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Metamorphism in the Olary Block, South Australia: Compression with Cooling in a Proterozoic Fold Belt

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Metamorphism in the Olary Block, South Australia: Compression with Cooling in a Proterozoic Fold Belt J. metamorphic Geol. 1987, 5, 291-306 Metamorphism in the Olary Block, South Australia: compression with cooling in a Proterozoic fold belt G.L. CLARKE, M. GUIRAUD, R. POWELL AND J.P. BURG*, Department of Geology, University of Melbourne, Parkville, Victoria, 3052, Australia Abstract. The sedimentary and igneous rocks Ky = kyanite comprising the lower Proterozoic Olary Block, Mu = muscovite South Australia, were deformed and meta- Qtz = quartz morphosed during the mid-Proterozoic ‘Olar- S = staurolite ian’ Orogeny. The area is divided into three Sill = sillimanite zones on the basis of assemblages in metapelitic rocks, higher grade conditions occurring in the INTRODUCTION south-east. Mineral assemblages developed during peak metamorphism, which accom- The anticlockwise pressure-temperature (P-T) panied recumbent folding, include andalusite paths of the early to middle Proterozoic in Zones I and I1 and sillimanite in Zone 111. domains of Australia (e.g. Phillips & Wall, Upright folding and overprinting of mineral 1981; Warren, 1983; Etheridge, Rutland & assemblages occurred during further com- Wybom, 1985) are incompatible with the rapid pression, the new mineral assemblages includ- crustal thickening followed by isostatic uplift ing kyanite in Zone I1 and kyanite and that is inferred for modem collisional orogens sillimanite in Zone 111. The timing relationships (England & Richardson, 1977; Thompson & of the aluminosilicate polymorphs, together England, 1984). Such a Proterozoic terrain is with the peak metamorphic and overprinting the Willyama Complex (Mawson, 1912; parageneses, imply an anticlockwise P-T path Vernon, 1969), which comprises meta- for the ‘Olarian’ Orogeny, pressure increasing sedimentary and meta-igneous sequences with cooling from the metamorphic peak. (Willis, Brown, Stroud & Stevens, 1983). It is informally divided into the Olary Block and Key-words: aluminosilicate polymorphs; per- the Broken Hill Block (Fig. l), arbitrarily along turbed geotherm; P-T path; Olary Block, the New South Wales-South Australia border Willyama Complex (Thompson, 1976), or along a geophysical boundary representing the probable south- List of abbreviations used in the text and western extension of the Pliocene Mundi Mundi figures Fault (Stevens, 1986). The Broken Hill Block is a low to inter- A = aluminosilicate mediate pressure terrain with regional meta- And = andalusite morphic grade increasing from the north-west, B = biotite where metapelitic rocks contain andalusite + C = cordierite muscovite, to the south-east where metapelitic Chl = chlorite rocks contain sillimanite + K-feldspar + garnet Ct =. chloritoid + cordierite and metabasic rocks contain ortho- Fib = fibrolitic sillimanite pyroxene + clinopyroxene (Phillips & Wall, G = garnet 1981). An anticlockwise P-T path, terminating Kf = K-feldspar in isobaric cooling has been inferred for the Broken Hill Block (Phillips & Wall, 1981). In *Present address: Centre Gtologique et GCophys- contrast, little has been published on the meta- ique, Universitt des Sciences et Techniques du Lan- morphism in the Olary Block. This paper pre- guedoc, Place Eugkne Bataillon, 34060 Montpellier sents the results of the metamorphic part of a Cedex, France. structurahetamorphic study of the Olary 292 G.L. Clarke et al. I I S1 And4 0 And-S I Q Sill- MU Izz] SiII-Kf 2 pyroxenes ct,, i s2 ,. s >I1::::::) / - . BROKEN HILL BLOCK Fig. 1. Sketch map of the Willyarna Complex showing the distribution of metamorphic zones. Zonation for the Broken Hill Block is after Phillips (1978). I, Ila, IIb and III refer to the zones defined in the text for the Olary Block. Block (see also Clarke, Burg & Wilson, 1986). ment (cf. Glen, Laing, Parker & Rutland, The regional metamorphic relationships within 1977). the Olary Block are outlined, the timing Resolution of the stratigraphy in the Olary relationships of metamorphism with respect to Block has allowed analysis of the regional struc- structure are discussed, and the likely P-T path ture; the outcrop geometry is explained in of metamorphism is inferred. The tec- terms of a refolded recumbent terrain (Clarke tonothermal consequences of these results are et al., 1986). An L, mineral and elongation also discussed. lineation, indicating a D1transport direction, has a consistent trajectory on a regional scale at a high angle to recumbent F1fold hinge lines. REGIONAL GEOLOGY OF THE OLARY Inverted limbs which are several kilometres BLOCK long suggest Flfolds of a similar scale. S1 clea- The gneisses and schists of the Olary Block vage-bedding relationships combined with stra- occur as a series of semi-isolated blocks which tigraphical younging directions indicate a south- are in fault contact with the upper Proterozoic eastward directed shear during Fl fold devel- ‘Adelaidean’ sedimentary succession (Mawson opment. The S1 mineral fabric associated with & Sprigg, 1950; Sprigg, 1952) to the west and the recumbent folds is folded about upright Fz south-west, but are onlapped by ‘Adelaidean’ folds. Sz is nearly vertical or is steeply dipping rocks to the north and north-east (Forbes & toward the north-west; it is consistent with F2 Pitt, 1980). The succession in the Olary Block folds being produced by continued compression has been divided into five conformable rock in the same sense and direction as inferred suites, which can be correlated with the lower for the recumbent deformation event, D1.This Proterozoic Willyama Supergroup of the continued compression giving rise to Dl and Broken Hill Block (Willis el al., 1983). It does Dzcomprises the ‘Olarian’ Orogeny. The sub- not contain any component of Archaean base- parallelism of the trend of F1and Fz axes, and Metamorphism in the Olary Block, South Australia 293 a consistent shear direction within the Olary morphic implications of possible mineral para- Block and Broken Hill Block, suggest that the geneses (Vernon, 1978), have been used to two areas are part of a single terrain (Clarke infer the S1 parageneses. The abundant matrix et al., 1986). minerals are included in both S1 and S2 para- At least two phases of granitoids intrude the geneses. metasediments. Syntectonic plutonism is most Three zones can be distinguished using prevalent in the southern part of the studied inferred S1 assemblages (Fig. 1). To the north, area, and includes albite-rich and lesser mic- Zone I is characterized by coexisting andalus- rocline-rich granitoids. Liverton (1967) con- ite, garnet and biotite; Zone I1 by coexisting sidered the microcline-rich granitoids to be andalusite, staurolite and biotite; and Zone 111, possibly syn-D,. However, P.M. Ashley (per- the poorly outcropping sequence surrounding sonal communication, 1986) considers them to the Mutooroo Copper mine to the south, by be synchronous with the albite-rich granitoids, coexisting sillimanite, staurolite and biotite. which post-date D, but contain an S2 foliation These assemblages also involve quartz and (Forbes & Pitt, 1980; 1987; Clarke et al., 1986). muscovite. This zonation suggests that the One of these albite-rich granitoids has been rocks exposed in the south were meta- dated at 1579 2 2 (U/Pb on zircons) by Ludwig morphosed at higher grade than those in the & Cooper (1984). Accompanying the syntec- north, although the Zone I assemblage appears tonic granitoids are numerous and widely dis- to be anomalous, as discussed later. This is tributed sodic-rich pegmatities. A later phase consistent with the previous interpretation by of granitic to granodioritic intrusions post-dates Spry (1977) and the south-eastward increase in D2 (Forbes & Pitt, 1980; 1987). metamorphic grade in the Broken Hill Block The area is cut by retrograde shear zones (Binns, 1964; Phillips & Wall, 1981). which post-date both the ‘Olarian’ orogeny and In Zones I and 11, S1 andalusite is pseudo- the latest granitoids but which began prior to morphed by Sz fibrolitic sillimanite except in deposition of the upper Proterozoic ‘Adelai- the vicinity of the syntectonic granitoids. Loca- dean’ sediments (Vernon & Ransom 1971; lized contact metamorphic zones of less than Etheridge & Cooper, 1981). The Cambrian 5 km radius occur around these granitoids and ‘Delamerian’ Orogeny caused the folding of are superimposed upon the S1 zonation. In the ‘Adelaidean’ sequence and resulted in the these contact zones prismatic sillimanite, development of discrete zones of high strain aligned within S2, replaces S1 andalusite. Zone within the lower Proterozoic gneisses (Clarke I1 can be subdivided on the basis of S2 chlor- et al., 1986). Movement on these zones resulted itoid (Zone IIa) and S2 staurolite (Zone IIb). in the outcrop pattern in the Olary Block. Staurolite is also present with S2 kyanite in Zone I11 (see also Thompson, 1976; Spry, 1977; Forbes & Pitt, 1980). METAMORPHIC GEOLOGY Attention is focused on the metapelitic rocks Petrography because they are the most useful in inferring the metamorphic history of the Olary Block. Zone I The metapelitic rocks contain two foliations, S1 The metapelitic rocks of Zone I contain cen- and S2, with S2 usually dominant. The domi- timetre-scale andalusite porphyroblasts, envel- nance of S2 makes the deduction of S1 mineral oped by a pervasive S2 mineral fabric defined assemblages a considerable problem. The min- by muscovite and chlorite, with less garnet and erals associated with S1 are considered by us to biotite or fibrolitic sillimanite. Biotite occurs as include those minerals that display apparent large euhedral flakes that define a relict S, reaction relationships
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