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.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.