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 follated 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 inter-leaved 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.