Late Cretaceous volcanic rocks in the Sangri area, southern Lhasa Terrane, Tibet: Evidence for oceanic ridge subduction
Late Cretaceous volcanic rocks in the Sangri area, southern Lhasa Terrane, Tibet: Evidence for oceanic ridge subduction
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DOI：
10.1016/j.lithos.2018.12.023
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发表时间：
2019-02
期刊：
影响因子：
3.5
通讯作者：
L. Zhang;Di‐Cheng Zhu;Qing Wang;Zhidan Zhao;Dong Liu;Jin‐Cheng Xie
中科院分区：
文献类型：
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作者：
L. Zhang;Di‐Cheng Zhu;Qing Wang;Zhidan Zhao;Dong Liu;Jin‐Cheng Xie
西藏拉萨地体南部晚白垩世岩浆活动对新特提斯大洋板块俯冲过程具有重要的制约作用。本文提供了拉萨南缘桑日县至桑耶寺(SS)晚白垩世早期火山岩的全岩地球化学、锆石单键铅年龄、微量元素含量和Hf同位素资料。SS火山岩包括玄武岩、安山岩和英安岩。SIMS和LA-ICPMS锆石USingle Bond-PbdPb数据表明，SS英安岩喷发时间约为95 Ma，与拉萨地体南部晚白垩世早期的“爆发”事件同时代。SS火山岩属钙碱性-高钾钙碱性系列，具有Nb、Ta、Ti负异常的俯冲火山岩地球化学特征。玄武岩具有与稀土元素配分模式相似的稀土元素配分模式，具有高的Zr丰度(65~190 ppm)和高的Zr/Y比值(3.3~6.5)。它们可能来自地幔橄榄岩的高度部分熔融，地幔橄榄岩在软流圈成分的输入下被俯冲相关的流体交代。安山岩全岩εNd(T)(+2.9~+3.2)正值及其变化的Mg#值(35-61)、镁含量(2.2 9-7.32 wt%)、铬(12-2 32 ppm)和镍(10-12 7ppm)可归因于地幔橄榄岩的中等程度部分熔融和单斜辉石和橄榄石的分离结晶。钙碱性SS英安岩与埃达克岩相似，具有高锶(6 63~1188 ppm)、低重稀土(HREE)和低Y(10 7~11 5 ppm)的特征。这些埃达克质英安岩具有正的全岩εNd(T)(+2.9~+3.4)和锆石εHf(T)(+8.4~+14.9)值，具有较高的Mg#(36~5 7)值和较高的相容元素含量，表明它们来自俯冲洋板的部分熔融，并与上升的岩浆与地幔楔橄榄岩相互作用。我们的新数据表明，同时代软流圈成分参与的玄武岩和板片派生的埃达克英安岩具有明显的岩石组合。这种组合与拉萨次地体南部碳质花岗岩围岩的同时代高温褐斑岩和高温麻粒岩相变质作用一起，指示了晚白垩世早期“爆发”事件中物质和热量的异常输入。这种现象可能归因于新特提斯洋脊向北俯冲，这使得上升软流层的贡献不同于法角俯冲或低角/平坦俯冲和随后的板块回滚的典型结果。
Late Cretaceous magmatism in the southern Lhasa subterrane, Tibet, provides critical constraints on the subduction processes of the Neo-Tethyan oceanic plate. Here, we provide data on the whole-rock geochemistry, zircon Usingle bondPb ages, trace element contents, and Hf isotopes of early Late Cretaceous volcanic rocks from Sangri County to Sangye Temple (SS) along the southern Lhasa subterrane. The SS volcanic rocks include basalts, andesites, and dacites. SIMS and LA–ICP–MS zircon Usingle bondPb data indicate that the SS dacites were erupted at ca. 95 Ma, coeval with the early Late Cretaceous “flare-up” event in the southern Lhasa subterrane. The SS volcanic rocks belong to the calc-alkaline to high-K calc-alkaline series and have the geochemical features of subduction-related volcanic rocks with negative Nb, Ta, and Ti anomalies. The basalts have similar rare earth element (REE) patterns toE-MORB and are characterized by high Zr abundances (65–190 ppm) and Zr/Y ratios (3.3–6.5). They were possibly derived by high-degree partial melting of mantle peridotite that had been metasomatized by subduction-related fluids with an input of asthenospheric components. Positive whole-rock εNd(t) (+2.9 to +3.2) values for the andesites, and their variable Mg# values (35–61) and MgO (2.29–7.32 wt%), Cr (12–232 ppm), and Ni (10–127 ppm) contents can be attributed to medium-degree partial melting of mantle peridotite followed by fractional crystallization of clinopyroxene and olivine. The calc-alkaline SS dacites are similar to adakites with high Sr (663–1188 ppm) and low heavy rare earth element (HREE) and Y (10.7–11.5 ppm) contents. These adakitic dacites have positive whole-rock εNd(t) (+2.9 to +3.4) and zircon εHf(t) (+8.4 to +14.9) values with relatively high values of Mg# (36–57) and high contents of compatible elements, indicating that they were derived from the partial melting of subducted oceanic slab, and with the ascending magma interacting with mantle wedge peridotite. Our new data indicate a distinct rock association of coeval asthenosphere-component-involved basalts and slab-derived adakitic dacites. Such an association, together with the coeval high-temperature charnockites and high-temperature granulite-facies metamorphism of the charnockitic country rocks in the southern Lhasa subterrane, indicates an anomalous input of materials and heat during the early Late Cretaceous “flare-up” event. Such an event can probably be attributed to the northwards subduction of a Neo-Tethyan oceanic ridge that allowed for contributions from upwelling asthenosphere that differed from the typical results of normal-angle subduction or low-angle/flat subduction and subsequent slab rollback.