西准噶尔谢米斯台山中段晚石炭世A型花岗岩地球化学特征及岩石成因

西准噶尔谢米斯台山中段发育大量晚石炭世A型花岗岩, 岩性主要为石英正长岩-正长花岗岩-正长花岗斑岩-二长花岗岩。通过LA-ICP-MS锆石U-Pb定年, 获得正长花岗斑岩、石英正长岩、正长花岗岩和二长花岗岩年龄加权平均值分别为301.7±2.5 Ma、302.1±3.6 Ma、301.6±3.1 Ma和305.3±2.5 Ma。岩石属高钾钙碱性和钾玄岩系列, 具有高硅、富碱、过铝质、贫钙、低镁、高TFeO/MgO值(>3.45)等特征, 同时岩石相对富集轻稀土元素(Eu除外)和大离子亲石元素(Rb、K), 相对亏损Ba、Nb、Ta元素, 强烈亏损Sr、P、Ti元素, 与典型铝质A型花岗岩特征一致。岩石εHf(t)均为正值(8.93~14.82, 平均12.64), 二阶段模式年龄(正长花岗岩平均452.01 Ma、二长花岗岩平均565.17 Ma)与形成年龄相近。认为该地区A型花岗岩形成于后碰撞伸展背景, 由于软流圈上涌, 诱发了俯冲流体交代过的下地壳发生部分熔融, 岩浆演化过程中经历了斜长石、辉石、磷灰石、榍石等矿物的结晶分异, 并在岩浆活动初期具有火山弧印迹(A2型), 随着板内进一步伸展, 出现了板内裂谷特征(A1型), 为区域地质演化提供了新的依据。

Geochemical characteristics and petrogenesis of Late Carboniferous A-type granites in the middle section of Xiemisitai Mountain,West Junggar

A large number of Late Carboniferous A-type granites developed in the middle of Xiemisitai Mountain, West Junggar.The main lithologies are quartz syenite-syenogranite-orthoclase granite porphyry-monzonitic granite.The LA-ICP-MS zircon U-Pb dating results show that the weighted mean ages of orthoclas granite porphyry, quart syenite, syenogranite and monzonitic granite are 301.7±2.5 Ma, 302.1±3.6 Ma, 301.6±3.1 Ma and 305.3±2.5 Ma.The rocks belong to the high potassium calc-alkaline and shoshonite series, they have high silicon, rich alkal, peraluminous, low calcium, low magnesium, high TFeO/MgO value(>3.45)and other characteristics.The rocks are relatively enriched in light rare earth elements(except Eu)and large ion lithophile elements(Rb, K), relatively depleted in Ba, Nb, Ta, and strongly depleted in Sr, P, Ti, which is consistent with the characteristics of typical aluminum A-type granite.The εHf(t)of the rocks is positive(8.93~14.82, average 12.64), and the two-stage model age(average 452.01 Ma of syenogranite and average 565.17 Ma of monzogranite)is similar to the formation age.Accordingly, it is considered that the A-type granite in this area is formed in the post-collision extension background.Due to the upwelling of the asthenosphere, the melting of the lower crust previously metasomatized by the subduction-related fluid occured.It has experienced the crystallization differentiation of minerals such as plagioclase, pyroxene, apatite and sphene, and showed volcanic arc imprinting(A2 type)in the early stage of magmatic activity.With the further extension of the plate, the characteristics of intraplate rift(A1 type)appear, which provides a new basis for regional geological evolution.