Metamorphosed Layered Igneous Complexes in

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Ann.Rev, EarthPlanet. Sci. 1981.9." 175-98 Copyright(~ 1981by AnnualReviewsInc. All rights reserved

METAMORPHOSED LAYERED IGNEOUS COMPLEXES IN ARCHEAN GRANULITE-GNEISS BELTS

10148

Brian F. Windley Department of Geology, University

of Leicester,

Leicester,

England LEI 7RH

Finley C. Bishop Department of GeologicalSciences,Northwestern University,Evanston,Illinois 60201 Joseph V. Smith Department of Geophysical Sciences,Universityof Chicago,Chicago,Illinois 60637 INTRODUCTION Archean granulite-gneiss belts are deeply eroded segments of crust formedbefore 2.7 GaB.P. (1 Ga= 1000m.y.). Theycontain remnants intrusive layere digneouscomplexesthat were highly deformedand metamorphosedto high amphibolite and granulite grades. The petrography and mineral chemistry of the complexescan provide someunderstanding of the igneousstratigraphy despite the metamorphic overprint, and selected geochemical features provide important constraints on the many tectonic modelsfor evolution of Archeancrust (summarizedby Windley 1977). Twocontrasting types of layered igneous complexesare recognized, Anorthosite-leucogabbro complexesoccur in manyhigh-grade Archean regions (Table i). Ultramafic-gabbrocomplexesin the Scourian belt northwest Scotland and the Limpopobelt of southern Africa (Table 1) are in different sub-belts than anorthosite-leucogabbrocomplexes(F. B. 175 0084-6597/81/0515-0175501.00

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Annual Reviews www.annualreviews.org/aronline 176 WINDLEYET AL

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177

Davis et al, unpublished data). This review concentrates on the anorthosite-leucogabbro complexes, which have been studied more thoroughly than the ultramafic-gabbro complexes.

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GEOLOGICAL

SETTING

Quartzo-feldspathic orthogneiss of tonalitic to granodioritic composition typically makes up 80-85~o of the surface of Archean granulite-gneiss belts. Older and younger gneisses, separated chronologically by amphibolite dike swarms, occur in West Greenland (McGregor 1973), Labrador (Bridgwater et al 1975), and the Limpopo belt (Barton et al 1977b). Metamorphic ages are: older gneisses, West Greenland 3.6 to 3.8 Ga B.P., Labrador 3.6, Limpopo3.86 _+ 0.12; younger gneisses, West Greenland 2.8 to 3.0, Labrador 2.8 _+ 0.2, Limpopo3.13 _+ 0.08 Ga B.P. Igneous tonalitic-granodioritic precursors can be seen in zones of low strain. The gneisses have calc-alkaline type of trace-element chemistry (Tarney 1976). Lowinitial strontium isotope ratios (~ 0.701) suggest derivation from low-Rb source, probably in the mantle, and a short time interval between emplacement and metamorphism (Moorbath 1975). Within the gneisses are conformable layers up to 1 km thick of metamorphosedsedimentary (mica schist, marble, quartzite) and volcanic rocks with rare relict pillows. Rare, but locally thick, banded iron formations occur. Deformedfragments of layered igneous complexes form lenses (1 m) layers (up to 1 kmthick and tens of kilometers wide) in the gneisses, and are commonly bordered by metamorphosed volcanic and sedimentary rocks (Figure 1). The fragmentation of the igneous complexes was caused partly by thinning during deformation and partly by intrusion of the tonalitic precursors of the gneisses. The complexesare usually conformable with their wall rocks but rarely, as at Fiskenaesset, West Greenland, discordant contacts demonstrate intrusion into supracrustal rocks (Escher & Myers 1975). The western part of the Fiskenaesset complex (Figure 1) was intruded into pillow-bearing basaltic lavas which are chemically akin to modern oceanic basalts (Rivalenti 1976, B. L. Weaveret al, unpublished data). Marble and schist units occur sporadically along the top. The complex was isoclinally folded together with bordering basaltic rocks in an early episode (Figure 2) associated with thru~sting; this probably occurred in nappe-thrust regime (Bridgwater et al 1974). Twolater phases of tight isoclinal folding produced complex interference patterns (Figure 2). Granulite-facies metamorphism of the region has a Pb-Pb whole-rock isochron age of 2.81 _ 0.10 Ga B.P. (Black et al 1973) and a U-Pb zircon

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178

WINDLEY ET AL

N

Figure 1 Geological mapof western part of Fiskenaesset region, West Greenland. Approximately 50030’ W, 630 N. After Kalsbeek & Myers (1973).

age of 2.80 _+ 0.01 Ga B.P. (Pidgeon &Kalsbeek 1978). Amphibolite-facies metamorphismhas zircon ages not older than 2.66 _ 0.02 Ga B.P. In the Limpopobelt of southern Africa the Messina layered intrusion was emplaced at 3.16 _+ 0.5 Ga B.P. (Rb-Sr isochron of Barton et al 1979) into near-surface rocks, now metamorphosedto interbedded iron formation, quartzite, pelitic gneiss, and hornblendite. The Sand River Gneisses of 3.86 Ga B.P. age form an elongated strip close to and partly bordering the intrusion (Figure 3). The exact details of the subsequent deformation are uncertain because of different structural interpretations (Hor et al 1975, Cowardet al 1976, Barton et al 1979); however, there is agreement on an early phase of nappe-like isoclinal folding and several phases of refolding. Repetition of individual layers of the complex suggests the

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179

ARCHEAN LAYERED COMPLEXES

0~ 1 km 2

AXIAL TRACES Fl ....... x..... syncline

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F2form "’x.. anti -’#~ syn form

F3 "-x" anti form ".o.-. synform

Figure 2 Successive deformations in Fiskenaesset region. The stratigraphic the Fiskenaesset complexis duplicated by the F1 isoclinal folding.

succession of

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WINDLEY ET AL

presence of early isoclines like those in the Fiskenaesset complex(Barton et al 1977a, 1979). Layered complexes of leucogabbro and anorthosite are commonin the high-grade gneisses of Tamil Nadu (Windley & Selvan 1975) and Karnataka (Ramakrishnan et al 1978) in southern India. They occur as concordant sheets, elose to but not usually bordered by metamorphosed sedimentary and volcanic rocks. The Sittampundi complex is known best (Subramaniam 1956, Ramadurai et al 1975, Janardhanan & Leake 1975). The layered complexeslie in those parts of the Archeangneisses that have undergone only an amphibolite grade of metamorphism. In NWScotland lithological units in the Scourian mobile belt have a NEtrend (F. B. Davies et al, unpublished data). On the mainland metamorphosed ultramafic-gabbro complexes contain only minor anorthosite (see,. for exampl.e,.~Boweset al 1964, Davies 1974), whereas in the Outer Hebrides.’thereareseveral anorthosite-leucogabbro complexes including Ness (Wa-tson 1969) and Rodil(Dearnley 1963, Wittey 1975). The ultramafic-gabbro complexes have been so deformed that they occur in the gneisses as relict conformable lenses up to 100 m thick and 1 kmlong; they are:preserved best in the Scourie region (Figure 4).

I

1~0

2~)0 ’/~1,

~1 Ultramofic I complexes

- gobbro

[~ 5upracrustats ~ Gua.r tzo- fetdsp~thic ~ gneiss ~’~Scourie dikes ccnd ~ metadolerites

N

0

1

2 I Kms

Badcattl

Figure 4 Geological mapof Scourie region, North Scotland. After Davies (1976).

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ARCI-IEAN LAYERED COMPLEXES

181

places they are borderedby mica schists of presumedsedimentaryorigin. Early isoclinal folding caused repetition of igneous stratigraphy and symmetricaldistribution of schists. In the northern marginalzone of the Limpopomobile belt of southern Africa there are poorly exposed remnants of metamorphosedand deformed ultramafic-gabbro complexes. The Neshuro complexcomprises serpentinite, tremolite-orthopyroxenerocks, pyroxenegranulite, chromitite, and metanorite (Robertson 1974). The Chingwa-Ma-Karoro complex,4 kmlong and 600 m thick, consists of layers of hornblende-two pyroxene-plagioclase, hornblende-orthopyroxene-plagioclase,and tremolite-orthopyroxene. The ultramafic layers contain chromitite seams up to 30 mthick andthere is a 100 mthick layer of anorthosite(Robertson 1973). Thecomplexes wererecrystallized in the granulite facies andlocally retrogressed. There are no modernmineralogical and petrochemicaldata on these Limpoporocks. STRATIGRAPHY Anorthosite-Leucooabbro Complexes The stratigraphic successions of the Fiskenaesset complex,WestGreenland at Majorqapq~va and 30 kmwest on Qeqertarssuatsiaq Island are given in Table 2. Ultramafic rocks range from dunite and peridotite to pyroxenite and hornblendite. Thin ultramafic interlayers, 30-100 cm thick and up to 30 m long, occur betweenand within manyof the above zones. Chromitite forms major seamsup to 20 m thick in anorthosite, and magnetite is a major cumulusmineral in both plagioclase and ultramafic layers (Myers1976a). Abundantrelict igneous textures prove that the zonesformedby differential precipitation of cumulusolivine, orthopyroxene, clinopyroxene, and plagioclase; the origin of hornblendeis uncertain (see below). Minerallayering and megacrysts(usually plagioclase) whichpreserve the original cumulatetextures are common. Trough layering and channel deposits demonstratethe action of currents on the floor of the magmachamber(Myers1976b). The rocks respondedvariably to the successive superimposedtectonic strains dependingon position relative to limbthinning, degreeof mineral layering, and viscosity contrast with respect to surroundingor adjacent rocks. Deformationtended to accentuate igneous layering. In the Messinalayered intrusion, anorthosite and gabbroicanorthosite makeup over 90~ of the succession (Hor et al 1975, Barton et al 1977a, 1979). Anorthositic gabbro, gabbro, melagabbro,and ultram~tfic rocks including serpentinites and pyroxenites form layers 10-103 cmthick.

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ARCHEAN LAYERED COMPLEXES 183 Layers of chromitite up to 60 cm thick and magnetitite also occur. The stratigraphic succession dependson the structural interpretation: that for one section (Hot et al 1975)is given in Table2. Cumulateplagioclase megacrysts, size-graded bedding, and mafic to felsic grading in some pyroxenitic and mafic layers were used to deduce youngingdirections (Bartonet al 1977a,1979). In southern India the layered complexesconsist predominantly of anorthosite and leucogabbro with minor gabbro and ultramafic rocks; relict cumulate textures and graded mafic and felsic layers occur (Ramakrishnan et a11978).Thestratigraphic successionof the Sittampundi complexis given in Table2. The Rodil complexin Scotland has a stratigraphic succession similar to that of the Fiskenaessetcomplexexcept for absenceof the lowerultramafics and the chromitites (Wittey 1975).

Ultramafic-Gabbro Complexes TheScouriebodies(J. D. Sills et al, unpublished data) consist of ultramafic units, composedmainly of amphibolepyroxenites, and of gabbroic units composedof garnet, two pyroxenes, plagioclase, amphibole,and spinel. Locallythere is a cyclic repetition of ultramafic and gabbroicunits with an intervening horizon of sulfide minerals. Someof the Scourie bodies contain minoranorthosite (Davies 1974). Theultramafic units have pronouncedmineralogical layering, and serpentinized bands up to 5 m thick mayhave been dunites. Increase of plagioclase in gabbro layers up to 50 m thick providesa criterion for determinationof youngingdirection. Anearly interpretation that the gabbroicunits werea metasomaticcorona betweenultramafic and gneissic rocks has been discarded in favor of a consanguinousigneous origin of gabbroic and ultramafic units (Bowes et al 1964). MINERALOGY Althoughthere is local preservation of igneoustextures, almost all the igneousmineralshaverecrystallized in responseto granulite, amphibolite, or greenschist grades of metamorphism. At least someamphibolereplaced pyroxene,thereby requiring influx of hydrogen,but most chemicaltrends appear to have been preserved. Mineral compositionsmust have changed during metamorphism, e.g. Mg/Feexchangebetweensilicates and oxides, and NaSi/CaA1exchange between plagioclase and amphibole. Because

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WINDLEY ET AL

TaMe3 Mineralogy of the Fiskenaesset complex at Qeqertarssuatsiaq ~ Mineralogy

Zone

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Upper anorthosite Chromitite Lower anorthosite Upper leucogabbro Gabbro Lower leuco~abbro Ultramafic

Hb+Pl _Gt_ Bi+ I1 + Mt +Po_ Py Hb+P1 _+ Cpx!Op__+Bi_ Mu+AC+Rt_I1 +Py+ Cp + Mi+ Pn Hb+P1 + Cpx + Sph_Qt_ Ep_ Mt_+ I1 _ Py + Cp Hb+P1+ Op + Bi_+ Qt + Ep_ Mt + Rt + Py +__ Cp Hb+P1 + Op_Bi_+Kf+O1 + Cpx_Qt_ Sph_ Zr___ Mt +AC_+_Pn +Po+Cp_+Py Hb+Pl +Cpx_Op+Bi+Mt Hb+O1+ Cpx_+Op +Tr___ Bi+Qt___ Se+ Ac__+ At+ I1 +Cm+Mt +Sp+_ AC +Cm+ Pn+ Po+Cp_ Py+ Mi+ Hz+ Cp+ Pd+ Gd +Vi+Cb+Di

a Abbreviations:Acactinolite, ACAl-chromite,At anthophyllite,Bi biotite, Cbcubanite,Cmchromite, CMCr-magnetite, Cpchalcopyrite, CPCo-pentlandite, Cpxclinopyroxene, Di digenite, Epepidote, Gt garnet, Hbhornblende,Hz heazlewoodite,I1 ilmenite, Kf K-feldspar, Mimillerite, Mtmagnetite, Mumuscovite,O1olivine, Oporthopyroxene,PI plagioclase, Pd polydymite,Pn pentlandite, Po pyrrhotite, Pypyrite, Qtquartz,Rt rutile, Seserpentine,Spspinel,Sphsphene,Tr tremolite,Viviolarite, Zr zircon.

of poorer exposure, the mineralogical variations of the Messina layered intrusion and Sittampundi complexcannot be correlated with stratigraphic position as reliably as in the Fiskenaesset complex. Anorthosite-Leucogabbro

Complexes

Table 3 showsthe principal minerals as a function of stratigraphic position in the Fiskenaesset complex. Plagioclase and amphibole are dominant, but about 40 minerals have been described. SILICATESAmphibole occurs throughout the .Fiskenaesset complex, usually as mosaic aggregates, rarely as inclusions in plagioclase megacrysts (Windley et al 1973), sometimes surrounding diopside and hypersthene relicts, and also as rims adjacent to peridotite layers (Myers&Platt 1977). Most amphiboles from the ultramafic and gabbro zones are magnesiohornblendes. In plagioclase-free rocks, the m~/ ratio [---Mg/(Mg + Fe) moll varies between 0.95 and 0.79. In plagioclase-bearing rocks, including the chromitites, the rn~7 ratio tends to be lower 0.82-0.48 (Steele et al 1977), and locally displays a systematic trend with stratigraphic position (Myers &Platt 1977). The TiO2 and Cr203 contents of hornblende are lower (mostly < 1 wt~) in the ultramafic and gabbro zones than in the chromitite zone ( --~ 1.2 wt~ TiO2; ~ 1.5 wt~ Cr2Oa), and the A1203content tends to increase from 2-11 wt~ in the ultramafic zone

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ARCHEAN LAYERED COMPLEXES

185

to 10-16 Wt~o in the chromitites and upper anorthosites. Coexisting tremolite and hornblende occur in the ultramafic zone. Calcic plagioclase is the most abundant mineral in the Fiskenaesset complex,occurring as rare megacrysts partly recrystallized along rims and fractures (Myers & Platt 1977), but mainly as polygonal mosaics. Compositions range from An9~to AnT0(Steele et al 1977) with considerable scatter in each zone, and only a weak tendency for the An content to decrease with increasing stratigraphic height. Althoughsecondary plagioclase grains have similar composition to primary megacrysts (Myers Platt 1977), a weaktendency for the Na content of plagioclase to increase with the amphibole/plagioclase ratio suggests transfer of Na from amphibole to plagioclase during metamorphism(J. V. Smith et al, unpublished data). Olivine has mosaic textures and ranges from Fo87 to Fo72 with no obvious correlation with stratigraphic position (Steele et al 1977). Olivine also occurs in cores of coronas in the gabbro zone (Myers &Platt 1977). Orthopyroxene (En86 62, CaO