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Abstract/Summary
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<jats:title>Abstract</jats:title><jats:p>Accessory mineral thermometry and thermodynamic modelling are fundamental tools for constraining petrogenetic models of granite magmatism. U–Pb geochronology on zircon and monazite from S-type granites emplaced within a semi-continuous, whole-crust section in the Georgetown Inlier (GTI), NE Australia, indicates synchronous crystallisation at 1550 Ma. Zircon saturation temperature (<jats:italic>T</jats:italic><jats:sub>zr</jats:sub>) and titanium-in-zircon thermometry (<jats:italic>T</jats:italic><jats:sub>(Ti–zr)</jats:sub>) estimate magma temperatures of ~ 795 ± 41 °C (<jats:italic>T</jats:italic><jats:sub>zr</jats:sub>) and ~ 845 ± 46 °C (<jats:italic>T</jats:italic><jats:sub>(Ti-zr)</jats:sub>) in the deep crust, ~ 735 ± 30 °C (<jats:italic>T</jats:italic><jats:sub>zr</jats:sub>) and ~ 785 ± 30 °C (<jats:italic>T</jats:italic><jats:sub>(Ti-zr)</jats:sub>) in the middle crust, and ~ 796 ± 45 °C (<jats:italic>T</jats:italic><jats:sub>zr</jats:sub>) and ~ 850 ± 40 °C (<jats:italic>T</jats:italic><jats:sub>(Ti-zr)</jats:sub>) in the upper crust. The differing averages reflect ambient temperature conditions (<jats:italic>T</jats:italic><jats:sub>zr</jats:sub>) within the magma chamber, whereas the higher <jats:italic>T</jats:italic><jats:sub>(Ti-zr)</jats:sub> values represent peak conditions of hotter melt injections. Assuming thermal equilibrium through the crust and adiabatic ascent, shallower magmas contained 4 wt% H<jats:sub>2</jats:sub>O, whereas deeper melts contained 7 wt% H<jats:sub>2</jats:sub>O. Using these H<jats:sub>2</jats:sub>O contents, monazite saturation temperature (<jats:italic>T</jats:italic><jats:sub>mz</jats:sub>) estimates agree with <jats:italic>T</jats:italic><jats:sub>zr</jats:sub> values. Thermodynamic modelling indicates that plagioclase, garnet and biotite were restitic phases, and that compositional variation in the GTI suites resulted from entrainment of these minerals in silicic (74–76 wt% SiO<jats:sub>2</jats:sub>) melts. At inferred emplacement <jats:italic>P–T</jats:italic> conditions of 5 kbar and 730 °C, additional H<jats:sub>2</jats:sub>O is required to produce sufficient melt with compositions similar to the GTI granites. Drier and hotter magmas required additional heat to raise adiabatically to upper-crustal levels. S-type granites are low-<jats:italic>T</jats:italic> mushes of melt and residual phases that stall and equilibrate in the middle crust, suggesting that discussions on the unreliability of zircon-based thermometers should be modulated.</jats:p>
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Hinweis
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