Middle Permian Seamount from Xiahe Area, Gansu Province, Northwest China: Zircon U-Pb Age, Biostratigraphy and Tectonic Implications

摘要

The well-preserved seamount buildups are documented from the northwestern Qinling （秦岭） orogenic belts, Northwest China. The study sections are located in the Ganjia （甘加） area of the Xiahe （夏河） County, Gansu （甘肃） Province. The dark basalt and overlying massive reef carbonate characterize the Xiahe seamount buildup. Basalts are dominated by the olivine type of rocks and bear distinct porphyritic textures, and fumarole and amygdaloidal structures. The basalts are dominated by SiO2 （up to 48.49 wt.%–52.29 wt.%） followed by （Na2O+ K2O） （3.80 wt.%–4.96 wt.%） and TiO2 （2.04 wt.%–2.52 wt.%）. They are featured by considerably high content of Ti. The tholeiiteseries rocks dominate the basalts, while calc-alkali-series rocks are also present. The REE of the basalts shows the LREE-enrichment type with distinct positive Eu abnormal. The trace elements of the basalts are characterized by the lack of P and high content of Ti. These geochemical signals suggest that the Xiahe basalts were formed in an ocean-island setting. The LA ICP-MS zircon U-Pb age of the basalts is267.6±5 Ma, which is reinforced by the presnce of the fusulinid Neoschwagerina Zone of the Wordian （Middle Permian） in the limestone interbeds of the basalts. Integration of petrological and geochemical studies of seamount basalts and lateral correlation of seamount buildups reveals that the Qinling-Qilian-Kunlun orogenic belts were probably the archipelagic oceans during the Permian.