Ultrahigh-temperature metamorphism in the Achankovil Zone: Implications for the correlation of crustal blocks in southern India
Ultrahigh-temperature metamorphism in the Achankovil Zone: Implications for the correlation of crustal blocks in southern India
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DOI：
10.1016/j.gr.2005.11.019
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发表时间：
2006-08
影响因子：
6.1
通讯作者：
S. Ishii;T. Tsunogae;M. Santosh
中科院分区：
文献类型：
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作者：
S. Ishii;T. Tsunogae;M. Santosh
印度南部的阿尚科维尔带是一条宽 8-10 公里、长超过 100 公里的西北-东南走向的地貌线，是一个在运动学上有争议的地壳特征，被认为是北部马杜赖麻粒岩块和南部特里凡得琅麻粒岩块之间的边界。尽管折返方式明显不同，但这两个地壳块都显示出泛非造山运动期间超高温变质作用的证据。阿尚科维尔带的特征是不连续的含堇青石片麻岩，其组合为堇青石+石榴石+石英+斜长石+尖晶石+钛铁矿+磁铁矿±斜方辉石±黑云母±钾长石±硅线石。岩性保存了多个峰期和峰后变质组合，包括：（1）斜方辉石+石榴石，（2）长辉石和/或反长辉石，（3）石榴石周围的堇青石±斜方辉石冠，（4）石榴石后的堇青石+石英辛辉石。我们使用含斜方辉石的地温气压计和长石固溶线估计了这些岩石的峰值变质条件，产生 8.5–9.5 kbar 和 940–1040 °C，这是迄今为止阿昌科维尔带记录的最高 P-T 条件。逆行条件是通过带有堇青石的地温气压计在 3.5–4.5 kbar 和 720±60 °C 下获得的。根据斜方辉石化学，我们记录了这些岩石的多阶段折返历史，这与马杜赖麻粒岩块最近研究报告的结果非常相似，但与特里凡得琅麻粒岩块记录的结果不同。对岩石学和地质年代学数据的评估以及折返路径的性质使我们提出，阿坎科维尔带可能是马杜赖麻粒岩块的南翼，而不是目前认为的特里凡得琅麻粒岩块的一个单位。沿马杜赖麻粒岩地块边缘和阿尚科维尔带内形成一系列“缝合岩体”的构造后碱性花岗岩，但在特里凡得琅麻粒岩地块中不存在，表明马杜赖麻粒岩地块和特里凡得琅麻粒岩地块之间的边界可能位于阿尚科维尔带南端的 Tenmalai 剪切带。
The Achankovil Zone of southern India, a NW–SE trending lineament of 8–10 km in width and >100 km length, is a kinematically debated crustal feature, considered to mark the boundary between the Madurai Granulite Block in the north and the Trivandrum Granulite Block in the south. Both these crustal blocks show evidence for ultrahigh-temperature metamorphism during the Pan-African orogeny, although the exhumation styles are markedly different. The Achankovil Zone is characterized by discontinuous strands of cordierite-bearing gneiss with an assemblage of cordierite+garnet+quartz+plagioclase+spinel+ilmenite+magnetite±orthopyroxene±biotite±K-feldspar±sillimanite. The lithology preserves several peak and post-peak metamorphic assemblages including: (1) orthopyroxene+garnet, (2) perthite and/or anti-perthite, (3) cordierite±orthopyroxene corona around garnet, and (4) cordierite+quartz symplectite after garnet. We estimate the peak metamorphic conditions of these rocks using orthopyroxene-bearing geothermobarometers and feldspar solvus which yield 8.5–9.5 kbar and 940–1040 °C, the highest P–T conditions so far recorded from the Achankovil Zone. The retrograde conditions were obtained from cordierite-bearing geothermobarometers at 3.5–4.5 kbar and 720±60 °C. From orthopyroxene chemistry, we record a multistage exhumation history for these rocks, which is closely comparable with those reported in recent studies from the Madurai Granulite Block, but different from those documented from the Trivandrum Granulite Block. An evaluation of the petrologic and geochronologic data, together with the nature of exhumation paths leads us to propose that the Achankovil Zone is probably the southern flank of the Madurai Granulite Block, and not a unit of the Trivandrum Granulite Block as presently believed. Post-tectonic alkali granites that form an array of “suturing plutons” along the margin of the Madurai Granulite Block and within the Achankovil Zone, but are absent in the Trivandrum Granulite Block, suggest that the boundary between the Madurai Granulite Block and the Trivandrum Granulite Block might lie along the Tenmalai shear zone at the southern extremity of the Achankovil Zone.