. of the Qorqut

in the Archaean

et al. (1981) have shown

studies that unlike the parents complex was derived by anatexis

of West by Rb-Sr

granite

Greenland and

Pb-Pb

of the earlier acid gneisses the Qbrqut granite from pre-existing sialic material. Apart from minor

granite sheets and pegmatite veins. the granite is contained within the outcrop of Amitsoq gneisses. Trace element geochemistry suggests that the Qorqut granite was derived

by a small percentage

although

field

contributors.

evidence

of partial

suggests

At all events.

that

melting

of both Amitsoq

the Amitsoq

this component

gneisses

of the plutonic

and Nuk gneisses, were

the dominant

history

of the area

presents further problems in petrogenesis. The evidence shows that it was generated ca. 2500 Ma from gneisses w:hose isotopes give isochrons ca. 3750 Ma even though they had been

involved

in a ma_jor crust

forming

event

(CADS:

Moorbath

and

Taylor.

1981) at 2800 Ma. It must be presumed that the activity at 2500 Ma involved a significant increase in P,, () but an increase that was localised within the area of Amitsoq gneisses because it is unknown in the Nhk gneisses of either amphibolite 01

granulite

facies, except very locally in northwest

Buksefjorden.

ACKNOWLEDGEMENTS

In this paper we have discussed some of the new developments which have sprung largely from work in the Buksefjorden region in the central western part of the Archaean block in southern West Greenland. The field work was undertaken in 1972-1977

with a team of research

University of Exeter. gen, and the Natural port is gratefully

acknowledged.

by kind permission on the Peninsular development

assistants

from the Department

of Geology,

supported by the Geological Survey of Greenland, Environment Research Council, United Kingdom. The summary

of results presented

here is published

of the Director, Geological Survey of Greenland. Gneiss in southern India appears to be largely

comparable

with that on the voluminous

gneisses

CopenhaThis sup-

Current work at a stage of

of broadly

similar

age in West Greenland about 15 years ago. We hope our review of problems and progress of Archaean investigations in Greenland will be of value to the burgeoning studies of the Peninsular Gneiss, whom this volume is dedicated.

many

of which were initiated

by Hari Narain

to

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