PLAGIOCLASE 87SR/86SR RATIOS AND ZIRCON U-PB AGES FOR YOUNG VOLCANIC ROCKS FROM NW TIBETAN PLATEAU: EVIDENCE FOR OPEN-SYSTEM MAGMATIC PROCESSES AND CONDITIONS

The Ashikule Volcanic Cluster (AVC) represents the youngest post-collisional volcanism of the northwestern Tibetan Plateau. Two of the most prominent members of the AVC, Ashishan and Wulukeshan, are investigated for magmatic processes. Here the ⁸⁷Sr/⁸⁶Sr ratios of plagioclase are compared with bulk rock ⁸⁷Sr/⁸⁶Sr ratio for both of these volcanoes, since mineral-scale isotope compositions are more sensitive in detecting magma assimilations compared with whole-rock composition. If we consider potential magma heterogeneity in ⁸⁷Sr/⁸⁶Sr of 0.001 within a closed-system magma chamber, then the plagioclase differing from the whole rock by >0.001 in ⁸⁷Sr/⁸⁶Sr values reflect open-system processes (mantle-derived magma recharge or crustal contaminations). Sr isotopic compositions for 12 plagioclase grains for two samples from Ashishan volcano have been obtained and more plagioclase grains are being analyzed for Sr isotopic compositions. Multiple plagioclase grains of Ashishan have ⁸⁷Sr/⁸⁶Sr values higher than their whole-rock values by more than 0.001 (up to 0.006), clearly indicating open-system process. Sr isotopic compositions of pyroxene grains are also being analyzed for a more comprehensive assessment of magmatic processes.

Zou et al. (2020) reported U-series (²³⁸U-²³⁰Th) ages (70 ka, 105 ka, and 290 ka) of the youngest zircons and found abundant xenocrystic zircons with ages beyond the range for U-series (>375 ka). Here we report their U-Pb zircon ages and zircon trace element compositions from the Ashishan volcano. U-Pb ages from Ashishan indicate multiple age populations with young phenocrystic (0.23±0.33 Ma and 0.45±0.46 Ma) and older (109-1,732 Ma) xenocrystic zircons. The older zircons likely were entrained in the magma during ascension through open system processes. Zircon trace element compositions of both zircon populations are used to constrain magmatic conditions such as temperature, oxygen fugacity, and crustal thickness.