ABSTRACT

The Triassic igneous complexes in the east flank of the East Kunlun orogenic belt (E-KOB) provide a window through which to investigate the Mesozoic magmatism in the E-KOB. Here, we report the precise timing of Triassic magmatism and magma petrogenesis of different rock types in the east flank of the E-KOB based on the petrology, mineral chemistry, geochemistry, zircon geochronology and Lu-Hf isotopes. Zircon U-Pb dating reveals that the Daheba complex was crystallized in 242–239 Ma. The rocks display enrichment in large-ion lithophile elements (LILEs) and LREEs and depletion in high field strength elements (HFSEs) and HREEs, which are typical of subduction-related magmatic rocks. The fine-grained granites (FGs) are characterized by a high SiO₂ content (72.25–74.28 wt. %), a high differentiation index (DI = 95) and marked depletion in Sr, Ba and Eu, which indicates that the FGs are highly fractionated I-type granitoids. The zircon ε_Hf(t) values from −3.6 to −1.1 with two-stage model ages (T_DM2) of 1496–1340 Ma, which suggests that they were derived from an ancient Mesoproterozoic lower crust. The similarity in the REE patterns of the mafic microgranular enclaves (MMEs) and host granites (HGs) indicates that the mixing between mafic and felsic magmas could have contributed to the chemical variation of the MMEs. The mafic melts were derived from the partial melting of the mantle that had been metasomatized by slab-derived fluids. The MMEs show typical igneous textures, acicular apatites, sieve texture of plagioclase and reverse zoning of plagioclase, which correspond to magma mixing during petrogenesis. Our geochemical and isotopic data suggest that the Daheba complex formed in an active continental margin setting closely related to the northward subduction of the Palaeo-Tethys Ocean Plate beneath the E-KOB.

摘要

东昆仑造山带(E-KOB)东翼的三叠纪火成岩杂岩为研究东昆仑造山带中生代岩浆作用提供了一个窗口。在这里，我们基于岩石学、矿物化学、地球化学、锆石年代学和 Lu-Hf 同位素报告了 E-KOB 东侧不同岩石类型的三叠纪岩浆作用和岩浆岩石成因的精确时间。锆石 U-Pb 测年显示大河坝杂岩结晶形成于 242-239 Ma。岩石显示大离子亲石元素 (LILE) 和轻稀土元素富集，高场强元素 (HFSE) 和重稀土元素贫化，这是与俯冲有关的岩浆岩的典型特征。细粒花岗岩 (FGs) 的特征是高 SiO ₂含量 (72.25–74.28 wt. %)、高分化指数 (DI = 95) 和 Sr、Ba 和 Eu 的显着损耗，这表明 FG 是高度分馏的 I 型花岗岩。锆石 ε _Hf (t) 值从-3.6 到-1.1，两阶段模型年龄 (T _DM2) 的 1496-1340 Ma，这表明它们来自古老的中元古代下地壳。镁铁质微粒飞地 (​​MME) 和寄主花岗岩 (HG) 稀土元素模式的相似性表明，镁铁质和长英质岩浆之间的混合可能导致 MME 的化学变化。镁铁质熔体源自地幔的部分熔化，地幔已被板块衍生的流体交代。MMEs表现出典型的火成岩结构，针状磷灰石，斜长石的筛状结构和斜长石的反分带，与成岩过程中的岩浆混合相对应。我们的地球化学和同位素数据表明，大河坝杂岩形成于与 E-KOB 下方的古特提斯洋板块向北俯冲密切相关的活跃大陆边缘环境中。