西天山特克斯北中酸性火成岩地球化学特征及成因意义

Geochemistry and geodynamics of intermediate-acid igneous rocks in northern Tekesi,Western Tianshan Mountains

新疆西天山特克斯县城北部伊特公路沿线和库勒萨依出露大量中酸性火成岩,伊特公路沿线为石英钠长斑岩,库勒萨依为石英闪长斑岩和花岗闪长斑岩。岩石地球化学和同位素组成研究表明,前者为典型的岛(陆)弧带火成岩,而后者具有埃达克岩的成分特征,两者均为古亚洲洋壳在俯冲过程中岩浆活动的产物。早先俯冲的较冷洋壳板片在深处脱水诱发上覆地幔楔熔融,熔体上升并经历壳幔相互作用等过程引发伊特公路一带弧岩浆活动;由于洋壳持续俯冲,后来新形成的靠近洋脊的年轻板片由于高热在较浅处直接发生部分熔融形成埃达克岩浆,并上侵至库勒萨依一带。库勒萨依斑岩体SIMS锆石U-Pb年龄为342.5±2.3Ma,属于早石炭世。两组中酸性火成岩的地球化学特征表明,古亚洲洋(南天山洋)在早石炭世还未完全闭合,洋壳向北的持续俯冲过程造成伊犁-中天山板块南缘广泛的岩浆活动,此时西天山陆壳增生方式主要为侧向增生,增生物质主要为洋壳板片(埃达克岩)和洋壳板片流体交代的地幔楔成分。

A large number of intermediate-acidic igneous rocks are exposed along Yi-Te Road and in Kulesayi area, the north of Tekesi County, Western Tianshan Mountains, Xinjiang, China. These rocks are quartz-albite porphyry along Yi-Te Road and quartz diorite and granodiorite in Kulesayi area. They were produced during subducting of the Paleo-Asian oceanic crust. According to their geochemical features and isotopic compositions, the quartz-albite porphyries are typical island/continental arc rocks, and quartz diorite and granodiorite have the compositions of adakite characteristics and may have been produced by slab melting. The earlier cold subducted slab dehydrated before it could begin to melt, and the released fluid ascented into the mantle wedge, which caused melting of mantle wedge and succedent crust-mantle interaction, giving rise to the typical arc magmatism in Yi-Te Road area. As the subduction of oceanic crust continued, the newly formed plate close to the ridge would be hot enough to melt in a shallower depth leaving garnet and rutile as residual phases. This process gave rise to adakitic magmatism in Kulesayi area. Secondary ion microprobe（SIMS） zircon U-Pb dating indicates the Kulesayi porphyry crystallized at the age of 342. 5±2. 3Ma, which means the Paleo-Asian Ocean（ South Tianshan Ocean） had not been completely closed in the Early Carboniferous. These resuhs show that the massive magmatism along southern margin of Yili-Central Tianshan plate was triggered by continuing subduetion of South Tianshan Oceanic crust. As thus in Carboniferous, crust growth in Western Tianshan Mountains was possibly mainly horizontal accretion, and the accretion materials may contain oceanic slab（adakite） and mantle wedge comoonents.