ABSTRACT

The volcanic rocks of the Elashan Formation provide important evidence for the Late Triassic post-collisional magmatism in the East Kunlun Orogenic Belt. This study focused on the geochronology, geochemistry, and petrogenesis of volcanic rocks of the Elashan Formation in the Dulan–Xiangride Basin. The 227.9 ± 0.4 Ma dacite exhibits calc-alkaline I-type characteristics, with low Sr/Y and (La/Yb)_N ratios and prominent negative Eu anomalies. We suggest that the dacite was derived from the partial melting of lower crust at normal crustal thickness. The crystalline tuff (225.0 ± 0.4 Ma) has negative ε_Hf(t) values, implying that it was likely formed by the partial melting of the ancient crust. The rhyolites (221.3 ± 1.1 and 221.9 ± 0.6 Ma) are peraluminous and calc-alkaline to high-K calc-alkaline. The high SiO₂ and Na₂O + K₂O contents, and high K₂O/Na₂O and Ga/Al ratios of these rhyolites have an affinity with A-type granite. The rhyolites were formed by the partial melting of Late Paleoproterozoic–Mesoproterozoic lower crust induced by mantle-derived mafic magma underplating. The andesite and basaltic andesites formed at 221.1–217.9 Ma. The zircon Hf isotopic characteristics show that the andesite was likely derived from the partial melting of the Late Paleoproterozoic mafic lower crust. The combination of our new data and previously obtained geological data suggests that the East Kunlun Orogenic Belt underwent three magmatic activities corresponding to three tectono-magmatic events (slab breakoff, the delamination of unstable lower lithospheric mantle, and the intense asthenosphere upwelling with further delamination and continuous crustal thinning) during the Late Triassic.

摘要

伊拉山组火山岩为东昆仑造山带晚三叠世后碰撞岩浆作用提供了重要证据。本研究的重点是都兰—向日日盆地伊拉山组火山岩的年代学、地球化学和岩石成因。227.9 ± 0.4 Ma英安岩呈现钙碱性I型特征，Sr/Y和(La/Yb) _N比值低，Eu负异常明显。我们认为英安岩来源于正常地壳厚度下地壳的部分熔融。结晶凝灰岩 (225.0 ± 0.4 Ma) 具有负_εHf(t) 值，表明它很可能是由古代地壳的部分熔化形成的。流纹岩（221.3 ± 1.1 和 221.9 ± 0.6 Ma）为过铝质和钙碱性至高钾钙碱性。高 SiO ₂和 Na ₂ O + K ₂ O 含量，高 K ₂ O/Na ₂这些流纹岩的O和Ga/Al比与A型花岗岩具有亲和力。流纹岩是晚古元古代—中元古代下地壳在幔源基性岩浆底侵作用下部分熔融形成的。安山岩和玄武质安山岩形成于 221.1-217.9 Ma。锆石Hf同位素特征表明该安山岩很可能来源于晚古元古代基性下地壳的部分熔融。我们的新数据和先前获得的地质数据相结合表明，东昆仑造山带经历了与 3 次构造-岩浆事件相对应的 3 次岩浆活动（板片断裂、不稳定的岩石圈下层地幔的分层，以及伴随进一步分层的强烈软流圈上涌和地壳持续变薄）在晚三叠世。