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Source composition and melting temperatures of orogenic granitoids: constraints from CaO/Na₂O, Al₂O₃/TiO₂ and accessory mineral saturation thermometry

¹ Philipps-Universität Marburg, Fachbereich Geowissenschaften, Lahnberge/Hans-Meerwein-Straße, 35032 Marburg, Germany
² TU Bergakademie Freiberg, Institut für Geologie, Bernhard von Cotta Straße 2, 09599 Freiberg, Germany

^* Corresponding author, e-mail: jungs{at}staff.uni-marburg.de

Granitoid melts, generated experimentally from various materials by fluid-absent melting, show characteristic major-element compositions that can be used to infer source characteristics and melting temperatures. CaO/Na₂O ratios distinguish between pelite-derived melts (CaO/Na₂O < 0.5) and melts derived from greywackes or igneous sources (CaO/Na₂O: 0.3–1.5). Distinctly more mafic melts (granodiorites and quartz diorites) generated by fluid-absent melting of amphibolite can show even higher CaO/Na₂O ratios, up to 10, although the majority of the melts have CaO/Na₂O ratios between 0.1 and 3. Al₂O₃/TiO₂ ratios reflect the melting temperature, and mathematical formulations are presented that allow using this ratio as a geothermometer for given source compositions. A comparison of temperatures from melting experiments with corresponding Al₂O₃/TiO₂ values indicate a reasonably good correlation (r²: 0.70–0.91), demonstrating the usefulness of temperature estimates in granitoid rocks based on Al₂O₃/TiO₂ systematics. Application to well investigated S-type and A-type granites and quartz diorites from the Damara Belt (Namibia) shows different CaO/Na₂O and Al₂O₃/TiO₂ ratios for all rock types, supporting their origin from different sources at different temperatures. For the quartz diorites, temperature estimates derived from Al₂O₃/TiO₂ ratios, and those derived from apatite solubility in mafic rocks, agree within ± 20 °C. On the other hand, temperature estimates for A-type and S-type granites derived from Al₂O₃/TiO₂ ratios are systematically higher by 50–150 °C compared with those from accessory mineral saturation, suggesting disequilibrium during partial melting of the lower crust.

Key-words: granite, major element ratios, accessory phase thermometry, major element thermometry, geochemistry, partial melting.