Petrogenesis of highly fractionated rhyolites in the southwestern Okinawa Trough: Constraints from whole-rock geochemistry data and Sr-Nd-Pb-O isotopes

作者：Chen, Zuxing; Zeng, Zhigang^*; Yin, Xuebo; Wang, Xiaoyuan; Zhang, Yuxiang; Chen, Shuai; Shu, Yunchao; Guo, Kun; Li, Xiaohui

来源：Geological Journal, 2019, 54(1): 316-332.

DOI：10.1002/gj.3179

• fractional crystallization
• magma chamber
• magma mixing
• rhyolite
• southwestern Okinawa Trough
• Sr-Nd-Pb-O isotopes

In the southwestern Okinawa Trough, a cluster of approximate 70 submarine volcanoes is located at the cross back-arc volcanic trail (CBVT), with the eruptive products dominated by rhyolites. New whole-rock major and trace element and Sr-Nd-Pb-O isotopic data and previously available geochemistry data of volcanic rocks from this region are presented to determine the petrogenesis of these rhyolites. The rhyolites have high SiO2 (>70.01 wt.%), K2O + Na2O (6.71-7.16 wt.%) contents and differentiation index (DI) values (DI > 93), and are metaluminous to weakly peraluminous (A/CNK = 0.99-1.11) and medium-K calc-alkaline in composition. The relatively low 10(4)*Ga/Al (2.06-2.17) and FeOMgO ratios (4.50-6.62), as well as Zr (91-174 ppm) and Zr + Ce + Nb + Y (175-271) contents suggest that the rhyolites, which contain hornblende, are generally similar to those of typical highly fractionated I-type granitoids. The rhyolites show arc-related features such as enrichment in LILEs (Rb, Ba, K), Th, U, Pb, and LREEs ((La/Yb)(N) = 6.21-6.74) and strong depletion in HFSEs (Nb, Ta, P, Ti) with both nearly identical and more enriched isotopic compositions, including epsilon Nd (from -3.2 to -3.5), Sr-87/Sr-86 ratios (0.705552 to 0.705713), delta O-18 values (+8.1 parts per thousand to +8.6 parts per thousand), and Pb isotopic compositions (Pb-206/Pb-204 = 18.568-18.589, Pb-207/Pb-204 = 15.641-15.656, and Pb-208/Pb-204 = 38.848-38.904). Based on these characteristics, we propose that subduction-related basalt magma and crust-derived magma mixed and homogenized within a shallow magma chamber with a homogeneous "base-level" isotopic signature of >0 center dot 7055 (Sr-87/Sr-86) and > -3 (epsilon Nd). This magma would have then experienced the extensive fractional crystallization of minerals, including plagioclase, hornblende, apatite, sphene, and Fe-Ti oxides, to form the CBVT rhyolites. Our geochemical interpretation of the existence of a long-lived, convecting magma chamber beneath the CBVT is also consistent with seismic tomography results.

• 出版日期2019-1
• 单位中国科学院海洋研究所; 中国科学院大学