Source composition and melting temperatures of orogenic granitoids: constraints from CaO/Na2O, Al2O3/TiO2 and accessory mineral saturation thermometry
Jung, Stefan; Pfänder, Jörg Albert
European Journal of Mineralogy Volume 19 Number 6 (2007), p. 859 - 870
published: Dec 17, 2007
ArtNo. ESP147051906008, Price: 29.00 €
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.