Australian Journal of Earth Sciences (1999) 46, 355–363

INTRODUCTION

Shear zones are important structures in many metamor-phic terrains. Because shear zones are commonly activefollowing the peak of regional metamorphism (Beach 1980;Dirks et al. 1991; Dipple & Ferry 1992), they are potentiallyimportant in the exhumation of metamorphic belts. Theyalso may be conduits for fluid flow (Beach 1980; Selverstoneet al. 1991; Dipple & Ferry 1992; McCaig 1997; Cartwright &Buick 1999), the loci of mineralisation (Mikuchi & Ridley1993), and sites where deformation-enhanced metamorphicreactions occur (Knipe & Wintsch 1985). Additionally, inpolymetamorphic terrains that have undergone early high-grade metamorphism, the effects of later lower grade meta-morphic events may be best documented from shear zonesas these are regions where new mineral growth (promotedby deformation and fluid flow) is most likely. Thus, docu-menting the age and conditions of shearing is importantfor understanding metamorphism and tectonics. In thispaper, we discuss the age of shear zones from the ReynoldsRange, central Australia, and examine the implications forthe tectonics of this area. Fluid flow in these shear zonesis the subject of a companion paper (Cartwright & Buick1999).

LOCAL GEOLOGY

The Reynolds Range (Figure 1) is a multiply deformed andmetamorphosed Proterozoic terrain within the AruntaInlier of central Australia (Black et al. 1983; Stewart et al.1984). The Reynolds Range consists of two stratigraphic

associations. A suite of pelitic and psammitic metasedi-ments, the Lander Rock beds, together with early graniticplutons form the basement to metasediments of theReynolds Range Group, which includes metamorphosedquartzites, pelites, marls and marbles (Stewart et al. 1980;Stewart 1981; Dirks 1990). The Reynolds Range Group wasintruded by a later suite of granitic plutons. The meta-morphic and structural history of the Reynolds Range hasbeen described by Clarke et al. (1990), Dirks and Wilson(1990), Dirks et al. (1991), Clarke and Powell (1991), Vry andCartwright (1994) and Buick et al. (1998). In this study, thegeochronological framework of Vry et al. (1996), Williamset al. (1996) and Buick et al. (1998) is used. An alternativegeochronological framework is presented by Collins andWilliams (1995). The earliest metamorphic–deformationcycle, D1–M1, affected some of the Lander Rock beds beforethe emplacement of S-type granites at ca 1.82 Ga (Collins& Williams 1995). Following the deposition of the ReynoldsRange sedimentary rocks there was a period of graniteintrusion and contact metamorphism at ca 1.78 Ga. Themain regional structural–metamorphic event to effect theReynolds Range, D2–M2, involved northeast–southwestshortening to form tight to isoclinal northwest-trending,upright, isoclinal folds and a penetrative, steep, northwest-trending S2 foliation. M2 metamorphism occurred atmedium to low pressures (400–500 MPa) and M2 grades varyfrom greenschist (~400°C) in the northwest to granulite(750–800°C) in the southeast at approximately the samestructural level (Dirks et al. 1991; Vry & Cartwright 1994; Buick et al. 1998). SHRIMP U–Pb zircon ages suggestthat M2 occurred at ca 1.6 Ga (Vry et al. 1996; Williams et al. 1996).

Alice Springs age shear zones from the southeasternReynolds Range, central AustraliaI. CARTWRIGHT1, I. S. BUICK2, D. A. FOSTER2 AND D. D. LAMBERT1

1Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences, Monash University,Clayton, Vic. 3168, Australia.

2Victorian Institute of Earth and Planetary Sciences, School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083, Australia.

The southeast Reynolds Range, central Australia, is cut by steep northwest-trending shear zones thatare up to hundreds of metres wide and several kilometres long. Amphibolite-facies shear zones cutmetapelites, while greenschist-facies shear zones cut metagranites. Rb–Sr and 40Ar–39Ar data suggestthat both sets of shear zones formed in the 400–300 Ma Alice Springs Orogeny, with the shearedgranites yielding well-constrained 40Ar–39Ar ages of ca 334 Ma. These data imply that the shear zonesrepresent a distinct tectonic episode in this terrain, and were not formed during cooling from the ca 1.6 Ga regional metamorphism. A general correlation between regional metamorphic grade andthe grade of Alice Springs structures implies a similar distribution of heat sources for the two events.This may be most consistent with both phases of metamorphism being caused by the burial ofanomalously radiogenic heat-producing granites. The sheared rocks commonly have undergonemetasomatism implying that the shear zones were conduits of fluid flow during Alice Springs times.

Key words: Alice Springs Orogeny, argon–argon dating, Arunta Inlier, metamorphism, Reynolds Range,rubidium–strontium dating, shear zones, tectonics.

356 I. Cartwright et al.

The Reynolds Range is cut by a system of majornortheast-dipping thrust faults and associated shear zones (Collins & Teyssier 1989) (Figure 1). Several of thekilometre-scale structures (e.g. the Aileron Shear Zone,Figure 1) extend into the deep crust (Lambeck et al. 1988;

Golby et al. 1990). The grade of the assemblages in theReynolds Range shear zones are locally as high as upperamphibolite facies with kyanite, staurolite and sillimanitedeveloped in sheared metapelites in the southeast of theterrain (Dirks & Wilson 1990; Dirks et al. 1991). There is a

Figure 1 (a) General tectonicmap of part of the Arunta Inlier showing the location ofmajor faults and shear zones (after Collins & Shaw 1995). Shear zones: DMSFZ,Delny–Mount Saint Hill Fault Zone; CS, Harry Creek ShearZone; MIFZ, Mt Ida Fault Zone;RTZ, Redbank Thrust Zone;WFZ, Woolanga Fault Zone.Geological regions: A, AnmatjiraRanges; AB, Amadeus Basin; AN,Arltunga nappe complex; ED,Entia Dome; GB, GeorginaBasin; MC, Mt Chappel; MH, MtHay; HR, Harts Ranges; NB,Ngalia Basin; ON, Ormistonnappe complex; RR, ReynoldsRange; S, Strangways Ranges.ASP, Alice Springs. (b) Geo-logical map of the southeasternReynolds Range (after Dirks et al.1991). Shear zones discussed inthis paper are from Sandy Creek(SC) and Mt Boothby (MB).(c) Distribution of regional M2

metamorphic grade and thegrade of mineral assemblages inshear zones and faults (afterDirks et al. 1991 and authors’observations).

Reynolds Range shear zones 357

broad correlation between the grade of these late structuresand the regional M2 metamorphic grade, with high-gradeshear zones only present in the southeast of the terrain(Figure 1). In the northwest of the terrain, structures withsimilar orientation are brittle faults with little associatedalteration.

While much attention has been paid to determining the ages of contact and regional metamorphism in theReynolds Range, the age of the shear zones is poorly known.Dirks and Wilson (1990) envisaged that the shear zones inthe southeast Reynolds Range formed in two events. Theyproposed that amphibolite-facies shear zones formedduring cooling from the regional metamorphism, whilegreenschist-facies shear zones, which they interpreted tolocally cut or reactivate the amphibolite-facies shear zones,formed during the much later 400–300 Ma Alice SpringsOrogeny. By contrast, based on dating elsewhere in theArunta Inlier, Collins and Shaw (1995) raised the possibilitythat all shearing may have occurred during the AliceSprings Orogeny. Here, we present new Rb–Sr and 40Ar–39Ardata from amphibolite- and greenschist-facies shear zonesthat yield Alice Springs ages, which we interpret as the age of shear zone formation in the southeastern ReynoldsRange.

DESCRIPTION OF REYNOLDS RANGE SHEARZONES

In this study we present data from rocks within: (i) a30–40 m-wide greenschist-facies shear zone from the Mt Airy Orthogneiss at Sandy Creek that occurs within the granulite-facies region of the Reynolds Range(Figure 1); and (ii) a 5–10 m-wide kyanite1staurolite-bearing amphibolite-facies shear zone cutting granulite-facies pelites of the Aileron Metamorphics (which areprobable Lander Rock bed equivalents) from Mt Boothby(Figure 1).

Sandy Creek granite-hosted shear zones

The unsheared granitic orthogneisses have a weak foliationthat at Sandy Creek trends ~300–320°/70–90°E. This fabricis interpreted to be the peak regional metamorphic S2

schistosity that is well developed in the surrounding peliticrocks. Unsheared granitic orthogneisses are coarse-grainedrocks that typically contain megacrysts of microcline (up to 5 cm long) in a coarse-grained (0.25–0.5 cm) plagio-clase+quartz+biotite+microcline matrix (Figure 2a). Theshear zone has a reverse fault sense of movement andcontains rocks with a strong foliation that is oriented~270–300°/70–90° NE and a strong down-dip lineation. Theshear zones and centimetre- to decimetre-scale shear bandsthat occur at the margins of the main shear zones cut, andhence post-date, the S2 schistosity. Within the shear zones,the granitic orthogneisses are progressively transformedinto micaceous schists that comprise quartz+muscovite6

biotite6chlorite with very little feldspar. In the shearedrocks, muscovite is typically fine grained (< 0.1 mm) andforms millimetre-wide layers that anastomose aroundaugen of polycrystalline quartz (Figure 2b). Chlorite occurs

within the micaceous layers and locally forms millimetre-wide seams oriented parallel to the shear fabric. The mosthighly sheared rocks show intense grainsize reduction(down to < 1 mm) and are dominated by quartz and musco-vite with only minor chlorite. Rocks in the shear zones lostNa, Ca, Fe, and Mg and gained K and Si during shearing(Cartwright & Buick 1999), implying that the shear zoneshosted significant fluid flow.

Mt Boothby sheared pelites

The shear zones at Mt Boothby that cut the metapelites of the Lander Rock beds – Aileron Metamorphics rangefrom a few centimetres to a few metres in width and arenorthwest-striking with steep, generally northeast-dipping,foliations and steeply plunging lineations. The shearedrocks from this area were described by Dirks and Wilson(1990) and Dirks et al. (1991) and only brief details are givenhere. The sheared metapelites are coarse-grained (milli-metre diameter) rocks with locally-aligned, but predomin-antly unaligned, centimetre-sized kyanite and stauroliteporphyroblasts in a foliated biotite+quartz+muscovite

Figure 2 Photomicrographs of granitic rocks showing varyingdegrees of shearing from Sandy Creek (see Figure 1). (a)Unsheared granite (95RR2) with large K-feldspar (Ksp) withplagioclase exsolution lamellae, plagioclase (Pl), quartz (qtz) and biotite (Bt) that defines a weak S2 fabric. Width of photographis 12 mm. (b) Sheared sample (94RR07) showing strong fabricdefined by elongate quartz, biotite and chlorite with a few relictquartz augen. Width of photograph is 3.8 mm.

358 I. Cartwright et al.

matrix. The foliation and lineation in these shear zoneshave a similar orientation to those in the granites at SandyCreek, although these pelites have not undergone as intenseshearing as the granitic rocks at Sandy Creek. The presencewithin these shear zones of quartz veins attests to the

presence of fluids during shearing. However, the shearedpelites do not appear to have undergone the intense alter-ation that was experienced by the sheared granites,suggesting that these shear zones may have been lessimportant fluid conduits.

Table 1 Ar–Ar data from sheared granites at Sandy Creeka.

Step Temp (°C) 39Ar (mol) Cumulative % 39Ar 40Ar/39Ar Age (Ma) Error (s) Ca/K

94RR8 Total fusion age 5 336.1 6 3.1 Ma; J 5 1.0921 3 1022

1 550 7.85E-14 1.53 8.809 165.7 1.8 0.452 650 1.56E-13 2.69 19.810 353.3 2.5 0.743 700 1.72E-13 7.01 19.877 354.4 2.6 1.094 730 2.10E-13 10.62 19.511 348.5 2.6 0.615 760 2.97E-13 15.72 18.945 339.3 1.9 0.526 800 4.64E-13 23.72 18.690 335.1 3.4 1.397 830 5.31E-13 32.87 18.901 338.5 2.5 0.588 860 6.94E-13 44.81 19.491 348.2 2.3 2.389 900 3.50E-13 50.84 18.420 330.7 6.7 3.13

10 950 1.00E-12 68.06 18.597 333.6 2.4 0.8811 1000 7.70E-13 81.32 18.863 337.9 2.9 0.7912 1050 3.67E-13 92.28 18.721 335.6 3.6 1.0813 1100 4.13E-13 99.29 18.623 334.0 4.7 0.1314 1150 2.96E-14 99.90 16.910 305.7 13.3 5.5615 1450 5.93E-15 100.00 11.190 208.0 4.4 5.2694RR6 Total fusion age 5 333.3 6 2.0 Ma; J 5 1.0900 3 1022

1 600 4.24E-14 1.10 15.822 278.0 3.1 0.722 700 1.07E-13 3.86 19.066 340.6 2.9 1.263 750 1.45E-13 7.60 18.921 338.3 1.9 0.034 800 2.19E-13 13.28 18.672 334.2 1.5 1.155 830 2.91E-13 20.82 18.376 329.4 2.6 0.976 860 1.35E-13 24.33 17.926 322.0 1.9 2.487 900 6.65E-13 41.55 18.347 328.9 1.6 1.418 950 8.96E-13 64.75 18.594 332.9 2.4 0.989 1000 1.50E-13 68.63 18.926 338.4 1.4 0.01

10 1050 5.26E-13 82.25 18.947 338.7 1.6 0.1611 1100 5.47E-13 96.42 18.944 338.7 1.6 0.7612 1450 1.38E-13 100.00 18.626 333.5 2.9 1.7694RR5 Total fusion age 5 332.8 6 2.0 Ma; J 5 1.0879 3 1022

1 600 4.47E-14 1.25 16.753 302.1 2.1 0.692 700 1.22E-13 4.66 19.022 339.3 2.5 0.643 750 1.68E-13 9.35 18.728 334.5 2.4 1.194 800 2.64E-13 16.73 18.560 331.8 1.3 0.725 830 3.13E-13 25.47 18.415 329.4 2.0 0.386 860 3.78E-13 36.02 18.478 330.5 2.6 0.187 900 4.77E-13 49.35 18.556 331.7 1.8 0.668 950 6.05E-13 66.24 18.737 334.7 1.9 1.229 1000 5.21E-13 80.78 18.762 335.1 1.9 0.48

10 1050 3.77E-13 91.31 18.758 335.0 2.1 1.2311 1100 2.44E-13 98.14 18.699 334.1 1.9 0.8512 1450 6.68E-14 100.00 18.039 323.3 1.4 0.9394RR4 Total fusion age 5 333.1 6 2.5 Ma; J 5 1.0858 3 1022

1 600 5.46E-14 1.36 14.851 269.7 1.7 0.832 700 1.21E-13 4.40 18.883 336.5 2.1 0.873 750 1.48E-13 8.09 18.807 335.2 2.5 0.344 800 2.45E-13 14.20 18.671 333.0 2.6 0.415 830 3.41E-13 22.72 18.672 333.0 3.5 0.966 860 4.12E-13 33.01 18.456 329.5 2.5 0.797 900 5.80E-13 47.51 18.629 332.3 2.7 0.618 950 6.24E-13 63.11 18.680 333.2 2.5 1.529 1000 4.57E-13 74.54 18.762 334.5 3.2 1.39

10 1050 4.44E-13 85.64 18.960 337.7 1.9 0.7011 1100 4.94E-13 97.99 18.939 337.4 2.1 0.1612 1450 8.03E-13 100.00 18.440 329.3 1.7 1.82

a Samples sites at GR 005154 on Reynolds Range 1:100 000 Geological Map Sheet 5453.

Reynolds Range shear zones 359

Conditions of shearing

By comparison with petrogenetic grids, Dirks et al. (1991)estimated that the kyanite+staurolite-bearing shear zonesin the Mt Boothby metapelites formed at amphibolite-faciesconditions (550–600°C and 500–600 MPa). The metaso-matised granitic rocks have high-variance mineral assem-blages from which it is difficult to quantify pressures andtemperatures. A combination of oxygen isotope geothermo-metry, average pressure and average temperature calcu-lations, and phengite geobarometry suggests that shearingoccurred at 420–535°C and 420–630 MPa (Cartwright &Buick 1999).

ANALYTICAL TECHNIQUES

Mineral separates for radiogenic isotope analyses werehand picked from cubes of rock (~1 cm on a side) that hadbeen coarsely crushed and sieved. Whole-rock samplesrepresent powder from ~3–5 kg of rock that were selectedon the basis of having little visible weathering and typicalmineralogies. Rb–Sr isotopes were analysed at La TrobeUniversity. Ten to fifty milligrams of sample powders were spiked with a mixed 87Rb–84Sr tracer. Rb and Sr wereseparated using standard cation exchange techniques andanalysed on a Finnigan MAT 262 mass spectrometer. Sr wasrun as phosphate on a single Ta filament in static collectionmode. Rb was run as chloride on a double Re filament insingle collector peak-switching mode. Procedural blanks forRb and Sr are < 0.5 ng. Typical internal precision for Srisotopes is ≤ 0.00003, comparable to or less than the externalprecision. Reproducibility of 87Rb/86Sr is 6 0.5%.

40Ar–39Ar analyses of mineral separates were performedat La Trobe University as described by Foster and Fanning(1997). Samples were irradiated in a core position at the IRR-1 reactor, Soreq Nuclear Research Center, Israel(Heimann et al. 1992) for 40 or 50 hours along with the fluxmonitor GA1550 biotite (McDougall & Harrison 1988). Gasextraction was performed in a computer-controlled double-vacuum resistance furnace with a tantalum crucible andno liner (to ensure uniform and accurate heating of thesample). The temperature precision of the furnace in thelocation of the samples is ~2°C and accuracy is ~5°C. Thegas was expanded into a stainless steel line and purifiedwith two 10 L/s Zr–Ti getters. Data were corrected formachine background determined by measuring systemblanks, and mass discrimination was determined byanalysing atmospheric argon. Extraction line blanks weretypically < 2 3 10216 mol 40Ar. Correction factors for inter-fering isotopes were determined by analysing K2SO4 andCaF2 salts irradiated with the samples.

40Ar–39Ar RESULTS: SANDY CREEK GRANITES

Results of 40Ar–39Ar analyses of muscovite from foursheared granites from Sandy Creek are given in Table 1 andage spectra are shown in Figure 3. All these rocks have verylittle relict feldspar and strongly-developed shear zonefabrics, implying that they were thoroughly recrystallisedduring shearing. The micas that were dated formmillimetre-sized polycrystalline aggregates, with numer-ous individual sub-millimetre grains. All four samples givesimilar results. The age spectrum of sample 94RR4 exhibits

Figure 3 39Ar–40Ar age spectrafrom muscovite from shearedgranites at Sandy Creek (seeFigure 1). These data suggestthat the age of muscovitegrowth, and hence shear zoneformation, was 334 Ma. Datafrom Table 1.

360 I. Cartwright et al.

a plateau over >95% of the gas release with an age of334.1 6 2.6 Ma (all uncertainties are 1s). A plateau age of334.8 6 2.0 Ma is given by ~50% of the gas from sample94RR5; a mean age of steps 2–11 for this sample (> 95% ofthe 39Ar release) is 333.4 6 2.0 Ma. Sample 94RR6 yields a slightly discordant spectrum with a mean age of333.3 6 2.0 Ma. The age spectrum for muscovite in 94RR8 isdiscordant over the ~40% gas released and flattens to a well-defined plateau age of 334.5 6 3.5 Ma. Overall, these datasuggest an age of ca 334 Ma. The estimated temperaturesof deformation (see above) may be close to, or above, theclosure temperature of Ar in muscovite (Kirschner et al.1996; Lister & Baldwin 1996), raising the possibility that theages are cooling ages. However, due to the lack of older agespreserved in any of the age spectra and the good agreementbetween the individual samples, we conclude that theseages are more likely to represent the time that mineralgrowth or recrystallisation occurred within the shearzones.

The younger ages given by very small fractions of thegas (< 2%) in initial steps of all spectra would generally beconsidered to have little geological significance when inter-preting mica results. This is because the apparent ages ofthese steps are likely to be mixtures between some mini-mum age on the rims and least retentive sites of grains andthe plateau age. It is interesting to note that the minimumages ca 300–260 Ma are similar to apatite fission track agesfrom northern and central Australia (Shaw et al. 1992;Spikings et al. 1997). Even so the amount of gas is insig-nificant so this similarity should be considered with caution.

Rb–Sr RESULTS: MT BOOTHBY PELITES

Figure 4 and Table 2 show Rb–Sr data from 3–5 kg whole-rock samples from sheared metapelites from MtBoothby (Figure 1) that give an age of 333 6 16 (1s) Ma(MSWD 5 32.6). While this age is relatively imprecise, it isidentical to within error of the 40Ar–39Ar ages discussedabove for the granite-hosted shear zones. The samples werecollected from an area of ~20 m2, and we interpret the ageto be that of shearing, rather than being due to later iso-topic resetting by simply heating the region, which isunlikely to have caused isotopic homogenisation over suchlarge distances. The scatter in the data may reflect variableisotopic resetting during shearing. The intensity of shear-ing and degree of recrystallisation in the shearedmetapelites was less than that in the metagranites and thesheared rocks may contain relict minerals that did not fullyequilibrate isotopically with the bulk of the rock.

DISCUSSION

The 40Ar–39Ar and Rb–Sr data imply that the shear zones inboth the metapelites and metagranites in the southeasternReynolds Range formed during the Alice Springs Orogenyas proposed by Collins and Shaw (1995). The increase ingrade of Alice Springs structures from high-level, lower- orsub-greenschist facies thrusts in the south of the AruntaInlier to amphibolite-facies shear zones that record

temperatures of ~500°C in parts of the Central Province hasbeen previously recognised (Collins & Shaw 1995; Dunlap& Teyssier 1995). However, the extent and grade of the AliceSprings overprint in the Northern Province was generallyunknown. This study implies that the grade of AliceSprings deformation and metamorphism in the ReynoldsRange was locally at least as high as that recorded else-where. North of the Reynolds Range, the grade of AliceSprings structures decreases (Stewart et al. 1980; Stewart1981) and the southeastern Reynolds Range probably repre-sented a local metamorphic high at that time. Many of theAlice Springs age shear zones in the Reynolds Range dipsteeply (45–55°) to the north or northeast with reversemovement senses and steep north- to northeast-plunginglineations. This is consistent with sinistral transpres-sional movement during the Alice Springs Orogeny. Northof the Reynolds Range, the geometry of the structures isreversed and the Reynolds Range occupies the centre of alarge scale ‘pop-up’ structure (Collins & Teyssier 1989). Thisgeometry is consistent with the highest grade Alice Springsrocks occurring in the southeastern Reynolds Range.

Figure 5 summarises a revised geological history of theReynolds Range. The grade of D1–M1, which affected theLander Rock beds and the early 1820 Ma granites, is notwell-known due to the overprint of later events. The areawas exhumed prior to the deposition of the Reynolds Range

Figure 4 Rb/Sr isochrons for sheared metapelites at Mt Boothby(see Figure 1). These data suggest that shear-.zone formation wasduring the Alice Springs Orogeny. The scatter in ages may be dueto the presence of relict (pre-shearing) minerals. Data fromTable 2.

Table 2 Rb–Sr data from Mt Boothby sheared pelitesa.

Sample Sr(ppm) Rb(ppm) 87Sr/86Sr Rb/Sr

95RR87 2.06 134.6 1.9002 211.695RR88 2.1 93.3 1.5262 139.295RR89 0.98 84.3 2.2673 289.895RR90 2.54 194.9 2.0476 251.695RR93 12.59 65.1 0.9467 15.30495RR94 21.54 128.9 0.9664 17.756

a Samples sites at GR 240064 on Reynolds Range 1:100 000Geological Map Sheet 5453.

Reynolds Range shear zones 361

sedimentary rocks in the interval 1820–1780 Ma. Contactmetamorphism associated with the ca 1780 Ma graniteslocally achieved amphibolite facies and producedandalusite-bearing assemblages (Dirks et al. 1991), althoughmetamorphic conditions are again not well-constrained. Itis unknown whether the region was at the surface prior tothe start of the D2–M2 tectonometamorphic cycle. The peakof D2–M2 at 1594 6 6 Ma occurred at 750–800°C and 500 MPain the southeast Reynolds Range, and the initial stages ofcooling from the D2–M2 peak were probably accompaniedby limited decompression (Vry & Cartwright 1994; Buick et al. 1998). A period of retrogression at 1568 6 4 Ma (at650–700°C and 300–400 MPa: Buick et al. 1997, 1998) wascaused by fluids exsolved from crystallising partial meltsthat were formed during M2 (Cartwright et al. 1996;Williams et al. 1996).

Ngalia Basin sediments were deposited ~20 km south ofthe Reynolds Range (Figure 1) during Neoproterozoic toCarboniferous times. However, as no similar age sedimen-tary rocks are present in the Reynolds–Anmatjira Ranges,it is uncertain whether those areas were close to the surfaceat that time. There is no indication that major subhoriz-ontal movements occurred during the early stages of theAlice Springs Orogeny prior to the formation of the shearzones that could have buried near-surface rocks in thesoutheast Reynolds Range to depths of > 15 km. In theabsence of such structures, it is probably more likely thatthe rocks in the southeastern Reynolds Range were at mid-crustal levels at the onset of the Alice Springs Orogenyand that any reburial was relatively modest.

The shear zones discussed in this paper were clearlyactive during the Alice Springs Orogeny, rather thanduring the D2–M2 tectonometamorphic cycle. However,while it is clear that the shear-zone assemblages were

formed at lower grades than the peak-metamorphic mineralassemblages in the surrounding rocks, it is less certainwhether they are retrograde with respect to the AliceSprings Orogeny. No widespread Alice Springs age over-printing mineral assemblages are developed in the rocks outside the shear zones, and the timing of formationof new minerals within the shear zones was controlled byrecrystallisation and fluid flow (rather than reflecting anyspecific point on the Alice Springs P–T path). The problemof whether Alice Springs age shear zone assemblages wereformed during prograde or retrograde evolution is ageneral one for the Arunta Inlier. Ballevre et al. (1998) inter-preted mineral reaction textures (garnet replacing horn-blende and plagioclase and staurolite+hornblende breakingdown to form plagioclase+gedrite) from Alice Spring age (381 6 7 Ma) shear zones in amphibolites from theStrangways Ranges as forming during prograde meta-morphism. While there are no similar textures recorded inthe Reynolds Range shear zones, the possibility remainsthat shearing may have occurred at different times duringthe Alice Springs Orogeny. The difference in temperaturesrecorded by the granite- and pelite-hosted shear zones couldeither represent a difference in temperature between thetwo localities or a slight difference (within the limits of theprecision on the ages) in the timing of deformation withrespect to the thermal evolution. The higher temperatureshear zones in the metapelites could represent structuresthat were active during the earlier stages of deformationwhile the lower temperature granite-hosted shear zonesmay represent structures along which later movementoccurred. Overall, the lack of partial melting at that timeimplies that the temperatures during the Alice SpringsOrogeny in the Reynolds Range could not have been muchhigher than those recorded by the shear zones in themetapelites.

The geological history depicted in Figure 5 raises a num-ber of questions. First, how much deformation occurred in the Reynolds Range during the 1.4 Ga ‘Anmatjira UpliftPhase’. 1.4 Ga Sm–Nd, 40Ar–39Ar and Rb–Sr ages from theRedbank Thrust Zone (which separates the Southern andCentral Provinces of the Arunta Inlier), sheared granitesin the Anmatjira Ranges, and sheared rocks in the EntiaDome in the Harts Ranges (Figure 1), have been interpretedas indicating significant tectonism at that time (Black et al.1983; Shaw & Black 1991; Shaw et al. 1992; Foden et al. 1995).However, none of the shear zones investigated in this study(and many other shear zones in the southeastern ReynoldsRange) appear to be polyphase making it most likely thatthey are solely Alice Springs age. The 1.4 Ga ages in theReynolds Range and Redbank regions may represent stagesof cooling and exhumation following regional metamor-phism that occurred at ca 1.6 Ga in both of those areas.Tectonic movement at that stage may have be accommo-dated along the major structures (such as the RedbankThrust or the Aileron Shear Zone) (Figure 1) with littledeformation in the intervening rocks.

A second question is why the grade of the shear zonesis broadly correlated with the much earlier M2 metamor-phic grades (Figure 1). Kyanite-bearing shear zones arerestricted to the southeast of the Reynolds Range, whichunderwent upper-amphibolite to granulite-facies M2 meta-morphism at 1.6 Ga. The assemblages in these shear zones

Figure 5 Revised P–T–time paths for the Reynolds Range. Theearly contact metamorphism, M2 regional metamorphism, andmetamorphism in the shear zones are separate events. P–T andage data for contact and M2 metamorphism from Dirks et al.(1991), Vry & Cartwright (1994), Collins & Williams (1995),Williams et al. (1996), Vry et al. (1996), Buick et al. (1997, 1998). Datafor the shear zones from Cartwright & Buick (1998) and Dirks et al. (1991).

record lower temperatures (550–600°C) than those of the M2

peak (750–800°C), so wholesale overprinting of peak M2

assemblages in the southeastern Reynolds Range duringAlice Springs times would not be expected. However, wide-spread overprinting Alice Springs age assemblages are notknown from the northwest Reynolds Range where M2

metamorphism occurred at temperatures as low as 400°C.Even in the absence of deformation, metamorphism ofthose rocks at similar conditions to those recorded in thesoutheastern Reynolds Range would be expected to causewidespread prograde metamorphic recrystallisation. Thatthis has not happened indicates that Alice Springs tem-peratures in that part of the terrain were never as high asthose in the southeast, regardless of the relative timing ofdeformation and thermal evolution. Late structures in thenorthwest Reynolds Range, which are probably contempor-aneous with the shear zones, are mainly faults with minorassociated greenschist-facies alteration and high-gradeshear zones are absent. Thus the distribution of meta-morphic grades in the Alice Springs Orogeny mirrors thatduring M2 even though the two events were separated by~1150 million years. It is possible for unrelated externalheat sources (e.g. suites of igneous rocks) to have similardistributions if they are emplaced along major structuralfeatures. However, in the Reynolds Range there are no volu-minous 1.6 Ga or 400–300 Ma igneous rocks. Nor is there anystructural evidence for Alpine-style crustal thickening atthose times. The similarity in grade distributions may alsobe explained if the heat sources were internal to theterrain. Hand et al. (1995, 1996) and Sandiford and Hand(1998) showed that many of the ca 1.8 Ga granites in theReynolds Range have anomalously high concentrations ofradioactive elements and proposed that the 1.6 Ga meta-morphism was caused by heat generated by radioactivedecay in these rocks during burial. In that model, the spatialvariation in metamorphic grade at that time reflects tosome extent the distribution of the radiogenic granites andthe thickness of the cover (especially basin sediments thatare good insulators). Reburial of the granites during theAlice Springs Orogeny may have also provided the heat forthat event, in which case the similar spatial distribution ofmetamorphic grade would be expected, but, due to thereduction of heat production time, the Alice Springs eventwould also be expected to everywhere be of lower grade. Infact, Hand et al. (1996) predicted, on the basis of such heat-flow calculations, that the shear zones were of AliceSprings age. While this model remains somewhat specu-lative, it does remove the need for explaining why externalheat sources would have the same distribution in twotemporally unrelated events.

Comparison with ages elsewhere in the AruntaInlier

Alice Springs ages are recorded from throughout theArunta Inlier (see summaries by Collins & Shaw 1995 andDunlap & Teyssier 1995). While there are some uncertain-ties over interpretation, 40Ar–39Ar, K–Ar, and Rb–Sr datasuggest that much of the southeastern Arunta Inlier cooledthrough ~350°C at 340–330 Ma (defined largely by muscovite40Ar–39Ar ages). U–Pb and Sm–Nd ages from shear zonesalso indicate orogenesis at that time (Ballevre et al. 1998;

Mawby et al. 1998a). The ages documented from theReynolds Range clearly fall within a similar time span, indi-cating that this was a major period of tectonism through-out the Arunta Inlier. Older (ca 400 Ma) ages (largely fromhornblendes in amphibolite facies rocks from the HartsRanges) have been interpreted as recording higher tem-perature cooling during the Alice Springs Orogeny (Collins& Shaw 1995; Dunlap & Teyssier 1995). However, recentSm–Nd and U–Pb studies of granulite facies rocks in theHarts Range region (Mawby et al. 1998b; Miller et al. 1998)indicated that at least part of the Arunta Inlier underwenthigh-grade metamorphism and deformation at 480–460 Ma.U–Pb titanite ages of 420–410 Ma and Sm–Nd ages of450–445 Ma in the Harts Range are interpreted as recordingthe cooling following the peak of that high-grade event.Thus, there is the possibility that the ca 400 Ma hornblende40Ar–39Ar ages reflect cooling ages from the earlierOrdovician high-grade metamorphism rather than a phaseof the Alice Springs Orogeny. It is as yet unclear what thedistribution of the Ordovician tectonometamorphic eventwas and whether and how it relates to the Alice SpringsOrogeny.

The Reynolds Range illustrates the importance in exam-ining shear zones in polymetamorphic terrains. Little effectof the Alice Springs Orogeny has been recognised else-where in the terrain, presumably because conditions neverexceeded those of the earlier regional metamorphismresulting in little new mineral growth except in regions ofactive deformation or fluid flow. Many of the analogousstructures in the Arunta Inlier (Figure 1) are probably also,at least partly, Alice Springs in age and the tectonics of thatevent will probably be best deciphered from those discretestructures.

ACKNOWLEDGEMENTS

This research was supported by ARC grants A39231141 andA39701890 (to IC). ISB was supported by an ARC AustralianResearch Fellowship and ARC small grants. E. Curl helpedwith the mineral separation, M. Jane and R. Maas with the Rb–Sr analyses, and S. Szczetanski with the Ar–Aranalyses. We are indebted to L. Black, M. Hand and R. Shawfor informative and helpful reviews.

REFERENCES

BALLEVRE M., MOLLER A. & HENSEN B. J. 1998. An Alice Springs (380 Ma)age for a prograde amphibolite-facies shear zone in theStrangways Metamorphic Complex, Arunta Block. GeologicalSociety of Australia Abstracts 49, 19.

BEACH A. 1980. Retrogressive metamorphic processes in shear zoneswith special reference to the Lewisian complex. Journal ofStructural Geology 2, 257–263.

BLACK L. P., SHAW R. D & STEWART A. J. 1983. Rb–Sr geochronology ofProterozoic events in the Arunta Inlier, central Australia. BMRJournal of Australian Geology & Geophysics 8, 129–138.

BUICK I. S., CARTWRIGHT I. & HARLEY S. L. The retrograde P–T–t path for low-pressure granulites from the Reynolds Range,central Australia: petrological constraints and implications for LP/HT metamorphism. Journal of Metamorphic Geology 16,511–530.

BUICK I. S., CARTWRIGHT I. & WILLIAMS I. S. 1997. High-temperatureretrogression of granulite-facies marbles from the Reynolds

362 I. Cartwright et al.

Range Group, central Australia: phase equilibria, isotopic reset-ting and time-integrated fluid fluxes. Journal of Petrology 37,1097–1124.

CARTWRIGHT I. & BUICK I. S. 1999. Meteoric fluid flow within AliceSprings age shear zones, Reynolds Range, central Australia.Journal of Metamorphic Geology (in press).

CARTWRIGHT I., BUICK I. S. & VRY J. K. 1996. Polyphase metamorphic fluidflow in the Lower Calcsilicate Unit, Reynolds Range, centralAustralia. Precambrian Research 77, 211–229.

CLARKE G. L., COLLINS W. J. & VERNON R. H. 1990. Successive earlyProterozoic deformation and metamorphic events in theAnmatjira Range, central Australia. Journal of MetamorphicGeology 8, 65–88.

CLARKE G. L. & POWELL R. 1991. Proterozoic granulite facies meta-morphism in the southeastern Reynolds Range, central Australia:context, P–T path and overprinting relationships. Journal ofMetamorphic Geology 9, 267–281.

COLLINS W. J. & SHAW R. D. 1995. Geochronological constraints onorogenic events in the Arunta Inlier: a review. PrecambrianResearch 71, 315–346.

COLLINS W. J. & TEYSSIER C. 1989. Crustal-scale ductile fault systems inthe Arunta Inlier, central Australia. Tectonophysics 158, 49–66.

COLLINS W. J. & WILLIAMS I. S. 1995. SHRIMP ion probe dating of short-lived Proterozoic tectonic cycles in the northern Arunta Inlier,central Australia. Precambrian Research 71, 69–89.

DIPPLE G. M. & FERRY J. M. 1992. Metasomatism and fluid flow in ductilefault zones. Contributions to Mineralogy and Petrology 112, 149–164.

DIRKS P. H. G. M. 1990. Intertidal and subtidal sedimentation during amid-Proterozoic marine transgression, Reynolds Range Group,Arunta Block, central Australia. Australian Journal of EarthSciences 37, 409–422.

DIRKS P. H. G. M., HAND M. & POWELL R. 1991. The P–T deformation pathfor a mid-Proterozoic, low-pressure terrane: the Reynolds Range,central Australia. Journal of Metamorphic Geology 9, 641–661.

DIRKS P. H. G. M. & WILSON C. J. L. 1990. The geological evolution ofthe Reynolds Range, central Australia: evidence for three distinctstructural–metamorphic cycles. Journal of Structural Geology 12,651–665.

DUNLAP J. W. & TEYSSIER C. 1995. Paleozoic deformation and isotopicdisturbance in the southeastern Arunta Block, central Australia.Precambrian Research 71, 229–250.

FODEN J., MAWBY J., KELLEY S., TURNER S. & BRUCE D. 1995. Metamorphicevents in the eastern Arunta Inlier. Part 2, Nd–Sr–Ar isotopic con-straints. Precambrian Research 71, 207–227.

FOSTER D. A. & FANNING C. M. 1997. Geochronology of the northernIdaho batholith and Bitterroot metamorphic core complex:magmatism preceding and contemporaneous with extension.Geological Society of America Bulletin 109, 379–394.

GOLBY B. R., KENNETT B. L. N., WRIGHT C., SHAW R. D. & LAMBECK K. 1990.Seismic reflection profiling in the Proterozoic Arunta Block, cen-tral Australia: processing for testing models of tectonic evolution.Tectonophysics 173, 257–268.

HAND M., FANNING C. M. & SANDIFORD M. 1995. Low-P high-T meta-morphism and the role of high-heat producing granites in thenorthern Arunta Inlier. Geological Society of Australia Abstracts40, 60–63.

HAND M., SANDIFORD M. & BINGEMER A. 1996. High heat production,low-P high-T metamorphism and crustal deformation. GeologicalSociety of Australia Abstracts 41, 179.

HEIMANN A., STEINITZ G. & ZAFRIR H. 1992. Irradiation of samples for40Ar/39Ar dating using the Soreq Nuclear Research Center IRR-1reactor, Israel. Nuclear Geophysics 6, 273–286.

KIRSCHNER D. L., COSCA M. A., MASSAN H. & HUNZIKER J. C. 1996.Staircase 40Ar/39Ar spectra of fine-grained white mica: timing andduration of deformation and empirical constraints on argondiffusion. Geology 24, 747–750.

KNIPE R. J. & WINTSCH R. P. 1985. Heterogeneous deformation, foliationdevelopment, and metamorphic processes in a polyphasemylonite. In: Thompson A. B. & Rubie D. C. eds. MetamorphicReactions, pp. 180–210. Springer, New York.

LAMBECK K., BURGESS G. & SHAW R. D. 1988. Teleseismic travel-timeanomalies and deep crustal structure in central Australia.Geophysical Journal of the Royal Astronomical Society 94, 105–124.

LISTER G. S. & BALDWIN S. L. 1996. Modelling the effect of arbitraryP–T–t histories on argon diffusion in minerals using theMacArgon program for the Apple Macintosh. Tectonophysics 253,83–109.

MAWBY J., HAND M., FODEN J., KELLEY S. P., KINNY P. & MCDOUGALL I.1998a. U–Pb, Sm–Nd, 40Ar–39Ar and K–Ar constraints on thethermal history of the Alice Springs Orogeny in the Harts Range,southeastern Arunta Inlier, central Australia. Geological Societyof Australia Abstracts 49, 295.

MAWBY J., HAND M., FODEN J. & KINNY P. 1998b. Ordovician granulitesin the southeastern Arunta Inlier: a new twist in the Palaeozoichistory of central Australia. Geological Society of AustraliaAbstracts 49, 296.

MCCAIG A. M. 1997. The geochemistry of volatile fluid flow in shearzones. In: Holness M. B. ed. Deformation-Enhanced FluidTransport in the Earth’s Crust and Mantle, pp. 227–266. ChapmanHall, London.

MCDOUGALL I. & HARRISON T. M. 1988. Geochronology andThermochronology by the 40Ar/39Ar Method. Oxford UniversityPress, New York.

MIKUCHI E. J. & RIDLEY J. R. 1993. The hydrothermal fluid of Archaeanlode gold deposits at different metamorphic grades—compo-sitional constraints from ore and wallrock alteration assemblages.Mineralium Deposita 28, 469–481.

MILLER J. A., BUICK I. S., WILLIAMS I. S. & CARTWRIGHT I. 1998. Re-evaluating the metamorphic and tectonic history of the easternArunta Block, central Australia. Geological Society of AustraliaAbstracts 49, 316.

SANDIFORD M. & HAND M. 1998. Australian Proterozoic high-temperature, low-pressure metamorphism in the conductive limit.In: Treloar P. J. & O’Brien P. J. eds. What Drives Metamorphismand Metamorphic Reactions? pp. 23–46. Geological Society ofLondon Special Publication 138.

SELVERSTONE J., MORTEANI G. & STAUDE J. M. 1991. Fluid channellingduring ductile shearing: transformation of granodiorite intoaluminous schist in the Tauern Window, eastern Alps. Journal ofMetamorphic Geology 9, 419–431.

SHAW R. D. & BLACK L. D. 1991. The history and tectonic implicationsof the Redbank thrust zone, central Australia, based on structural,metamorphic and Rb–Sr isotopic evidence. Australian Journal ofEarth Sciences 38, 307–332.

SHAW R. D., ZEITLER P. K., MCDOUGALL I. & TINGATE P. R. 1992, ThePalaeozoic history of an unusual intracratonic thrust belt incentral Australia based on 40Ar–39Ar, K–Ar and fission trackdating. Journal of the Geological Society of London 149, 937–954.

SPIKINGS R. A., FOSTER D. A. & KOHN B. P. 1997. Phanerozoic denudationhistory of the Mount Isa Inlier, Northern Australia: a record ofthe response of a Proterozoic mobile belt to intraplate tectonics.International Geology Review 39, 107–124.

STEWART A. J. 1981. Reynolds Range region. Bureau of MineralResources 1:100 000 Geological Map Series, Canberra.

STEWART A. J., OFFE L. A., GLIKSON A. J., WARREN R. G. & BLACK L. P.1980. Geology of the northern Arunta Block, Northern Territory.Bureau of Mineral Resources Record 1980/83.

STEWART A. J., SHAW R. D. & BLACK L. P. 1984. The Arunta Inlier: a com-plex ensialic mobile belt in central Australia. Part 1: stratigraphy,correlations and origin. Australian Journal of Earth Sciences 31,445–455.

VRY J. K. & CARTWRIGHT I. 1994. Sapphirine–kornerupine rocks fromthe Reynolds range, central Australia: constraints on the uplifthistory of a Proterozoic low pressure terrane. Contributions toMineralogy and Petrology 116, 78–91.

VRY J., COMPSTON W. & CARTWRIGHT I. 1996. SHRIMP II dating of zirconsand monazites: reassessing the timing of high-grade meta-morphism and fluid flow in the Reynolds Range, northern Arunta Block, Australia. Journal of Metamorphic Geology 14,335–350.

WILLIAMS I. S., BUICK I. S. & CARTWRIGHT I. 1996. An extended episodeof early Mesoproterozoic metamorphic fluid flow in the ReynoldsRange, central Australia. Journal of Metamorphic Geology 14,29–47.

Received 18 May 1998; accepted 30 October 1998

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