The petrogenesis of the Devonian granites of the east Tasmania terrane

The granites (sensu lato) of the East Tasmania Terrane (ETT) were emplaced over 20 Myr period during the Devonian period (Black et al., 2005) towards the end of a major period of crustal growth. This thesis investigates aspects of the petrogenesis of the granites to better constrain the processes that led to this crustal growth. A general review of the chemistry of the granites, geochronology of the granites and new zircon O and Lu/Hf isotopes were used to confirm and consider previous findings. <br>The topics have been addressed in 3 manuscripts, the first of these is published and titled “Zircon O and Lu-Hf isotope evidence of mantle and supracrustal origins of Tasmanian Devonian granites”. This part of the thesis aimed to measure O and Lu-Hf isotopes in zircons from the granites to constraint the relationships between the sources of the magmas and to determine if these zircon isotopic compositions can be used to inform granite-related ore endowment. Granites that were previously dated by U-Pb geochronology using SHRIMP had the zircon pits from the dating analyses targeted for O and later in Lu-Hf isotope analyses. The zircon δ18OVSMOW results from the Devonian I-type and S-type granites were similar to those documented by other studies of the Lachlan Fold Belt. Low, mantle-like, zircon δ1⁸O results for the Lisle granodiorite and the Tombstone Creek and Mt Stronach exclude significant amounts of mixing with crustal material and are consistent with extreme fractionation for the latter two granites. The difference in the zircon εHfi for the Tombstone Creek, Lisle and Mt Stronach granites demonstrate they evolved from at least two different mantle/lower crustal reservoirs. The maximum median value of +1 εHf_i for Lisle and Mt Stronach would be compatible with the melting of Cambrian oceanic crust similar in chemistry to MORB. The lower εHf_i (-2) for Tombstone Creek would require an older mafic rock or perhaps a more fractionated arc andesite protolith. The granite zircon εHf_i–δ¹⁸O results can be modelled as two distinct paths from more infracrustal rock values to more supracrustal values that are consistent with melt mixing or assimilation. Result correlations with granites hosting ore deposits suggest different lower crustal/mantle components may be a factor in the apparent ore mineralization difference between the granites evolving from each source. <br>The manuscript “Geochemical and O isotope constraints on the differentiation of Tasmanian Devonian granites” investigates the contribution of late wall rock assimilation by magma. The contribution of assimilation to pluton construction was examined using comparisons of whole-rock (WR) δ¹⁸O calculated from modal mineralogy and both zircon δ₁₈O at zircon crystallization temperatures and quartz δ¹⁸O (δ¹⁸O_qz) at the quartz closure. In 8 of 12 Tasmanian Devonian granites there was no late magmatic contamination with higher or lower δ18O material, two fractionated granites were found to have accumulation of almost 20 % higher δ¹⁸O material, presumably from Mathinna Supergroup rocks, and both these fractionated granites, Mt Stronach and Tombstone Creek have the mantle-like δ¹⁸O_zir indicating at most minimal assimilation of crustal material prior to zircon closure. Two granites showed converse results indicating that the mechanism of late pluton construction varies considerably in the Tasmanian granites. <br>The third manuscript “Genesis of the Early Devonian Haleys New Country mafic and metamorphic complex, eastern Tasmania” investigated magma genesis and mineral geochronology of the Haleys New Country (HNC) pluton in the Blue Tier batholith of the East Tasmania Terrane (ETT). This pluton contains a mafic and basic suite of igneous and metamorphic rocks, the Hogans Road Diorite (HRD), within hornsfelsed Mathinna Supergroup and Devonian granodiorite. Petrography, geochronology, whole-rock Rb/Sr and Sm/Nd and zircon Lu/Hf isotope studies were used to characterize the igneous and metamorphic events and place these events within the regional tectonic regime. Extensive dating studies showed the HRD rocks had crystallized immediately prior to the emplacement of the first Devonian granites. The petrology and chemistry of the rocks suggests they were mixtures between an oceanic or lower crustal olivine-hornblende pyroxenite cumulate xenolith and a basic, high Ca and Al magma. <br>Zircon xenocrysts in the HRD rocks have ages range from 465 to 392 Ma (n = 238) and the ages are clustered around three time intervals: 423.2 ± 2.1 to 419.8 ± 2.1 Ma, 413.5 ± 1.9 to 410.1 ± 1.2 Ma, and 403.6 ± 0.8 to 402.2 ± 0.7 Ma. The time intervals are coincident with periods of extension and sedimentation in the ETT, the Melbourne and the Tabberabbera Zones of the Lachlan Fold Belt, and peaks in the inherited zircon age profiles of the I- and S-type granites in the ETT, the Melbourne Zone and Bendigo Zone granites.