甘肃北山小西弓金矿区石英正长斑岩LA-ICP-MS锆石U-Pb年龄和地球化学特征

摘要

通过高精度的LA-ICP-MS锆石U-Pb测年，获得北山南带小西弓金矿区石英正长斑岩的形成年龄为247.5±2.2Ma，属中三叠世。地球化学分析表明，该岩体为准铝质、钾玄岩系列，主量元素高硅（SiO2=65.8%~66.2%）、富碱（Na2O+K2O=8.99%~9.41%）、低钙（CaO=1.72%~2.19%）、贫镁（MgO=0.63%~0.70%）；轻稀土元素富集，重稀土元素亏损，弱负Eu异常。大离子亲石元素Rb、Th、U、K和Pb富集，同时Ba、Sr、P、Ti和Eu亏损，并具高104∗Ga/Al值和高Zr+Nb+Ce+Y含量，表现出A型花岗岩的特征。北山南带三叠纪岩浆活动强烈，高分异I型-A型花岗岩大量发育，暗示区域内三叠纪处于造山后伸展环境。%The Xiaoxigong quartz syenite porphyry is located in the south of the Beishan orogenic belt, Northwest China. LA-ICP-MS zircon U-Pb dating yielded a weighted mean 206Pb/238U age of 247.5±2.2Ma for the quartz syenite porphyry, implying middle Tri⁃assic intrusion. Geochemical analyses show that the porphyry is enriched in Si, K, Na, and REE, and depleted in Mg and Ca, with SiO 2 being 65.8%-66.2%, Na2O+K2O 8.99%-9.41%, CaO 1.72%-2.19%, and MgO 0.63%-0.70%. REE are characterized by negative Eu anomalies. The porphyry is enriched in Rb, Th, U, K, Pb, Nb, Zr and Hf and depleted in Ba, Sr, P, Ti and Eu. The quartz syenite por⁃phyry has high Zr+Nb+Ce+Y abundances and 104∗Ga/Al ratios. Petrologic and major and trace elements data all indicate that the Xia⁃oxigong quartz syenite porphyry is a typical A-type granite. The magma probably originated from partial melting of the crust due to underplating mantle-derived magmas and was emplaced in a post-orogenic extensional tectonic setting.