新疆哈尔里克山琼祖尔开地区变质安山岩锆石U-Pb年龄和地球化学特征

Zircon U-Pb age and geochemical characteristics of meta-andesite in Qiongzuerkai area of Harlik Mountain, Xinjiang

新疆哈尔里克山广泛出露古生代火山岩,由于缺少可靠的年龄和地球化学数据,其成因及地球动力学背景长期存在争议。为探讨这一问题,选择琼祖尔开地区一套层状分布的火山岩作为研究对象,采集其中的变质安山岩进行U-Pb同位素定年,得到2个样品的^(206)Pb/^(238)U年龄分别为309.9±4.6Ma(MSWD=2.6,n=9)和304.6±1.8Ma(MSWD=0.84,n=11),代表火山岩的结晶年龄。变质安山岩SiO_2含量为49.96%~62.22%,具有低TiO_2含量(0.94%~1.7%)和高Al2O3含量(14.76%~16.25%)的特征。轻、重稀土元素分馏明显,富集轻稀土元素,具负Eu异常(δEu=0.88~0.94);岩石地球化学研究表明,变质安山岩富集大离子亲石元素Th、U、Ce、Rb、Ba,亏损高场强元素Nb、Ta、Ti,具岛弧型火山岩特征。原始岩浆源区可能为地壳熔体和俯冲洋壳熔体组成的混合物,岩浆在上升过程中有地壳物质的加入,岩石成因可能与大洋板块俯冲作用有关。哈尔里克山石炭纪火山活动可能与其南侧康古尔洋盆的演化相关。

The studied volcanic rocks are mainly distributed in Qiongzuerkai area of Harlik Mountain. There are no accurate ages and geochemical data for these rocks, The genesis and the geodynamical background of these volcanic rocks have been controversial for a long time. In order to tackle this problem, the authors chose layered volcanic rocks in Qiongzuerkai area of Harlik Mountain as the study object. LA-ICP-MS zircon U-Pb ages of 309.9±4.6Ma(MSWD=2.6,n=9)and 304.6±1.8Ma(MSWD=0.84,n=11)from two meta-andesites suggest that these volcanic rocks erupted in Late Carboniferous. The meta-andesite is characterized by SiO_2 content from 49.96%~62.22%, low TiO_2(0.94%~1.7%)and high Al_2O_3(14.76%~16.25%). The meta-andesite forms a LREE-enriched distribution pattern with a strong contrast between LREE and HREE and the existence of negative Eu anomalies(δEu=0.88~0.94).Geochemical characteristics of meta-andesite suggest that this rock shows island-arc volcanic characteristics, being enriched in LILE such as Th, U, Ce, Rb and Ba and depleted in HFSE such as Nb, Ta and Ti. The mantle source was made up of crust melt, subducted ocean crust melt and so on, with the addition of crustal material during plate subduction. Combined with regional data, the authors consider that the evolution of the Carboniferous volcanic activity might have been associated with the Kanggurtag basin.