Metamorphic and geochronological evolution in the Musgrave Ranges, Central Australia

Abstract

Contrasting PTt paths obtained from high grade metamorphic rocks in Musgrave Ranges of Central Australia near Amata, indicate that Gneiss Terrane north of the Woodroffe Thrust and the granulites which outcrop south of the thrust represent two distinct crustal blocks whose tectonothermal histories only converged 1n the Middle Cambrian. The granulites record a complex, well preserved metamorphic history which is characterised by a sigmoidal PTt path consisting of an initial phase of near isobaric cooling at high pressure, followed by near isothermal decompression. The near isothermal decompression was in turn terminated by near isobaric cooling at a lower pressure. The oldest recognisable metamorphic event, Ml, in the granulites is represented by two pyroxene-bearing mineral assemblages which equilibrated at PT conditions of 12 kb and -860 °C. The Ml event was coeval with the emplacement of anatectic granitoids and mafic bodies of noritic to anorthositic composition. The M1 event is dated by U-Pb zircon ages of -1200 Ma. Subsequent to the Ml event, the granulites experienced an episode of near isobaric cooling which resulted in the formtion of JVI2 garnet-bearing mineral assemblages at PT conditions of 11 kb and -720 °C. Isothem1al and, possibly, isobaric partial hydration of the granulites resulted in the formation of M3 hornblende and biotite-bearing mineral assemblages. The M3 evem was followed by an episode of near isothermal decompression, M4, which is divided into two stages. During the first stage of M4, M3 hornblende and biotite were partially dehydrated to form coronas of garnet with or without clinopyroxene at PT conditions of -8 kb and -720 °C. During the terminal phase of M4, total consumption of M3 hornblende locally led to the formation of a second generation garnet-clinopyroxene mineral assemblage at PT conditions of 5-6 kb and -640 °C. An anatectic adamellite emplaced during the terminal stage of M4 yields a zircon U-Pb age of 1193 ± 8 Ma which is indistinguishable from the age of the Ml event. Measurements by the ⁴⁰Ar-³⁹ Ar dating technique on hornblende, biotite, K-feldspar and muscovite indicate that the post-metamorphic cooling of the granulites was very slow. Following peak metamorphism, hornblendes and biotites from the granulites did not start to accumulate argon quantiatively till after ~930 Ma and ~690 Ma respectively. The slow, near isobaric cooling that started during the terminal stage of M4, continued till the Middle Cambrian when the granulites were uplifted from mid-crustal levels and thrust onto cold rocks of the Gneiss Terrane along the Woodroffe Thrust. Frictional heating associated with the thrusting precipitated the last metamorphic event, M5, which affected both the granulites and the structurally underlying gneisses. During M5, muscovite, epidote and rarely actinolitic hornblende and chlorite grew in shear zones at PT conditions of 3.8 kb to 4.4 kb and 340 °C to 400 °C. Syn-M5 muscovites from both granulites and gneisses yield flat ⁴⁰Ar-³⁹ Ar age spectra which date the event at between 530 Ma and 540 Ma. Initial cooling from the M5 thermal maximum in both granulites and gneisses was very rapid. The rapid cooling was, however, followed by prolonged slow cooling which is reflected in age minima of between -490 Ma and -340 Main steadily rising ⁴⁰Ar-³⁹ Ar age spectra of K-feldspars from the granulites and in minima as young as -300 Main the spectra of K-feldspars from the Gneiss Terrane. These ages broadly indicate the time at which the terranes cooled below temperatures of between 200 °C and 70 °C subsequent to the 1\15 event. The initial phase of high pressure isobaric cooling in the granulites is interpreted as reflecting the decay of a short-lived thermal perturbation induced by the syn-Ml emplacement of mantle-derived magmas in the lower crust. These magmas probably recrystallised into the metanorites/metaanorthosites which form scattered outcrops throughout the axial belt of the Musgrave Block. The near isothermal decompression, on the other hand, is best interpreted in terms of rapid, possibly tectonically-driven uplift aimed at re-establishing isostatic balance following a crustal thickening episode that preceded granulite facies metamorphism. The cessation of the uplift and the long-lived isobaric cooling that ensued suggest that isostatic equilibrium was achieved after about 20 km of unroofing. This, in turn, suggests that granulite facies metamorphism in the area was preceded by a tectonic event during which the crust was thickened by ~20 km. The amphibolite facies Gneiss Terrane records a very different metamorphic history from the granulites. Because of the lack of appropriate mineral assemblages, however, details of gneisses PTt path are rather sketchy. The oldest recognisabie metamorphic event in the gneisses records temperatures of -650 °C ai1d is dated by zircon U-Pb ages of -1600 Ma. This event was followed by an episode of granitoid magmatism which is dated by zircon U-Pb ages of about 1500 Ma. Subsequently, the Gneiss Terrane experienced a second metamorphic event at -1400 Ma which records temperatures of -560 °C. The post-metamorphic thermal history of the Gneiss Terrane is constrained by age maxima of -1320 Ma and -1260 Ma obtained from . steadily rising 40 Ar-39 Ar age spectra of hornblende and K-feldspar respectively. The two ages set younger time limits to the commencement of intracrystalline retention of argon in hornblendes and K-feldspars respectively, following the -1400 Ma metamorphic event. The latter age also shows that rocks of the Gneiss Terrane were at or very close to the earth's surface when the granulites were experiencing peak metamorphic conditions. The paucity of mafic volcanics in the granulite sequence, Sm-Nd model ages of -1800 Ma which far exceed the metamorphic age, and the ubiquity of premetamorphic, inherited zircons in the granulites all suggest that the entire tectonic history of the granulite terranes took place in an intracratonic environment. The PTt path and the intracratonic setting of the metamorphism are consistent with a tectonic scenario involving crustal thickening, possibly prompted by stresses transmitted from distant plate margins, delamination of the mantle part of the lithosphere and concomitant asthenospheric upwelling resulting in the emplacement of large volumes of mafic material in the lower crust. It is proposed that rapid heating of the lower crust by the mantle-derived magmas weakened the thickened crustal segment, prompting lateral extension under its own weight. The crustal extension, in conjunction with normal, isostatically-driven uplift resulted in the rapid exhumation of the granulites which is reflected in the post-M2/M3 near isothermal decompression. The prolonged near isobaric cooling from -1200 Ma to -540 Ma is interpreted as indicating the gradual decay, under a tectonically quiescent regime, of the thermal perturbation responsible for granulite facies metamorphism. There is no evidence for a major plate collision involving the closure of an ocean basin between the granulites and the Gneiss Terrane during the Middle Cambrian, nor evidence of a major deformation event in the granulites in the time interval between M4 and M5. Under the circumstances, the absence of a -1200 Ma thermal overprint in the Gneiss Terrane and the contrasting thennal histories of the gneisses and granulites during the Proterozoic are reconciled by postulating the existence of a -1200 Ma geosuture between the two units, located well south of the presently exposed trace of the \Voodroffe Thrust. It is proposed that the main . movement on the geosuture was strike slip such that the thermal and deformational aureole associated with the suturing process was only confined to a narrow zone and did not, therefore, extend sufficiently far to affect the rocks now exposed north of the Woodroffe Thrust. The Mann Fault which outcrops about 20 km south of the Woodroffe Thrust may represent such a suture which was reactivated during M5 time. Show more