First report of garnet–corundum rocks from southern India: Implications for prograde high-pressure (eclogite-facies?) metamorphism
First report of garnet–corundum rocks from southern India: Implications for prograde high-pressure (eclogite-facies?) metamorphism
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DOI：
10.1016/j.epsl.2005.11.042
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发表时间：
2006-02
影响因子：
5.3
通讯作者：
M. Shimpo;T. Tsunogae;M. Santosh
中科院分区：
文献类型：
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作者：
M. Shimpo;T. Tsunogae;M. Santosh
我们在这里报告的第一次发生在Sevitturangampatti（Namakkal区）在Palghat Cauvery剪切带系统（PCSS），印度南部的镁铝丰富的岩石石榴石和石榴石。岩石中含有石榴子石-镁硅线石-堇青石-尖晶石-富镁十字石、石榴子石-钠硅镁石-堇青石-硅线石/蓝晶石、石榴子石-富镁十字石-硅线石/蓝晶石、钠硅镁石-富镁十字石-堇青石-黑云母-尖晶石-黑云母、钠硅镁石-尖晶石-堇青石等稀有矿物组合。这些岩石中的石榴石和石榴石都是以粗粒（1 mm ~ 10 cm）变斑晶的形式出现在硅线石、堇青石和钙铁榴石的基质中。蓝晶石是石榴石中常见的包裹体，但基质铝硅酸盐主要是硅线石。目前为止，金伯利岩捕虏体、铝质榴辉岩和超高压变质岩以及高压实验中都有报道发现稀有石榴石+石榴石的存在，这表明该组合是异常高压事件的一个指标，这在印度南部以前的研究中没有记录。石英缺失MAS系统的相分析也表明该组合具有高压稳定性。高压变质作用的推断也得到了富Mg [Mg/（Fe+Mg）=0.51]十字石存在的支持，这已被报道来自高压岩石，包括来自粗粒石榴石和镁铁榴石的核心。石榴子石+榴辉岩以及十字石和蓝晶石包裹体的斑状变晶产出表明，该地区可能在榴辉岩区经历了逆冲高压变质作用。岩石随后经历了940至990 °C的持续加热，表明超高温（UHT）变质作用沿着顺时针轨迹进行。石榴子石和硅线石之间的蓝宝石+堇青石和尖晶石+堇青石合晶表明峰值事件后的近等温减压。这种高压变质作用之后，超高温条件沿着一个单一的顺时针折返路径的证据是第一次报告，从印度南部。Namakkal地区位于PCSS的中央区域，该区域确定了北部太古代克拉通和南部元古宙马杜赖地块之间可能的碰撞缝合线。我们建议，在这项研究中报道的岩石证明变质地幔深处（榴辉岩相）和他们随后的加热在极端的热条件下碰撞造山事件和随后的快速折返与最后的冈瓦纳组装在泛非晚期的最后一个。本文所报道的高压变质作用和随后的超高温事件对印度南部大陆深部地壳的构造演化具有重要意义。
We report here for the first time the occurrence of garnet and corundum in Mg–Al-rich rocks at Sevitturangampatti (Namakkal district) in the Palghat-Cauvery Shear Zone System (PCSS), southern India. The rocks contain several rare mineral assemblages such as garnet–corundum–sillimanite–cordierite–sapphirine–spinel–Mg-rich staurolite, garnet–corundum–sodic gedrite–cordierite–sillimanite/kyanite, garnet–Mg-rich staurolite–sillimanite/kyanite, sodic gedrite–Mg-rich staurolite–corundum–sapphirine, biotite–corundum–sapphirine and sodic gedrite–sapphirine–spinel–cordierite. Both garnet and corundum in these rocks occur as coarse-grained (1 mm to 10 cm) porphyroblasts in the matrix of sillimanite, cordierite and gedrite. Kyanite is common as inclusions in garnet, but matrix aluminosilicates are mainly sillimanite. The presence of rare garnet+corundum, which has so far been reported from kimberlite xenoliths, aluminous eclogites and ultrahigh-pressure metamorphic rocks as well as in high-pressure experiments, suggests that the assemblage is an indicator of an unusually high-pressure event, which has not been recorded in previous studies from southern India. Phase analysis of quartz-absent MAS system also suggests high-pressure stability of the assemblage. The inference of high pressure metamorphism is also supported by the presence of Mg-rich [Mg/(Fe+Mg)=0.51] staurolite, which has been reported from high-pressure rocks, included from cores of coarse-grained garnet and gedrite. Porphyroblastic occurrence of garnet+corundum as well as staurolite and kyanite inclusions suggests that the area underwent prograde high-pressure metamorphism, probably in the eclogite field. The rocks subsequently underwent continuous heating at 940 to 990 °C, suggesting ultrahigh-temperature (UHT) metamorphism along a clockwise trajectory. Sapphirine+cordierite and spinel+cordierite symplectites between garnet and sillimanite suggest near isothermal decompression after the peak event. Evidence for such high-pressure metamorphism followed by UHT conditions along a single clockwise exhumation path is the first report from southern India. The Namakkal area occurs within the central domain of the PCSS, which defines the probable collisional suture between the Archean craton in the north and the Proterozoic Madurai Block in the south. We propose that the rocks reported in this study testify to metamorphism at mantle depths (eclogite-facies) and their subsequent heating at extreme thermal conditions during collisional orogenic event and subsequent rapid exhumation associated with the final suturing of the Gondwana assembly in the late Pan-African. The high-pressure metamorphism and subsequent UHT event reported in this study have important bearing on the tectonic evolution of the continental deep crust in southern India.