Ultrahigh-temperature metamorphism and decompression history of sapphirine granulites from Rajapalaiyam, southern India: implications for the formation of hot orogens during Gondwana assembly
Ultrahigh-temperature metamorphism and decompression history of sapphirine granulites from Rajapalaiyam, southern India: implications for the formation of hot orogens during Gondwana assembly
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DOI：
10.1017/s0016756809990100
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发表时间：
2009-07
影响因子：
2.3
通讯作者：
T. Tsunogae;M. Santosh
中科院分区：
文献类型：
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作者：
T. Tsunogae;M. Santosh
摘要印度南部马杜赖地块南部Rajapalaiyam产出的含蓝宝石镁铝麻粒岩为新元古代-寒武纪晚期超高温变质作用提供了重要证据。在石英-石英质麻粒岩和泥质麻粒岩中的绿榴石石榴石含有与石英伴生的细粒次自变晶至他变晶石榴石包裹体，表明岩石经历了T > 1000°C的峰值变质作用。石榴石中尖晶石中的石英包裹体也被认为是超高温条件的明确证据。含辉绿石岩石中变斑石榴子石中的斜方辉石包裹体显示最高的Al 2 O3含量达10.3wt%，表明T = 1050-1070°C和P = 8.5-9.5 kbar。根据三元长石和其他地质温度计估算的温度（T = 950-1000°C）进一步支持了该地区的极端热变质作用。与石英共生的尖晶石包体表明，尖晶石+石英组合的形成时间早于尖晶石+石英组合，可能是从T ~ 1050°C开始冷却或经历了一条逆逆时针P-T路径，而尖晶石+石英+石榴石→斜方辉石+硅线石的FMAS反应则表明峰期变质作用后从尖晶石+石英稳定场开始冷却。石榴石周围的斜方辉石+堇青石（±堇青石）、斜方辉石+堇青石+堇青石和堇青石+尖晶石的电晕结构表明随后的近等温减压，然后向T = 650-750°C和P = 4.5-5.5 kbar的减压冷却。本文所描述的锆石-石英组合及相关结构对马杜赖地块的超高温变质作用和折返历史以及印度南部大陆深部地壳的构造演化具有重要意义。我们的研究提供了一个典型的例子，极端变质作用与碰撞构造在晚新元古代-寒武纪组装的冈瓦纳超大陆。
Abstract Sapphirine-bearing Mg–Al granulites from Rajapalaiyam in the southern part of the Madurai Block provide critical evidence for Late Neoproterozoic–Cambrian ultrahigh-temperature (UHT) metamorphism in southern India. Poikiloblastic garnet in quartzo-feldspathic and pelitic granulites contain inclusions of fine-grained subidioblastic to xenoblastic sapphirine associated with quartz, suggesting that the rocks underwent T > 1000°C peak metamorphism. Quartz inclusions in spinel within garnet are also regarded as clear evidence for a UHT condition. Inclusions of orthopyroxene within porphyroblastic garnet in the sapphirine-bearing rocks show the highest Al2O3 content of up to 10.3 wt%, suggesting T = 1050–1070°C and P = 8.5–9.5 kbar. Temperatures estimated from ternary feldspar and other geothermometers (T = 950–1000°C) further support extreme thermal metamorphism in this region. Xenoblastic spinel inclusions in sapphirine coexisting with quartz suggest that the spinel + quartz assemblage pre-dates the sapphirine + quartz assemblage, probably implying a cooling from T ~ 1050°C or an anticlockwise P–T path. The FMAS reaction sapphirine + quartz + garnet → orthopyroxene + sillimanite indicates a cooling from the sapphirine + quartz stability field after the peak metamorphism. Corona textures of orthopyroxene + cordierite (± sapphirine), orthopyroxene + sapphirine + cordierite, and cordierite + spinel around garnet suggest subsequent near-isothermal decompression followed by decompressional cooling toward T = 650–750°C and P = 4.5–5.5 kbar. The sapphirine–quartz association and related textures described in this study have an important bearing on the UHT metamorphism and exhumation history of the Madurai Block, as well as on the tectonic evolution of the continental deep crust in southern India. Our study provides a typical example for extreme metamorphism associated with collisional tectonics during the Late Neoproterozoic–Cambrian assembly of the Gondwana supercontinent.