Petrogenesis of the post-collisional oxidized felsic potassic rocks: A case study from the Jianchuan trachytes in eastern Tibetan plateau

Abstract

Petrogenesis of post-collisional felsic potassic rocks and major controls on their magmatic redox states are still debated. To address this, we have carried out an integrated petrological-geochemical study for the trachytes in Jianchuan area, eastern Tibetan plateau. Zircons from the trachytes yield a U-Pb age of ~35 Ma. These rocks are characterized by significant LREE and LILE enrichments, pronounced negative Nb-Ta-Ti-P anomalies and enriched Sr-Nd isotopic compositions, similar to those of the nearby, coeval mafic potassic volcanic rocks (e.g., Wozhong shoshonites). The trachytes have zircon ε Hf from 0.04 to 1.48 and δ 18 O from 7.06 to 7.77 clinopyroxene ( 87 Sr/ 86 Sr) i from 0.70571 to 0.70746. The above-mentioned similarities, together with uniform whole-rock and mineral isotopes and modeled parental magma compositions using clinopyroxene trace elements, strongly support the interpretation that the trachytes were formed by fractional crystallization of mantle-derived mafic potassic parental melts similar to the Wozhong shoshonites. Using the amphibole, iron-titanium oxide and sphene-magnetite-quartz oxybarometers, magmatic oxidation states of the trachytes are estimated to be at ΔFMQ between 1.49 and 2.86, similar to the previously reported values for the Wozhong shoshonites. Our results demonstrate that source is the first-order control on magmatic oxygen fugacity of the Jianchuan trachytes, implying such mantle-derived felsic potassic rocks can also track redox conditions of mantle source. Our results also suggest that, Eocene post-collisional mantle-derived potassic magmas in western Yangtze Craton are oxidized in nature, which are crucial for post-collisional porphyry Cu-Au formation.