High-grade reworking of central Australian granulites. Part 1: Structural evolution

Tect~o~hysics, 204 (1992) 361-399 Elsevier Science Publishers B.V., Amsterdam

361

High-grade reworking of central Australian granulites, Part 1: Structural evolution Ben Goscombe Geology~~~~rne~t, U~ive~si~of ~ei~~e,

’ Park&e 3052, V&t.,Australia

(Received October 29,199O; revised version accepted June 4,1991)

ABSTRACT Goscombe, B., 1992. High-grade reworking of central Australian granulites. Part 1: Structural evolution. Tectonophysics,204: 361-399. Four distinct deformational events (Dz-Ds) accompanying a granulite facies metamorphic cycle (Mz-Ms), are shown to st~cturally overprint (rework) pre-existing granuiites (MI) of the northeast Stran~ays Range in the central Arunta Block. The first metamorphic cycle (M,) at 1800 Ma, involved an anti&&wise P-T path peaking at 8.50-950 *C at 8-9 kbar. M, metamorphism involved widespread partial melting that produced stromitic migmatites and map-scale concordant granitic gneisses. Isobaric cooling after the peak, and accompanying hydration, gave rise to a wide variety of coronitic and symplectitic reaction textures that enclose and replace Mt mineral parageneses. No kinematic structuring is associated with M,, and the period encompassing the thermal peak and immediately subsequent are thought to have been absent of deviatoric stress. The second metamo~hi~ cycle (Mz-M,), possibly at lo-15~ Ma, involved a &&wise P-7’ path with a m~mum P of > 9-10 kbar and a thermal peak of approximately 800” C. This metamorphic cycle accompanied the major ductile deformations recognized (Dz-Ds), and has been labelled the “Proterozoic Reworking”. The Proterozoic Reworking has been divided into two thermo-barometric and structurally distinct periods. D,-D, involved regionally inclined, ENE-WSW non-coaxial shear of high bulk shear strains, giving rise to an intense (often mylonitic) pervasive fabric (Sz-I.,) and isoclinal and sheath folds on all scales (Fz, F,). Sz-L, is the first recognized kinematic fabric and does enclose and partially recrystallize M, mineral parageneses, including late-M, metamorphic reaction textures. Dz-Ds deformation was due to crustal shortening and gave rise to crnstal per-thickening (loading) accompanying prograde metamo~hism (MT). D,-D, involved upright, open and asymmetrical folding (Fe) and E-W-trending shear zone development (Ss) within a regionally extensive system of inclined, oblique, sinistral transpression. Both Dz-D, and D,-D, episodes occurred under the same app~~mately E-W-directed compressive stress, and are considered sequential episodes in the one tectonothe~a1 cycle. F4 folding accompanied significant melt formation and the peak of metamorphism of the Mz-Ms metamorphic cycle. Ss shear zones accompanied 3-4 kbar of decompression with cooling, presumably during uplift in isostatic response to crustal over-thickening in Dz-D,. Progression from inclined non-coaxial shear to transpression occurred in response to an increase in the relative buoyancy forces limiting crustal thickening, as is typical of many compressional orogens. However, the Proterozoic Reworking is atypical of simple linear or arcuate mountain belts because late-stage shearing (S,) is aligned sub-parallel, not at a high angle, to the transport vector of the earlier ductile defo~ations (Dz-Ds). Consequently, this region is modelled in terms of crustal shortening directed along the length of an E-W-trending orogen confined to the north and south by relatively stable crustal blocks. When E-W shortening could no longer be accommodated by crustal thickening, strain was partitioned by sinistral transpression between the north and south bounding crustal blocks.

Introduction The Arunta Inlier of central Australia has a 2000 Ma history of multiple ductile deformation

Correspondence to: B. Goscombe, Tasmanian Department Mines, Rosny Park 7018, Tasmania, Australia. ~-1951/92/$05.~

of

and multiple high-grade metamorphic cycles and dissection by at least three generations of shear zones. Like most high-grade Proterozoic blocks preserved in the Earth’s crust, the boundary conditions of the interacting tectonic elements (plates?) affecting the Arunta Block during the Proterozoic are not known. As a consequence, kinematic analysis and the P-T-t paths experi-

Ca 1992 - Eisevier Science Publishers B.V. Ail rights reserved

362

enced become crucial in constraining the Proterozoic tectonic evolution of the Arunta Block. This paper is part one of two papers that together present the tectonothermal history, during the Early-Middle Proterozoic, of the Strangways Orogenic Belt of granulites (James and Ding, 19881 in the central Arunta Block (Fig. 1). Presented here is the structural and textural analysis of the major Proterozoic events recorded in the highest-grade granulites in the Strangways Orogenie Belt and which are located in the northeast Strangways Range (NESWR) (Fig. 11. The accompanying (Part 2) metamorphic analysis is presented in Goscombe (1992b). The most immediate problem associated with the tectonothermal analysis of Strangways Orogenie Belt granulites is the relative chronology of deformational structures and metamorphic parageneses and reaction textures. Associated with this problem is the correlation of published geochronological data with tectonothermal events. Surmounting these problems is crucial for meaningful tectonothermal studies from any high-grade Proterozoic metamorphic complex. Previous workers in the Arunta Block (Warren, 1983; Shaw et al., 1984a; Norman and Clarke, 1990) interpret the pervasive ductile deformation of the Strangways Orogenic Belt to accompany the first and highest-grade metamorphic cycle (M,, Table 1). Norman and Clarke (1990) propose that the second high-grade metamorphic event was defined by coronitic and symplectitic metamorphic reaction textures and post-dated the major ductile deformational events experienced in region (D2D,, Table 1). In this paper and Goscombe (1991~) it is argued that the peak of the first metamorphic cycle (M, 1, and the subsequent cooling phase defined by coronitic and symplectitic reaction textures, was not accompanied by pervasive deformation. In addition, the second metamorphic cycle CM,-M,, Table 1) overprinted and accompanied the major ductile deformations CD,-D,) that structurally reworked the M, granulites. Mineral parageneses and deformational fabrics associated with the second tectonothermal cycle overprint and recrystallize M, and late-M, mineral parageneses and so these events are considered to have structurally reworked the earlier

8. GOSCOMl31:

Ml textures. Consequentially, this second tectonothermal cycle CM,-M, and D,-D,) has been labelled the Proterozoic Reworking (Goscombe, 1987). The term reworking has been expanded from its more typical structural application to the reworking of shear zones, to encompass the kinematically and thermally induced partial but pervasive recrystallization of granulites throughout the Strangways Orogenic Belt. This reworking occurred at approximately the same crustal level that the pre-existing granulites were formed at, that is, M, granulites were not exhumed prior to reworking. The Proterozoic Reworking has been divided into two structurally distinct episodes (Table 1). The initial episode involving pervasive non-coaxial shear (Dz-D3) during compressional tectonics (Goscombe, 1991) and gave rise to crustal overthickening during prograde metamorphism CM,) (Goscombe, 1992b). The latter episode involved inclined, oblique transpression CD,-D,) accompanying decompression CM,). Consequently, the Proterozoic Reworking involved a clockwise P-T path (Goscombe, 1992b) entirely compatible with compressional tectonics (England and Thompson, 1984). The Early-Middle Proterozoic history covered by this paper is discussed in two parts, one each for the two tectonothermal cycles recognized; M, metamorphism and the later Proterozoic Reworking CM,-M,). Part 1 describes the body of rocks that existed, as well as M, metamorphism, prior to the intense Middle Proterozoic Reworking. Tectonic models for M, metamorphism are discussed in detail in Goscombe (1992a, b). Recognition that the major ductile deformational episodes of the Strangways Orogenic Belt over-printed and reworked a pre-existing granulite facies terrain is important in understanding the wider tectonic evolution of the Arunta Block. Particularly in the absence of knowledge of the tectonic plate boundary conditions during the Early-Middle Proterozoic. This study is of wider importance in illustrating that multiple granulite facies events can be recognized in Proterozoic high-grade blocks, as well as discussing the nature of deformation in the middle-lower crust during crustal shortening. Additionally, the tectonic

363

‘Cadnty

100

IQ0 km

1 tlordar Proterozoic Late

and Pataeotcur

Proterozoic

to Devonian

shear

zones

El

dykes.

\’

Amadeus Basin

Igneous

Complex

L tll8Oflat

Granulite-facies gneisscs. extent o? Stranqvays Orogcnlc

Belt

sediments Anphibolite-facier

Basaltic

Gntltr

gneisses

Protwozorc

Fig. 1. Regional geological features (based on Shaw et al., 1979, 1984) and the locality of area studied. Inset shows the tectonic provinces of the Arunta Block (Shaw et al., 1984a) and adjacent Proterozoic-Palaeozoic sedimentary basins,

1

8-9 kbar

I Mt 850-950

oC

< 7-8 kbar

and

crustal

geotherm

to “normal”

Possibly cooled

hydration

Cooling

J-

aric cooling

morphism

J Peak of meta-

crys Gzation

M, melt

migmatisation

stromatic

stromatic S, -gneissic

_

_

S,,-1eucosomes

layering

,

in M,

*

formation

;,_,.-,,__

biotite

random

textures

reaction

Extensive

and symplectite

textures

+

Corona

polygonal

porphyroblastic

Inchtsions phases

Extensive

layering

lithological

5, -gross

region

Textures

Range

parageneses

P - Tpath)

Strangway

Fabrics

in the Northeast

Deformation

events recorded

Prograde

First metamorphic

Melts

Proterozoic

cycle (M,) (anticlockwise

of tectonic-metamorphic

Metamorphism

Summary

TABLE

.,,“,

+ Q

exsolution

__,

gn, opx 2 phl

perthitic

I

+ opx opx * sill-Q-gd-phl

gn * sill-Q-gd

sa = sill-opx

cd * sill-Q-opx-gd

opx-cd-gn-Q-kfld

sa-cd-opx

sp-sa-sill-Q

Parageneses

.,

,

I

stress

_...-

of deviatoric

in the absence

then cooling,

T perturbation

age (Ma)

setting

-

1670-1720

[X6,71

1700-1775

[VI

1750-1800

L&WI

2000-2100

Possible

Tectonic

T

Decompression + cooling

I Peak of Ma-M, metamorphism

Prograde crystallization during D2

1

with

Alice

phl-sill pegs.

Mafic dykes

the Palaeozoic

Heating event

gn-phi granite

opx-phl and tm-sill peg.

opx-leucosome

gn-sill-phl leucosome

Springs

D6

l-

Orogeny

inclined and asymmetrical folds

, F,-open,

D, Fs-isoclines,

D, F,-isoclines, sheats

S, -N-S-trending normal, E down shear zones

S, -E-ESEtrending shear zones

S4 -E-ESEtrending weak foliation

S, -pervasive tectonic fabric

Mylonites

Ultra-mylonites

Discontinuous quartz aggregate

Post-M, garnet overgrowing S, fabrics

Annealed S, fabrics

Intense mylonitic fabric enclosing all M 1 parageneses

gn-hn-Q-pl+

phl

opx-gn-sill-Q opx-cpx-gn-me-p1

gn-opx-sill-Q

opx-sill-gd-phl-Q sill over-growths

Possible lithospheric extension

NE over SW oblique, sinistral tri mspression

I

I

1

coaxial shear

E over W non-

]2,3,7>8,93

900-1050

[lOI

t3,91 1400-1500

1424-1490

L4,81

1400-1500

associated

quartzofeldspathic

* Events

and mafic gneisses.

Metamorphic

conditions

from Goscombe

are omltted.

Parageneses are for alummous rock types Goscombe, 1992bL except MS and M6 are (1992b). The possible age correlations are justified in the text. ill Allen and Stubbs (1982); 121 Windrim et al. (1984); [3] Black et al. (1983); [4] Iyer et al. (1976); [5] Black and McCulloch (1984); [6] Windrim and McCulloch(l986); [7] Mortimer et al. (1985); 181Woodford et al. (1975); 191Black (1980); [lo] Shaw and Black (1991).

650-720 o C 7 kbar

M6

Later tectonic events

to 550°C to 6 kbar

M5

750-800 ’ C 8-9 kbar

> 9-10 kbar

W

Second tectonothemaal cycle (hf2 - h4*) (clockwke P - Tpath)

2

L

9

E

E Fi

$,

T

366

H. (iOSCOMHt

NOTATION 1 Abbreviations NESWR sill opx gn gd sa SP Phi

tm kfld PI cpx me hn

Q cd

Northeast Strangways Range sillimanite orthopyroxene garnet gedrite sapphirine spine1 phlogopite tourmaline K-feldspar plagioclase clinopyroxene meionite hornblende quartz cordierite

model proposed for the Proterozoic Reworking of the Strangways Orogenic Belt has many aspects shared in common with the structural history of many other high-grade Proterozoic orogens as well as having possible modern analogies. For abbreviations in this paper see Notation 1. Previous work and tectonic framework The relative chronology of tectonic fabrics, structures and mineral parageneses recognized in the NESWR is shown in Table 1. The abbreviations for structural elements, as presented in Table 1, are referred to throughout the paper. This chronology is compared with that of previous work from the Strangways Orogenic Belt (Fig. 1) in the following discussion. The correlation of structural and metamorphic events recognized within the NESWR with geochronology from the central Arunta Block (Tables 1 and 2) is discussed and justified throughout the paper. Lithologies in all tectonic provinces have been recognized as having supracrustal origins (Windrim, 1983; Stewart et al., 1984). The protoliths are thought to have formed in an intracontinental rift (Shaw et al., 1984a; Stewart et al., 1984). Nd-Sm mantle separation ages for the metamorphosed mafic and felsic volcanics, that comprise the majority of the Strangways Range, span 1980-2085 f 120-190 Ma (Table 2).

Granulite-facies peak metamorphism (M i, Table 1) is recognized throughout the entire Strangways Orogenic Belt (Warren, 1983; Windrim, 1983; Shaw et al., 1984a; Oliver et al., 1988; Norman and Clarke, 1990). Rocks of the Strangways Orogenic Belt were totally recrystallized to granulite-facies grade at 1780-1820 Ma (Table 2). Peak metamorphic conditions are narrowly constrained to 840-950” C and 8-9 kbar from throughout the Strangways Range (Allen and Stubbs, 1982; Warren, 1983; Goscombe, 1992b). Warren (1983) recognized isobaric cooling after M, metamorphism, whereas Norman and Clarke (1990) proposed an increase in pressure during cooling after M, metamorphism. M, metamorphism was responsible for extensive partial melting and migmatite formation (Warren, 1983; Norman and Clarke, 1990). M, partial melting gave rise to large scale concordant bodies of granitic gneiss (Windrim, 1983; Collins et al., 1988). Granite gneisses were deformed by the pervasive tectonic foliation (S,, Table 1) (Windrim, 1983) and thus pre-date this first recognized tectonic deformation in the Strangways Range (Table 1). Melt crystallization and consequent fluid release is inferred to have been responsible for auto-retrogression of surrounding gneisses (Hensen and Warren, 19851, giving rise to the crystallization of biotite (the “biotite stage” of Warren, 1983) and whole-rock metasomatic trace-element signatures (Allen, 1979; Goscombe, 1984, 1989). Woodford et al. (1975) and Iyer et al. (1976) recognized two granulite facies tectonothermal events in the NESWR; M, the highest temperature metamorphism and a later M, metamorphic event at higher P and lower T.These metamorphic events were dated respectively at 1800 Ma and 1400-1500 Ma by Rb-Sr whole-rock (Iyer et al., 1976) and 40Ar/39Ar mineral (Woodford et al., 1975) methods. Subsequent workers (Warren, 1983; Shaw et al., 1984a; Norman and Clarke, 1990) have correlated the pervasive ductile deformation of the region CD,-D,, Table 1) and dominant fabric development (S,) to have accompanied and immediately post-dated M, metamorphism of 1800 Ma age. It is argued in this paper and the accompanying metamorphic analysis

HIGH-GRADE

REWORKING OF CENTRAL AUSTRALIAN

367

GRANULITES

(Goscombe, 1992b), that M, granulites were structurally reworked in a second tectonothermal cycle (M,-M,) during pervasive defo~ation of the Strangways Orogenic Belt (D,-D,) at approximately 1400-1500 Ma. This is based on outcrop and microscopic textural over-printing relationships, contrasting P-T paths during M, (anticlockwise) and M,--M, (clockwise) metamorphic cycles (Goscombe, 1992b) and the available ge~hrono~ogical data. All previous workers have recognized multiple isoclinal fold events (F2-F3, Table 1) with an accompanying pervasive tectonic foliation (S,), and later upright, open and asymmetrical folds (F,, Table 1). Norman and Clarke (1990) have

interpreted these deformational events to immediately post-date M, metamorphism and that D2-D4 defo~ational elements are over-printed by a second granulite-facies metamorphism. However, in the NESWR D,-D, deformational events clearly overprint and recrystallize both M, granulites and reaction textures formed during cooling after M, (see M, metamorphism section). In addition, mineral parageneses of the second metamo~hic cycle (M,-M,) are correlated with the deformational elements of the Proterozoic Reworking (D,-D,), as discussed throughout this paper. E-W-trending granulite-facies ultramylonite shear zones are recognized in the NESWR (S,,

TABLE 2 Summary of Early-Middle

Proterozoic geochronological data largely from the Strangways Orogenic Belt

Tectonic period

Rock, mineral or fabric dated

Location

Method

Protolith geochronology

mantle separation age, felsic metavolcanic , mafic granulites , mafics and felsics , mafics and felsics

NW Strangways R. NW Strangways R. NW Strangways R. regional sampling

Sm-Nd Sm-Nd Sm-Nd Sm-Nd

Peak M, metamorphism

peak of metamo~hism,

S Strangways R. NW Strangways R. NW Stran~a~ R. NW Strangways R. NW Strangways R.

4oAr/39Ar hn. Rb-Sr (TR) Rb-Sr (TR) Rb-Sr (TR) Rb-Sr (TR)

1780-1820 1800 + 25 1780 f 40 1790 + 35 1820 f 78

[l]

Mt. Ida Granite migmatite deformed granitoid Bruna gneiss emplacement

S of Harts Range NW Strangways R.

Rb-Sr (TR) Rb-Sr, model

131 [6] 161

Harts R.

U-Pb, zircon.

1690-1670 1777, 177.5 1704,170O 1748*4

Early Proterozoic hydration (“ biotite stage”)

secondary hornblende crystallization “biotite stage”, cd-Q pelite , sapphirine pelite , total pelite

S Strangways R. NW Strangways R. NW Stran~ays R. NW Strangways R.

‘%r,/39Ar hn Rb-Sr (TR) Rb-Sr (TR) Rb-Sr (TR)

169%1720 1670 + 45 1700 f 30 1690 + 25

[11

Middle Proterozoic recrystallization

secondary hornblende and biotite

S Strangways R.

1450-1500

[ll

syntectonic (5,) recrystallization

NE Strangways R. NE Strangways R. Redbank Zone

‘%r/ =sAr hn-phl ‘%r/ 39Ar phl Rb-Sr (TR) Rb-Sr (TR)

1479 1400 + 59 1400-1500

[SIC t41 c [IO]

W Strangways R. Reynolds Range Reynolds Range Reynolds Range

Rb-Sr Rb-Sr Rb-Sr Rb-Sr

1426 _I 81 1491 f 103 1424 f 58 1459 f 10

131 [31

Early Proterozoic granite crystallization

E-W-trending Middle Proterozoic granite crystallization

granulites

mylonite

Wuluma granitoid Wangala granite Napperby granite Unca granite

a TR = total rock. b Sources as number in Table 1. Recalculated:

(1984a).

Age a (Ma) (TR) (TR) (TR) (TR)

(TR) (TR) (TR) (TR)

2015 * 2085 f 2070 f 1980 +

Ref. b 120 175 125 190

121 [21 [61 [S]

[21

f61 131 [41f

I71

[61 161 [61

131 [91

source Windrim and McCulloch (19861, ’ Source Shaw et al.

368

Table 1) and in the granulites to the east (Norman and Clarke, 1990). Large-scale (up to 10 km wide) steeply dipping, E-W-trending amphibolite-facies shear zones dissect the Arunta Block, for example, the Redbank and Harry Creek Deformed Zones and the Wallaby Knob Schist Zone (Fig. 1). Marjoribanks and Black (1974) have identified a Proterozoic pre-history to the Redbank Deformed Zone and Shaw and Black (1991) present 1400-1500 Ma Rb-Sr isotopic dates from mylonites in this zone. Dolerite dykes are recognized in the NESWR and as a rare component in other parts of the Strangways Orogenic Belt (Allen and Stubbs, 1982; James and Ding, 1987). In the south Strangways Range, dolerite dykes have been dated at 1057 f 19 Ma (Allen and Stubbs, 1982). Dolerite dykes in the NESWR are overprinted by steep NNW-NNE-trending amphibolite-facies (M,: 6.4-7.5 kbar, 640-730 o C) shear zones (S,) with east down normal movements (Goscombe, 1989). The Strangways Orogenic Belt is dissected and bound by E-W-trending N-dipping retrograde shear zones. North over south thrusting along these shear zones in the Middle Palaeozoic is responsible for uplift and exhumation of the Strangways Orogenic Belt of granulites (Forman, 1971; Allen and Black, 1979; Shaw et al., 1984a). Nature of the terrain prior to the Proterozoic Reworking Supracmstal protoliths The NESWR consists of many compositionally distinct granulite-facies gneisses that are concordantly inter-layered on all scales (centimeters to hundreds of metres) (Fig. 2). Gross lithological layering is designated 5, (Table 11, though in the absence of sedimentary structures has not been proved to be bedding. Finer-scale (millimetre to centrimetre scale) concordant compositional layering, defined by gneissic layering (S,) and partial melt segregations (S,,), is developed within most rock types. Intimately associated with the granmite-facies gneisses are numerous concordant granitic gneisses and pegmatites.

h7.GOSCOMBE

Approximately 65-75% of the NESWR is composed of two-pyroxene-hornblende mafic gneiss and biotite-garnet-orthopyroxene-bearing quartzofeldspathic gneiss. These gneisses have low-Ti tholeiitic and rhyolitic compositions respectively (Goscombe, 1989). Intimately associated with the low-Ti meta-tholeiites are l-100 m wide lenses of quartz-plagioclase (An,,_,,) gneisses with chemistry equivalent to plagiogranites (Coleman, 1977). Both meta-tholeiite and meta-plagiogranite have chemical affinities with oceanic crust (Goscombe, 1989). Despite the geochemical affinities with oceanic crust, there is evidence that the meta-tholeiites and meta-rhyolites constitute a supracrustal bimodal volcanic suite (i.e. they were not sills). This is based on fine (centimetre scale) concordant inter-layering of these units, the absence of both ultrabasic gneisses and cross-cutting relationships and the large proportion of meta-rhyolite versus metatholeiite (2 : 1). Meta-tholeiites show extreme Feenrichment, with no evidence of hornblende-biotite fractionation. Thus they are incompatible with talc-alkaline basalts from modern convergent environments. Consequently, the bimodal volcanics are thought to have originated in an extensional and rifting environment (Shaw et al., 1979, 1984a; Warren, 1983) possibly within continental crust, with no oceanic crust development (Bergh and Torske, 1988). Such an environment is supported by the similarity between NESWR meta-tholeiites and those from the Proterozoic intra-continental rifting environments in north Norway (Bergh and Torske, 1988), Namibia (Breitkopf, 1986) and Thessalon, Canada (Jolly, 1987). Intimately inter-layered with the metavolcanics are a variety of aluminous gneisses containing cordierite, orthopyroxene, garnet, sapphirine, kornerupine, pleonastic spinel, sillimanite, quartz, phlogopite and feldspars. Aluminous gneisses have a wide variety of compositions from highly migmatized potassium-rich metapelites to more magnesian quartz-rich granulites and orthopyroxene-rich granulites. These aluminous gneisses have protoliths of clay/mudstones and hydrothermally altered rhyolites and basalts respectively (Warren and Shaw, 1985; Goscombe, 1989). The metamorphosed hydrothermally al-

HIGH-GRADE

REWORKING

OF CENTRAL

AUSTRALIAN

GRANULITES

Middle

Pakeozoic

611 --‘& Ezl

Amphibolite and qreen schist-faties schists and mylonites formed during the Alice Springs Orogeny.

Garnet

and biotite

orthogneiss

rich granitic

(s~~-D~

I

EarlyProterozoic

cl xxx xxx

Syn-Ml granitk orthogneiss and undifferentiated migmatic quarftofe~dspathjt gneiss

cl Puartzo-feldspathic gneisses Mafic gneisses and meta-plagioqranites. ...,,.. ...*. .III ...‘. .. . ... Meta-pelitic gneisscs wifh minor quartzite,

banded

iron formation,

orthopyroxene-rich

granulite

and quartzo-feldspathic

Fig. 2. Simplified

lithological

gneisses.

map of the northeast

Strangways

Range,

after Goscombe (1991).

370

HIGH-GRADE

REWORKING

OF CENTRAL

AUSTRALIAN

GRANULITES

tered rock types are concordant, laterally extensive and intimately inter-layered with all other rock types. Such relationships and the presence of boron-rich protoliths (kornerupine-bearing gneisses) are indicative of stratigraphically controlled hydrothermal alteration in a volcanically active subaqueous environment (Goscombe, 1989). Other laterally extensive concordant gneisses characteristic of supracrustal subaqueous environments are; metaquartzite, banded iron formation, dolomitic marble and talc-silicate rock (Warren and Shaw, 1985). Ml metamorphism M, metamorphism was responsible for the total recrystallization of rocks in the Strangways Orogenic Belt to granulite-facies assemblages. M, is the highest temperature event recorded in the Arunta Block and the highest temperatures are recorded in the Strangways Range, 850-950°C at 8-9 kbar (Warren, 1983; Goscombe, 1992b). The peak of metamorphism is characterized by gn-cd-opx-Q-sill and sa-opx-cd-sill k Q assemblages in aluminous and magnesian gneisses. M, assemblages are recognized, in all rock types, as phases constituting a medium- to coarsegrained polygonal framework and also includes sub-idioblastic porphyroblastic phases such as opx, cd, sill, gn, sa, cd and kornerupine. M, metamorphism is considered responsible for the development of millimetre-centimetre scale compositional gneissic layering 6,). There is no alignment of mineral phases within this gneissic layering, and no folding is associated with S,. Thus there is no expression of deformation associated with the peak of M, metamor-

371

phism. Furthermore, all inclusions in M, phases are randomly oriented, suggesting the absence of deviatoric stress immediately prior to M,. Norman and Clarke (1990) report aligned sillimanite inclusions within garnet and correlate these to a syn-M, deformational fabric. No aligned inclusions are observed in M, garnet porphyroblasts in the NESWR, thus it is felt the sillimanite inclusions reported by Norman and Clarke (1990) are contained within post-M, garnet (Table 1) that over-grows S, fabrics (Goscombe, 1992b). Post-M, garnet is distinct from M, porphyroblastic garnet by its aggregate nature enclosing elongate domains within the S, fabric. In the more “typical” metapelites (i.e. not the Mg-rich hydrothermally altered protoliths), post-M, garnet is volumetrically significant and over-grows earlier M, garnet porphyroblasts, essentially absent of inclusions, as well as the S, fabric (Goscombe, 1992b). Consequently, it is felt there is no textural support for the proposed deformation accompanying M, metamorphism reported by Norman and Clarke (1990). All lithologies, except the most magnesian aluminous granulites, develop extensive stromitic quartzofeldspathic segregations (discussed later). These are rhythmic, parallel to S, gneissic layering and only rarely discordant to S,. They are considered to have been partial melt segregations developed during M,, and because they accentuate the gneissic layering, are labelled S,,. M, minerals are often enclosed by, and partially replaced by a multitude of coronitic and symplectitic reaction textures (Fig. 3A, B, D). The vast majority of these textures formed immediately after the peak of M, metamorphism and so called late-M, reaction textures (Goscombe,

Fig. 3. (A) Sillimanite corona development between peak metamorphic (M,) sapphirine and K-feldspar in magnesian metapelitic gneiss. Both sapphirine and corona are enclosed by a well developed S, tectonic fabric defined by fine-grained phlogopite, sillimanite and orthopyroxene. (B) M, sapphirine and late-M, sillimanite corona both enclosed by S, foliation of phl-sill-opx. Note fine-grained S, sillimanite over-growths on coarse late-M, sillimanite corona. (C) S, foliation in quartz-rich granulite enclosing partially retrogressed M, cordierite. (D) Close-up detail of the central cordierite porphyroblast in (0. Note cordierite is replaced by randomly oriented sill-opx and that the mineral grains on the outer margins of the retrogressed cordierite have been re-oriented into parallelism with the enclosing finer-grained S, foliation. All plates in plane polarized light and all bar scales are 1 mm.

372

B. GOSCOMBE

HIGH-GRADE

REWORKING

OF CENTRAL

AUSTRALIAN

GRANULITES

1992a, b). Late-M, reaction textures are commonly enclosed by the S, (M,, Table 1) tectonic fabric and nowhere do such reaction textures enclose, overprint or replace S, mineral parageneses. Consequently, both M, minerals and the late-M, reaction textures pre-date the pervasive S, tectonic fabric. This is further substantiated by the following. (1) Phases constituting the reaction textures are randomly oriented and so did not form during S, development (Fig. 3A, B, C, D). (2) Margins of corona textures are recrystallized and have under-gone grainsize reduction and mineral re-orientation within the enclosing S, tectonic fabric (Fig. 3C, D). (3) M, porphyroblasts (Fig. 4A, B) and late-M, reaction textures associated with them are totally enclosed by S, fabrics (Fig. 3A, B, C), but these same metamorphic reaction textures are not seen to enclose or over-print S, fabrics. (4) Late-M, coronitic sillimanite is kinked and develops undulose extinction where enclosed by the S, fabric (Fig. 4C). (5) M, sillimanite and M, orthopyroxene and K-feldspar including exsolution lamelli, are kinked, sheared and develop undulose erection (Figs. 4D and 5). These phases are also asymmetrically boudinaged and rotated in sympathy with the S, shear fabric (Fig. 5). Thus D, deformation post-dates both M, and the later re-equilibration that formed the exsolution lamelli. Furthermore, grain-size reduction and reorientation of late-M, phlogopite into parallelism with S, foliation, implies that hydrous retrogression also occurred before D, deformation. The textural interpretation presented here is in distinct contrast to that of Norman and Clarke

373

(1990) that report coronitic and replacive reaction textures formed during a second granulite facies metamorphism after the development of the pervasive tectonic fabric (S,). There is no textural evidence in the NESWR rocks to support such a chronology. Nor do Norman and Clarke present any evidence of coronitic and symplectitic reaction textures over-printing the S, foliation. The late-M,, me-M, metamorphic reaction textures formed in response to isobaric cooling and concomitant hydration, the “biotite stage” of Warren (1983). Late-M, reaction textures preserve evidence of cooling from 950 o C to at least 750 “C at 8-9 kbar (Goscombe, 1992b). Typical reaction textures in aluminous magnesian granulites include; gd-opx-sill-0 jr phl aggregate replacing cordierite (Fig. 3C, D), sapphirine enclosed by sill-opx symplectites and garnet and orthopyroxene replaced by opx-sill-Q rtr gd aggregate. In mafic and quartzofeldspathic gneisses M, orthopyroxene and ilmenite are enclosed by hornblende and/or garnet coronas and M, garnet and orthopyroxene is replaced by randomly oriented phlogopite. Perthitic exsolution lamelli in M, K-feldspar and kyanite over-growing sillimanite within reaction textures replacing M, cordierite, both suggest isobaric cooling continued to at least 700 ’ C (Goscombe, 1992b). It is inferred that cooling after M, continued until the the crust was thermally equilibrated, that is to approximately 550600 “C at 8-9 kbar. Thermal equilibration of a perturbed crustal geotherm typically occurs within the order of 50-70 Ma (England and Thompson, 1984). Thermal equilibration of the NESWR after M, (or at least cooling to < 700 ‘C) occurred prior

Fig. 4. (A) Intense fine-grained S, foliation asymmetrically enclosing MI orthopyroxene porphyroblasts in quartz-rich granulite. This rock is consequently dominated by fine-grained M, mineral parageneses (Q-gd-sill-opx) with only isolated relicts of M, parageneses (opx po~yroblasts). (B) Asymmetrically enclosed M, orthopyroxene porphyroblast in orthopyroxene-rich granulite, defining sinistral shear sense. S, is defined by quartz-aggregate (clear) and orthopyroxene- aggregate (grey) ribbons. In plane polarized light. (0 Bent and undulose extinction in late-M, coronitic sillimanite (indicated by arrows) after M, sapphirine. This photo is a closeup of the bottom left hand portion of Fig. 3A in crossed polarized light. (D) Kinking of peak metamo~hic (Mr) orthop~oxene in mafic gneiss and and grain boundary rec~stallization of this same porphyroblast by an insipient S, fabric. Crossed polarized light. All bar scales are 1 mm.

374

to D, deformation and the development of the pervasive S, fabric. This is supported by the following. (1) S, over-prints all late-M, coronitic, symplectitic and replacive reaction textures and these reaction textures do not over-print the S, foliation (Fig. 3A, B, C, D). (2) Exsolution lamelli within M, K-feldspar (formed by re-equilibration at temperatures < 700 o C; Morse, 1969) are sheared by consistently inclined microfaults in K-feldspar porphyroblasts boudinaged by D, non-coaxial shear (Fig. 5). (3) The late-stage development of the S, tectonic fabric is supported by the available geochronological data from S, minerals and intense S, tectonites (Woodford et al., 1975; Iyer et al., 1976) as discussed later. Consequently, in contrast to previous workers (Shaw et al., 1984a; Warren, 83; Norman and Clarke, 1990), M, metamorphism is interpreted to pre-date the regionally pervasive deformation (D,) of the Strangways Orogenic Belt by a significant period of time. The textural relationships discussed above can only be rationalized by M, and late-M, mineral parageneses being structurally reworked by D, deformation (producing the S, foliation and M, mineral assemblages) during a later tectonic period. M, partial melts and granitic gneisses

Coarse-grained (2-35 mm), homogeneous quartzofeldspathic granitic gneiss units are common throughout the NESWR. These consist of perthitic K-feldspar augen, quartz, plagioclase, garnet (or hornblende) and biotite. The granitic gneisses are intimately inter-layered with quartzofeldspathic and metapelitic gneisses on metre to tens of metre scale, as concordant stromitic sheets (Fig. 2). They also occur as conformable, highly elongate lens shaped bodies (from 15 by 80 m to 500 by 3500 m) within quartzofeldspathic, metapelitic and less commonly mafic gneiss units (Fig. 2). Granitic gneiss units only rarely have discordant contacts with the surrounding gneisses and then only on a very localized scale. Biotitequartz-rich xenoliths are rare, small ( < 20 by 40 cm) and lensoidal.

Planar, laterally extensive two-feldspar-quartz leucocratic segregations (0.5 to 10’s cm wide) are developed in all lithologies except the most magnesian aluminous granulites. Segregations are largely stromitic and concordant to both gneissic and gross lithological layering (Fig. 6A). Segregation assemblages in migmatized gneisses reflect the host-rock composition, for example, orthopyroxene porphyroblasts develop within segregations in mafic gneisses (Fig. 6C) and garnet porphyroblasts form in segregations in metapelitic (Fig. 6A) and quartzofeldspathic gneisses. This suggests that both the leucocratic segregations and incorporated porphyroblasts formed coevally by incongruent partial melting reactions (Powell, 1983) during M,. Granitic gneiss bodies are closely associated with, and grade across strike to, highly migmatized quartzofeldspathic and metapelitic gneisses. This suggests that the large granitic gneiss units also formed as partial melts during widespread migmatization (M, >. Gradations along strike, from granitic gneiss to highly migmatized gneisses and then progressively to less migmatized gneisses, are observed. This gradation in the proportion of partial melting, in conjunction with the concordant nature of leucocratic segregations, suggests an in-situ partial melting origin for the granitic gneisses. Both stromitic segregations and granitic gneiss units are boudinaged and folded on all scales by all fold generations (Table 1, Figs. 2 and 7) and they develop the regional S,-L, fabric (Fig. 6B,D). Thus segregations and granitic gneisses had crystallized prior to the earliest deformational event recorded in the NESWR CD,). Granitic gneisses define a well constrained minimum melt granite composition (Vielzeuf and Holloway, 1988) rich in SiO, (72-78%) and K,O (4.1-6.2%) and low in CaO, TiO,, Al,O, and MgO (Goscombe, 1989). The near minimum melt composition suggests that the granitic gneisses were among the earliest melts formed and that it is unlikely that large volumes of earlier formed melt was removed from the region. This is supported by the large volume of partial melt segregations, on all scales, still preserved within the terrain (Figs. 2 and 6B). For example, the proportion of leucocratic segregations in quartzofeld-

_. ^

I

Fig. 5. M, perthitic K-feldspar

_

is asymmetrically

has undulosz extinction

-

_

_.“__

bent and offset during S, formation.

boudinaged,

reductmn

.

7

_

durmg S, formation.

In crossed polarized light, bar scale IS i mm.

and undergone pmin-size

-

,_

Note exsolution

._

_

ldmelb are similarly

._

,

,

.

-

--

.”

_

-

..,

-.

La

-

I

_

_

.>

_

_*

-

(6) Stramatically

migmatized

(D) kucacrat~

metapelitic

in quartwfeldspathic

wthin

partial melt segregatmn

(bottom

S, f&km

gamd,

developed

in metawlite.

orthopymxcne

rtght hand comer).

necking of segregation

in

S,

Pale area on bottom right is freshly er~nsed rack. The segregation is sheared and displaced by the S2 fabric.

in leucosome.

is presented

and weakly developed layermg (close-up of the same relationship

garnet in leucosome. at low angles Lo migmatmc

gneisr. Note pmphyroblastic

mafic gneiss. note porphyrobiaslic

xgregations

containing porphyroblasbc

wthm

leucrrratic

gnelss, note intense penetrative

parflal melt segregation

plrphymbla*b

Lens cap 1s 6 cm m diameter.

(C) Quartz-plagx&re

Fig. 6D). AIK, note garnrl

development

to leuwsome.

Fig. 6. (AI Partial melt segregation

tectonic fabrxc sub-parallel

HIGH-GRADE

REWORKING OF CENTRAL AUSTRALIAN

-----

GRANULITES

Lithological (So) layering trace

Fold axial

trace

-

Fr

--m-

F3

----+

and dip

Plunge axis

*

52

and plunge direction

shear

of fold

sense

Sense of fold closure h

”

z “1

cs>

Antiform Overturned

Overturned

Arrow

to

of sheath, horizontal

t..:_:“’

antiform

Synform synform

terminal dOSUre ~I’tose”) projected onto plane

Retrogressive shear zones reactivated duripg the Alice Springs

Orogeny kliddle Palaeozoicf.

Fig. 7. Simplified D,-D,

structural map of the northeast Strangways Range.‘te

379

spathic and metapelitic migmatites range from 25% up to approximately 100%. Near 100% insitu melting of quartzofeldspathic gneisses to form map-scale granitic gneiss units is illustrated by the presence of indistinct “ghosted” gneissic layering in portions of the granitic gneiss bodies. Near 100% in-situ melting is further supported by the granitic gneisses of the Strangways Range having very similar composition to that shown by the host quartzofeldspathic gneisses (Collins et al., 1988; Goscombe, 1989). No pre-D, plutonicintrusive granitic gneiss bodies are recognized in the Strangways Range, all are intimately associated with and have not separated from, their source rocks (Collins et al., 1988; Goscombe, 1989). The concordant nature and tabular to lensoidal shape (Fig. 2) of granitic gneiss bodies is due to near complete melting of gneisses closest to minimum melt compositions (i.e. quartzofeldspathic gneiss). Granitic melts typically rise as plutons to higher crustal levels as a result of their lower viscosity and density with respect to the surrounding gneisses. Thus the preservation of large volumes of concordant sheets of melt at depths of 25-28 km during and.immediately subsequent to M,, is enigmatic. Their preservation suggests that the viscosity contrast between them and the surrounding gneisses was not great. This may be due to the high metamorphic temperatures during M {. giving rise to quasi-solid host gneisses. Because of the similar composition of granitic gneisses and host quartzofeldspathic gneisses, the density contrast between melt and host is unlikely to have been large. This concept is illustrated by the more discordant nature of partial melt segregations in mafic gneisses (Fig. 6C), where presumably a greater density and viscosity contrast existed between the host gneiss and partial melt. Granitic melts act as viscous fluids and so flow under deviatoric stress and are thus typically mobilized from their site of origin. The preservation of large volumes of concordant planar granitic gneiss units in the Strangways Range, suggests the region was not under any deviatoric stress (i.e. anorogenic) during M, and until these melts had crystallized. ~ternatively, their planar stability may have been preserved in an environment of

non-coaxial shear. However, this is not supported by the coarse-grained granoblastic and porphyroblastic nature of M, textures in all gneisses including Ieucocratic segregations and granitic gneisses. Furthermore, there is an absence of aligned kinematic fabrics pre-dating the pervasive S,--L2 tectonic fabric that overprints and partially recrystallizes leucocratic segregations and granitic gneisses. The randomly oriented nature of secondary biotite and phases within late-M, coronitic and symplectitic reaction textures is also suggestive of anorogenic conditions continuing for some period subsequent to the peak of M, metamorphism. The A-type geochemistry of Stran~ays Range granitic gneisses (Goscombe, 1989) further supports anorogenic stable cratonic settings (Collins et al., 1982). Early Proterozoic geochronology

Sm-Nd whole-rock isochrons, from both mafic and quartzo-feldspathic gneisses, consistently range between 2000 and 2100 Ma (Table 2) and are interpreted as mantle separation ages (Windrim et al., 1984). Rb-Sr whole-rock isochrons from the supracrustal granuhte-facies gneisses range 1780-1820 Ma (Table 1) and are considered representative of ~~stal~izat~on of these gneisses during M, (Black et al., 1983). Rb-Sr 1670-1775 Ma whole-rock ages are obtained from migmatites and granitic gneisses (Black et al., 1983; Mortimer et al., 1985; Windrim and McCulloch, 1986) (Table 2). These ages are not considered the age of melt formation (in-situ melting during M,), but are interpreted by the respective authors as minimum ages of the crystallization of the melts. The 1670-1775 Ma minimum ages are consistent with the crystallization of M, granitic melts occurring during the period immediately after the peak of M, metamorphism (1780- 1820 Ma). Consequently, there is no need to invoke a new tectonic event, the Aileron Event (Shaw et al., 1984a), for these isotopic ages because M, metamo~hism and the formation of partial melts followed by their crystallization are all of the one tectonothermal cycle. 1670-1720 Ma Rb-Sr whole-rock isochrons from a variety of metapelitic gneisses (Windrim

HIGH-GRADE

REWORKING

OF CENTRAL

AUSTRALIAN

GRANULITES

and McCulloch, 1986) (Table 2) can, at best, only be interpreted as the minimum age of biotitephlogopite crystallization because the majority of K and Rb is contained within phlogopite-biotite. Similarly, 40Ar/39Ar degassing ages (1690-1720 Ma) from secondary hornblende (Allen and Stubbs, 1982), represent either the age of secondary hornblende crystallization (Dallmeyer, 1979) or a cooling age due to a blocking temperature of approximately 600 ’ C, depending on grain size and rate of cooling (Harrison, 1981). Consequentially, the 1670-1720 Ma isotopic ages in Table 1 are considered to be the minimum age of crystallization of secondary hydrous phases, the “biotite stage” of Warren (1983). The 1670-1720 Ma minimum age range for hydrous retrogression overlaps with the minimum age of granitic gneiss crystallization. Thus the hydrous retrogression event is compatible with being due to fluids released during and immediately after crystallization of M, granites and partial melt segregations (Hensen and Warren, 1985) . The Proterozoic Reworking tectonothermal cycle D,-D,

ductile deformation

D, structural elements

Mesoscopic (centimetre to meter scale) F2 isoclinal folds are both intra-folial and fold lithological layering (S,,). F2 folds of all scales and styles are defined by the presence of a strong fold axial planar fabric (S,) (Fig. 8D). They plunge steeply (60-80”) to the east-southeast, are steeply inclined to the southeast (Fig. 9) and have rounded and thickened hinges with attenuated limbs. Mesoscopic Fz sheath folds with flattened ellipse shaped sections in the plane orthogonal to their long axis are common (Goscombe, 1991). Elliptical sections orthogonal to the long axis of the sheaths have dimensions of l-8 cm by 2-20 cm. Sheaths are highly elongate parallel to the regionally pervasive mineral elongation lineation CL,) and mesoscopic sheaths are up to 50 cm long. Map scale F2 structures occur as SE-plunging and inclined isoclinal folds with axial surfaces orientated parallel to the regional S, foliation

381

and with fold axes sub-parallel to the regional L, orientation. F, structures also occur as kilometrescale closed outcrop patterns that have flattened ellipse shaped horizontal sections (Figs. 2 and 7). These structures define an elongate flattened sheath fold geometry in three dimensions, with sub-parallel marginal hinges (Goscombe, 1991) (Figs. 2, 7, 10). Like mesoscopic sheath folds, the plane that these structures are flattened in is parallel to the regional S, foliation and their axis of elongation is parallel to L,. Boudinage contained wholly within the plane of S, is common on all scales, and for example involved separation of gross lithological units as well as porphyroblasts of garnet, feldspars and pyroxenes formed during Mr. Boudins are separated and ductilely stretched into elongate lozenges (X/Z 2 4 to 7 in boudinaged M, porphyroblasts), along a vector parallel to the mineral elongation lineation (LJ; thus defining L, as the vector of maximum elongation. Asymmetrical boudinage (Etchecopar, 1977) is also encountered on outcrop and mineral grain scale (Fig. 5). Asymmetrical boudinage is accommodated by normal displacements along consistently inclined microfaults, with the major axis of extension being parallel to L,. The regional elongation lineation CL,) is defined by elongate quartz-aggregate lozenges, boudinaged feldspar, orthopyroxene and garnet porphyroblasts, aligned sillimanite needles and elongate pressure shadows of fine-grained granoblastic aggregates enclosing M, porphyroblasts (Fig. 5). Despite two subsequent fold generations, L, orientation is tightly constrained throughout the region; plunging 60” towards 100” (Fig. 9). Thus L, accurately outlines the direction of transport during D,. The regionally extensive tectonic foliation (S,) is defined by intense grain-size reduction of preexisting coarse-grained M, minerals (Fig. 4B). S, is defined by preferred shape orientation of biotite and sillimanite and by laterally persistent ribbons of fine-grained quartz-, feldspar-, gametand pyroxene-aggregate (Fig. 8A). S, typically parallels the gneissic and lithological layering (S&r) (Fig. 9), but overprints S,-S, in 1;2 fold hinges. S, is coeval with F2 folding because of its

Y

HIGH-GRADE

REWORKING

So-S1

F2:

&

Iyer

OF CENTRAL

parallel

axial , plane

AUSTRALIAN

S2, poles.

I

plane

F3: fold axis. I

Fig. 9. Summary of structural data from throughout the region mapped. All data plotted on lower hemisphere equal area stereonets, contour intervals are 0.5, 2.5 and 4.5% per 1% area. Note that data has not been unfolded of F, folding effects. For example, the majority of S, data is from the southwest portion of the map area, the whole area of which is contained on one limb of a major F4 fold (Figs. 2 and 12). In contrast, few F, fold axial plane data are from this same region, thus the S, data are more widely distributed than the F2 fold axial plane data.

fold axial (Fig. 8D) mylonitic gneisses,

whole region mapped. Grain refinement is so intense in metapelites, quartzofeldspathic gneiss and orthopyroxene-rich and quartz-rich granulites, that S, is commonly the only recognizable planar fabric. Such samples have a texture of isolated M 1 porphyroblasts asymmetrically enclosed by a fine-grained foliated and lineated matrix (Figs. 3A, 4A,B). D,, type-l S-C mylonites (Berthe et al., 1979) are common in the least deformed tectonites, the most intense S, fabrics developed are interpreted as having coplanar S,- and C-planes. In addition to grain boundary recrystallization of pre-existing phases, new fine-grained minerals are developed as aggregates within the S, fabric. The typical S, CM,) assemblages in orthopyroxene-rich granulite, quartz-rich granulite and other aluminous gneisses are sill-opx-Q k phl and sill-opx-gd-Q f phl (Fig. 3C,D). In mafic gneisses the M, assemblage is typically opx-cpxhn-pl-Q f phl and in quartzofeldspathic gneiss, gn-Q-pl-phl-kfld f opx and gn-Q-pl-phl-kfld f hn. M, mineral grains in the S, fabric are annealed to fine-grained, aligned, polygonal textures with no internal deformational features (such as undulose extinction and kinking) preserved in individual S, mineral grains. Thus, though formed in response to D, deformation, M, assemblages were subsequently annealed at the peak of the second metamorphic cycle CM,M,). Fine-grained idioblastic garnet envelops S, fabrics in some aluminous granulites. This garnet has been labelled post-M, garnet (Table 1) (Goscombe, 1992b) and has not been specifically correlated with either of D,, D, or D, deformations. Post-M, garnet is considered to have formed near the peak of the second metamorphic cycle (Goscombe, 1992b), which possibly coincides with D4 deformation (as discussed later).

...: . . ~:~~* * ,. . 0 F2: fold a,xis.

n=37

F3: axial

383

GRANULITES

planar development on both outcrop and map scale (Fig. 9). S, fabrics are to proto-mylonitic in the majority of except mafic gneiss, throughout the

D, structural elements

D, deformation is characterized by tight to isoclinal folds without penetrative fabric development. F3 folds occur on all scales (millimetres to kilometres), have rounded hinges and less pronounced hinge thickening than F, folds, F3 sheath folds, of all scales (mm-km), are present

3x4

in the area mapped. Mesoscopic F3 sheath folds with irregular cross-sectional shapes, are most abundant in the tightest macroscopic F3 hinges. Like F2, map scale F3 sheaths have flattened ellipse shaped cross-sections. F3 folds are nearly co-planar and co-linear to F, folds (Fig. 9). Both generations of fold axes are contained within the plane of S,. Like F2 folds, F, fold axes are scattered symmetrically around the average L, orientation. Macroscopic refolding of F2 folds by F3 folds, both having extremely non-linear fold axes (in the case of the sheath folds) within co-planar fold axial planes, gave rise to complex fold interference patterns (Figs. 2 and 7) (Goscombe, 1991). Fold interference patterns are broadly similar, but far more complex, to a dome and basin pattern that has been flattened in the S, plane and stretched parallel to L, so that all limbs and hinges of F2 and F3 structures are sub-parallel, except in the region of the terminal closure of sheaths (Goscombe, 1991). No new mineral phases crystallized during D,, nor was there any grain refinement. Aligned biotite platelets, quartz-aggregate and feldspar-aggregate ribbons, all defining S,, are isoclinally folded by F3 folds. However, no undulose extinction or kinking of S, mineral grains is apparent. Furthermore, there is no alignment of S, mineral grains with the F3 fold axial plane (Goscombe, 1991a). Thus, S, mineral grains cannot have been totally recrystallized during D, and the deformational features they presumably inherited during D, must have been annealed out subsequent to F3 folding. D,-D,

partial melts

Coarse-grained (l-6 mm) anhydrous hvofeldspar-quartz leucocratic segregations are developed in a variety of structural relationships. All of which cross-cut earlier formed layer parallel partial melt segregations (S,,) and gneissic layering (S,). These post-M, leucocratic segregations occur as; thin (3-12 mm) planar segregations that are sub-parallel to the axial surface of mesoscopic F2 folds (Fig. 8B), as elongate shaped bodies aligned parallel to the long axis of mesoscopic sheaths and as infilling between boudins

(Fig. 10). As such, all these segregations are associated with D, deformation and are aligned largely within the XY plane of the strain ellipsoid. Alignment of all these post-M, segregations with D, structural elements, suggests that minor anhydrous melting occurred during D,. Furthermore, some of these segregations develop S2-L,, fabrics, while the majority do not, suggesting the melts crystallized after D,. Coarse-grained (l-20 mm), opx-kfld-pl-Q leucocratic segregations in quartzofeldspathic gneiss, cross-cut F3 structures and lack S, fabrics (Fig. 80 These segregations are interpreted to be late-D, partial melts. Their presence suggests that partial melting continued subsequent to D,D,. Thus, the peak of metamorphism of the Proterozoic Reworking metamorphic cycle CM,-M,) is interpreted to post-date D,-D,. Consequently D,-D, deformation occurred during a prograde P-T path (Goscombe, 1992b). Nature of D,-D,

deformation

A detailed analysis of the strain regime of D,-D, deformation has been presented by Goscombe (1991). D, and D, structures, on all scales, have preserved evidence for non-coaxial (rotational) shear (Fig. 10). Boudinage is commonly governed by slip along high angle antithetic microfaults with normal displacements. Such a geometry must involve rotation of the individual boudins (Etchecopar, 1977) in response to non-coaxial shear (Simpson and Schmid, 1983; Lister and Snoke, 1984). S, asymmetrically encloses u- (Fig. 4b) and S-type M, porphyroblasts, both cases involve relative rotation between the porphyroblast and enclosing foliation and are thus indicative of non-coaxial shear (Passchier and Simpson, 1986). Type-l S-C fabrics are also indicative of non-coaxial shear (Berthe et al., 1979). The majority of intensely foliated gneisses are interpreted to have co-planar S-C fabrics that have formed by high bulk shear strains in a non-coaxial deformational environment (Simpson, 1984). On a regional scale, both F2 and F3 fold axes are symmetrically scattered around L, and contained within the S, plane (Fig. 9). Fold axis distribution, as well as the presence of sheath

HIGH-GRADE

REWORKING

OF CENTRAL

AUSTRALIAN

GRANULITES

Fig. 10. Schematic block diagram summarizing mesoscopic (insert) and macroscopic D, structural elements. Axes of the D, strain ellipsoid (X-Y-Z) is related to D, structural elements (after Nicolas and Poirier, 1976). Crosses are partial melt, dotted regions are porphyrohlastic M, mineral phases and solid (black) units are compitent rock types, for example mafic gneiss. Modified after Goscombe (1991).

folds, is consistent with extreme rotation of fold axes towards L, with progressive non-coaxial shear of high bulk shear strains (Cobbold and Quinquis, 1980; Berthe and Brun, 1980; Lacassin and Mattauer, 1985). Furthermore, non-sheath like mesoscopic F2-F3 folds are asymmetrical and commonly intra-folial, thus implying noncoaxial shear (Berthe et al., 1979). Co-planarity and co-linearity of F2 and F3 fold axial planes and fold axes (Figs. 7 and 9) suggest that both D, and D, are of the same progressive shear deformation and that L, represents the vector of transport. Shear sense during the development of S,-L, fabrics is consistently east over west after unfolding F4 folds (Fig. 7). Sense of shear is defined by S-C relationships (Simpson, 1984) and by asymmetrically enclosed and boudinaged porphyroblasts (Passchier and Simpson, 1986). Bulk shear strain during D,-D, is inferred to have been very high because of the common presence of sheath folds on all scales (Quinquis et al., 1978; Berthe and Brun, 1980), the co-lin-

385

earity of F2 fold axes with L, (Coward and Potts, 1983) and the flattened ellipse shaped horizontal sections through the sheaths (Lacassin and Mattauer, 1985). High bulk-shear strains are further supported by co-planar S-C fabrics (Berthe et al., 1979). Extreme asymmetrical stretching of garnet, orthopyroxene and feldspar porphyroblasts (Fig. 5) and the development of ribbons consisting of aggregates of these typically more competent minerals, illustrates the high temperature, high shear strain and ductile nature of D, deformation. The high shear strains were inhomogeneously partitioned across the region mapped. Metapelite, quartz-rich granulite and quartzofeldspathic gneiss experienced intense grain refinement and are typically mylonitic to proto-mylonitic throughout the whole region. Mafic gneisses have only incipient S, fabrics (Fig. 4D) and partitioned much less strain. Significantly, all lithological boundaries are mylonitized, suggesting zones of layer parallel shear between units of differing rheology. M, partial melt segregations (S,,) and S, gneissic layering are displaced along L, within the S, plane (Fig. SD). Consequently, layer parallel S, mylonites are considered true mylonites (Lister and Snoke, 1984) and thought to be zones of layer parallel displacement. The steeply inclined nature of the entire Strangways Orogenic Belt is interpreted to be the result of D,-D, deformation (Goscombe, 1991). F,-F, fold repetition was responsible for rotating previously recumbent M, gneisses into steep (average 60 o in the NESWR) easterly inclined orientations. Such a mechanism is in contrast to a model of steeply inclined orientation resulting from late-stage tilting or mega-warping. Reorientation of gneisses by F2-F3 folding gave rise to a steeply inclined terrain while these gneisses remained essentially at the same crustal level (Fig. 11). As a corollary of D,-D, producing inclined structures, deformation can only have occurred with concomitant crustal shortening and so must have involved compressional tectonics and crustal thickening (Goscombe, 1991). M,-M, mineral parageneses record > 3-4 kbar of decompression subsequent to D,-D, (Goscombe, 199213). Thus it is estimated that a > 45-48 km

thick crustal column was supported during D,-D,, if the isostatically equilibrated “normal” crustal thickness is assumed to have been 35 km (Turcotte and Schubert, 1982) prior to deformation. L, lineations define a consistent W- to SWtrending vector of transport throughout the Strangways Orogenic Belt as far east as the South Harts Ranges (Shaw et al, 1984b; Goscombe, 1984; Norman and Clarke, 1990). Consistent transport direction and sense of shear, in conjunction with identical fold style, fold orientation and nature of tectonic fabrics throughout the Strangways Orogenic Belt (Goscombe, 1984; Shaw et al., 1984b; Shaw and Langworthy, 1984; Norman and Clarke, 19901, suggest that D,-D, was responsible for the major ductile deformations of, at least, the Strangways Orogenic Belt part of the central Arunta Block (Fig. 1). Consistent shear sense over such a large region can only be rationalized by some form of crustal scale over-riding of the east-northeast over the west-southwest. This model of crustal over-thickening is supported by the accompanying metamorphic analysis of the region (Goscombe, 1992b). D,-D,

transpressional episode

D, structural elements

D4 involved large-scale open to tight folding (F,) with wave-lengths of 0.5 m to 3.0 km. F4 structures are upright to steeply (80-90”) inclined either north-northeast or south-southwest and plunge 50-80” to the east-southeast (Fig. 12). Such folds contribute significantly to the map outcrop patterns of the Strangways Orogenic Belt. In the most northern area mapped, F4 folds are open and more symmetrical. In the south, F4 folds are tighter and asymmetrical with steep southern limbs. Fold asymmetry defines a fold vergence (Bell, 1981) to the south-southwest and is suggestive of the transport during D, being from north-northeast to south-southwest. Mesoscopic F4 folds are developed in hinge regions and on the steepest limbs of macroscopic F4 folds. A weak, discontinuous quartz-aggregate fabric (S,) is developed co-planar to the axial surface of

Fig. 11. Schematic cartoon illustrating the Proterozoic evolution of the crustal lithosphere from the peak of metamorphism CM,) through the initial stages CD,-D,) and latter transpressional phase CD,-D,) of the Proterozoic Reworking. Solid line in D,-D, diagram, represents the palaeo-M, 8-kbar isobar after being folded during D, -D, deformation. Dashed line in final diagram represents the 8-kbar isobar at the thermal peak CT,,) of the Proterozoic Reworking metamorphic cycle (Mz-Ms). Star represents the relative vertical position of the NESWR rocks presently exposed at the surface.

mesoscopic F4 structures (Fig. 13A). No other mineral phases are aligned parallel to S,, thus S, is only developed in quartz-rich lithologies such as leucocratic segregations and quartzofeldspathic gneisses. This penetrative fabric trends 90-120 o and on a regional scale is parallel to the fold axial trace of macroscopic F4 folds (Fig. 12). Surface lineaments in quartzofeldspathic gneiss units, observable on aerial photographs, trend approximately 100” (Fig. 12); these are expressions of S, controlling the weathering pattern. S,, though very steep, dips predominantly to the north, this reflects the asymmetry of F4 folds in the southern part of the map area. S, is best developed in the tighter F4 folds of the south. Steep, E-plunging intersection lineations CL,) between S, and S, surfaces, parallel F4 fold axes.

HIGH-GRADE

REWORKING

OF CENTRAL AUSTRALIAN

\(

-__--_ -__ ‘4

b4 % -B--H+

&

Sg-St

& S2 trace.

S4

inclined,

s4

I race.

s5

inclined, ‘4.

S5

shear

F4

d0 Fq

vertical.

vertical..

sense.

axial

trace,

fold

axis,

Ketrogrtssive ated

387

S2

% -3

‘St

GRANULITES

during

inclinatioll. vertical.

shear the

zones Alice

reactive Springs

Orogeny. 0

l

Fig. 12. Simplified D,-D,

F4

axis

F4

plane

S4

pole

structural map of the northeast Strangways Range. D4 structural data is presented on a equal area lower hemisphere stereonet.

HIGH-GRADE

REWORKING

OF CENTRAL

AUSTRALIAN

389

GRANULITES

Syn-D, igneous bodies Syn-D, pegmatite veins, of wideiy varying compositions, are scattered throughout the NESWR. They are small (5-50 cm wide), vertical discordant veins consisting dominantly of coarse-grained (0.5-4 cm) perthitic K-feldspar with interstitial quartz, tourmaline, plagioclase, biotite, sillimanite and orthopyroxene. These veins trend 90-100 * and are so-tectonically emplaced into E-Wtrending zones of ductile shear (Fig. 13B). Discontinuous E-W-trending fabrics (S,), defined by elongate quartz-aggregate lenses, are well developed in syn-D, pegmatites. All primary mineral grains display minor rec~stall~ation and quartz grains preserve undulose extinction. A large (300-800 m), E-W-trending elongate garnet-biotite-rich granitic orthogneiss body is enclosed within and dissected by the Wallaby Knob Schist Zone (Fig. 2). This granitic orthogneiss is composed of quartz (40%), K-feldspar (25%), plagioclase (25%), garnet (58%) and green-brown biotite (5%) and minor magnetite. Block shaped inclusions of plagioclase within euhedral K-feldspar phenocrysts, in conjunction with the homogeneous and unlayered nature of this body, attests to a magmatic origin. No migmatites have been found associated with this body. It is thus interpreted to have moved from its site of generation and was emplaced as a pluton, though the margins with surrounding gneisses have been obliterated by the development of the Wallaby Knob Schist Zone. An E-W-trending vertical tectonic fabric within this granitic orthogneiss, parallel to the regional S, fabric, is defined by aligned biotite platelets. Elongate garnet-biotite-aggregates are aligned with S, and attest to the syn-(D,) emplacement and crystallization of this granitic orthogneiss. Late-stage leucocratic K-feldspar-rich

dykes (up to 35 cm wide) are vertical and also develop S, fabrics. Assuming that these dykes were originally emplaced vertically, this granitic orthogneiss body is inferred to have not been significantly rotated subsequent to crystallization. Thus this body must post-date D,-D, and emplacement must have occurred prior to, or syntectonically with, D4. The lenticular shape of this granite body, prior to dissection by the Wallaby Knob Schist Zone, in conjunction with the development of a vertical E-W-trending syn-tectonic fabric, is typical of granites emplaced into environments of oblique shear (Hutton, 1982; Guineberteau et al., 1987). Furthermore, its composition (Goscombe, 1989) is very similar to post-collisional granites emplaced during latestage strike-slip shearing in the Alaskan Cordillera and New England and Lachlan Fold Belts (Sylvester, 1989). Elsewhere throughout the Strangways Orogenie Belt, granitic orthogneiss bodies that have been emplaced into the cores of upright open folds (F,) have been recognized; their emplacement has been interpreted as being syn-tectonic with Ir;k folding (Shaw et al., 1979). All syn-D, granitic melts were intrusive plutons. Thus these granites were sourced at lower crustal levels than presently exposed and are suggestive of a perturbed geothermal gradient at this time. The large volume of D4 melt recognized in the Stran~ays Orogenic Belt, relative to syn-D,-D, melt, is suggestive of the peak of the Proterozoic Reworking metamorphic cycle CM,-M,) being coeval with D4.

E-W- to SE-NW-trending S, shear zones occur as steep (70-90 ’ 1, N- and S-dipping (Fig. 14) planar mylonite and ultra-mylonite zones of gran-

Fig. 13. (A) F4 refolding of a S2 mylonitized metapetitic gneiss. S, is defined by garnet-aggregate and quartz-aggregate ribbons. Note discontinuous quartz-aggregate fabric (&I, developed parallel to F4 fold axial plane. (3) E-W-trending pegmatitic vein in layered quartzo- feldspathic gneiss, emplaced syn-tectonically with D,,-Ds shear. 0 Plane-polarized ~~ophotograph illustrating the mylonitic fabric, and large degree of grain-size reduction, in Ds shear zones in mafic gneiss. Relic Mt ~~h~bl~ts are a~etri~ly enclosed by S,. (D) Region of tight, upright Fe folding in metapelitic gneiss and the development of a relatively strong fold axial planar fabric (S,). Parallel to S, is thin localized zones of intense recrystallization and grain refinement with small displacements; these are Ss.

Fig. 14. (A) D, structural data plotted on a equal area lower hemisphere stereonet. Dots are L, and planes are S, with sense of shear indicated. (B) Palaeo-D, stress directions derived by fault analysis of movement vectors (I,,) within S, shear planes, after the method of Angelier ef al., (1982) and Angelier (1984). Shaded regions are the range of 2a errors.

ulite to upper-amphibolite-facies grade. They are typically thin, ranging in width from 0.2 to 20 cm and are common throughout the NESWR (Fig. 12) particularly in mafic and quartzofeldspathic gneisses. Internal foliation (S,) parallels the boundary of these zones. S, mylonites and ultramylonites display high degrees of grain refinement (70-lOO%), with marked grain-size reduction to between 0.005 and 0.08 mm (Fig. 13C). S, fabrics in mafic gneisses are defined by elongate ribbons consisting of granoblastic aggregates of hornblende and pyroxenes (Fig. 130, these abut syn-tectonic, fine-grained sub-idioblastic garnet grains. In quartzofeldspathic gneisses, S, is defined by quartz- and feldspar-ribbons composed of fine-grained equigranular granoblastic subgrains, and by biotite platelets that both abut and enclose fine-grained sub-idioblastic garnet grains. Typical assemblages are: cpx f opx-gn-me-hnpl-Q in mafic gneiss and gn-opx-sill-hn-pl-Q and gn-opx-cpx-hn-pl-Q in quartzofeldspathic gneiss (Goscombe, 1992b). A mineral elongation lineation CL,) occurs within the shear zone fabric. L, plunges steeply (50-60 Q) towards the northeast to east-northeast

(Fig. 14) and defines the movement vector for the last episode of D, movement. Displacement parallel to L, is typically small, ranging from 0.5 cm to 50 cm. S, shear zones are extremely abundant, though the true spatial density in the NESWR is unknown, the concentration is conservatively estimated at being one shear zone per metre along a N-S section. Thus, despite such small displacements, the high spatial frequency of S, shear zones suggests the accumulative effect of strain during D, could have been large. Sense of oblique shear, defined by asymmetrically enclosed a-type porphyroblasts and finite displacements, is typically reverse and sinistral (Figs. 12 and 14). Predominance of the one shear sense, lack of conjugate shear planes and well constrained co-planarity and co-linearity of S, and L, orientations (Fig. 141, suggests that bulk non-coaxial deformation (Harris and Cobbold, 1984) operated during DY Significantly larger (tens of metre- to kilometre-scale width) E-W-trending steep shear zones of amphibolite-facies grade are distributed within and bounding the Strangways Orogenic Belt, for example the Redbank and Harry Creek Deformed zones and the Gough Dam and Wallaby Knob Schist Zones (Fig. 1). Though these shear zones were all extensively re-activated during the Mid-Palaeozoic Alice Springs Orogeny (Marjoribanks, 1976; Shaw et al., 1984; Teyssier, 1985), the Redbank and Harry Creek Deformed Zones have been shown, both by structural over-printing and by isotopic evidence, to have a Proterozoic pre-history (Marjoribanks and Black, 1974; Shaw and Black, 1991). The Harry Creek Deformed Zone initially experienced north over south thrusting, of up to 10 km, with a component of sinistral transcurrent movement of possibly 60 km prior to emplacement of the Gumtree Granite at 990 f 13 Ma (Allen and Black, 1979). Hydration accompanying mylonitization in the Redbank Deformed Zone has been dated at approximately 1400-1500 Ma by Rb-Sr isotopic dating (Shaw and Black, 1991). S, shear zones of the NESWR are correlated with the larger E-W-trending shear zones of the central Arunta Block on the basis of the common E-W trend, steep north inclination and oblique sinistral thrusting shear sense in the

HIGH-GRADE

REWORKING

OF CENTRAL

AUSTRALIAN

Proterozoic. Though no pre-Palaeozoic kinematic fabrics have been found in the Wallaby Knob Schist Zone; this zone is thought to have also been localized by pre-existing S, shear zones (Goscombe, 1989). Nature of D,-D,

transpression

S, shear zones cross-cut

391

GRANULITES

F4 structures and so post-date D,. However, near co-planarity of S, with F4 axial surfaces and progression from intense S, fabrics to S, shear bands (Fig. 13D), at least in outcrop, infer a close temporal relationship between these two events. On a larger scale of observation, regions with relatively strong S, development have a higher spatial concentration of S, shear zones. These relationships suggest that S, shear zones are localized developments of S, in zones of high shear strain. Thus D, and D, are interpreted to have formed together in the same progressive non-coaxial deformational environment. Relative orientation between S, and S, fabrics as well as fold asymmetry and S, shear sense, are all consistent with sinistral oblique transpression (Fig. 15) within an E-W-trending regionally extensive zone of shear (Sanderson and Marchini, 1984). The inclined nature of these structures and oblique movement vectors suggest an inclined, oblique transpression system involving northeast over southwest over-thrusting. The tightly constrained co-planarity of S, planes suggest, as a corollary of bulk non-coaxial deformation, that S, represents primary shear bands CD) and that Riedel (R) and thrust shears (P) (Tchalenko, 1968; Mandl et al., 1977) have not been developed or recognized. Thus the average S, orientation is considered representative of the orientation of the boundary of the regional zone of transpression during the D,-D, episode (Fig. 15). The low angle between the average F4 fold axial surface and the average S, orientation (30 “), is consistent with models of sinistral wrenching with a component of shortening across the zone of shear, this is transpression (Harland, 1971) (Fig. 15A, B). Fault analysis (Angelier et al., 1982; Angelier, 1984) of all S,-L, data for which the sense of shear is available, is presented in Figure 14. De-

spite the small data set (n = 111, resultant palaeoD, stress axis orientations are very well constrained using 70% minimization of the data. ui is horizontal and trends 105 O, a, is essentially vertical and a, plunges < 8 o towards 015 ‘. The palaeo-D, a,-orientation is considered representative of the regional scale or stress direction at this time. This is based on there being no evidence for significant rotation of the NESWR subsequent to D,-D, (as previously discussed). The relationship between the derived palaeoD, stress directions and the corresponding strain ellipsoid is presented in Figure 16. The D, strain ellipsoid, as defined by Nicolas and Poirier (19761, is X parallel to L, and Z parallel to the axis of D,-D, shortening (defined by F4 folding), that is, the XY plane is co-planar with S, (Fig. 15). F4 fold morphology and orientation, S, and S, orientation and sense of shear and the palaeostress-strain relationships described above, can only be rationalized by oblique sinistral transpression during D,-D, (Fig. 15). A plunging a, orientation implies that transpression was not in an upright orientation, but must have been inclined and involved over-riding from the northeast to southwest, as suggested by the asymmetry of F4 structures and consistent D, sense of shear. Age of Proterozoic Reworking

The broadest constraints on the age of the Proterozoic Reworking (D,-Ds) are: deformation must have been younger than the minimum age of crystallization of deformed granitic gneisses (1670-1775 Ma) (Black et al., 1983; Windrim and McCulloch, 1986) and older than the Gumtree Granite, of 990 Ma age (Allen and Black, 1979), that post-dates the initiation of the Harry Creek Deformed Zone. Analysis of published geochronological data from the Strangways Orogenic Belt further constrains the Proterozoic Reworking to be of approximately 1400-1500 Ma age. 1400-1500-Ma Rb-Sr whole-rock isochrons have been obtained from D, tectonized quartzofeldspathic gneisses in the NESWR by Iyer et al. (1976) (Table 2). Quartzofeldspathic gneisses in the NESWR have experienced high degrees of D, recrystallization (70-100%) to give rise to

A.

unto

horiranlal T.”

ss

s Trend

shear

of boundary of 2cme of regional as defined by Sg.

.._:.,. .._,: Orientation of zone of transpression _::.::.__ as defined by F4-S4. ___ . .. Average S. trend.

-.-.-

Average

F4 axial

-

Average

Sg treild.

B. Transpressiou. oc’<

I.

c. Classical Wrenching. Q&

1

a Transtension. o

I

-

trace.

HIGH-GRADE

REWORKING

OF CENTRAL

AUSTRALIAN

(Parallel

GRANULITJSS

to X)

Fig. 16. Schematic block diagram summarizing D,-Ds structural elements, and the Ds stress directions derived from 5s shear zones (Fig. 14) are indicated by round arrows. The D,-D, structural elements are related to the general strain ellipsoid (X-Y-Z; Nicolas and Poirier, 19761, and the inferred shear regime of northeast over southwest inclined, oblique transpression is indicated by blocky arrows.

intense S2 fabrics (as previously discussed). Near total recrystallization of these samples is considered to have given rise to near total isotopic re-equilibration. Thus, these 1400-1500 Ma ages are considered representative of the pervasive S, fabric in the NESWR. Mafic gneisses in the same region, which underwent minimal recrystallization during D,, retain a 1800 Ma (M,) Rb-Sr whole-rock isochron (Iyer et al., 1976). The disturbance of peak M, metamorphic isotopic systems (1780-1820 Ma) by a tectonother-

393

ma1 event of approximately 1400-1500 Ma age is further supported by 4oAr/39Ar stepwise degassing mineral spectrums from the south Strangways Range (Allen and Stubbs, 1982) and in the NESWR (Woodford et al., 1975) (Table 2). Phlogopite-biotite spectrums define good plateau ages of 1479 Ma. Phlogopite 4oAr/39Ar spectrums have been shown to be reset at temperatures of 550600 ’ C in fast cooling terrains (Harrison and McDougall, 1980). Thus, phlogopite plateau ages are interpreted, after the method of Turner (1968), to be the blocking age due to cooling through 550600 ‘C after a tectonothermal event that either totally degassed or recrystallized this phlogopite. Primary (M,) hornblende and pyroxene grains preserve 1800 Ma plateau ages with well defined 1450-1500 Ma saddles (Allen and Stubbs, 1982). Dallmeyer (1979) suggests that 40Ar/39Ar homblende ages are not totally reset without complete hornblende recrystallization. Thus these saddle ages represent partial degassing of M, hornblende and pyroxenes as a result of a significant tectonothermal event at approximately 1450-1500 Ma. Consequently, Rb-Sr isochrons and 40Ar/39Ar degassing ages from both the NESWR and south Strangways Range, define the age of D, recrystallization to be approximately 1400-1500 Ma. Such an age for the Proterozoic Reworking is supported by plutonic granite ages of 1426-1491 Ma from the West Strangways and Reynolds Ranges (Black, 1980; Black et al., 1983) (Table 2). These large bodies of granite are interpreted to be higher crustal equivalents of syn-D, granites found throughout the Strangways Orogenic Belt.

Fig. 15. Figure illustrating the relationship between D,-D, structural elements and general wrench, transpression and transtension models (Sanderson and Marchini, 1984). (A) Summary of the possible boundary orientations of the regional zone of shear during D,-Ds, projected onto the horizontal plane and presented as the shaded zones. The zone of regional shear defined as being parallel to the average S, orientation (diagonal shading), is approximately parallel to the zone of model transpression (B), the orientation of which is constrained by the average S,-F, orientation (dotted shading). Note that in the case of the regional zone of D,-D, shear being parallel to the average S, orientation, the 5s shear zones are considered to be the primary shear planes (D-shear) after Tchalenko (1968). Average orientations of D,-D, structures and the general D,-D, strain ellipsoid are projected onto the horizontal plane and presented in the centre of diagram. (B-D) Idealized models of transpression, classical wrenching and transtension respectively. Open arrows indicate a shortening or dilation component in the shear system. C = axis of maximum compression ((or); E = axis of maximum extension (a,); R-R’ = Riedal shear band orientation with sense of shear; D = dominant shear band orientation (primary shear). (~-l = degree of shortening across the zone.

394

1400-1500

Ma Rb-Sr isotopic ages are derived from mylonites in the Redbank Deformed Zone (Shaw and Black, 1991). The pre-Palaeozoic E-W-trending mylonites of the Redbank Deformed Zone are correlated with the S, shear zones in the NESWR (as previously discussed). Thus there is isotopic evidence for S, shear zones as well as the intense S, fabrics and D, granites to have all formed in the period approximately spanning 1400-1500 Ma. Proterozoic Reworking-discussion Both D,-D, and D,-D, tectonic episodes involved east-northeast over west-southwest tectonic transport under an approximately E-Wtrending compressive stress (a,); thus, both these deformational episodes occurred in the one tectonic cycle. During this cycle there was a distinct change in deformational style from inclined ductile over-riding and fold repetition (D2-Ds) to very steeply inclined transpressional tectonics CD,-D,). Progression from inclined non-coaxial shear to upright folding (F,) with the axis of shortening being at high angles to the initial (D2-DJ transport direction, is a common feature of many high-grade Precambrian terrains. Examples include; Southern Peninsular India (Naquis et al., 1978), the Napier Complex, Antarctica (Sandiford and Wilson, 1984), west Uusimaa, southwest Finland (Schreurs and Westra, 1986), the Olary Block, Australia (Clarke et al., 1986), the Rayner Complex, Antarctica (Clarke, 1988) and the Limpopo Belt (Van Reenen et al., 1987). It is considered highly unlikely that the “apparent” 90 o rotation of the axis of shortening in all these regions, including the NESWR, was due to a coincidently similar change in the configuration of interacting crustal plates. Though upright folding (F,) was at high angles to the D,-D, transport direction, F4 has been shown to be a product of D,-D, transpression under the same E-W-trending principle compressive stress as D,-D,. Thus, considering an essentially constant 1+, orientation between both periods of the Proterozoic Reworking, the change in tectonic style experienced was possibly the result of changes in response by the terrain to

deviatoric stress, with time (Houseman and England, 1986; England, 1987; Molnar and LyonChen, 1988), rather than a change in the orientation of the tectonic stress field (i.e., crustal plate configeration). An additional feature of tectonics associated with the Proterozoic Reworking is the progression from regionally extensive ductile folding (F,-FJ to open folding (FJ and finally to the partitioning of bulk shear strain into very localized zones of deformation @,-shear zones) (Fig. 11). Such a progression in the localization of strain partitioning is very common in Precambrian orogens (Clarke et al., 1986; Van Reenen et al., 1987; Clarke, 1988). Strain localization is the result of potentially all the following mechanisms: (1) rheological strain hardening of the region, (2) uplift during deformation moving the terrain into progressively less ductile zones, (3) increase in strain rate and (4) localization of deformation by pre-existing zones of weakness. The structural information available cannot constrain the most significant causative mechanism, though decompression (“uplift”) accompanying D, has been recognized by the accompanying metamorphic analysis (Goscombe, 1992b). Despite the general similarities of the Proterozoic Reworking to compressional orogenesis, there are significant differences to typical mountain belts with shortening orthogonal to a linear orogen, such as the Himalayan-Tibetan orogen (Tapponnier et al., 1982). In compressive orogens that have been active for a long time, crustal thickening is limited because of an increase in the relative buoyancy forces associated with isostatic compensation (Molnar and Lyon-Caen, 1988). This results in widening of the orogen and eventually a predominance of strike-slip deformation, such as post-Tertiary tectonics in the Himalayas (Tapponnier et al., 1982; England, 1987) and Eastern Anatolia (Dewey et al., 1986). Strike-slip movements in the Himalayas and Eastern Anatoha are directed along the length of the orogen (orthogonal to the transport direction of convergence). In contrast, late-stage shearing during transpression in the NESWR CD,) was directed parallel to the early (Dz-Da) transport direction. In this respect, the NESWR is more analogous to

HIGH-GRADE

REWORKING

OF CENTRAL

AUSTRALIAN

395

GRANULITES

the Limpopo Belt (Van Reenen et al., 1987) than the Himalayan orogen. This distinctly different behaviour to typical linear or arcuate ~llision~ orogens is possibly the result of confinement of the Proterozoic Reworking orogen between stable continental crust to both the north and south. Relative left-lateral movement between such crustal blocks potentially resulted in D,-D, transpression (Fig. 17). As presented in Goscombe (19911, D,-D3 ductile deformation of similar style and NE to E plunging L, elongation lineations are recognized throughout the Strangways Erogenic Belt (Goscombe, 1984; Shaw et al., 1984b; Norman and Clarke, 1990) and as far west as the Mount Hay Granulites (Glikson, 1984) (Fig. 1). Thus the E-W width of deformation of the Proterozoic Reworking orogen can be confidently assumed to be a rnin~~ of 500 km wide. The orogen may well have had a width as great as that of the Arunta Block (1000 km). Though, with the paucity of structural data from exposures in the very west of the Arunta Block, this cannot be confidently established. Comparison of the minimum estimate of the width of the Proterozoic Reworking orogen (500 km) with typical linear compressional orogens yields additional problems in making comparison with such orogens. Taking the simplest case of an orogen oriented essentially orthogonal to the axis of convergence (i.e. the Himalayas), the length of the Proterozoic Reworking orogen would have trended N-S. Convergent orogens have been theoretically modelled to typically have lengths in the order of appro~mately eight times their width (England, 1987). There is an absence of exposed tectonites, of both the same deformational age and E-W transport direction as the Strangways Orogenic Belt, to both the north and south of the Arunta Block. This casts doubt as to the existence of a ~-8~ km long N-S-trending orogen having existed within the Australian continent. Consequently, a tectonic model for the Proterozoic Reworking, involving a more limited N-S length than is theoretically predicted, must be considered. A result of these arguments is that the Proterozoic Reworking orogen cannot have been a mountain belt with a straight margin,

A. Initirtiaaof Proterozoic

Fig. 17. Time sequence of schematic plan views of horizontal sections through the middle-lower crust (the crustal level represented by the NESWR) for a postulated crustal plate ~n~guration during the Proterozoic Reworking (see text), (A) Progressive ductile over-riding and fold repetition under an E-W-directed compressive stress, resulting from the impinging non-linear margin of a continental crustal block in the east. (B) Continued crustal shortening and propagation of Dz-Ds deformation along the length of an E-W-trending orogen confined to both the north and south by relatively more stabie (less tectonized) continental crust. (CT)Locking up of the orogen and partitioning of strain into sinistral transpression of the region, while still under approximately E-Wdirected principle compressive stress.

parallel convergence and unimpeded lateral length, but was possibly an orogen confined to both the north and south by relatively more stable continental crust in a confi~ration broadly similar to an “aulacogen”. Adopting such a model, deformation during the Proterozoic Reworking was the result of impingement of an irregular

shaped crustal block from the east, in contrast to a straight or arcuate erogenic front (Fig. 17). Modelling of the palaeo-crustai plate configuration associated with the Proterozoic Reworking cannot be uniquely constrained because of the absence of a recognized basement to the Arunta Block (Shaw et al., 1984a). As a result, it is not known if the Proterozoic Reworking was collisional (between once isolated crustal plates) or intra-continental. There is an absence of evidence for ophiolites and sutures throughout the Arunta Block, thus a collisional orogeny is not considered responsible for the Proterozoic Reworking (Shaw et al., 1984a). The only proposed suture in the Arunta Block is between Division 1 and 2 gneisses in the Harts Ranges (James and Ding, 1988). However, emplacement of the granitic Bruna gneiss (prior to crystallization at 1748 Ma; Mortimer et al., 1985) throughout the whole length of this “suture” suggests that these two divisions

were juxtaposed prior to 1400-1500 Ma deformation Consequently, for the want of more definitive constraints, D,-D, deformation of the Strangways Orogenic Belt is considered to have occurred in an intra-continental environment (Shaw et al., 1984a). The Proterozoic Reworking is proposed to have involved abduction of continental crust in the east giving rise to D,-D, compression directed along (not orthogonal to) the length of an E-W-trending orogen confined to both the north and south. Crustal thickening as a result of deformation and crustal shortening was eventually limited by isostatic compensation. At this stage of orogenesis, deformational strain is typically partitioned into widening of the orogen (Molnar and Lyon-Caen, 1988). In the case of the Strangways Orogenic Belt, the E-W-directed (Dt--D3) compressive stress eventually could no longer be accommodated by E-W crustal short-

Tethyan

Asian

Series.

Terrains.

Fig. 18. Simplified map of the Tien Shan plateau-Karakoram region (after Mattauer, 1986) for comparison with the proposed mode1 of crustaf pIate ~n~guration during the I&-D5 transpre~~naI episode in the Strangways Orogenic Belt (Fig. 17). This map has been rotated for comparison with the crustai plate configumtion presented in Fig. 17. Note, however, the dissimilarity of subduction of the Indian continental plate in the Himalayan orogen and abduction of the east in the proposed model for orogenesis of the Strangways Orogenic Belt.

HIGH-GRADE

REWORKING

OF CENTRAL

AUSTRALIAN

GRANULITES

ening and thickening. As a result, compressive stress was accommodated by strike-slip movement between the north and south bounding crustal blocks (D,-D,). This gave rise to sinistral transpression throughout the orogen (Fig. 17). This model of the tectonics during the Proterozoic Reworking is very similar to that experienced by the Karakoram-Tien Shan region (Tapponnier et al., 1982; Coward et al., 1986). In the Karakoram-Tien Shan region the impingement of a corner of the Indian plate resulted in an elongate zone of deformation and shortening orientated approximately parallel to (not orthogonal to) the vector of convergence (i.e. the Tien Shan Plateau) (Fig. 18). Late-stage shortening in this region is accommodated by sinistral transcurrent movement along one margin of the Indian plate and transpression within .the Karakoram-Tien Shan region. Acknowledgments

Dr.% Chris Wilson, Roger Powell, Geoff Clarke, Pat James, Ron Vernon and two anonymous reviewers are acknowledged for their constructive comments. This research was undertaken at Melbourne University, under the supervision of R. Powell and C. Wilson, with the support of an Australian Research Council grant to Powell and Wilson (# A385153781 and a Commonwealth Government post-graduate research scholarship. References Allen, A.R., 1979. Metasomatism of a depleted granulite facies terrain in the Arunta Block, Central Australia. 1. Geochemical evidence. Contrib. Mineral. Petrol., 71: 8598. Allen, A.R. and Black, L.P., 1979. The Harry Creek Deformed Zone, a retrograde schist zone of the Arunta Block, Central Australia. J. Geol. Sot. Aust., 26: 17-28. Allen, A.R. and Stubbs, D., 1982. An *Ar/39Ar study of polymetamorphic complex in the Arunta Block, Central Australia. Contrib. Mineral. Petrol., 79: 319-332. Angelier, J., 1984. Tectonic analysis of fault slip data sets. J. Geophys. Res., 89: 5835-5848. Angelier, J., Tarantola, A., Valette, B. and Manoussis, S., 1982. Inversion of field data in fault tectonics to obtain the regional stress, 1. Single phase fault populations: a new

397 method of computing the stress tensor. Geophys. J. R. Astron. Sot., 69: 607-621. Bell, A.M., 1981. Vergence: an evaluation. J. Struct. Geol., 3 (3): 197-202. Bergh, S.G. and Torske, T., 1988. Palaeovolcanology and tectonic setting of a Proterozoic metatholeiite sequence near the Basaltic shield margin, Northern Norway. Precambrian Res., 39: 227-246. Berthe, D. and Brun, J.P., 1980. Evolution of folds during progressive shear in the South Armorican Shear Zone, France. J. Struct. Geol., 2: 127-133. Berthe, D., Choukroune, P. and Jegouzo, P., 1979. Orthogneiss, mylonite and noncoaxial deformation of granites: the example of the South Armorican Shear Zone. J. Struct. Geol., 1: 31-42. Black, L.P., 1980. Rb-Sr geochronology of the Jervois Range area in the eastern portion of the Arunta Block. N.T. BMR., J. Aust. Geol. Geophys., 5: 265-270. Black, L.P and McCulloch, M.T., 1984. Sm-Nd ages of the Arunta, Tennant Creek and Georgetown inliers of Northern Australia. Aust. J. Earth. Sci., 31: 49-60. Black, L.P., Shaw, R.D. and Stewart, A.J., 1983. Rb-Sr geochronology of Proterozoic events in the Arunta Inlier, central Australia. Aust., Bur. Miner. Resour., Geol. Geophys. J., 8: 129-137. Breitkopf, J.H., 1986. Iron formations and associated amphibolites in the southern part of the Damara Orogen of South West Africa/Namibia. Ph.D. Thesis, Univ. Witwatersrand, 350 pp. (unpublished). Clarke, G.L., 1988. Structural constraints on the Proterozoic reworking of Archaean crust in the Rayner Complex, MacRobertson and Kemp Land coast, East Antarctica. Precambrian Res., 40/41: 137-156. Clarke, G.L., Burg, J.P. and Wilson, C.J.L., 1986. Stratigraphic and structural constraints on the Proterozoic tectonic history of the Olary Block, South Australia. Precambrian Res., 34: 107-137. Cobbold, P.R. and Quinquis, H., 1980. Development of sheath folds in shear regimes. J. Struct. Geol., 2: 119-126. Coleman, R.G., 1977. Ophiolites. (Minerals and Rocks Series, 12.) Springer, Berlin, 229 pp. Collins, W.J., Beams, SD., White, A.J.R. and Chappell, B.W., 1982. Nature and origin of A-type granites with particular reference to south eastern Australia. Contrib. Mineral. Petrol., 80: 189-200. Collins, W.J., Flood, R.H. and Vernon, R.H., 1988. Granite generation in the Strangways metamorphic complex, Arunta Block, Central Australia - Implications for the origin of high temperature, low pressure granulite facies terranes. Conf. Geol. Sot. Aust. Abstr. Brisbane,l980. Coward, M.P. and Potts, G.J., 1983. Complex strain patterns developed at the frontal and lateral tips to shear zones and thrust zones. J. Struct. Geol., 5: 383-399. Coward, M.P., Windley, B.F., Broughton, R.D., Luff, I.W., Petterson, M.G., Pudsey, C.J., Rex, D.C. and Asif Khan, M., 1986. Collision tectonics in the NW Himalayas. In: M.

Coward and A.C. Ries (Editors), Collisional Tectonics. Geol. Sot. London, Spec. Pub]., IY: 203-220. Dalimeyer, R.D., 1979. 4”Ar/“‘Ar dating: principles, techniques, and applications in erogenic terranes. In: E. Jaeger (Editor), Lectures in Isotope Geology. Springer, Berlin, pp. 77- 104. Dewey, J.F.. Hempton, M.R., Kidd, W.S.F., Saroglu, F. and SengGr, A.M.C., 1986. Shortening of continental lithosphere: the neotectonics of Eastern Anatolia-a young collision zone. In: M. Coward and A.C. Ries (Editors), Collisional Tectonics. Geol. Sot. London, Spec. Publ., 19: 3-36. England, P.C., 1987. Diffuse continental deformation: length scales, rates and metamorphic evolution. Philos. Trans. R. Sot. London, Ser. A, 321: 3-22. England, P.C. and Thompson, A.B., 1984. Pressure-temperature-time paths of regioal metamorphism, 1. Heat transfer during the evolution of regions of thickened continental crust. J. Petrol., 25: 894-928. Etchecopar, A., 1977. A plane kinematic model of progressive deformation in a polycrystalline aggregate. Tectonophysics, 39: 121-139. Forman, D.J., 1971. The Arltunga Nappe Complex, Macdonnell Ranges, Northern Territory, Australia. J. Geol. Sot. Aust., 18(2): 173-182. Glikson, A.Y., 1984. Granulite-gneiss terrains of the southwestern Arunta Block, Central Australia: Glen Helen, Narwietooma and Anburla 1: 100,000sheet areas. Austr., Bur. Miner. Resour. Geol. Geophys., Rec., 22. Goscombe, B.D., 1984. The structure, metamorphism, petrology and geochCmistry of the Paradise Well area in the Harts Ranges, eastern Arunta Block. Honours thesis (unpublished). Goscombe, B.D., 1987. Regional rotational shear accompanying high grade metamorphism in the East Strangways Range, Arunta Block. Geol. Sot. Aust. Abstr., 19: 58-59. Goscombe, B.D., 1989. Structural and metamorphic study of the northest Strangways Range, Central Australia. Ph.D. thesis, Melbourne Univ. (unpublished). Goscombe, B.D., 1991. Intense non-coaxial shear and the development of mega-scale sheath folds in the Arunta Block, Central Australia. J. Struct. Geol., 13: 299-318. Goscombe, B.D., 1992a. Petrogenesis of silica-undersaturated and kornerupine-bearing granulites from the Strangways Range, Central Australia. J. Metamorph. Geol., 10 (in press). Goscombe, B.D., 1992b. Tectonic reworking of granulites from Central Australia. P. 2: Metamorphic evolution. J. Petrol., 4 (in press). Guinerberteau, B., Bouchez, J.L. and Vigneresse, J.L., 1987. The Mortagne granite pluton (France) emplaced by pullapart along a shear zone: structural and gravimetric arguments and regional implication. Geol. Sot. Am. Bull., 99: 763-770. Harland, W.B., 1971. Tectonic transpression in Caledonian Spitsbergen. Geol. Msg., 108: 27-42. Harris, L.B. and Cobbold, P.R., 1984. Development of conju-

gate shear bands during bulk simple shearing. J. Struct. Geol., 7( 1): 37-44. Harrison, T.M., 1981. Diffusion of ‘“Ar in hornblende. Contrib. Mineral. Petrol., 78(3): 324-331. Harrison, T.M. and McDougall, I., 1980. Investigation of an intrusive contact, northwest Nelson, New Zealand, 1, Thermal, chronological and isotopic constraints. Geochim. Cosmochim. Acta., 44: 1985-2003. Hensen, B.J. and Warren, R.G., 1985. Partial melting during granulite metamorphism a mechanism for control of fluid composition? BMR, Proterozoic Tectonics Conf., Darwin, Abstr. Houseman, G. and England, P., 1986. A dynamic model of lithosphere extension and sedimentary basin formation. J. Geophys. Res., 91: 719-729. Hutton, D.H.W., 1982. A tectonic model for the emplacement of the Main Donegal Granite. N.W. Ireland. J. Geol. Sot. London, 139: 615-631. Iyer, S.S., Woodford, J. and Wilson, A.F., 1976. Rb-Sr isotopic studies of a polymetamorphic granulite terrane, Strangways Range, central Australia. Lithos, 9: 211-224. James, P.R and Ding, P.. 1987. Field guide to the Harts Ranges area and the Harts Range detatchment zone from the Arunta Inlier of Central Australia. Arunta Field Workshop, Univ. Adelaide, 1987, f22-f31. James, P.R and Ding, P., 1988. “Caterpillar tectonics” in the Harts Range Area: A kinship between two sequential Proterozoic extension- collision erogenic belts within the Eastern Arunta Inlier of Central Australia. Precambrian Res., 40/41: 199-216. Jolly, W.T., 1987. Lithophile elements in Huronian low-Ti continental tholeiites from Canada, and evolution of the Precambrian mantle. Earth Planet. Sci. Lett., 85: 401-415. Lacassin, R. and Mattauer, M., 1985. Kilometre-scale sheath fold at Mattmark and implications for transport direction in the Alps. Nature, 315: 739-742. Lister, G.S. and Snoke, A.W., 1984. S-C mylonites..J. Struct. Geol., 6(6): 617-638. Mandl, G., deJong, L.N.J. and Maltha, A., 1977. Shear zones in granular materials; an experimental study of their structure and mechanical genisis. Rock Mech. (Vienna), 9: 95-144. Marjoribanks, R.W., 1976. Basement and cover relations on the northern margin of the Amadeous Basin, Central Australia. Tectonophysics, 33: 15-32. Marjoribanks, R.W. and Black, L.P., 1974. Geology and geochronology of the Arunta Complex north of Ormiston Gorge, Central Australia. J. Geol. SOC.Aust., 21: 291-299. Mattauer, M., 1986. Intracontinental subduction, crust-mantle decollement and crustal-stacking wedge in the Himalayas and other Collision belts. In: M. Coward and A.C. Ries (Editors), Collision Tectonics. Geol. Sot. Spec. Publ., 19: 3-36. Molnar, P. and Lyon-Caen, H., 1988. Some simple physical aspects of the support, structure and evolution of mountain belts. Geol. Sot. Am., Spec. Pap., 218: 179-207.

HIGH-GRADE

REWORKING

OF CENTRAL

AUSTRALIAN

GRANULITES

Morse, S.A., 1969. Alkali feldspars with water at 5 kbar pressure. J. Petrol., 11: 221-251. Mortimer, G.E., Cooper, J.A. and James, P.R., 1985. U/Pb and Rb/Sr geochronology of Proterozoic supracrustals in the vicinity of the Harts Range Ruby Mine, Arunta Inlier, Central Australia. BMR. Proterozoic Conf., Darwin, Abstr. Naquis, M., Divakara, R.V. and Narain, H., 1978. The primative crust evidence from the Indian shield. Precambrian Res., 6: 323-345. Nicolas, A. and Poirier, J.P., 1976. Crystalline Plasticity and Solid State Flow in Metamorphic Rocks. Wiley-Interscience, New York, N.Y., 444 pp. Norman, A.R. and Clarke, G.L., 1990. A barometric response to late compression in the Strangways Metamorphic Complex, Arunta Block, central Australia. J. Struct. Geol., 12: 667-684. Oliver, R.L., Lawrence, R., Goscombe, B.D., Ding, P., Sivell, W.J. and Bowyer, D.G., 1988. Metamorphic and crustal considerations in the Harts Range and neighbouring regions, Arunta Inlier. Central Australia. Precambrian Res., 40/41: 277-295. Passchier, C.W. and Simpson, C., 1986. Porphyroclast systems as kinematic indicators. J. Struct. Geol., 8(8) 831-843. Powell, R., 1983. Fluids and melting under upper amphibolite facies conditions. J. Geol. Sot. London, 140: 629-633. Quinquis, H., Audren, Cl., Brun, J.P. and Cobbold, P.R., 1978. Intense progressive shear in ile de groix blueschists an compatibility with subduction or abduction. Nature, 273: 43-45. Sanderson, D.J. and Marchini, W.R.D., 1984. Transpression. J. Struct. Geol., 6: 449-458. Sandiford, M. and Wilson, C.J.L., 1984. The structural evolution of the Fyfe Hills-Khmara Bay region, Enderby Land, East Antarctica. Aust. J. Earth Sci., 31: 403-426. Schreurs, J. and Westra, L., 1986. The thermotectonic evolution of a Proterozoic, low pressure, granulite dome, West Uusimaa, SW Finland. Contrib. Mineral. Petrol., 93: 236250. Shaw, R.D. and Black, L.P., 1991. The history and tectonic implications of the Redbank Thrust Zone, central Australia, based on structural, metamorphic and Rb-Sr isotopic evidence. Aust. J. Earth Sci., 38: 307. Shaw, R.D. and Langworthy, A.P., 1984. Strangways Range region, Northern Territory. Aust., Bur. Miner. Resour., Geol. Geophys., 1: 100,000 Geological Map Commentary. Shaw, R.D., Langworthy, A.P., Offe, L.A., Stewart, A.J., Allen, A.R., Senior, B.R. and Clarke, D.B., 1979. Geological report on 1: 100,000scale mapping of the southeastern Arunta Block, Northern Territory. Aust., Bur. Miner. Resour., Geol. Geophys. Rec., 1979/47. Shaw, R.D., Stewart, A.J. and Black, L.P., 1984a. The Arunta Inlier: a complex ensialic mobile belt in central Australia. P. 2: Tectonic history. Aust. J. Earth Sci., 31: 457-484. Shaw, R.D., Stewart, A.J. and Rickard, M.J., 1984b. Arltunga-Harts Range region, Northern Territory. Aust., Bur. Miner. Resour., Geol. Geophys., 1: 100,000 Geological Map Commentary.

399 Simpson, C., 1984. Borrego Springs-Santa Rosa mylonite zone: A Late Cretaceous west-directed thrust in southern California. Geology, 12: 8-11. Simpson, C. and Schmid, SM., 1983. An evaluation of criteria to deduce the sense of movement in sheared rocks. Geol. Sot. Am. Bull., 94: 1281-1288. Stewart, A.J., Shaw, R.D. and Black, L.P., 1984. The Arunta Inlier: a complex ensialic mobile belt in central Australia. P. 1: stratigraphy, correlations and origin. Aust. J. Earth Sci., 31: 445-455. Sylvester, P.J., 1989. Post-collisional alkaline granites. J. Geol., 97: 261-280. Tapponnier, P., Peltzer, G., LeDain, A.Y. and Armijo, R., 1982. Propagating extrusion tectonics in Asia: New insights from simple experiments with plasticine. Geology, 10: 611-616. Tchalenko, J.S., 1968. The evolution of kink bands and the development of compression textures in sheared clays. Tectonophysics., 6: 159-174. Teyssier, C., 1985. A crustal thrust system in an intracratonic tectonic environment. J. Struct. Geol., 7: 689-700. Turcotte, D.L. and Schubert, G., 1982. Geodynamics: Applications of continuum physics to geological problems. Wiley, New York, N.Y., 450 pp. Turner, G., 1968. The distribution of potassium and Argon in chondrites. In: L.H. Ahrens (Editor). Origin and distribution of the Elements. Pergamon, London, pp. 387-398. VanReenen, D.D., Barton, J.M.Jr., Roering C., Smith C.A. and VanSchalkwyk, J.F., 1987. Deep crustal responce to continental collision: the Limpopo Belt of southern Africa. Geology, 15: 11-14. Vielzeuf, D. and Holloway, J.R., 1988. Experimental determination of the fluid absent melting relations in the pelitic system. Contrib. Mineral. Petrol., 98: 257-276. Warren, R.G., 1983. Metamorphic and tectonic evolution of granulites Arunta Block, Central Australia. Nature, 305: 300-303. Warren, R.G. and Shaw, R.D., 1985. Volcanogenic Cu-Pb-Zn bodies in granulites of the central Arunta Block, central Australia. J. Metamorph. Geol., 3, 481-499. Windrim, D.P., 1983. Chemical and thermal evolution of Strangways granulites Central Australia. PhD. Thesis, The Australian National University, Canberra (unpubl.). Windrim, D.P. and McCulloch, M.T., 1986. Nd and Sr isotopic systematics of central Australian granulites: chronology of crustal development and constraints on the evolution of lower continental crust. Contrib. Mineral. Petrol., 94: 289-303. Windrim, D.P., McCulloch, M.T., Chappell, B.W. and Cameron, W.E., 1984. Nd isotopic systematics and chemistry of central Australia sapphirine granulites: an example of rare earth mobility. Earth Planet. Sci. Lett., 70: 27-39. Woodford, P.J., Mateen, A., Green, D.C. and Wilson, A.F., 1975. 4oAr/39Ar geochronology of a high-grade polymetamorphic terrain, northeastern Strangways Range, central Australia. Precambrian Res., 2: 375-396.