A burst of middle-late Jurassic high-Mg andesitic magmatism in central Tibet: types, magmatic process, and subduction dynamics

High-Mg andesites (HMAs) formed at convergent plate margins are of special importance in deciphering subduction processes and dynamics. Although abundant, diverse Middle-Late Jurassic (ca. 170–148 Ma) HMAs have been documented in Central Tibet (SW China), and their detailed spatiotemporal evolution and exact genetic and dynamic connections with the subduction of the Bangong-Nujiang Tethyan Ocean (BNTO) are not well constrained. Here, we report newly discovered 157–155 Ma sanukitic HMA dykes in the Qieli Tso ophiolite, central Tibet. Geochemical characteristics and simulation suggest they were generated by low-degree melting (3–6%) of sediment melt-fluxed mantle wedge at 1.8–2.2 GPa, 1230–1260°C, resembling those of sanukite from SW Japan. Comprehensive geochemical analysis of the HMAs in central Tibet divides them into three types with distinctive magmatic processes: (1) boninitic HMAs, generated by melting of fluxed depleted mantle; (2) sanukitic HMAs, generated by melting of sediment melt-fluxed mantle; and (3) adakitic HMAs, generated by melting of subducted slab and lower crust. Moreover, significant spatiotemporal variation is displayed among these HMAs according to their current geography and inferred tectonic settings, i.e. ca. 170–160 Ma FAB-boninitic magmatism in the southern forearc region, ca. 169–155 Ma dominant sanukitic magmatism in the northern continental arc region, and ca. 155–148 Ma dominant adakitic magmatism in between them. These variations attest to a northward and then southward migration of the magmatism front. This migration, together with other geological evidence, is consistent with successive transitions of subduction dynamics of BNTO, likely from subduction initiation through ridge-trench collision to ridge subduction. The prolonged high heat flux during the whole transitioning process results in a burst of high-Mg magmatism.

汇聚板块边缘形成的高镁安山岩（High-Mg andesites, HMAs）对于解读俯冲过程与动力学机制具有特殊重要意义。尽管已有研究在西藏中部（中国西南）报道了大量发育、类型多样的中晚侏罗世（约170–148 Ma）高镁安山岩，但这类岩石的精细时空演化，以及其与班公湖-怒江特提斯洋（Bangong-Nujiang Tethyan Ocean, BNTO）俯冲作用的确切成因与动力学联系，目前仍未得到很好约束。本文报道了在西藏中部切列错蛇绿岩中新发现的157–155 Ma萨努基型高镁安山岩岩脉。地球化学特征与模拟实验表明，该岩脉形成于1.8–2.2 GPa、1230–1260 ℃条件下，由经沉积物熔体交代的地幔楔发生低程度熔融（3%–6%）形成，其地球化学特征与日本西南部的萨努基岩相似。对西藏中部高镁安山岩的综合地球化学分析将其划分为三类，各自具有独特的岩浆成因机制：（1）玻安岩型高镁安山岩，由交代亏损地幔熔融形成；（2）萨努基型高镁安山岩，由沉积物熔体交代地幔熔融形成；（3）埃达克质高镁安山岩，由俯冲板片与下地壳熔融形成。此外，根据当前的地理分布与推断的构造背景，这类高镁安山岩呈现出显著的时空分异：约170–160 Ma的弧前玄武岩-玻安岩型岩浆作用发育于南部弧前带；约169–155 Ma的萨努基型岩浆作用为主导，分布于北部大陆弧带；约155–148 Ma的埃达克质岩浆作用为主导，分布于二者之间的中部区域。上述时空变化表明岩浆作用前锋先向北迁移，随后向南迁移。该迁移过程与其他地质证据均支持班公湖-怒江特提斯洋的俯冲动力学经历了连续演化阶段，即从俯冲起始、洋脊-海沟碰撞直至洋脊俯冲。整个演化过程中持续的高热流背景，最终造就了高镁岩浆作用的爆发式发育。