Occurrence and petrogenesis of diverse S-type granites in an extensional tectonic setting: a case study from the Wongwibinda Complex, eastern Australia

The high-temperature–low-pressure (HTLP) Wongwibinda Metamorphic Complex (WMC) hosts four distinct types of early Permian S-type granitoids belonging, or related, to the Hillgrove Supersuite. We use field relationships, age data and petrogenetic modelling to determine the petrogenesis of these granitoid types. Biotite-only bearing granitoids are the most voluminous (∼50% of the WMC area). A newly characterised second type (&lt;1% of the WMC), here defined as the Wongwibinda monzogranite, has nearly equal proportions of muscovite and biotite, and a distinct chemical signature. The third and fourth types (&lt;1% of the WMC) include pegmatite dykes and a garnet-bearing leucogranite. THERMOCALC modelling of water-fluxed and dehydration partial melting of local metasedimentary rock compositions indicates that the biotite-only granitoids are consistent with crustal melts produced by 30–40% dehydration partial melting of accretionary complex metasedimentary rocks at 6–9 kbar, with an admixture of ∼30% mafic magma. In contrast, the Wongwibinda monzogranite is a nearly pure S-type granite with its composition influenced by assimilated migmatite xenoliths. One viable model for its geochemistry involves a mix of 45% water-fluxed 5–10% partial melt of metasedimentary rocks at 3–6 kbar, 65% assimilated local migmatites and inclusion of 10% residual biotite following the extraction of 20% haplogranite melt, although alternative scenarios are plausible. The leucogranite can be modelled through both dehydration and water-fluxed melting of metasedimentary rocks at 3–6 kbar, while the pegmatite composition is consistent with 5% water-fluxed or dehydration partial melts at 3 kbar. It is plausible that the pegmatites, leucogranite and Wongwibinda monzogranite formed coevally with peak metamorphism in the complex. In contrast, the emplacement of the biotite-only Hillgrove Supersuite plutons occurred in the later stages of thermal perturbation, as melts from the deep crust ascended <i>via</i> the Wongwibinda Shear Zone. Small volumes of two-mica Wongwibinda monzogranite type granitoids are commonly associated with HTLP metamorphism in extensional tectonic settings. Four granitoid types are recognised within the WMC. One is newly named and described. Petrogenetic modelling and isotope data indicate that three are pure S-types formed from local metasedimentary rocks. The most voluminous plutons are concluded to be a mix of S-type melts with minor mafic magma.