Abstract
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 (Tzr) and titanium-in-zircon thermometry (T(Ti–zr)) estimate magma temperatures of ~ 795 ± 41 °C (Tzr) and ~ 845 ± 46 °C (T(Ti-zr)) in the deep crust, ~ 735 ± 30 °C (Tzr) and ~ 785 ± 30 °C (T(Ti-zr)) in the middle crust, and ~ 796 ± 45 °C (Tzr) and ~ 850 ± 40 °C (T(Ti-zr)) in the upper crust. The differing averages reflect ambient temperature conditions (Tzr) within the magma chamber, whereas the higher T(Ti-zr) values represent peak conditions of hotter melt injections. Assuming thermal equilibrium through the crust and adiabatic ascent, shallower magmas contained 4 wt% H2O, whereas deeper melts contained 7 wt% H2O. Using these H2O contents, monazite saturation temperature (Tmz) estimates agree with Tzr 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% SiO2) melts. At inferred emplacement P–T conditions of 5 kbar and 730 °C, additional H2O 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-T 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.
Highlights
Crustal anatexis, accompanied by melt extraction and magma ascent to upper-crustal levels, constitutes the most important mechanism for geochemical differentiation1 3 Vol.:(0123456789) 110 Page 2 of 22Contributions to Mineralogy and Petrology (2020) 175:110 of the continental crust (e.g., Vielzeuf et al 1990)
Secondary high‐resolution ion microprobe (SHRIMP) U–Pb geochronology of monazite and zircon indicates synchronous crystallisation at c. 1550 Ma for granites that were emplaced from deep (6–9 kbar), through the middle (4–6 kbar), to upper (0–4 kbar) crustal levels
By applying zircon and monazite thermometry combined with phase equilibrium modelling to granites and volcanic rocks of the Georgetown Inlier (GTI), this study highlights the following:
Summary
Crustal anatexis, accompanied by melt extraction and magma ascent to upper-crustal levels, constitutes the most important mechanism for geochemical differentiation1 3 Vol.:(0123456789) 110 Page 2 of 22Contributions to Mineralogy and Petrology (2020) 175:110 of the continental crust (e.g., Vielzeuf et al 1990). Peraluminous, K-rich (S-type) granitic magmas that intrude at shallow crustal levels or that erupt, are widely modelled as hightemperature (> 800 °C) melt products form under fluidabsent melting conditions at granulite facies (e.g., Clemens et al 2020; Le Breton and Thompson 1988; Vielzeuf and Montel 1994). Field and petrological observations of large S-type granitic batholiths emplaced in high-grade migmatitic terranes (i.e., mid- to low- crustal levels) commonly point to lower temperatures (≤ 750 °C) at upper amphibolite facies, which is possible if waterfluxed melting conditions are considered (e.g., Weinberg and Hasalová 2015). Estimating the temperature and water content of granitic magmas is difficult. Clemens (1984) divided the commonly used methods into four main groups: (1) measurement of quenched magma (i.e., volcanic glass) by gravimetric means; (2) geological inference in a field area where both anatectic source rocks and granitoids crystallised from the partial melts can be studied; (3) experimental determinations (Clemens 1984 and reference therein); 4) Thermodynamic calculations (e.g., Harley 2008; Wheller and Powell 2014; White et al 2007)
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