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首页> 外文期刊>Contributions to Mineralogy and Petrology >Post-collisional, K-rich mafic magmatism in south Tibet: constraints on Indian slab-to-wedge transport processes and plateau uplift
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Post-collisional, K-rich mafic magmatism in south Tibet: constraints on Indian slab-to-wedge transport processes and plateau uplift

机译：藏南碰撞后富钾的镁铁质岩浆作用：印度板块到楔块的运输过程和高原隆升的制约

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摘要

Post-collisional (23–8 Ma), potassium-rich (including ultrapotassic and potassic) mafic magmatic rocks occur within the north–south-trending Xuruco lake–Dangre Yongcuo lake (XDY) rift in the Lhasa terrane of the southern Tibetan Plateau, forming an approximately 130-km-long semi-continuous magmatic belt. They include both extrusive and intrusive facies. Major and trace element and Sr–Nd–Pb isotopic data are presented for all of the known exposures within the XDY rift. The potassium-rich, mafic igneous rocks are characterized by high MgO (5.9–10.8 wt.%), K2O (4.81–10.68 wt.%), Ba (1,782–5,618 ppm) and Th (81.3–327.4 ppm) contents, and relatively high SiO2 (52.76–58.32 wt.%) and Al2O3 (11.10–13.67 wt.%). Initial Sr isotopic compositions are extremely radiogenic (0.712600–0.736157), combined with low (206Pb/204Pb) i (18.28–18.96) and (143Nd/144Nd) i (0.511781–0.512046). Chondrite-normalized rare earth element patterns display relatively weak negative Eu anomalies. Primitive mantle-normalized incompatible trace element patterns exhibit strong enrichments in large ion lithophile elements relative to high-field-strength elements and display strongly negative Ta–Nb–Ti anomalies. The combined major and trace element and Sr–Nd–Pb isotopic characteristics of the K-rich igneous rocks suggest that the primitive magmas were produced by 1–10 % partial melting of an asthenospheric mantle source enriched by both fluids and partial melts derived from Indian passive continental margin sediments subducted into the shallow mantle as a consequence of the northward underthrusting of the Indian continental lithosphere beneath Tibet since the India–Asia collision at ~55 Ma. The best-fit model results indicate that a melt with trace element characteristics similar to those of the K-rich rocks could be generated by 8–10 % partial melting of a metasomatized mantle source in the south and 1–2 % melting in the north of the XDY rift. Trace element and Sr–Nd–Pb isotopic modeling indicate that the proportion of fluid derived from the subducted sediments, for which we use as a proxy the Higher Himalayan Crystalline Sequence (HHCS), in the mantle source region increases from north (rear-arc) to south (front-arc), ranging from 0 to 5 %, respectively. Correspondingly, the proportion of the melt derived from the subducted HHCS in the source increases from north (2 %) to south (15 %). The increasing proportion of the fluid and melt component in the mantle source from north to south, together with a southward decreasing trend in the age of the K-rich magmatism within the XDY rift, is inferred to reflect rollback of the subducted Indian lithospheric mantle slab during the period 25–8 Ma. Slab rollback may be linked to a decreasing convergence rate between India and Asia. As a consequence of slab rollback at 25 Ma beneath the Lhasa terrane, its geodynamic setting was transformed from a convergent (55–25 Ma) to an extensional (25–8 Ma) regime. The occurrence of K-rich magmatism during the period 25–8 Ma is a consequence of the decompression melting of an enriched mantle source, which may signal the onset of extension in the southern Tibetan Plateau and provide a petrological record of the extension process.
机译：碰撞后（23-8 Ma），钾含量丰富（包括超钾和钾质）的镁铁质岩浆岩发生在青藏高原南部拉萨地带的南北向的Xuruco湖-Dangre Yongcuo湖（XDY）裂谷内，形成一条约130公里长的半连续岩浆带。它们包括挤压相和侵入相。提供了XDY裂谷内所有已知暴露的主要和微量元素以及Sr–Nd–Pb同位素数据。富含钾的镁铁质火成岩的特征是高MgO（5.9-10.8 wt。％），K2O（4.81-10.68 wt。％），Ba（1,782-5,618 ppm）和Th（81.3-327.4 ppm）含量，以及较高的SiO2（52.76–58.32 wt。％）和Al2O3（11.10–13.67 wt。％）。最初的Sr同位素组成具有极高的放射成因（0.712600–0.736157），再加上低（206Pb / 204Pb）i（18.28–18.96）和（143Nd / 144Nd）i（0.511781–0.512046）。球粒晶归一化稀土元素图案显示出相对较弱的负Eu异常。相对于高场强元素，原始地幔归一化不兼容的痕量元素模式在大型离子亲石元素中表现出强烈的富集，并显示出强烈的Ta-Nb-Ti异常负值。富含钾的火成岩的主要元素和微量元素以及Sr–Nd–Pb同位素特征表明，原始岩浆是由软流圈地幔源的1–10％部分融化而产生的，这些源由印度的流体和部分熔体所富集自印度-亚洲在约55 Ma碰撞以来，西藏以下印度大陆岩石圈向北俯冲，导致被动大陆边缘沉积物俯冲到浅层地幔中。最佳拟合模型结果表明，南部的交代化地幔源部分熔融有8–10％，北部的熔融部分有1-2％，可能会生成微量元素特征与富K岩石相似的熔体。 XDY裂痕痕量元素和Sr–Nd–Pb同位素模型表明，地幔源区中俯冲沉积物的流体比例（北高弧（后弧），我们将其用作代用喜马拉雅高结晶序列（HHCS））。 ）到南方（前弧），范围分别从0％到5％。相应地，源自俯冲HHCS的熔体在源中的比例从北部（2％）增加到南部（15％）。推测地幔源中流体和熔体成分的比例从北向南增加，以及XDY裂谷内富K岩浆作用年龄的向南下降趋势，反映了俯冲的印度岩石圈地幔平板的回滚。在25-8Ma之间。板坯回滚可能与印度和亚洲之间的收敛速度下降有关。由于拉萨地块以下25 Ma的平板回滚，其地球动力学环境从收敛的（55-25 Ma）转变为伸展的（25-8 Ma）体制。在25-8Ma期间发生富K岩浆作用是富集地幔源减压融化的结果，这可能标志着青藏高原南部开始扩展，并提供了扩展过程的岩石学记录。

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