西藏安多早侏罗世火山岩岩石成因及对班公湖-怒江特提斯洋俯冲消减的制约

发布时间：2018-09-05 17:25

【摘要】：拉萨地块广泛分布着中生代的岩浆活动,研究它们对于认识特提斯洋的演化过程有着重要的启示。本文报道安多地区新识别出的早侏罗世岛弧型火山岩LA-ICP-MS锆石U-Pb年龄、锆石原位Hf同位素、全岩主微量元素分析和Sr-Nd同位素结果。英安岩206Pb/238U年龄为180.7±4.7Ma(MSWD=0.22,n=10),指示其形成时代为早侏罗世。这些火山岩的SiO_2含量为62.50%~76.15%,MgO含量为0.33%~3.05%,Fe_2O_3T含量为2.35%~6.69%;所有样品都显示轻稀土元素相对富集,具有铕的弱负异常,具有富集大离子亲石元素、亏损高场强元素的地球化学特征;同时这些火山岩锆石原位εHf(t)值均为正值(+15.1~+17.1);全岩Sr-Nd同位素εNd(t)值也均为正值(+7.1~+13.2),指示这些火山岩很可能是特提斯洋俯冲消减过程中新生下地壳部分熔融的产物。结合区域地质资料,认为安多火山岩不可能是龙木错-双湖古特提斯洋或雅鲁藏布江新特提斯洋俯冲消减的产物,而是班公湖-怒江特提斯洋俯冲消减的产物,区域上很可能与改则北、日土地区的侏罗纪侵入岩一起构成了班公湖-怒江特提斯洋北侧的岩浆弧带。由此本文认为安多早侏罗世岛弧型火山岩是班公湖-怒江特提斯洋北向俯冲消减的产物,从而为班公湖-怒江特提斯洋的俯冲消减过程提供了直接的火山岩证据。
[Abstract]:The Mesozoic magmatic activities are widely distributed in Lhasa block, and their study has important implications for understanding the evolution of the Tethys Ocean. This paper reports the LA-ICP-MS zircon age of early Jurassic island arc volcanic rocks in Ando area, zircon in situ Hf isotopes, the analysis of main trace elements in the whole rock and the results of Sr-Nd isotopes. The 206Pb/238U age of dolomite is 180.7 卤4.7Ma (MSWD=0.22,n=10), indicating that the age of its formation is early Jurassic. The SiO_2 content of these volcanic rocks is 62.50 and 76.150.The SiO_2 content of these volcanic rocks is 0.33 and 3.05 respectively. The content of Fe2O3T is 2.35 ~ 6.696.690.All samples show relative enrichment of light rare earth elements, weak negative anomalies of europium, geochemical characteristics of enrichment of large ion lithophile elements and depletion of high field strength elements. At the same time, the zircon in situ 蔚 Hf (t) values of these volcanic rocks are positive (15.1-17.1), and the Sr-Nd isotopic 蔚 Nd (t) values of the whole rock are all positive (7.1 ~ 13.2), indicating that these volcanic rocks are probably the product of partial melting of the lower crust during the subduction of the Tethys Ocean. Based on the regional geological data, it is concluded that the Ando volcanic rocks can not be the product of subduction of the ancient Tethys ocean of Longmu Duo-Shuanghu Lake or the New Tethys Ocean of the Yarlung Zangbo River, but of the subduction of the Bangong Lake and the Tethys Ocean of the Nujiang River. It is very likely that the Jurassic intrusive rocks in the northern part of the Bangong Lake-Nujiang River formed the magmatic arc zone along the northern side of the Tethys Ocean of the Bangong Lake-Nujiang River. It is concluded in this paper that the early Jurassic island arc volcanic rocks in Ando are the product of northward subduction of the Bangong Lake-Nu River Tethys and thus provide direct evidence for the subduction process of the Bangong Lake-Nu River Tethys Ocean.
【作者单位】：中国地质调查局成都地质调查中心;四川省地质矿产勘查开发局106地质队;中国地质调查局青藏高原地质研究中心;中国地质大学;
【基金】：国家重点研究发展计划项目(2015CB452601、2016YFC0600305) 四川省基金项目(2014JQ0025) 国家自然科学基金项目(41603038) 中国地质调查局地质调查项目(DD20160016、1212011121262)联合资助
【分类号】：P542;P588.14

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