Low–temperature eclogite–facies bauxite from the Red River shear zone in Vietnam: Its evolution and significance
Low–temperature eclogite–facies bauxite from the Red River shear zone in Vietnam: Its evolution and significance
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DOI：
10.2465/jmps.150727
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发表时间：
2016-03
影响因子：
0.7
通讯作者：
N. Nakano;Y. Osanai;N. Nam;T. V. Tri
中科院分区：
文献类型：
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作者：
N. Nakano;Y. Osanai;N. Nam;T. V. Tri
本文首次报道了越南北方的红河剪切带中低温榴辉岩相变质作用的发现，该剪切带被称为古近纪高温变质带。该岩石为极铝质（50.13wt%Al2O3）和氧化[Fe 3 +/（Fe 3 + + Fe 2+）= 0.89]金刚石型岩石。其成分也具有非常低的SiO2、CaO和Na 2 O，高场强元素含量高的特征，表明原岩为红土型铝土矿。分析的岩石主要由斑状变晶钛磁赤铁矿、蓝晶石、褐帘石和绿帘石组成，基质中含有丰富的细粒矿物。矿物共生和相平衡模拟表明，铝质岩石经历了顺时针的压力-温度轨迹，峰值变质条件对应于低温榴辉岩相，温度约为520-550 °C，压力为1.2-1.5 GPa。独居石U-Th-Pb测年结果表明，变质作用的年龄大于214 Ma。我们的研究结果需要两个地质事件：1）热带风化和铝土矿化的侵蚀表面，和2）前晚三叠世俯冲的表壳物质最有可能在二叠纪-三叠纪碰撞事件在东亚和东南亚。我们的研究首次提供了印度支那和华南陆块碰撞过程中40-50 km深处陆外地壳物质俯冲的证据。
We report first finding of low–temperature eclogite–facies metamorphism from the Red River shear zone in northern Vietnam that has been known as the Paleogene high–temperature metamorphic belt. The rock is extremely aluminous (50.13 wt% Al2O3) and oxidized [Fe3+/(Fe3+ + Fe2+) = 0.89] emery–type rock. Its composition is also characterized by very low SiO2, CaO, and Na2O with high concentrations in high field strength elements, suggesting that the protolith is lateritic bauxite. The analyzed rock is composed mainly of porphyroblastic titanomaghemite, kyanite, allanite, and chloritoid with abundant fine–grained corundum in the matrix. The mineral paragenesis and phase equilibria modeling suggest that the aluminous rock underwent a clockwise pressure–temperature trajectory with the peak metamorphic condition corresponding to low–temperature eclogite–facies approximately 520–550 °C at 1.2–1.5 GPa. Monazite U–Th–Pb dating constrains the timing of the metamorphism to be older than 214 Ma. Our results require two geological events; 1) tropical weathering and bauxitization on the erosional surface, and 2) pre–Late Triassic subduction of supracrustal materials most likely during the Permo–Triassic collisional events in East and Southeast Asia. Our study provides the first evidence for the subduction of supra–continental crustal materials up to 40–50 km depth during the collision between the Indochina and South China blocks.