Middle Triassic volcanic rocks from the Gangdese belt, southern Tibet: petrogenesis and implications for Tethys tectonic evolution

The voluminous Middle Triassic to Early Jurassic magmatic rocks in the southern Gangdese belt of Tibet have received wide attention in tracing the subduction history of the Yarlung-Zangbo Neo-Tethys Ocean (YZNTO) and Bangong-Nujiang Tethys Ocean (BNTO). Here we report a suite of volcanic rocks from the Gangdese arc belt and present results from petrological, geochemical and zircon U-Pb-Lu-Hf isotopic studies with a view to constrain their timing and magma source in relation to the Tethys tectonic evolution. Zircon U-Pb data show ages in the range of 237.2–235.0 Ma for the andesitic rocks, representing their eruption ages during the Middle Triassic. Zircon Lu-Hf data display markedly positive εHf(t) values of 11.44–15.99 with the two-stage model ages (TDMC) of 535–241 Ma, suggesting that the magma was derived from depleted mantle or juvenile crust components. Geochemically, the andesitic rocks show low Sr/Y (20.26–43.74) and La/Yb ratios (8.52–14.21) in the absence of Eu anomalies, enrichment in LILEs (K, U, Rb, Ba) and LREEs, and depletion of HFSEs (Nb, Ta, Ti, Zr) and HREEs, suggesting normal arc andesite with emplacement in a continental island arc setting. Accordingly, we infer that these rocks were formed by partial melting of a young oceanic crust and sediments in the transitional zone between plagioclase and garnet stability within subduction-related setting, followed by fractional crystallization of amphibole and apatite. In conjunction with previous studies on the Tethys tectonic evolution, we argue that the southern margin of the Gangdese belt during the Middle Triassic might be unrelated with the northward subduction of the YZNTO and southward subduction of the BNTO. Our results suggest that the Middle Triassic magmatic suites in the southern margin of the south Gangdese belt were generated through the southward subduction of the Zhikong-Sumdo Tethys Ocean.

西藏冈底斯带南部广泛分布的中三叠世至早侏罗世巨量岩浆岩，在示踪雅鲁藏布江新特提斯洋（Yarlung-Zangbo Neo-Tethys Ocean, YZNTO）与班公湖-怒江特提斯洋（Bangong-Nujiang Tethys Ocean, BNTO）的俯冲演化历史领域受到广泛关注。本文报道了一套采自冈底斯弧带的火山岩，并开展了岩石学、地球化学及锆石U-Pb-Lu-Hf同位素研究，旨在约束其形成时代与岩浆源区，以阐明其与特提斯构造演化的关联。锆石U-Pb定年数据显示，这套安山岩的形成年龄介于237.2~235.0 Ma之间，代表其喷发时代为中三叠世。锆石Lu-Hf同位素数据呈现显著正的εHf(t)值（11.44~15.99），两阶段模式年龄（TDMC）为535~241 Ma，指示岩浆起源于亏损地幔或新生地壳组分。地球化学特征上，该安山岩具有较低的Sr/Y（20.26~43.74）与La/Yb比值（8.52~14.21），无明显铕异常，富集大离子亲石元素（Large Ion Lithophile Elements, LILEs，K、U、Rb、Ba）与轻稀土元素（Light Rare Earth Elements, LREEs），亏损高场强元素（High Field Strength Elements, HFSEs，Nb、Ta、Ti、Zr），表明其为形成于大陆岛弧环境的典型弧安山岩。据此，我们认为该套岩石形成于俯冲相关构造背景：年轻洋壳及沉积物在斜长石与石榴石稳定过渡带内发生部分熔融，随后经历了角闪石与磷灰石的分离结晶作用。结合此前关于特提斯构造演化的研究成果，我们提出中三叠世时期冈底斯带南部边缘与雅鲁藏布江新特提斯洋的北向俯冲、班公湖-怒江特提斯洋的南向俯冲均无关联。本研究结果表明，南冈底斯带南部边缘的中三叠世岩浆岩组合，是由直孔-松多特提斯洋（Zhikong-Sumdo Tethys Ocean）南向俯冲形成的。