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.

形成于汇聚板块边缘的高镁安山岩（High-Mg andesites, HMAs）对于解析俯冲过程及其动力学机制具有特殊重要意义。尽管已有研究在中国西南的西藏中部报道了大量赋存多样的中晚侏罗世（约170–148 Ma）高镁安山岩，但这类岩石的精细时空演化特征，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和动力学联系，迄今仍未得到明确约束。本文报道了在西藏中部乞力措蛇绿岩中新发现的157–155 Ma sanukitic型高镁安山岩（sanukitic HMA）岩脉。地球化学特征与模拟实验结果表明，该类岩脉形成于1.8–2.2 GPa、1230–1260 ℃的温压条件下，由受沉积物熔体交代的地幔楔发生低程度熔融（3–6%）而成，其地球化学特征与日本西南部产出的sanukite岩（sanukite）相似。通过对西藏中部高镁安山岩的综合地球化学分析，可将其划分为三类具有独特岩浆演化机制的类型：（1）玻安岩型高镁安山岩，由经交代的亏损地幔熔融形成；（2）sanukitic型高镁安山岩，由受沉积物熔体交代的地幔熔融形成；（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，依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：南部弧前区域发育约170–160 Ma的FAB-玻安岩型岩浆作用，北部大陆弧区域以约169–155 Ma为主的sanukitic型岩浆作用为特征，而二者之间的区域则以约155–148 Ma为主的埃达克质岩浆作用为特征。上述时空分异证实了岩浆作用前锋先北向、后南向的迁移过程。该迁移过程与其他地质证据均表明，班公湖-怒江特提斯洋的俯冲动力学过程经历了连续转变，即从俯冲起始阶段，历经洋脊-海沟碰撞，最终演变为洋脊俯冲。整个转变过程中持续的高热流环境，引发了高镁岩浆作用的爆发式发育。