Timing of Proterozoic deformation and magmatism in a tectonically reworked orogen, Rayner Complex, Colbeck Archipelago, east Antarctica

Precambrian Research, 63 ( 1993 ) 1-26

1

Elsevier Science Publishers B.V., Amsterdam

Timing of Proterozoic deformation and magmatism in a tectonically reworked orogen, Rayner Complex, Colbeck Archipelago, east Antarctica Richard W. White and Geoffrey L. Clarke Department of Geology and Geophysics, University of Sydney, Sydney, NSW 2006, Australia Received January 21,1992; revised version accepted November 5, 1992

ABSTRACT The high-grade metamorphic rocks of the Colbeck Archipelago comprise granulite facies metasedimentary gneisses and charnockitic gneisses of three ages. A suite of rocks previously mapped as the Colbeck Gneiss is shown to be composite in age and origin, consisting of metasedimentary and orthogneiss that are intruded by granitic gneiss and charnockitic gneiss. An (7) Archaean granulite facies Sl gneissosity is preserved in the metasedimentary gneisses and early orthogneiss; the development of S: was followed by isobaric cooling from peak metamorphic conditions of T>~750°C and P = 5.1 + 0.8 kbar. Younger intrusions cut rocks containing $1 structures, but were deformed by a ~ 1200 Ma granulite facies D~ event that resulted in reclined, isoclinal F2 folds oriented parallel to a pervasive east-trending1.2 mineral and stretching lineation. Rocks bearing $2 structures are cut by the areally extensive ~ 960 Ma Mawson Charnockite, which was affected by two upright folding events D3 and D4 at ~ 920 Ma. Events D2-4 comprise the Proterozoic Rayner Structural Episode, which is characterized regionally by the retrogression of Archaean assemblages. The two pulses of extensive intermediate to felsic magmatism accompanied or immediately preceded the Proterozoic orogenies evident as D2-4, and it is tempting to infer causal links. However, field relationships are consistent with intrusion having been contemporary with anomalously high conductive heat fluxes, which could be due to advective heating from larger intrusions not exposed at the present structural level. The formation of extensional, granulite facies Ds ultramylonite+ pseudotachylite zones is the last deformation event in the area.

Introduction

High-grade gneisses forming the 1.2-1.0 Ga Rayner Complex (Kamenev, 1972; Sheraton et al., 1980) in Kemp and MacRobertson Lands, east Antarctica involve Archaean rocks that were tectonically reworked at amphibolite to granulite facies conditions during the Proterozoic Rayner Structural Episode (Sheraton et al., 1980; Sandiford and Wilson, 1984; Grew and Manton, 1986; Clarke, 1988; Grew et al., 1988 ). Mineral assemblages developed during Correspondence to: G . L Clarke, Department of Geology and Geophysics, University of Sydney, Sydney, NSW 2006, Australia.

the reworking resulted from the hydration of rocks previously metamorphosed at granulite facies conditions (Sheraton et al., 1980; Clarke, 1988 ); a source of heat, a source of water, and a mechanism to pervasively recrystallize very dry rocks are called for. Detailed mapping of Rayner Complex rocks forming the Colbeck Archipelago (Fig. 1 ) suggests that the Proterozoic tectonism was preceded and accompanied by extensive charnocldtic and granitic magmatism that culminated in intrusion of the Mawson Charnockite batholith (Crohn, 1959). The role of magmatism early in the Proterozoic reworking has not been previously described, yet it presents the most likely source of heat to facilitate the observed recrystalliza-

0301-9268/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

2

R.W. WHITE AND G.L. CLARKE

67 S Fold Island

Kemp Peak e,.Ti /// i ", f

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, Stillwell Hills

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• Robinson Group

~ / - / fS,; % #~/]//l~Mawson

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Station

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~11Mt Henderson

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Rock outcrop

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Approximate coastline

~

~

Framnes Mountains

68 3

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50 kms 62 E

Fig. 1. Map showing the distribution of rock exposures along the MacRobertson and Kemp Land coast.

tion. The evolution of high-grade rocks forming the Colbeck Archipelago is described below, a quantitative P - T - t path inferred for the Rayner Structural Episode, and features of the recrystallization discussed.

Regionalgeology High-grade metamorphic rocks exposed in Enderby, Kemp and MacRobertson Lands comprise upper amphibolite to granulite facies Precambrian gneisses that have been divided into the Napier and Rayner zones (Kamenev, 1972 ) or equivalent Napier and Rayner Complexes (Sheraton et al., 1980). Archaean rocks of intermediate pressure granulite facies form the Napier Complex in Enderby Land, which is bounded to the south, east and west by extensive Proterozoic upper amphibolite to granulite facies rocks forming the Rayner Complex (Kamenev, 1972). The two com-

plexes have been distinguished on the basis of metamorphic grade (Kamenev, 1972; Ravich and Kamenev, 1975), the presence of undeformed 1190 + 200 Ma Amundsen Dykes in the Napier Complex, considered to be equivalent to metamorphosed and deformed relics of mafic dykes in the surrounding Rayner Complex rocks (Sheraton et al., 1980), and numerous isotopic dates (Sheraton and Black, 1981; Black et al., 1983; Black et al., 1987). In east Kemp Land, Archaean ages are preserved in felsic gneisses of the Rayner Complex (Clarke, 1988; Grew et al., 1988) that host metamorphosed and deformed relics of dykes, interpreted as being equivalent to the Amundsen Dykes (Sheraton et al., 1980; Clarke, 1988; James et al., 1991 ). The deformation and recrystallization of these rocks occurred during the Proterozoic Rayner Structural Episode (Sheraton et al., 1980; Sandiford and Wilson, 1984; Sheraton et al., 1988 ), which was a con-

TIMING OF PROTEROZOIC DEFORMATION AND MAGMATISM IN A TECTONICALLY REWORKED OROGEN

sequence of the ductile thrusting of Archaean gneiss westward toward the stable Archaean Napier Complex (Clarke, 1988). Whereas these features suggest that this part of the Rayner Complex represents, at least in part, tectonically reworked Archaean gneisses, rocks forming the Rayner Complex in Enderby Land are interpreted to be rocks with sources of mainly Mesoproterozoic age (Black et al., 1987). Trail ( 1970: after Crohn, 1959; McCarthy and Trail, 1964; Trail et al., 1967 ) subdivided the high-grade metamorphic rocks of MacRobertson and Kemp Lands into four compositionally distinct units. The western part of the Rayner Complex in Kemp Land (Fig. 1 ) is dominated by deformed Archaean felsic and marie orthogneiss called the Stillwell Gneiss (Trail, 1970; Clarke, 1988 ). Whereas felsic orthogneisses within the Stillwell Gneiss have given Rb-Sr whole rock ages of 2692 +_48 Ma (Clarke, 1988 ), minor metapelitic horizons in the same area give a broad spread of whole rock Rb-Sr ages between 1400 and 1100 Ma that is most probably related to recrystallization during the Rayner Structural Episode (Clarke, 1988 ). Rock exposures eastward from Taylor Glacier (Fig,. 1) are of unknown age, and comprise garnet-rich felsic gneiss and metapelitic rocks of the Colbeck Gneiss. The Proterozoic Mawson Charuockite intrudes the Colbeck Gneiss and dominates rock exposures in MacRobertson Land (Crohn, 1959; Trail, 1970; Sheraton, 1982) to the east of Colbeck Gneiss exposures. The crystallization age of the charnockite is constrained by a Rb-Sr whole rock isochron age of 1084-+ 37 Ma (Sheraton 1982) and ion-probe U-Pb dates of 954_+ 12 Ma and 985-+29 Ma obtained from zircons with an igneous aspect within the charnockite (Young and Black, 1991 ). The charnockite contains metre-scale to kilometre-scale feldspar-rich, calc-silicate and pelitic xenoliths of unknown age that are collectively termed Painted Gneiss (Trail, 1970). Evidence of four or five deformation events

3

(Di-5) can be recognized in the Stillwell, Colbeck and Painted gneisses (Clarke, 1988; James et al., 1991 ). An early foliation (S1) in the Archaean Stillwell Gneiss is cut by marie dykes that are interpreted to be the equivalent of the Amundsen Dykes (Sheraton et al., 1980; Clarke, 1988), and Sl is taken to be a relic of Archaean deformation (Clarke, 1988; James et al., 1991 ). Although an equivalent foliation is observed in the Colbeck and Painted gneisses (Clarke, 1988; see below), there are only rare marie dykes and few isotopic data to confirm any structural correlations. As described below, the Rayner Structural Episode involved at least three episodes of deformation, D2..4 (after Clarke, 1988). Di and D2 pre-dated the Mawson Charnockite and resulted in reclined to recumbent, tight to isoclinal F2 folds with a pervasive, high-grade axial planar $2 gneissosity. Rb-Sr whole rock isochron ages of 1254+ 31 and 1153_+47 Ma obtained from samples of the Painted Gneiss (P.A. Arriens, unpublished data, cited in Young and Ellis, 1991 ) may reflect recrystallization during these events (Young ahd Ellis, 1991 ); no Archaean ages have been reported from the Colbeck or Painted gneisses. Successive mineral parageneses comprising S~ and $2 in granulite facies metapelitic gneisses of the Painted Gneiss suggest that deformation occurred during cooling at approximately constant pressure of 5.6_+ 0.4 kbar (Clarke et al., 1989; Clarke and Norman, 1992). Intrusion of the Mawson Charnockite resulted in the migmatization and partial to complete resorption of metasedimentary inclusions, and average pressure calculations using an orthopyroxene-garnetbearing assemblage from a recrystallized part of the Painted Gneiss suggests that the terrain was at a slightly lower pressure (than SI and $2) of 3.9_+ 1.0 kbar (Clarke and Norman, 1992 ). East-trending pegmatite dykes of identical age (0.94 _+0.08 Ga, Grew et al., 1988 ) to the Mawson Charnockite cut $2 in the Stillwell Hills (Clarke, 1988). The D3 and D4 deformation events resulted in the open to tight, up-

4

R.W. WHITE

right crenulation of the pervasive $2 in the metasediments, and steeply dipping retrograde foliations that partially recrystallized igneous textures in the porphyritic charnockitic gneiss. Young and Black ( 1991 ) estimate that this recrystallization occurred at 921 _+19 Ma, using U-Pb ion probe analyses of metamorphic zircon rims. Coarse-grained, massive pegmatite dykes cut all penetrative tectonic fabrics, but pre-date late ultramylonites (Trail, 1970). Rb-Sr biotite dates from one pegmatite dyke in the Stillwell Hills (Fig. l ) provides a minimum age of 718_+ l0 Ma (Clarke, 1988). These dykes are cut by abundant ultramylonite _+ pseudotachylite zones of unknown age that were probably partly responsible for returning the rocks to the earth's surface (Clarke, 1988 ). In the vicinity of Mawson, the ultramylonites comprise re-

AND G.L. CLARKE

crystallized granulite facies mineral assemblages that constrain ultramylonitization to have occurred at T>~700°C and P~<7.3_+0.5 kbar (Clarke and Norman, 1992). Ultramylonitization could have occurred during cooling of the terrain from D4 soon after ~ 900Ma, or it could be related to the widespread reequilibration of biotite Rb-Sr isotopic systems in MacRobertson Land at approximately 500 Ma (Tingey, 1982).

Geology of the Coibeck Archipelago The Colbeck Archipelago consists of a series of small ice-free islands and partially ice covered coastal exposures immediately west of a large northeast-trending rock exposure called Chapman Ridge (Fig. 2). Trail et al. (1967)

Py,oxe.e~ 4~

0

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A/. N o r r i s Is

? Mawson Charn~kilc, porphymhla~li¢ Mawson Charnockilc, cvcn grained Biotltc±pyroxcncgneiss Charnockiuc gneiss

Layeredgaeiss Granitic Gncis,s Pl~aclamalionGnei~ Garnet felsic gneiss

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E

Fig. 2. Geological map the Colbeck Archipelago and Chapman Ridge area; the location is shown in Fig. 1.

Intermediate to mafic intrusion High grade gneissic layering in Stillwell Gneiss and ?metasediments of Colbeck Gneiss Rare pyroxone dykes chamockite and granite intrusion

Intrusion of charnockitic gneiss DI Intrusion of mafic dykes. Intrusion of biotite:tpyroxene gneiss granite gneiss, and layered gneiss

?Archaean

2692d:48 Ma$

i11903_200 Ma&

E-trending, north dipping normaJ faults. N - S extension

N-trending upright folds affecting the F ~ Mtns ~md Stillwell Hills

ENE-trending open to isoclinal folds

Large composite batholith with related felsic pegrnatites

P<:7.3:L-'0.5 kbar, T>700°C

P= 3.9-a:1.0 Kbar

P=5.1:1: 0.8 kbar, T>_750°C Approximately isobaric cooling

P-T conditions

R S E = Rayner Structural Episode. l-event not observed in study area $ - Clarke (1988), &- Sheraton and Black (1981), @- P.A.Arriens (unpublished data), ^- Grew et al. (1988), § -Young and Ellis. (1991), *Tingey, (1982).

D5

?~500Ma*

D41

Intrusion of garnet+biotite pegmatite dykes

E

D3

Intrusion of Mawson Chamockite & pegmatites

>718"t"10 Ma$

)21±19Ma§

S

intrusion of pegmatites in the Stillwell Hills

0.94±0.08 Ga ^

954±12Ma, 985:f.29Ma§

D2

1254±31,1153±47Ma@

N-trending isoclinal reclined folds developed during east-directed simple shear

Mainly psammites with some pelites

Deposition of sediments

?Archaean

R

Style of e v e n t

Age

Event

Summary o f the geological evolution o f high-grade rocks comprising the Coibeck Archipelago

TABLE l

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6

and Trail (1970) produced detailed sketch maps of parts of the area, whilst undertaking regional mapping, and reported that the Colbeck Gneiss comprises garnet-biotite gneiss, sillimanite-bearing gneiss, clinopyroxenebearing gneiss, granitic gneiss, quartzite and biotite gneiss. Outcrops previously mapped as the Mawson Charnockite (Trail, 1970) include porphyritic and even-grained charnockites of the same age and an earlier biotite+pyroxene gneiss. The porphyritic and even-grained Mawson Charnockite show intrusive contacts with the biotite _+pyroxene gneiss and the Colbeck Gneiss. The biotite + pyroxene gneiss was in part mapped as Colbeck Gneiss by Trail (1970), who made no temporal distinctions for rocks comprising the Colbeck Gneiss. However, rocks comprising the Colbeck Gneiss in the Colbeck Archipelago are subdivided below into six lithological units of at least three distinct ages. The oldest rocks in the area are the metasedimentary gneisses, which were intruded by a layered orthopyroxene +_clinopyroxene gneiss before the first deformation event, D~. An S~ gneissosity is cut by a second intrusive phase involving a biotite ___pyroxene gneiss and rare mafic dykes. The pervasive gneissosity in these rocks is axial planar to reclined, isoclinal F2 folds that are oriented parallel to an intense east-trending mineral and stretching lineation, L2. With the exception of F2 hinges, S~ has been everywhere rotated into parallelism with a retrogressive $2 and it is difficult to discriminate the two fabrics in many outcrops. The $2-L2 fabric is cut by the intrusive Mawson Charnockite and related pegmatite dykes, with all rocks affected by open to tight, east-trending F 3 folds that have a steeply dipping, retrogressive, axial planar fabric. In the Colbeck Archipelago, $3 is cut by east-trending D5 ultramylonite + pseudotachylite zones. The geological evolution of the area is summarized in Table 1, and field relationships illustrated in Fig. 2. Informal unit names are italicised below to distinguish them from compositional terms.

R.W. WHITE AND G.L. CLARKE

Rock units

Proclamation Gneiss The Proclamation Gneiss is a leucocratic felsic gneiss named after the locality near Cape Bruce (to the west of the Colbeck Archipelago ) where Mawson claimed much of the east Antarctic coastline for Britain in 1931 (British, Australian and New Zealand Antarctic Research Expedition). It comprises quartz, Kfeldspar, plagioclase, biotite and garnet in various proportions and is interpreted as metasedimentary in origin. The gneiss is characteristically banded in appearance, with 1-50 cm wide garnet-biotite-rich layers alternating with quartz-feldspar-rich layers. Both the composition and spacing of the layering are variable, consistent with an origin involving a modified sedimentary layering; the compositional variations now define S 1 (see below ). Feldspar-rich layers form the dominant host, suggesting that the unit was an impure arkose. Biotite-garnet-rich layers are subordinate in occurrence to the felsic layers and are not present in all outcrops. Poikiloblasts of garnet up to 2-3 cm wide contain numerous rounded and kidney-shaped inclusions of quartz, feldspar and opaque oxide minerals. Garnet is enveloped by a composite $1 and $2 gneissosity comprising biotite, quartz, plagioclase and Kfeldspar. Late fine-grained biotite cuts $1 and $2. The quartz-feldspar-rich layers comprise large perthitic orthoclase grains ( > 0.6 m m diameter) and clusters of quartz grains (0.2-0.3 mm diameter). Plagioclase, often showing antiperthitic textures is present but subordinate to orthoclase. Fine-grained sillimanite and spinel occur in minor quantities within the quartzo-feldspathic layers, with spinel being mantled by a thin corona of plagioclase. The occurrence and form of the spinel suggests that the prograde mineral assemblage included quartz and spinel. Common 2-10 cm-wide, coarse-grained quartz-feldspar-rich leucocratic segregations locally transgress the com-

TIMING OF PROTEROZOIC DEFORMATION AND MAGMATISM IN A TECTONICALLY REWORKED OROGEN

positional layering and are interpreted to have formed during partial melting of the rock. These segregations exhibit two relationships with respect to S~: common early leucosomes define S~, but some later leueocratic segregations cut SI in the hinges of isoelinal F2 folds and define $2 (see below).

Orthopyroxene +_clinopyroxene gneiss The orthopyroxene + clinopyroxene gneiss is a dark, medium to coarse-grained banded orthogneiss. The gneiss is predominantly intermediate in composition with the dominant primary mineral assemblage comprising hypersthene, K-feldspar, plagioclase, quartz and ilmenite. In places it is considerably more marie containing both orthopyroxene and elinopyroxene as well as significantly less quartz and K-feldspar than the dominant assemblage. Biotite and hornblende commonly occur as retrograde $2 and $3 minerals. The gneiss contains a miUimetre to centimetre scale S~ gneissic layering that is defined by alternations in the proportions of marie and felsic minerals, and by distinct feldspar-rich leucocratic segregations that define S~. The marie orthopyroxene and clinopyroxene-bearing gneiss often occurs as discrete, discontinuous lenticular bodies (within the orthopyroxene +_clinopyroxene gneiss) up to 20 m long and 1-2 m wide; boundaries between the marie and intermediate layers are sharp. A single large body of marie gneiss forms Pyroxene Island (Fig. 2). The marie gneiss layers comprise SI orthopyroxene, clinopyroxene, plagioclase, biotite, hornblende and ilmenite, with or without K-feldspar and quartz. Stromatie to nebulitie plagioelase-rich leueoeratie segregations (Fig. 3a) occur in the marie gneiss with complex structural relationships; they are interpreted as indicating partial melting. Most leucoeratie segregations define S~ and are commonly folded into F2 folds (Fig. 3b) to give a stromatie appearance, but some cut S1 and are only weakly affected by $2 thus having a he-

7

bulitic appearance (Fig. 3a). The leucocratic segregations comprise coarse-grained SI orthopyroxene, plagioclase and quartz, with or without K-feldspar and $2 and $3 biotite. In places, the leucocratic segregations are deformed by F 3 folds, and $2 and $3 biotite, with or without hornblende, partially to completely pseudomorph coarse-grained S~ and $2 orthopyroxene (Fig. 3c). Garnet-bearing gneiss layers that are distinctly richer in K-feldspar and quartz have gradational contacts with a orthopyroxene+ clinopyroxene gneiss host on Norris Island (Fig. 2 ). They are lenticular 1-2 m wide bodies with a pervasive SI gneissosity defined by large ( > 4 mm diameter) embayed almandine-rich garnets, plagioclase, K-feldspar, quartz, ilmenite and biotite. These lentieular bodies have compositions similar to the Proclamation Gneiss, and they are interpreted as being altered metasedimentary xenoliths within the orthogneiss body.

Granitic gneiss Outcrops of weakly layered coarse-grained quartz-K-feldspar-plagioclase-biotite granitic gneiss occur on Norris Island and in the North Ridge area (Fig. 2). The gneiss is generally pink and forms low lying recessive outcrops that are commonly partially obscured by glacial erratics and coarse-grained sand. A weak $2 gneissie layering is defined by ram-scale alternations of K-feldspar-rich and plagioclaserich gneiss. Biotite and quartz are evenly distributed throughout the unit. Coarse biotite cuts $2 and defines $3. Crohn ( 1959 ) and Trail (1970) interpreted this gneiss to be a deformed and metamorphosed granite. On the basis of structural relationships, intrusion predated the D2 event.

Layered gneiss The layered gneiss occurs as distinctive outcrops of heterogeneous banded rock predomi-

8

R.W. WHITE AND G.L. CLARKE

TIMINGOF PROTEROZOICDEFORMATIONAND MAGMATISMIN A TECrONICALLYREWORKEDOROGEN

9

f Fig. 3. (a) Stromatic to nebulitic leucocratic segregations in the orthopyroxene+_ clinopyroxenegneiss on t'yroxene Island. These two exhibit textures suggesting that they developed contemporaneously. The cross-cutting relationship suggests melt was present during and after D2. Note the nearly vertical retrograde $3 foliation-cutting leucosomes in the lower part of the photo. (b) 1:2 structure in the Proclamation Gneiss. S, leucosomes are commonly boudinaged and have irregular shapes. (c) Sparse coarse-grained orthopyroxene in nebulitic leucocratic segregations in the orthopyroxene +_clinopyroxene gneiss on Pyroxene Island. The pen is aligned parallel to the S i and $2 gneissic layering. (d) Tight F3 structures in a pegmatite cutting the Mawson Charnockite. A weak $3 foliation is developed in the charnockite. (e) Large irregularly shaped pegmatites from Long Island, immediately to the north of the area shown in Fig. 2. ( f ) Open F3 antiform deforming isoclinal 1:2 folds, Chapman Ridge. The nearly recumbent nature of $2 in the F3 hinge is an apparent dip; $2 is actually dipping toward where the photograph was taken from.

nantly comprising quartz, K-feldspar, plagioclase, biotite, garnet and ilmenite. Layering is defined by garnet-biotite-rich layers alternating with feldspar-quartz-rich layers on a centimetre to metre scale; the feldspar-quartz-rich

layers form the bulk of the rock. Metapelitic gneiss occurs as discontinuous metre-scale boudins and layers within the garnet-biotiterich layers. The metapelitic gneiss comprises a granoblastic network of SI and $2 garnet, silli-

10

R.W. WHITE AND G.L. CLARKE

manite, biotite, quartz, perthitic K-feldspar, ilmenite, magnetite and cordierite. The presence of abundant isoclinal F2 folds means that the pervasive gneissosity in the metapelitic gneiss can be confidently identified as S]. Garnet and cordierite contain numerous inclusions, predominantly quartz or sillimanite, although hercynitic spinel, ilmenite and biotite also occur; in garnet these are concentrated towards the core of individual grains. The sillimanite and biotite inclusions in garnet commonly show a preferred orientation and outline rootless isoclinal folds oblique to the enveloping S~ foliation. Retrograde symplectites and

coronas of biotite, ilmenite, magnetite and fibrolitic sillimanite enclose hercynitic spinel, or occur along the margins ofS~ cordierite grains. The origin of this unit is interpreted to have involved the intrusion of a felsic magma similar to the protolith of the granitic gneiss, into metapelitic gneiss. The boudins and discontinuous layers are xenoliths. The partial assimilation of xenolith material, and subsequent recrystallization during D 2 , could have given rise to the garnet-biotite-rich layers in the felsic host. Intrusion is inferred to have occurred before D2.

/

C

f

F 2 h,ld axis S 2 dip and s n k c

f

f

,

F~tolda×is ~

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~[racc ol S 2 l(}hatlon

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"Irate ol S l Iolimu,n

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t hlam~lorulc pwud,,~aciwhtc lime

Fig. 4. Structural map of the Colbeck Archipelago and Chapman Ridge area. (A) to (D) are equal area projections of structural data contoured at various percentages per 1% area. (A) 132 poles to S~/$2 foliation contoured at l, 2, 5 and 10% per 1% area. (B) 40 L2 lineations and F2 axes contoured at 3, 10, 15 and 20% per 1% area. (C) 43 F3 axes contoured at 3, 10, 30 and 50% per 1% area. ( D ) 51 poles to $3 foliation contoured at 2, 5, 15 and 20% per 1% area. The map covers the same area as Fig. 2.

TIMING OF PROTEROZOIC DEFORMATION AND MAGMATISM IN A TECTON1CALLY REWORKED OROGEN

Biotite + pyroxene gneiss Medium- to coarse-grained biotite + pyroxene gneiss dominates the coastal exposures adjacent to Norris Island (Fig 2). At Coffee Rocks numerous dykes of biotite+pyroxene gneiss originating from the main body intrude the surrounding metasedimentary gneiss. The dykes exhibit a discordant relationship with S~ in the metasedimentary gneiss suggesting that the biotite+pyroxene gneiss represents a postD1, pre-D2 intrusion. It contains a weak $2 gneissosity defined by approximately equal proportions of coarse-grained perthitic K-feldspar and plagioclase, biotite, quartz and ilmenite, with or without orthopyroxene or hornblende. In places, narrow, coarse-grained feldspar-rich leucocratic segregations occur in $2 and give the rock a layered appearance. Kfeldspar is more abundant than plagioclase in these segregations, which contain less biotite and orthopyroxene than the host gneiss. Where present, orthopyroxene occurs as clusters of 13 m m diameter grains that are partially retrogressed to $2 and $3 biotite, ilmenite and hornblende. The unit is similar in many respects to both the orthopyroxene+clinopyroxene gneiss and the Mawson Charnockite but can be distinguished both compositionally and temporally. The biotite + pyroxene gneiss is less mafic than the orthopyroxene +_clinopyroxene gneiss, and usually only contains one pyroxene. $2 in the biotite +_-pyroxenegneiss is cut by the intrusive contact of the Mawson Charnockite on Chapman Ridge.

Mafic dykes Several discontinuous, boudinaged and recrystallized, 1-2 m wide mafic dykes transgress $1 in metasedimentary gneisses but contain $2. Immediately east of Norris Island (Fig. 2) a fine-grained equigranular pyroxene-bearing dyke has a weak $2 defined by hypersthene, bytownite, hornblende, biotite, ilmenite and

I1

quartz with accessory zircon and rutile. Biotite and hornblende may form retrogressive syn-D2 rims on pyroxenes and ilmenite. The temporal relationship between these dykes and the postDI pre-D2 orthogneiss bodies is unclear, as the dykes are not observed in these bodies.

Mawson Charnoclcite The Mawson Charnockite is a large intrusive body that outcrops along much of the Mawson Coast. Massive porphyritic and evengrained hypersthene granite or charnockite forms most of Chapman Ridge (Fig. 2) and shows intrusive relationships with the surrounding metasedimentary gneisses and orthogneisses. Trail (1970) defined the charnockite along Chapman Ridge to be the westernmost outcrop of the Mawson Charnockite body. The porphyritic variety of the Mawson Charnockite contains euhedral megacrysts of K-feldspar ( < 6-7 cm in length) in a coarse-grained matrix of quartz, K-feldspar, plagioclase, hypersthene, biotite and rare garnet. The megacrysts are commonly aligned parallel to a retrograde biotite foliation that is equivalent to the regional $3 foliation. Outcrops of this variety occur mainly at Byrd Head and along the lower region of Chapman Ridge (Fig. 2). The even-grained charnockite is identical in mineralogy, and only distinguished from the porphyritic variety by the absence of the coarse K-feldspar. It outcrops along Chapman Ridge, where a transitional boundary between the two varieties of charnockite is observed. A zone of alteration persists for 10-20 m into the Mawson Charnockite along intrusive contacts with metasedimentary gneisses where biotite is more common and orthopyroxene less common than elsewhere. Magmatic orthopyroxene is commonly pseudomorphed by biotite and garnet. There has been a subtle alteration of plagioclase with plagioclase in unaltered charnockite being andesine (An35-38) and plagioclase in pervasively altered charnockite

12 being oligoclase (An25_29). This localized alkali metasomatism of the outer margin of the intrusion is interpreted as having involved fluids derived from hydrous assemblages in the surrounding metasedimentary gneisses during contact metamorphism. Young and Ellis ( 1991 ) suggest that the Mawson Charnockite was emplaced at temperatures above 1000 ° C. Similar contact effects were observed around many of the large inclusions within the charnockite in the Framnes Mountains. Contacts between the charnockite and metasedimentary gneisses are also marked by numerous narrow (5-100 cm wide) pegmatite dykes, which cut both the charnockite and metasedimentary gneiss. The pegmatites comprise quartz, K-feldspar, plagioclase and biotite, and contain occasional xenoliths of metapelitic gneiss. In the charnockite, the pegmatites are bordered by an alteration halo approximately one third the width of the dyke, where garnet and biotite are markedly enriched compared with surrounding charnockite. The dykes are common only where the charnockite intrudes metasediments, and are distinct from large younger pegmatite dykes that intrude all rock types (see below). They are interpreted to have formed from the partial melting of the surrounding metasediments during contact heating from the intrusion of the Mawson Charnockite.

Pegmatite dykes Post-D2, pre-D3 pegmatite dykes (Fig. 3d) intrude all the above rock types. They range in width from several centimetres to several metres and are commonly folded. Irregular web-shaped masses of pegmatite intrude orthogneiss (Fig. 3e) on Long Island just north of the mapped area (Fig. 2). The pegmatites comprise coarse-grained quartz and K-feldspar, with less plagioclase and biotite, and rare garnet. K-feldspar is commonly perthitic and forms phenocrysts up to 2 cm in length. Myrmekitic intergrowths of quartz and plagioclase

R.W. WHITE AND G.L. CLARKE

are common around the margins ofplagioclase grains. These pegmatites are probably related to the intrusion of the Mawson Charnockite but differ from the dykes described above by being of a late-stage highly differentiated magmatic origin.

Structural geology The Rayner Structural Episode was a series of deformation events that, on the basis of available isotopic information and regional correlations, began at approximately 1200 Ma with D2 and ended at approximately 900 Ma with D4. The biotite+pyroxene gneiss, the granitic gneiss , the layered gneiss and the ~950 Ma Mawson Charnockite are interpreted to have intruded during the Rayner Structural Episode.

D I event Evidence for the earliest tectonic event is preserved in metasedimentary gneisses and the orthopyroxene+_ clinopyroxene gneiss unit of the Colbeck Gneiss, which contain a penetrative $1 foliation that is deformed by isoclinal F2 folds. $1 is a high-grade granoblastic gneissose layering that represents recrystallization at or near peak conditions of granulite facies metamorphism; there is no evidence of any earlier structures. It is defined by abundant stromatic leucocratic segregations that are rich in feldspar, and has mineralogies consistent with the development of abundant partial melt at peak conditions. Some leucocratic segregations cut S 1, suggesting that melt was also present after D I. The intensity of recrystallization during D2 was such that, with the exception of F: hinges, $1 is now everywhere parallel to $2. Thus, structural features associated with the development of $1 are difficult to interpret. Several mafic dykes cut $1 but are deformed by D2.

TIMING OF PROTEROT-,OIC DEFORMATION AND MAGMATISM IN A TECTON1CALLY REWORKED OROGEN

Rayner Structural Episode (1)2_4) 1)2 event. The D2 event is characterized by mesoscopic to macroscopic, isoclinal F2 folds (Fig. 3f) with an axial surface, $2, commonly defined by a transposed SI foliation and the retrograde growth of biotite and sillimanite or hornblende axial planar to F2 folds. Where unaffected by later folding events the folds trend northerly and plunge steeply to the east, and have a strong, steeply east-dipping $2 axial planar fabric. Some coarse-grained leucocratic segregations cut SI in F2 hinges, and are aligned with $2. A pervasive mineral and stretching lineation (L2) is present within $2 and is oriented parallel to F2 hinges. Only minor reodentation of this lineation has occurred during subsequent deformation, and it is a prominent feature of most outcrops. Whereas in most outcrops $2 dominates over L2 to form an $2L2 composite fabric, in some outcrops L2 dominates to define an L2 tectonite (Fig. 5a). The intensity of recrystallization associated with L2 development was such that it occurs everywhere in reoriented S~ fabrics. It is commonly defined by rodding of S1 and $2 felsic aggregates (Fig. 5a), and by aligned sillimanite in metapelitic rocks. Macroscopic F2 folds are well developed on the northern half of Norris Island (Figs. 2 and 4), where they have a wavelength of 20-40 m and control the lithological distribution. Mesoscopic F2 folds are well developed within all metasediments and the orthopyroxene +clinopyroxene gneiss, although $2 is the earliest fabric that can be identified in the biotite+pyroxene gneiss. In the other homogeneous orthogneiss units $2 is well developed, and the presence of S I cannot be unambiguously established. Thin section examination of a metapelitic gneiss comprising mostly $2 sillimanite and quartz shows that the intensity of D2 strain was heterogeneous, and microscopic $2 and C2 shear planes (usage after Berth6 et al., 1979) occur at low angles defining an axial planar foliation to asymmetric F2 folds. The

|3

shear planes are generally less than 1 mm in width and are composed of fine-grained recrystallised quartz, sillimanite, opaque oxides and minor biotite. Their presence suggests that the D2 folding event involved a significant component of simple shear (see, for example, Bell, 1978). D3 event. The D3 event is characterised by angular, open to isoclinal mesoscopic to macroscopic (Fig. 3f), east-northeast-trending F3 folds. F 3 folds plunge mostly at moderate angles to the east and have moderately to steeply south-dipping $3 axial planes, which are defined by reoriented $2 foliations and retrogressive $3 biotite and hornblende (Fig. 3a) that envelop $2 pyroxene. Some leucocratic segregations transgress $2 but define $3, suggesting that partial melting occurred during D3. The intensity of recrystallization associated with D3 varies significantly throughout the area. On the northern tip of Norris Island and on the islands to the north, the effects of D3 have been minor and have resulted in open, angular to rounded F3 crenulations of S~ and $2 with a 520 cm wavelength; large-scale F3 structures are rare to absent. On Pyroxene Island (Fig. 2) $2 leucocratic segregations have suffered no rotation during D3 but an Sa biotite foliation envelopes SI and $2 pyroxene (Fig. 3a). On the southern portion of Norris Island and in coastal exposures near Chapman Ridge (Fig. 2), the effects of D3 are more intense: large open to tight F3 folds, with a wavelength of 20-50 m, reorient S~ and $2 into parallelism with $3. $3 is the earliest foliation that can be recognised in the Mawson Charnockite, where it comprises reoriented K-feldspar and plagloclase megacrysts that are enveloped by finegrained biotite, quartz, orthopyroxene and ilmenite with or without clinopyroxene and hornblende. Numerous pegmatite dykes occur in the Mawson Charnockite and the surrounding gneisses, where they cut rocks with $2 structures. Whereas many of these pegmatites are intensely folded (Fig. 3d) others exhibit only a planar $3 foliation.

m >

TIMING OF PROTEROZOIC DEFORMATION AND MAGMATISM IN A TECTON1CALLY REWORKED OROGEN

15

d_..L.,_

Fig. 5. (a) L2 mineral and stretching lineation in S 1 leucosomes transposed into $2; orthopyroxene +_clinopyroxene gneiss, Pyroxene Island. (b) Mesoscopic F2-F 3 interference pattern in the Proclamation Gneiss; this interference is F2 dominated. (c) Mesoscopic F2-F3 interference pattern in the Proclamation Gneiss; this interference is F3-dominated. The pencil is aligned parallel to $3. (d) Narrow ultramylonites with limited sinistral displacement of SI in the Proclamation Gneiss. (e) Ultramylonite-pseudotachylite zone on Chapman Ridge illustrating brecciated clasts of country rock bounded by thin mylonite zones.

The mesoscopic and macroscopic outcrop pattern in the area is principally controlled by F2 and F3 fold interference, but variations are observed due to the heterogeneousnature of D3 strain. Fold interference patterns are best developed in the metasedimentary gueisses where a strong S~ gueissose layering is present. In their original orientations the trends of the F2 and F3 folds are nearly orthogonal. The F2 and F3 fold axes are nearly collinear, and thus only minor reorientation of F2 and L2 occurred during D3. Two forms of type 3I (Ramsay 1967) F2-F3 fold interference pattern are observed in outcrops of metasedimentary gneisses (Figs. 5b, 5c): these are called F2 dominant and F3 dominant. F2 dominant patterns occur in areas with minor effects of D3, and are commonly open F3 crenulations overprinting large isoclinal F2 folds that essentially retain their original orientation. F3 dominant patterns occur where F3 folds are of a size and style which causes the F2 folds to be significantly deformed and/or reoriented. Cross-sections il-

lustrating these inferred geological relationships are shown in Fig. 6. D4 event. North-trending folds with a steeply dipping axial planar fabric have been described from outcrops along the Mawson Coast and in the Framnes Mountains (Fig. l; Trail, 1970; Clarke, 1988). Clarke (1988) considered this deformation event to be the first that affected the Mawson Charnockite in this area, but observed a deformed foliation which was interpreted as a magmatic flow foliation in many places. This earlier foliation was re-examined during numerous field trips to the area, and found to be tectonic on the basis of fractured megacrysts (in the charnocldte) aligned within the foliation, and the observation that the foliation was continuous across contacts between metasedimentary xenoliths and charnockite. Outcrops that are unambiguous are rare, because of the intensity of recrystallization associated with the north-trending folds. Where unaffected by the north-trending folds, the earlier foliation is east-trending and thus

16

R.W. WHITE AND G.L. CLARKE

;-'\

\\ \-J

/

1 km

f\ \ 1

l

~/,

i~//'11'1\'

I

~,~ ~, %%:, ~,\ ,\, \, \, 1,\\\/\\ \, ~ ,/1\\\,,/ ~t

'1\

""i'?',!:

Fig. 6. Geological cross-sections through portions of the Colbeck Archipelago--Chapman Ridge area showing approximate topography. The location of the section lines and key to the symbols are shown in Fig. 2.

correlated with the $3 fabric observed in the Colbeck Archipelago. This makes the northtrending folds in the Mawson area F4, and the pervasive foliation in the Mawson Charnockite $4. The effects of this event in the Colbeck Archipelago were very minor and a possible weak $4 foliation was only recognised at a few localities.

D5 ultramylonite event Discontinuous shear zones containing ultramylonite, with or without pseudotachylite, cut all the deformation fabrics described above. Individual zones vary in width from a few millimetres to approximately 2 m, with the wide zones having greater strike lengths and being continuous for several hundred metres (Figs. 2, 4). Numerous ultramylonites occur on a lo-

cal scale that is too small to map (Fig. 5d). Large ultramylonite___ pseudotachylite zones are usually a 0.1-1.5 m wide zone and comprise pseudotachylite breccia (Fig. 5e; after Sibson, 1975 ) that contains large (1-20 cm) brecciated clasts of country rock, bounded by thin (0.4-3 cm wide) ultramylonites. Discrete veins of pseudotachylite commonly intrude gneisses surrounding the shear zone. The ultramylonites form an anastomosing network of shear zones resulting in the width and strike of individual ultramylonite + pseudotachylite zones changing along strike. Individual ultramylonite zones principally contain a well-developed C-planar fabric (usage after Berth6 et al., 1979) most commonly defined by alternations in the proportions of quartz, feldspar, orthopyroxene, ilmenite and biotite with or without clinopyroxene and hornblende. The mineral

TIMING OF PROTEROZOIC DEFORMATION AND MAGMATISM IN A TECTONICALLY REWORKED OROGEN

assemblage comprising the ultramylonite is generally similar to that of the host rock, and where shear zones cut metapelitic rocks the ultramylonite assemblage may include garnet, sillimanite, biotite and cordierite. The prominent ultramylonite _+pseudo tachylite zones strike southeasterly and dip steeply to the north (Fig. 2). Numerous subsidiary ultramylonite and ultramylonite+_pseudotachylite zones show a variable geometrical relationship to these larger zones. A pervasive, down-dip mineral and stretching lineation is developed within the C and S fabrics of the large ultramylonite zones, indicating dip-slip movement. On the basis of offset pegmatite dykes and marker horizons, and CS fabrics (after Berth6 et al., 1979) movement on the large zones was normal and involved offsets of a few centimetres to several metres.

Metamorphism Although the interpretation of S~ and S 2 fabrics in many outcrops is difficult due to transposition Of Sl during D2, the two can be distinguished petrographically: whereas S assemblages are characteristically poor in hydrous minerals, Se commonly involves retrogressive biotite and/or hornblende enveloping SI minerals and textures. Thus, in the metapelitic and o r t h o p y r o x e n e + _ clinopyroxene gneiss units of the Colbeck Gneiss, the S~ gneissosity represents recrystallization near peak conditions, and the assemblages described earlier imply metamorphic conditions in the pyroxene granulite sub-facies of Turner and Verhoogen (1960). Although the origin of the abundant SI leucocratic segregations is complex, they suggest that significant partial melting occurred. However, the segregations are deformed and contain SI indicating that they were mostly solid during at least some part of D~. This suggests that the terrain was already cooling from a metamorphic peak during the development of S,, an interpretation that is supported by the observation of complex S I

17

mineral corona textures in metapelitic rocks. The most common coronas consist of garnet, sillimanite, cordierite, magnetite and/or ilmenite on cores of hercynite-rich spinel. The coronas are interpreted to have formed prior to D2 since they contain no hydrous minerals and appear to have been partially retrogressed during D2. They are similar to those described from near Mawson by Clarke et al. (1989), and reflect the instability of coexisting hercynitic spinel and quartz during essentially isobaric cooling of the terrain. Quantitative estimates of the pressure and temperature conditions of metamorphism can be made using microprobe analyses of the S I and $2 minerals and THERMOCALC following the method of Powell and Holland ( 1988 ) with the expanded internally consistent thermodynamic dataset of Holland and Powell (1990). The essence of this approach is the combination of the pressures, calculated for each reaction in an independent set at an estimated temperature, involving the mineral endmembers .shared between an (equilibrium) mineral assemblage and the dataset. Analyses were obtained using the University of Sydney ETEC autoprobe, operated with an accelerating voltage of 15kV, a beam width of I-3 #m and ZAF data reduction. Representative microprobe results are summarized in Table 2, and average pressure calculations in Table 3. Calculations made using microprobe analyses of sample 68731, a garnet-sillimanitecordierite-bearing metapelitic gneiss, give a statistically acceptable result of P=4.3_+0.6 kbar (2a) at T=700°C (see Powell and Holland, 1988, for a discussion of result diagnostics). The spinel and hercynite mineral endmembers were excluded from these calculations because the spinel grains are enclosed in $1 coronas, and the eastonite end-member was excluded on the basis of statistical diagnostics (Holland and Powell, 1990). The temperature estimate is a reliable minimum for the conditions of SI on the basis of similar assemblages

18

R.W. WHITE AND G.L. CLARKE

TABLE 2

Representative microprobe analyses of minerals used in the average pressure calculations Sample: Mineral

68731

68731

68731

68731

g

plag

cd

spinel

68731 cd

68731 bi

68731 li

68725 g

wt% SiO2 TiO2 AI203 Cr203 FeO MnO MgO CaO Na20 K20 ZnO

37.544 22.135 31.086 0.975 7.359 1.037 -

Total

100.136

No. oxygens

12

60.456 0.035 25.104 0 0.003 0.05 0.001 7.308 7.054 0.299

100.31 8

50.17 0 34.043 0.038 6.134 0.029 9.792 0.022 0.116 -

0.202 0.033 57.401 2.879 25.936 0.199 5.693 0 0 0 7.832

49.867 0.011 34.077 0.031 6.029 0.099 9.883 0.009 0.089 0.004

35.929 5.182 16.908 0.264 16.278 0.06 12.455 0 0.114 9.598

0.213 50.51 0.216 0.066 47.268 0.263 1.068 -

37.627 21.466 33.617 0.987 5.105 2.779 -

100.344

94.692

100.099

96.788

99.604

101.581

18

4

18

22

3

12

Si Ti AI Cr Fe Mn Mg Ca Na K Zn

2.9421 0.0000 2.0449 0.0000 2.0373 0.0647 0.8594 0.0871 0.0000 0.0000

2.6830 0.0012 1.3134 0.0000 0.0000 0.0019 0.0001 0.3475 0.6070 0.0169

5.0039 0.0000 4.0030 0.0030 0.5117 0.0025 1.4555 0.0024 0.0224 0.0000

0.0060 0.0007 1.9945 0.0671 0.6393 0.0050 0.2501 0.0000 0.0000 0.0000 0.1635

4.9866 0.0008 4.0174 0.0025 0.5042 0.0084 1.4729 0.0010 0.0173 0.0005

5.3291 0.5780 2.9566 0.0310 2.0192 0.0075 2.7532 0.0000 0.0328 1.8162

0.0054 0.9641 0.0065 0.0013 1.0033 0.0057 0.0404 0.0000 0.0000 0.0000

2.9559 0.0000 1.9881 0.0000 2.2087 0.0657 0.5977 0.2339 0.0000 0.0000

Total

8.0355

4.9711

11.0043

2.9693

11.0115

15.5236

2.0266

8.0500

0.96

0.79

XFe=Fe/(Fe+Mg) Xca=Ca/(Ca+Na+

0.70 K)

0.26

0.72

0.26

0.42

0.36

reported from elsewhere (Stiiwe and Powell, 1989a, b; Clarke et al., 1989; Clarke and Powell, 1991; Clarke and Norman, 1992). Calculations made using microprobe analyses of sample RWl 2, another garnet-sillimanitecordierite-bearing metapelitic gneiss, give a statistically similar result of P = 5.4 _+0.6 kbar (2tr) at T=700°C. Calculations made using microprobe analyses of sample 68725, a garnet-orthopyroxene-bearing gneiss that is interpreted to be a recrystallized metasedimentary xenolith to the pre-Dl orthopyroxene+_ clinopyroxene gneiss give a

statistically good result of P = 5.9 + 1.0 (2a) at T= 700 ° C. Results of calculations made using the same analyses following the method of Perkins and Chipera ( 1985 ) give a similar pressure result (Pus = 2.9 _+ 1 kbar and PFe = 5.9 _ 1 kbar) and P = 4.1 kbar for the method of Bohlen et al. (1983). Sample 90016 is a similar garnet-orthopyroxene-bearing gneiss, and resuits of calculations made using microprobe analyses from that rock and the method of Perkins and Chipera (1985) give PM~=3.8_+l kbar and PFe = 6.4 _+ 1 kbar and P = 4.7 kbar for the method of Bohlen et al. (1983). It is im-

TIMING OF PROTEROZOIC DEFORMATION AND MAGMAT1SMIN A TEC'IONICALLY REWORKED OROGEN

19

68725

68725

68725

RWI2

RW12

RW12

RW12

RWI2

RW12

plag

opx

bi

plag

il

cd

g

spinel

bi

8.658 7.005 0.51

49.151 1.833 32.429 0.392 16.19 0.279 -

34.773 5.45 14.362 0.168 19.446 0.076 11.334 0 0.115 11.563

60.653 0 24.098 0 0.227 0 0.028 5.738 8.602 0.205

0.074 48.583 0.174 0.109 49.049 0.095 1.123 0.026 0 0

38.735 0.085 21.087 0.066 32.867 0.99 5.716 0.965 0 0

0.166 0.033 55.766 1.249 31.326 0.208 4.534 0.025 0 0 5.806

35.255 5334 17.312 0.355 18.408 0 11.403 0 0.152 8.069

100.457

100.274

97.287

99.551

99.233

100.511

99.113

96.294

58.044 -

26.24 -

8

6

2.5958 0.0000 1.3835 0.0000 0.0000 0.0000 0.0000 0.4149 0.6074 0.0291

1.9222 0.0000 0.0845 0.0000 1.0606 0.0130 0.9436 0.0117 0.0000 0.0000

5.0307

4.0356 0.53

0.39

22

51.57 0 33.319 6.194 8.698 0.069 0.01 0.125 0.015 0

100.27

8

3

5.3108 0.6260 2.5860 0.0203 2.4838 0.0098 2.5798 0.0000 0.0341 2.2530

2.7142 0.0000 1.2713 0.0000 0.0085 0.0000 0.0019 0.2751 0.7464 0.0117

0.0019 0.9412 0.0022 1.0567

15.9036

5.0292

2.0532

0.82 0.27

0.96

0.49

possible to be certain as to what extent the minerals in either rock suffered chemical reequilibration during D2, and the range in pressure estimates may be partially due to re-equilibration. However, since the metamorphic textures are part of the $1 foliation, we infer that these pressure estimates reflect SI conditions giving an average estimate of P = 5.2 _+0.7 kbar at T = 7 0 0 ° C . Whereas $2 is characterized by the development of pseudomorphous biotite and hornblende, post-Sl marie dykes contain equilibrated two pyroxene-bearing $2 assemblages. $2 sillimanite and biotite, with or without quartz,

18

0.0021 0.0431 0.0007 0.0007 0.0000 0.0000

12

4

22

5.2406 0.0000 3.9917 0.4977 0.7622 0.0059 0.0015 0.0136 0.0030 0.0000

3.0411 0.0050 1.9518 0.0041 2.1581 0.0658 0.6688 0.0812 0.0000 0.0000

0.0050 0.0007 1.9678 0.0296 0.7842 0.0053 0.2023 0.0008 0.0000 0.0000 O. 1243

5.2669 0.6000 3.0491 0.0419 2.2999 0.0000 2.5388 0.0000 0.0440 1.5379

10.5162

7.9759

3.1199

15.3786

0.76

0.79

0.48

0.82

partially pseudomorph SI cordierite. In marie S 2 orthopyroxene-clinopyroxerie-biotite-hornblende-bearing assemblages are common; rocks that were strongly recrystaUised during D2 show equilibrium textures between these minerals. Rocks that were less strongly recrystallised during D2 contain $2 biotite and hornblende grains that partially pseudomorph S I pyroxene. These relationships indicate that a major period of retrogression occurred post-Dl and either pre-D2 or synDe. Although retrogressive in character, the mineral assemblages defining $2 indicate that the terrain was at the hornblende granulite subassemblages,

no effect on calculations by varying XH20 750 6.5 1.2 1.1

Calculated P(T) 8.3 2.4 5.4 5.8 9.7 9.3 4.4 2.4 5.5 4.9 4.4

all end-rot.tubers 1) 2sp + 5qtz = crd 2) 3crd = 2py + 4sill + 5qtz 3) aim + 2sill = fcrd+ herc 4) 2herc + 5qtztz = fcrd 5) 5enst + 3crd = 5phi + sp + 10sill 6) 16phi + 3herc + 30sill = ann + 15east + 9crd 7) 2~an + 3erd + 2ab = 2naph + 6herc + 2ksp + 15qtz

minus sp, herc, east 1) 3crd = 2py + 4sill + 5qtz 2) 3ferd = 2aim + 4sill + 5qtz 3) 2ann + 3ford = 2phi +2aim + 4sill + 5qtz 4) 2naph + 2aim + 2ksp + 4sill + 5qtz = 2ann + 3crd + 2ab

Independent reactions

py 0.00746 0.58265

0.78 0.57 0.65 1.32

-0.0003 0.0085 0.0083 0.0073

pressures at T=750"C O dT/dP 0.58 0.0062 0.78 -0.0003 0.53 0.0091 0.80 0.0064 1.70 0.0127 1.73 0.0127 1.85 0.0047

sp 0.420 0.10092

Calculated pressures at T=700°C P(T) O dT/dP 4.3 1.81 0.0014 6.2 1,07 0.0132 4.8 1.54 0.0152

gr 4.5e-4 0.76773

Average pressure calculations for sample 68731, 8-cd-sill-sp-bi-Kf-qtz 6neiss phi ann east naph crd ford Activities (a) 0.0754 0.0183 0.0217 0.00952 0.530 0.0654 O (In a) 0.31573 0.48797 0.43625 1.05042 0.06627 0.33236

O'fit

avP O

all end.members 650 700 5.3 5.9 1.0 1.0 0.9 1.0

average p r e s s u r e s

1) 3en + 3an = gr + 2py +3qtz 2) 3fs + 3an = gr + 2alto + 3qtz 3) 3en + 2arm + 3an = 2phi + gr + 2alto + 3q

Independent reactions

Average pressurecalc~ationsforsample 68725,~-opx-pla~-bi-qtz ~neiss en ~ phi ann naph Activities(a) 0.218 0.223 0.0708 0.0389 0.00728 O(Ina) 0.18346 0.16713 0.32318 0.39343 1.37363

-10.316 5.740 2.292 1.163

In K 1.100 -10.316 -2.051 - 1.638 7.265 -24.176 - 1.990

herc 0.580 0.05292

In K -10.148 -2.413 -1.147

aim 0.369 0.07195

py 0.00222 0.68050

ab 0.578 0.05343

aim 0.295 0.10059

an 0.394 0.11017

ab 0.625 0.04219

ksp 0.0277 0.36101

ksp 0.880 0.00432

q~ 1.00 0.0

sill 1.00 0

Average pressure calculations o n the m i n e r a l parageneses in s a m p l e s 68725 a n d 68731 following the a p p r o a c h o f Powell a n d H o l l a n d ( 1988 ) with the e x p a n d e d internally consistent dataset o f H o l l a n d a n d Powell ( 1990 )

TABLE 3

qtz

0

1.00

> z

m

bO

600 3.6 0.4 1.3

650 3.9 0.5 1.5

avP 0 Ofit

600 4.5 0.6 1.2

average pressures

650 4.9 0.6 1.2

700 5.4 0.6 1.2

750 5.8 0.7 1.2

no effect on calculations by varying XH20

In K 5.890 -2.716 5.994

4.9 8.3 3.6

I) 3fcrd = 5qtz + 2alto + 4sill 2) 2phl + 4sill = qtz + crd + 2east 3) 2ksp + 3fcrd +2naph = 2ab + 6qtz + crd + 2aim + 2east

0.0085 0.0130 0.0074

Calculated pressures at T=750°C P(T) o dT/dP

Independent reactions 0.53 1.94 1.74

0.0187 0.447

1.9

750 4.6 0.75

O. 4 0.365

700 4.3 0.6 1.7

minus sp, herc, east

no effect on calculations by varying XH20

Averaae pressure calculations for sample RWI2, 6-cd-sill-sp-bi-Kf-qtz 8neiss ab ksp qtz crd fcrd 0.0665 0.326 Activities (a) 0.722 0.900 1.00 0.443 0.3304 0.0875 O (In a) 0.0232 0.003 0.0 0.093

avP O O'fit

750 5.6 1.0 3

all end.members

average pressures

O. 1.054

sill 1.0 0.0

b,J

z

M

o

o

t'-

z

z

M

o

o

c~ o"11

22

facies during D2, at a slightly decreased temperature and/or increased water activity than those present during the development of $1. The extent of recrystallization associated with the development of $2 is patchy on a local and regional scale, suggesting that there were local variations in the availability of water. Water is unlikely to have been derived from the S i assemblages, since the rocks had been previously dehydrated by granulite facies metamorphism. The most notable area of retrogression occurs within and adjacent to the biotite + pyroxene gneiss, which is essentially a strongly retrogressed charnockitic gneiss. Rock exposures to the west of Taylor Glacier (Fig. 1 ) show significantly less retrogression than that observed in rocks of the Colbeck Archipelago, where the biotite+pyroxene gneiss is restricted in occurrence or absent. Retrogression in the biotite+_pyroxene gneiss is significantly more pervasive than that observed in the mineralogically similar pre-D1 orthopyroxene +__clinopyroxene gneiss, even though the two are, in many places, juxtaposed. Once crystallized, the protolith charnockite for the biotite + pyroxene gneiss could release water to allow auto-retrogression. Water is also likely to have moved into the surrounding gneisses via the numerous dykes that extend from the main body of the biotite +pyroxene gneiss, resulting in local increases in water activity and the observed patchy retrogression. Increased water activity in rocks at high temperatures presents the potential for renewed partial melting, and such a scenario could explain the $2 leucosomes that are observed to cut $1 in the metasedimentary gneisses of the Colbeck Archipelago. The mineral assemblages defining $3 are similar to those defining $2, suggesting that metamorphic grade was similar for the two events. The amount of recrystallisation associated with D 3 is variable, with the most pervasive recrystallisation occurring in and near the Mawson Charnockite. Again, this is in-

R.W. WHITE AND G.L. CLARKE

ferred to be the case due to a local thermal anomaly and/or water availability. Mineral assemblages comprising the ultramylonites include neoblastic siUimanite, garnet, biotite and cordierite where the zones cut metapelitic rocks, and pyroxene and garnet where the zones cut mafic gneisses. The mineral assemblages preserved in each mylonite zone are similar to those of the host gneiss, but the neoblastic minerals preserve compositions distinct to the 52_ 4 minerals and with sympathetic changes in, for example, Xve values. Moreover, garnet is not observed in $2-4 assemblages in mafic gneiss, yet it occurs as neoblasts where they are cut by ultramylonites. These observations suggest that the mineral assemblages forming the ultramylonites represent, at least locally, equilibrium mineral assemblages. Hydrous phases are only found where biotite and/or hornblende comprise a significant proportion of the host gneiss, indicating that mylonitisation was not accompanied by any significant increase in water activity. Neoblastic sillimanite indicates that the ultramylonites involved temperatures in excess of 520°C, but geothermometric and barometric estimates made from microprobe analysis of the ultramylonite assemblages suggests that conditions were not dissimilar to those of $2-4. To what extent the assemblages developed in the mylonites reflect regional temperature conditions is unclear. Granulite facies conditions are inferred for identical ultramylonites reported from near Mawson by Clarke and Norman ( 1992 ). Discussion

Three broad lithological groups are recognised within the Colbeck Archipelago: metasedimentary gneisses, charnockitic gneisses and minor mafic gneisses. The rocks preserve evidence of four phases of ductile deformation (Di-3, D5), which probably all occurred at granulite facies conditions. Much of the early history of the area has been obscured by re-

TIMING OF PROTEROZOIC DEFORMATION AND MAGMATISM IN A TECTONICALLY REWORKED OROGEN

crystallisation during intense deformation that accompanied these high-grade metamorphic conditions, which are probably of several distinct ages. The Colbeck Gneiss has been shown to be composite in age and origin; it comprises pre-D~ metasedimentary gneiss and orthogneiss, and post-D~, pre-D2 orthogneisses. No data are available to constrain the absolute age of the metasedimentary gneisses, which show a range in composition from arkosic to pelitic. A high-grade, layer parallel S~ gneissosity is characterised by a coarse-grained gneissic layering and quartz-feldspar-rich leucocratic segregations that represent mobilisate. S~ assemblages include garnet-cordierite-sillimanitebearing metapelitic gneisses and suggest conditions of T~>700°C and P=5.1_+0.8 kbar. The presence of mineral corona textures involving S1 spinel, sillimanite, garnet and cordierite in the metapelitic gneisses is consistent with DI having occurred during essentially isobaric cooling of the terrain (after Clarke et al., 1989). The orthopyroxene + clinopyroxene gneiss is inferred to have intruded the metasediments before DI. This gneiss is identical in mineralogy and habit to the Archaean Stillwell Gneiss, which outcrops 10 km west of the Colbeck Archipelago (Trail, 1970; Clarke, 1988 ) and is interpreted as preserving structural features that pre-date the Rayner Structural Episode (Clarke, 1988; James et al., 1991 ). This correlation, together with the structural correlations discussed below, suggests that S~ probably pre-dates the Rayner Structural Episode and could be Archaean in age. A second major phase of intrusion resulted in the biotite+_pyroxene gneiss, rare mafic dykes and possibly the granitic gneiss and layered gneiss, which cut S~ but pre-dated the development of isoclinal, reclined east-trending F2 folds that have a steeply east-dipping $2 axial planar foliation. An intense, east-trending L 2 mineral and stretching lineation is oriented parallel to most F2 axes, and is interpreted as representing an axis of transport during D2. Recrystallization associated with the develop-

23

ment of a transitional granulite facies S 2 involved pseudomorphous S 2 biotite and/or hornblende in the orthogneisses, and retrograde biotite and sillimanite in the metapelitic gneisses. Retrogression is patchy in the metasedimentary gneisses and orthopyroxene +_clinopyroxene gneiss, but more pervasive in the biotite+_pyroxene gneiss with water for the retrogression probably having been locally derived from the crystallizing igneous bodies. The general features of DE, including a pervasive L2 lineation and the interpreted easttrending axis of transport, can be correlated with those of recumbent fabrics in the Mawson area and the Stillwell Hills (termed DI by Clarke, 1988 ). The Mawson Charnockite, accompanied by signifcant amounts of pegmatite dykes, represents another major phase of intrusion that pre-dated the development of upright eastnortheast-trending F3 folds and a steeply dipping $3 foliation. The effects of D3 are identical to upright east-trending folds observed in the Stillwell Hills, and to poorly preserved fabrics in rocks of the Mawson area (DE in Clarke, 1988 ). Moreover, 0.94_+0.08 Ga pegmatites occur at Fold Island just to the north of the Stillwell Hills (Fig. 1; Grew et al., 1988), apparently with identical timing relationships to the Mawson Charnockite. On this basis, D E and D3 represent effects of the Rayner Structural Episode in the Colbeck Archipelago, with a later D4 event evident at both the Stillwell Hills and Mawson but absent in rocks of the Colbeck Archipelago. The biotite+_pyroxene gneiss, granitic gneiss, and the Mawson Charnockite could then represent significant Proterozoic magmatic contributions to reworked Archaean gneisses, with magmatism preceding and continuing throughout the Rayner Structural Episode. It is possible that the post-D4 pre-D5 pegmatites (Clarke, 1988 ) are also part of such a magmatic episode. The retrograde nature of the $2-4 fabrics is consistent with effects of the Rayner Structural Episode elsewhere (Kamenev, 1972; Sheraton et al., 1980;

24

Clarke, 1988; Sheraton et al., 1988), and the rare post-Dl pre-D2 mafic dykes could be equivalent to the strongly deformed mafic dykes in the Stillwell Hills that have been correlated with the Amundsen Dykes of the Napier Complex (Sheraton et al., 1980; Clarke, 1988 ). Large east-trending granulite facies D5 ultramylonite + pseudotachylite zones preserve a down-dip stretching lineation and consistent S-C fabrics indicating that the terrain underwent late north-south extension. These ultramylonite zones are mineralogically similar to those observed at Mawson (Clarke and Norman, 1991 ), and could have occurred in a setting involving crustal extension due to, and shortly after, continental overthickening related to the Rayner Structural Episode. In rocks comprising the Colbeck Archipelago, the Rayner Structural Episode is interpreted to have initiated when the rocks were at depths equivalent to ~ 5 kbar, which represents similar conditions to those inferred for rocks along the Mawson Coast (5.6 __0.4 kbar, Clarke and Norman, 1992). Mineral assemblages in the Mawson area are consistent with the terrain having been at depths equivalent to P = 3.9 +_ 1.0 kbar when the ~ 950 Ma Mawson Charnockite intruded (Clarke and Norman, 1992), and similar conditions are inferred for D 3 in rocks of the Colbeck Archipelago. Whereas limited decompression due to a terrain undergoing magmatic accretion would be consistent with these relationships, the inferred 280 Ma time gap between D2 ( 1200 Ma) and D 3 (920 Ma) is more consistent with the biotite+_pyroxene gneiss and Mawson Charnockite representing two discrete magmatic pulses. Deformation accompanied or postdated both phases of intrusion, but the D2-4 events that form the Rayner Structural Episode did not involve a single evolving orogeny. Instead, the P - T data correspond to points on at least two separate P - T loops for which we have little information outside the related magmatism. Nonetheless, successive low-pressure granulite facies events, which necessarily

R.W. W H I T E A N D G.L. C L A R K E

involved advective heating at such shallow crustal levels (e.g. England and Thompson, 1984; De Yoreo et al., 1989; Sandiford and Powell, 1991 ), were responsible for the recrystallization of Archaean rocks. If the succeeding metamorphism (e.g. $2 ) did not approach the temperature of the earlier metamorphism (e.g. S I ), then there would have been little recrystallization without an influx of water (Clarke and Powell, 1991b). Thus, it is tempting to consider the Proterozoic magmatic additions at 1200 Ma and 960 Ma as integral to the two main phases of deformation forming the Rayner Structural Episode, and possibly the single most important factor in facilitating recrystallization due to their supply of both heat and water (e.g. Sandiford et al., 1991 ). However, extensive retrogression and recrystaUization in rocks of the Colbeck Archipelago occurs only in proximity to the biotite+_pyroxene gneiss, indicating that water was only locally introduced from that intrusion. Also, contact relationships of the Mawson Charnockite suggest that it was actually extracting water from surrounding rocks rather than introducing it. In the absence of an unrecognized source of water, the processes of recrystallization would appear to be dependent on heating. Whereas the Mawson Charnockite is sufficiently large to have induced low-pressure metamorphism (e.g. Barton and Hanson, 1989), the pyroxene+_biotite gneiss is volumetrically insufficient at the presently exposed structural level to have been responsible for the metamorphism that accompanied the development of $2. The limited contact effects that accompany both periods of magmatism and the persistence of granulite facies S2-a assemblages distal to the intrusions are consistent with intrusion having been contemporary with anomalously high conductive heat fluxes. Given the transient nature of such elevated temperatures at shallow crustal levels (Barton & Hanson, 1989; Sandiford & Powell, 1991 ), these were most probably induced by advective heating from intrusions not be exposed at the present struc-

TIMING OF PROTEROZOIC DEFORMATION AND MAGMATISM IN A TECTONICALLY REWORKED OROGEN

t u r a l level ( S a n d i f o r d & Powell, 1991 ), a n d a causal link between deformation and magmatism seems probable for events of the Rayner Structural Episode.

Acknowledgements T h e A u s t r a l i a n A n t a r c t i c D i v i s i o n is t h a n k e d f o r logistic s u p p o r t p r o v i d e d to W h i t e a n d Clarke during the 1990/91 Australian National Antarctic Research Expedition. This work was supported by funding from the Antarctic Science Advisory Committee (Clarke, Powell a n d Wilson, A S A C P r o j e c t 177 ) a n d t h e Australian Research Committee small grants s c h e m e ( C l a r k e ) . J.P. B a r k a s , S. H a r l e y a n d J. P e r c i v a l are t h a n k e d for i m p r o v i n g t h e manuscript.

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Clarke, G.L. and Powell, R., 1991a. Decompressional coronas and symplectites in granulites of the Musgrave Complex, central Australia. J. Metamorph. Geol., 9: 441-440. Clarke, G.L. and Powell, R., 199 lb. Proterozoic granulite facies metamorphism in the southeastern Reynolds Range, central Australia: geological context, P - T path and overprinting relationships. J. Metamorph. Geol., 9: 267-282. Clarke, G.L., Collins, W.J. and Vernon, R.H., 1989. Successive overprinting granulite facies metamorphic events in the Anmatjira Ranges, central Australia. J. Metamorph. Geol., 8: 65-88. Crohn, P.W., 1959. A contribution to the geology and glaciology of the western part of the Australian Antarctic Territory. Bur. Miner. Resour., Geol. Geophys., Bull., 52, 103 pp. De Yoreo, J.J., Lux, D.R. and Guidotti, C.V., 1989. The role of crustal anatexis and magma migration in the thermal evolution of regions of overthickened continental crust. In: J.S. Daly, R.A. Cliffand B.W.D. Yardley (Editors), Evolution of Metamorphic Belts. Geol. Soc. Spec. Publ., 43:187-202. England, P.C. and Thompson, A.B., 1984. Pressure-temperature-time paths of regional metamorphism, I. Heat transfer during the evolution of regions of thickened continental crust. J. Petrol., 24: 894-928. Grew, E.S. and Manton, W.I., 1986. A new correlation of sapphirine granulites in the Indo-Antarctic metamorphic terrain: late Proterozoic dates from the Eastern Ghats Province of India. Precambrian Res., 33:123137. Grew, E.S., Manton, W.I. and James, P.R., 1988. U-Pb data on granulite facies rocks from Fold Island, Kemp Coast, east Antarctica. Precambrian Res., 42: 63-75. Holland, T.J.B. and Powell, R., 1990. An enlarged and updated internally consistent dataset with uncertainties and correlations: the system K20-Na20-CaOMgO-MnO-FeO-Fe2Oa-Al203-TiO2 -SIO2-C-H202. J. Metamorph. Geol., 8: 89-124. James, P.R., Ding, P. and Rankin, L., 1991. Structural geology of the early Precambrian gneisses of northern Fold Island, Mawson Coast, east Antarctica. In: M.R.A. Thompson, J.A. Crame and J.W. Thompson (Editors), Geological Evolution of Antarctica. Cambridge University Press, Cambridge, pp. 19-23 Kamenev, E.N., 1972. Geological structure of Enderby Land. In: R.J. Adie (Editor), Antarctic Geology. North-Holland Publishing Company, Amsterdam, pp. 579-583. McCarthy, W.R. and Trail, D.S., 1964. The high-grade metamorphic rocks of the MacRobertson Land and Kemp Land coast. In: R.J. Adie (Editor), Antarctic Geology. North-Holland Publishing Company, Amsterdam, pp. 473-481. Perkins, D. and Chipera, S.J., 1985. Garnet-orthopyroxene-plagioclase-quartz barometry: refinement and

26 application to the English River subprovince and the Minnesota River Valley. Contrib. Mineral. Petrol., 89: 69-80. Powell, R. and Holland, T.J.B., 1988. An internally consistent dataset with uncertainties and correlations, 3. Applications to geobarometry, worked examples and a computer program. J. Metamorph. Geol., 6:173-204. Ramsay, J.G., 1967. Folding and Fracturing of Rocks. McGraw-Hill, New York, N.Y. Ravich, M.G. and Kamenev, E.N., 1975. Crystalline Basement of the Antarctic Platform. John Wiley, New York, N.Y. Sandiford, M. and Powell, R., 1991. Some remarks on high temperature-low pressure metamorphism in convergent orogens. J. Metamorph. Geol., 9: 333-340. Sandiford, M.A. and Wilson, C.J.L., 1984. The structural evolution of the Fyfe Hills-Khmara Bay Region, East Antarctica. Aust. J. Earth Sci., 31: 403-426. Sandiford, M., Martin, N., Zhou, S. and Fraser, G., 1991. Mechanical consequences of granite emplacement during high-T, low-P metamorphism and the origin of "anticlockwise" P T paths. Earth Planet. Sci. Lett., 107: 164-172.exam Sheraton, J.W., 1982. Origin of charnockitic rocks of MacRobertson Land. In: C. Craddock (Editor), Antarctic Geoscience. University of Wisconsin Press, Madison, Wisc., pp. 489-497. Sheraton, J.W. and Black, L.P. 1981. Geochemistry and geochronology of Proterozoic tholeiite dykes of East Antarctica: evidence for mantle metasomatism. Contrib. Mineral. Petrol., 78:305-317. Sheraton, J.W., Offe, L.A., Tingey, R.J. and Ellis, D.J. 1980. Enderby Land, Antarctica--an unusual Precambrian high grade terrain. J. Geol. Soc. Aust., 27:1-18. Sheraton, J.W., Tingey, R.J., Black, L.P., Offe, L.A. and Ellis, D.J., 1988. Geology of Enderby Land and western Kemp Land, Antarctica. Bur. Miner. Resour., Geol. Geophys., Bull., 223. Sibson, R.H., 1975. Generation ofpseudotachylite by an-

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