Continental collisional effects on magma source redox state and water content: Insights from thermodynamic modeling

Abstract Arc magmas are typically more oxidized and hydrous than postcollisional magmas, but the geochemical signatures reflecting these critical differences remain poorly understood. This study examines variations in the redox state and water content of magmatism in the West Kunlun Orogenic Belt, northwestern Tibetan Plateau, using Rhyolite-MELTS and Perple_X thermodynamic modeling. We identified closely comparable samples from two distinct magmatic episodes: a Late Ordovician to Early Silurian episode (ca. 444–441 Ma), comprising Datong monzonite-syenites and Kangxiwa monzogranites; and a Late Silurian to Early Devonian episode (ca. 420–409 Ma) comprising Saitula monzonites and North Kudi syenogranites. Elemental and Sr-Nd-Hf-O isotopic data indicate that both intermediate rock series (i.e., Datong monzonite-syenites and Saitula monzonites) formed through fractional crystallization of mantle-derived basaltic magmas, while the granitic rock series (i.e., Kangxiwa monzogranites and North Kudi syenogranites) formed through partial melting of metasedimentary rocks. Despite their similar sources and petrogenetic processes, the two magmatic episodes display distinct compositional characteristics. For instance, the Datong monzonite-syenites exhibit lower TiO2 and higher Nb/Ta ratios, as well as depleted Zr and Hf than the Saitula monzonites. Similarly, the Kangxiwa monzogranites show lower K2O, higher CaO, and consequently lower K2O/CaO ratios than the North Kudi syenogranites. Thermodynamic and trace-element modeling indicate that such differences may arise from variations in oxygen fugacity and water content. Late Ordovician to Early Silurian magmas formed under more oxidizing and hydrous conditions (H2O = ~4 wt%, ΔQFM [quartz-fayalite-magnetite] = +1.0–+2.0), whereas Late Silurian to Early Devonian magmas crystallized in a more reduced, anhydrous environment (H2O = ~1 wt%, ΔQFM = −0.5 to +0.5). Combined with existing data, our results are consistent with continental collision in the West Kunlun Orogenic Belt at ca. 430–420 Ma. This study highlights how variations in the geodynamic setting influence magma differentiation, specifically through changes in the redox state and water content of magmas between subduction and postcollisional settings. Furthermore, our findings provide a potential timing constraint for continental collision in other orogenic belts.