C. M. Gray

A strontium isotopic traverse across the granitic rocks of southeastern Australia: Petrogenetic and tectonic implications

Three regional granite associations are resolved by a Sr isotopic traverse across the orogenic structures of southeastern Australia. The associations identified by geochronology and the distribution of maximum initial 87Sr/86Sr ratios are (1) western — ∼500 Ma granites of southeastern South Australia with initial ratios up to ∼0.718; (2) central — 400 and 370 Ma granites of central Victoria with maximum initial ratios of ∼0.712; and (3) eastern — 415 Ma granites of the batholiths which parallel the eastern seaboard and have highest initial ratios of ∼0.720. Strontium isotopic compositions preclude granite magma formation by simple melting of a concealed Proterozoic basement comparable with that exposed in the interior of the continent; the latter would be far too radiogenic at the time of magmatism (87Sr/86Sr> 0.73). However, the maximum initial ratios of the western and eastern granite associations correspond to mean values for their respective enclosing Palaeozoic (meta)sedimentary terrains. This link e...

Geochronology of granulite‐facies gneisses in the western Musgrave Block, Central Australia

Abstract Rubidium‐strontium total‐rock geochronology of granulite‐facies gneisses in the western Musgrave Block, Central Australia records distinct metamorphic and earlier events. The Tomkinson Ranges are subdivided into two regions with similar relative geological sequences of events, but different primary ages and lithologies; the boundary between the regions is the latitudinally‐trending Hinckley Fault. A 90‐km traverse along regional strike in the northern region produces consistent 1550 m.y. ages. Sampling at several structural levels over a slight range of metamorphic grade in the southern region reveals a single regional isochron with a 1330 m.y. age. Both ages are interpreted as the time of supracrustal genesis of the rocks. Subsequently, granulite‐facies metamorphism occurred throughout the area at 1200 m.y.: attribution of this age to the metamorphism is based upon evidence for outcrop scale isotopic homogenization, and the dating of a granitoid intrusion contemporaneous with the metamorphism.

Barium contents of granites: key to understanding crustal architecture in the southern Lachlan Fold Belt?

Victorian granites containing more than 750 ppm Ba are almost entirely confined to the region between a line from Geelong to Swan Hill and the Wonnangatta Fault Zone. Granite Ba contents normalised to 70% SiO2 range from 620 to 733 ppm in western Victoria, 719 to 1560 ppm in central Victoria and 493 to 689 ppm in eastern Victoria. Melting of Ba-rich (meta)sedimentary rocks in the lower – middle crust is implicated in the petrogenesis of central Victorian S types, at least. Thus, granite geochemistry supports the concept of some sort of (largely concealed) Ba-rich Selwyn Block beneath central Victoria, although the boundaries that have been proposed for the block are modified here. There is a strong possibility that the Selwyn Block is an exotic terrane emplaced by northwest-directed strike-slip movement during the Bindian Orogeny. Such movement appears to have been controlled by the previously postulated Baragwanath Transform and another fundamental fault here called the Ulrich Transform. Asthenospheric upwelling related to movement on the Ulrich and Baragwanath Transforms may be the explanation for the twin belts of 400 Ma plutonism occurring to the west of the former and to the east of the latter. The southern extension of the Ulrich Transform may be the Tamar – Tiers structure in Tasmania. Plate-tectonic models suggesting Ordovician – Silurian subduction in Victoria need to be carefully revisited given the possibility of Siluro-Devonian exotic terrane emplacement.

Provenance of Palaeozoic turbidites in the Lachlan Orogenic Belt: Strontium isotopic evidence

The sedimentary provenance of Palaeozoic turbidites in the southern Lachlan Orogenic Belt is determinable by comparing the mean Sr isotopic ratios of the turbidites with those of potential provenance areas at the time of sedimentation. The possible provenances encompass rocks of Precambrian to Cambrian age extending from central South Australia to western Tasmania and estimates of their isotopic compositions are obtainable by pooling data in the geochronological literature. Sr isotopic data exist for turbidites of Early Ordovician, Late Ordovician and Devonian age located in northeastern Victoria, southeastern New South Wales and northeastern Tasmania, respectively. All Precambrian provenance areas have mean 87Sr/86Sr ratios that are too high to be equated with those of the turbidites. The turbidites contain Sr with a relatively ‘juvenile’ isotopic composition and the only possible equivalent sediment sources are Cambrian sedimentary rocks, such as the Kanmantoo Group in South Australia, and igneous rocks...

Rb‐Sr isotopic systematics of an Archaean granite–gneiss terrain: The Mount Edgar Batholith, Pilbara Block, Western Australia

Rb‐Sr whole rock analyses describe the isotopic system of the Mount Edgar Batholith in the Archaean Pilbara Block, Western Australia. The batholith comprises a complex gneiss terrain intruded by ‘older’ variably deformed granites, minor aplite, and a ‘younger’ post‐tectonic granite, the Moolyella Suite. The ‘older’ granites occur as six suites with isotopically indistinguishable individual isochrons; the pooled mean age is 3202 ± 17 Ma with initial 87Sr/86Sr ratios in the range 0.7010–0.7025. This age is taken to record timing of granite emplacement, although it may represent a metamorphic resetting. If the latter is correct, resetting took place in the greenschist to lower amphibolite facies, less than 100 Ma after batholith formation. Localized Rb‐Sr resetting and aplite dyke emplacement record a ∼3000 Ma event also detected in other parts of the Pilbara Block. The ‘younger’ Moolyella suite is dated at ∼2800 Ma, but appears to have undergone minor isotopic disturbance. The gneiss complex records a compl...

Delamerian Glenelg tectonic zone, western Victoria: characterisation and synthesis of igneous rocks

Cambro‐Ordovician plutonic rocks of the Glenelg River Complex (western Victoria) are grouped into three structural associations based on intrusion timing relative to Delamerian deformational episodes. Early syn‐compressional plutons contain the regional S2 foliation and comprise the Wando, Wennicott and Deep Creek granitic types, which have low K affinity and are confined to the southwestern corner of the complex. Wando types include hornblende diorite to gneissic tonalite and contain numerous mafic igneous enclaves; high Al2O3 and increasing K2O with silica are characteristic. Hornblende tonalites of Wennicott type are less deformed and by contrast evolve towards lower K2O, whereas the geochemistry of Deep Creek type granodiorites suggests plagioclase accumulation. The nearby Caupaul Igneous Complex also developed at this time and encompasses hornblende tonalite through diorite, gabbronorite and pyroxenite. Tonalitic and dioritic rocks are geochemically equivalent and exhibit a steep increase in K2O with silica, implying compositional control by fractional crystallisation, though relationships to cumulate rocks are obscure. Crystallisation of foliated hornblende tonalites is estimated at ∼460–660 MPa from hornblende‐plagioclase geobarometry. These rocks were supplanted by late syn‐compressional plutons of the Tuloona, Harrow and Loftus Creek magma types, which post‐date D2, but are weakly deformed by D5. Tuloona types are predominantly tonalitic and comprise the elongate central granitic batholith. Mafic igneous enclaves are ubiquitous, although more felsic varieties have magmatic muscovite and are complexly interleaved with migmatites. Harrow type granitic rocks to the northeast contain abundant muscovite, metasedimentary enclaves and are transitional to migmatitic envelopes, consistent with derivation by in situ anatexis of the host metasedimentary sequence. The subsequent transition to higher K magmatism is marked by intrusion of Loftus Creek type hornblende granodiorite, of which biotite phenocrysts and a high Sr signature are diagnostic. Considerably shallower emplacement levels (∼200–360 MPa) require exhumation of up to 11 km of crust after formation of Tuloona types. Following cessation of compressional deformation, the final igneous activity in the Glenelg River Complex involved intrusion of distinctive K‐rich adamellite and granite in the far west. This coherent plutonic sequence is more elaborate than documented from the eastern Mt Lofty Ranges in South Australia and conveys a more dynamic impression of magmatism within the trans‐Gondwana Delamerian Orogen than has hitherto been appreciated.

Delamerian Glenelg tectonic zone, western Victoria: geology and metamorphism of stratiform rocks

The Cambro‐Ordovician Glenelg tectonic zone of western Victoria is a distinctive metamorphic‐igneous segment of the Delamerian Orogenic Belt comprising two northwest‐striking regional metamorphic segments of andalusite‐sillimanite type prograding towards an axial granitic batholith. The second of five deformations (D2) was most significant, producing isoclinal folds, transposition and a pervasive regional foliation (S2). Southwest of the central batholith, biotite to migmatite zones contain mainly quartzo‐feldspathic rock (turbiditic metagreywacke, quartzo‐feldspathic schist and migmatite), plus less common metaquartzite and calc‐silicate rocks and minor metapelite. Metagabbro, metadolerite and amphibolite typically have the chemistry of mid‐ocean ridge basalts. Serpentinite pods and sheets were tectonically introduced to low‐grade areas. Northeast of the central batholith, quartzo‐feldspathic rock occupies the sillimanite and migmatite zones exclusively, with a regional concentration of pegmatites adjacent to the zone boundary. Gross interleaving of quartzo‐feldspathic schist, migmatite, pegmatite and muscovite‐bearing granitic rock is characteristic. Peak metamorphic conditions of 550 MPa at 640°C leading to migmatite formation were established by D2 time and accompanied by tonalite‐granodiorite and pegmatite emplacement. Subsequently, the thermal high contracted to the northeast culminating in the more extensive syn‐, post‐D4 to pre‐D5 granitic magmatism.

Geological affinities of the Glenelg river complex, western Victoria

A detailed comparison between the Glenelg River Complex in the Wando Vale district of western Victoria and the South Australian Kanmantoo Group of the Mt Lofty Ranges reveals many common elements. As the basis of comparison the geology of Wando Vale is described concisely with emphasis on its magmatic and structural history. Both have similar metasedimentary rocks, which are siliciclastically dominated with a substantial calcareous component. Geochemically, these rocks are relatively rich in CaO and Na2O, and compositionally distinct from the mature turbidites of the Lachlan Orogenic Belt. Four deformational episodes are identified in the Glenelg River Complex and are comparable to the history of the Kanmantoo Group in the Tungkillo‐Palmer region. The regional metamorphic zonation of both units extends into the amphibolite facies and is of the andalusite‐sillimanite type with similar mineral crystallization sequences. Tholeiitic dykes with comparable timing, the geochemical affinities of mid‐ocean ridge b...

The parnell quartz monzonite: A proterozoic zoned pluton in the archaean pilbara block, Western Australia

The Parnell Quartz Monzonite in the Pilbara Block of Western Australia is a Proterozoic (1731 ± 14 Ma) pluton characterized by high modal K‐feldspar and a greater abundance of hornblende relative to biotite, as is typical of Phanerozoic monzonitic rocks in eastern Australia. The only geochemical features reflecting its setting in an Archaean terrain are high Na2O, Ni and Cr. The pluton is zoned, with an increase in K‐feldspar, quartz and biotite and a decrease in plagioclase and hornblende from margin to core. Chemically, this zoning is reflected by systematic variation of CaO, K2O, Na2O, Sr and Rb, but ferromagnesian elements have irregular trends, implying preferential extraction of feldspars relative to mafic minerals during differentiation of the magma. The unusual geochemical trends are explained by a model involving ‘in situ’ feldspar fractionation of a K‐rich residual liquid from a mafic crystalline mush. A parent magma similar to the average rock composition of the pluton is deduced because high f...