东天山小铺地区晚古生代中-低压变质带成因研究

Xiaopu Late Paleozoic progressive medium-low pressure metamorphic belt (M-LPMB) outcrop in the southern foot of the Harlik Mountains, eastern Tianshan, which involves a zonal sequence of biotite, garnet, staurolite, andalusite and sillimanite zones from the south to the north. The sillimanite zone had experienced partial melting, including various types of leucogranites. Integrated with the detailed field investigation, petrography, comprehensive analysis of elemental geochemistry, systematic Sr-Nd-Pb-Hf isotopic data and precise geochronology for the research, the aims of this project are to constrain the petrogenesis and genetic relationships between the metamorphic rocks and leucogranites, to estimate the metamorphic P-T-t paths, and to simulate changes of the mineral assemblage, metamorphic reaction and fluid behavior during the metamorphic stages. Additionally, we will present the geodynamic setting and magmatic mechanism of the M-LPMB. The results of this study not only constrain the orogenic evolution of Tianshan during the Late Paleozoic, but also shed new light on understanding the phase relations from lower temperature to higher of metapelite, partial melting of continental crust and origin of leucogranite.

东天山哈尔里克山南麓小铺地区出露一条晚古生代中-低压递增变质带，由南向北可划分为黑云母带、石榴石带、十字石带、红柱石带和矽线石带，其中矽线石带发生了深熔作用，并发育多种类型的淡色花岗岩脉。本项目拟通过野外地质、岩石学、矿物学、矿物/岩石地球化学、Sr-Nd-Pb-Hf同位素地球化学和年代学等方面的研究，查明不同类型变质岩及淡色花岗岩的岩石成因以及各类岩石之间的成因联系，确定各类变质岩石的P-T-t轨迹，模拟变质过程中的矿物组合变化、变质反应和流体行为，进而确定小铺中-低压递增变质带形成的动力学背景及相关的深熔和岩浆作用机理。该项研究不仅可为深入探讨天山造山带晚古生代造山过程提供制约，而且对进一步探讨中-低压泥质岩石从低温到高温过程的相平衡关系、陆壳部分熔融以及淡色花岗岩成因等地球科学问题具有重要意义。

本项目以东天山哈尔里克山南麓小铺地区中-低压递增变质带中的矽线石带及多种类型淡色花岗岩为重点研究对象，通过详细的野外地质调查、岩石学查明矽线石带发生了深熔作用形成了具条带状、云雾状、眼球状以及肠状构造的混合岩，同时带中发育四类淡色花岗岩，包括黑云母淡色花岗岩、二云母淡色花岗岩、石榴石淡色花岗岩以及电气石淡色花岗岩。锆石U-Pb年代学研究结果显示混合岩锆石源岩的形成年龄介于1952~392Ma之间，限定其沉积上限为~392Ma，峰期变质时代为309±2Ma，代表了深熔作用发生的时代；各类淡色花岗岩的侵位时代均为~308Ma，与矽线石带发生深熔作用的时代一致。岩石地球化学研究表明这些淡色花岗岩普遍具有高硅、高铝、高碱，低钛、铁、镁、钙的特点，为弱过铝质钙碱性-钾玄岩系列岩石。其中黑云母淡色花岗岩为砂质源岩经黑云母脱水熔融形成，二云母和电气石淡色花岗岩为泥质源岩经白云母脱水熔融形成，石榴石淡色花岗岩则为高分异型花岗岩。以上研究结果表明原划太古代小铺岩群为古生代地层，而哈尔里克地区在晚石炭世时期便已处于后碰撞构造背景下，为进一步探讨天山地区早古生代构造演化提供了重要依据。