中天山地块南缘两类混合岩的成因及其地质意义

中天山地块是位于中亚造山带西南缘的西天山造山带的重要组成块体,其基底演化和构造亲缘性对恢复西天山的增生造山方式和大地构造格局具有重要意义。混合岩在中天山地块的高级变质地体中广泛分布,是揭示中天山地块基底演化和构造属性的窗口。本文通过开展锆石U-Pb年代学和Hf同位素及岩石地球化学研究,确定了中天山地块南缘乌瓦门杂岩的两类条带状混合岩的原岩性质和形成时代以及混合岩化作用时代和成因机制。第一类条带状混合岩的原岩为中基性岩屑砂岩,混合岩化时代为～1. 8Ga,是在同期角闪岩相变质过程中通过变质分异形成的。第二类条带状混合岩的古成体包括黑云角闪斜长片麻岩和黑云斜长角闪片麻岩,原岩均形成于～2. 5Ga,并叠加～1. 8Ga角闪岩相变质作用,是洋陆俯冲背景下由俯冲洋壳或岩石圈地幔部分熔融形成。侵入古成体的变基性岩墙形成于～1. 72Ga,具有Fe-Ti玄武岩的地球化学特征,起源于后碰撞伸展背景下的软流圈地幔。该类混合岩的浅色体同时穿插古成体和变基性岩墙,呈现突变的野外接触关系,与区域内约787～785Ma混合岩化同期,即混合岩化作用是外来岩浆注入的结果,可能是造山带垮塌引发地壳深熔作用的产物。乌瓦门杂岩记录的～2. 5Ga岩浆活动、～1. 8Ga变质作用和～790Ma混合岩化作用可以和塔里木北缘进行对比,暗示中天山地块是一个具有确切新太古代-古元古代结晶基底的微陆块,并且和塔里木克拉通存在构造亲缘性。

Contrasting migmatites in the southern Chinese Central Tianshan: Petrogenesis and geological implications

The Central Tianshan Block is a major constituent of the Paleozoic Western Tianshan Orogen in the southwestern Central Asian Orogenic Belt, and its geological evolution and tectonic affinity is essential to reconstruction of the architectural framework and subduction-accretion processes of the Western Tianshan Orogen. The ubiquitous migmatites in high-grade metamorphic terranes provide an excellent window to the tectonic evolution and affinity of the crustal basement of the Central Tianshan Block. Here we present zircon U-Pb-Hf isotopic and whole-rock geochemical data for two contrasting migmatites from the Wuwamen complex in the southern Central Tianshan Block. The first type of stromatic migmatite with an intermediate-mafic greywacke protolith underwent migmatization at ca.1.8Ga, resulted from metamorphic differentiation associated with the coeval amphibolite facies metamorphic event. The paleosomes of the second type of stromatic migmatite are composed of interlayered orthogneisses with protolith formation ages at ca. 2.5Ga and an amphibolite facies metamorphic overprint at ca.1.8Ga. Geochemical data reveal that they were derived from partial melting of subducted oceanic crust or lithospheric mantle in an oceanic subduction setting. The meta-mafic dykes intruding the orthogneisses were formed at ca.1.72Ga and display geochemical affinity to Fe-Ti basalts, which are interpreted to be derived from partial melting of the asthenospheric mantle in a post-collisional setting. The leucosomes crosscut the orthogneisses and meta-mafic dykes with a sharp contact relationship, suggesting that migmatization was resulted from injection of external partial melts probably induced by anataxis during orogenic collapse. The~2.5Ga magmatism,~1.8Ga metamorphism and~790Ma migmatization recorded by the Wuwamen complex are comparable to those events in the northern Tarim Craton, and thus we conclude that the Central Tianshan Block is a microcontinent with a Neoarchean-Paleoproterozoic basement and has a close affinity with the Tarim Craton.