Genesis of the Eocene oxidized potassic felsic volcanic rocks in western Yunnan, China: Insights into mantle redox conditions and post-collisional porphyry Cu-Au formation

Cenozoic post-collisional potassic igneous suites with compositions varying from mafic to felsic members are widespread in western Yunnan, China, but the major controls on magmatic redox states of the felsic members remain not well understood. To answer this outstanding question, we have carried out an integrated petrological-geochemical study for the potassic felsic volcanic rocks in Jianchuan area, western Yunnan. The samples from Jianchuan are trachytes, containing 59.74-63.96 wt% SiO2 and 5.50-6.72 wt% K2O, with K2O/Na2O of 1.43-2.02. Zircon grains from the Jianchuan trachytes yield a U-Pb age of similar to 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, which are very similar to those of the mafic endmember in the region, such as the Wozhong shoshonites. The Jianchuan trachytes have zircon epsilon Hf from 0.04 to 1.48 and delta O-18 from 7.06 to 7.77 parts per thousand, apatite epsilon Nd from -3.5 to -2.5, and clinopyroxene (87Sr/86Sr)i from 0.70571 to 0.70746. The above-mentioned similarities, together with uniform whole-rock and mineral isotope compositions, as well as modeled parental magma compositions using trace elements of clinopyroxenes, strongly support the interpretation that the Jianchuan trachytes were formed by fractional crystallization of mantle-derived potassic mafic parental melts similar to the Wozhong shoshonites. Elements and Sr isotopes of clinopyroxenes suggest multiple magma recharge from the same source during the formation of the Jianchuan trachytes. Using the amphibole and sphene-magnetitequartz oxybarometers, magmatic oxidation states of the Jianchuan trachytes are estimated to be at Delta FMQ (FMQ = fayalite + magnetite + quartz redox buffer) between 1.74 and 2.66, similar to the previously reported values for the Wozhong shoshonites. Our new results show that source, rather than magmatic evolution processes, is the first-order control on magmatic oxygen fugacity of the Jianchuan trachytes. Our results also suggest that, Eocene post-collisional mantle-derived potassic magmas in western Yangtze Craton are most likely oxidized in nature, which are likely crucial for post-collisional porphyry Cu-Au formation in the region.