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Petrogenesis of Miocene ultrapotassic rocks in the western Lhasa terrane, Tibetan Plateau: evidence from in situ zircon u–pb dating, Hf–O isotopes, and whole-rock geochemistry

Petrogenesis of Miocene ultrapotassic rocks in the western L...

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2014年中国地球科学联合学术年会——专题34：特提斯—青藏高原地质演化与成矿

作者： Shihong Tian Zengqian Hou Xuanxue Mo Zhusen Yang Yue Zhao Wenjie Hu MLR Key Laboratory of Metallogeny and Mineral Assessment Institute of Mineral Resources Chinese Academy of Geological Sciences School of Earth Science and Mineral Resources China University of Geosciences Institute of Geology Chinese Academy of Geological Sciences Jiangxi Provincial Institute of Geological Survey

Major and trace elements, whole rock Sr–Nd–pb isotopic data, zircon u–pb geochronology, and in situ zircon Hf–O isotopic compositions have been determined for the ultrapotassic lavas from Chajiasi, Dangreyong lake, Xuru lake, and Chazi in the western Lhasa terrane of the Tibetan Plateau. The results provide new insights into the nature of the source region of the lavas and their petrogenesis, and also into the mechanism of crust–mantle interaction related to the India–Eurasia continental collision. The lavas are mainly trachyte and trachyandesite. Secondary ion mass spectrometer(SIMS) u–pb analyses of zircons from the lavas yield Miocene ages of 24–11 Ma, consistent with a range of previously published ages from other parts of the Tibetan Plateau. The lavas are characterized by high SiO2, K2 O, and MgO contents, and they are enriched in light rare earth elements and large ion lithophile elements, and depleted in high field strength elements. They have highly radiogenic Sr(87Sr/86Sr(i)=0.715922–0.732492) and pb(206pb/204pb=18.4547–18.8885, 207pb/204pb=15.7394–15.7848, 208pb/204pb=39.3997–39.6433), together with negative values of εNd(t)(–12.8 to –11.3). Altogether, these data indicate the lavas were derived from an enriched mantle with some crustal contamination. Magmatic zircons from the Chajiasi and Dangreyong lake areas display δ18O values of 6.7‰ to 11.3‰ and εHf(t) values of –17.0 to –7.9 with relatively old Hf crustal model ages of 1.61 to 2.19 Ga, indicating little or no involvement of juvenile crust and/or new mantle-derived materials. However, magmatic zircons from the Xuru lake and Chazi areas exhibit δ18O values in the range 6.8‰ to 8.5‰ and εHf(t) values from –13.1 to –6.3, with relatively old Hf crustal model ages varying from 1.50 to 1.93 Ga. The negative correlation between δ18O and εHf(t) values indicates a two-component mixing of mantle and crust-derived melts. All the εHf(t) and 176Hf/177 Hf values fall around the line of the lower crust, indicating

关键词： u–pb zircon ages geochemistry in situ zircon Hf–O isotopes ultrapotassic lavas magma source petrogenesis Lhasa terrane

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