Redox conditions and nature of Pan-African granites from the E–W trending Cauvery Shear Zone, Southern Granulite Terrain of Central Tamil Nadu, India: Constraints from magnetic susceptibility and biotite chemistry,Journal of Earth System Science

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Redox conditions and nature of Pan-African granites from the E–W trending Cauvery Shear Zone, Southern Granulite Terrain of Central Tamil Nadu, India: Constraints from magnetic susceptibility and biotite chemistry
Journal of Earth System Science ( IF 1.3 ) Pub Date : 2021-05-20 , DOI: 10.1007/s12040-021-01578-2
R Anshu , Anjali Solanki , S Ramasamy , Sonam Singh , S Raju

Abstract

Pan-African (ca. 620–390 Ma) granites (sensu lato) are exposed as small circular to elliptical intrusive bodies (stocks or plutons), viz., Karamadai (Kar), Sirumugai (Sir), Punjaipuliyampatti (Pun), Tiruchengodu (Tir), Sankari (San), and Idappadi (Ida) along the E–W trending Cauvery shear zone (CSZ) of Southern Granulite Terrain (SGT), central Tamil Nadu. Mineralogically all these plutons share a similar composition (plagioclase–K-feldspar–quartz–biotite–(±hornblende±garnet)–muscovite–apatite–zircon–magnetite–ilmenite–monazite) but differ in its relative proportions as well as textures. The observed magnetic susceptibility (MS) values (< 3.00 × 10^–3 SI unit) of these granites largely correspond to ilmenite (reduced type) granite series, probably intrinsic to the source region. Compositionally biotites of these granites are primary and co-precipitated with muscovite in a peraluminous (S-type) granitic magma. Although biotites from these granites are mostly Fe-biotites, however, three distinct compositional groups can be classified based on variable X_Mg content; Idappadi (Ida)–Tiruchengodu (Tir) granite biotites with low X_Mg, Sankari (San), Sirumugai (Sir) and Punjaipuliyampatti (Pun) granite biotites with high X_Mg and Karamadai (Kar) granite biotites with moderate X_Mg. Biotites of Ida and Tir granite exhibit 2Al↔3Fe substitution which is typical to a peraluminous (S-type) host magma with syn-collisional affinity. However, Kar, San, Sir and Pun granite biotites crystallized from transitional host magmas between metaluminous (I-type) and peraluminous (S-type) in syn- to post-collision tectonics. Mineral assemblage, biotite chemistry, ilmenite (reduced) series nature (Ida granite: log fO₂ = −15.76 to −16.18, T = 780°–690 °C; San and Sir granites: log fO₂ = −14.59 to −15.05, T = 820°–730°C; Kar granite; log fO₂ = −14.66 to −15.78, T = 790°–710 °C) of the studied granites indicate that these are derived from melting of mixed crustal sources (infracrustal and metapelitic). Therefore, the comprehensive studies of the discussed granite plutons of the Southern Granulite Terrain of central Tamil Nadu indicate that they were formed and evolved under differential reducing conditions, during a protracted thermal event related to amalgamation and growth of the Pan-African supercontinent.

Research Highlights

1.
Granites of pan-African age (650–320 ma) are exposed as isolated bodies within the E–W trending Cauvery Shear Zone in the central part of Southern Granulite Terrain, Tamil Nadu.
2.
Seven plutons were selected for petrogenetic studies during the Gondwana amalgamation.
3.
All the granitic variants share a common mineral assemblage (Qz + K-felspar + Na-plagioclase – Biotite ± Amphibole).
4.
Magnetic Susceptibility values of these granitic plutons are <3.00 × 10–3 SI units, indicating ilmenite granite series.
5.
The biotite composition is indicative of three distinct compositional fields, peraluminous (s-type) host magma formed in syn-collisional setting and origin from the heterogeneous crustal source.

中文翻译：

印度泰米尔纳德邦中部南部花岗石地形从E-W趋向于细碎屑剪切带的泛非洲花岗岩的氧化还原条件和性质：磁化率和黑云母化学的制约

摘要

泛非（约620–390 Ma）花岗岩（细苏拉托）呈小圆形暴露于椭圆形侵入体（砧木或母体），即，卡拉玛代（Kar），Sirumugai（Sir），旁遮普利耶安帕蒂（Pun），Tiruchengodu （Tir），Sankari（San）和Idappadi（Ida）沿着泰米尔纳德邦中部南部Granulite地形（SGT）的E-W趋势的切线切变带（CSZ）。从矿物学上讲，所有这些云母具有相似的成分（斜长石-钾长石-石英-黑云母-（±角闪石-石榴石）-白云母-磷灰石-锆石-磁铁矿-钛铁矿-独居石），但其相对比例和质地不同。观察到的磁化率（MS）值（<3.00×10 ^–3这些花岗岩的SI单位在很大程度上对应于钛铁矿（还原型）花岗岩系列，可能是源区固有的。这些花岗岩的黑云母成分是原生的，并与白云母（S型）花岗岩岩浆中的白云母共沉淀。尽管这些花岗岩中的黑云母大部分是铁黑云母，但是，可以根据可变的X _Mg含量对三个不同的组成组进行分类。Idappadi（IDA）-Tiruchengodu（TIR）黑云母花岗岩具有低X_镁，Sankari（SAN），西鲁穆盖（爵士）和Punjaipuliyampatti（双关语）黑云母花岗岩具有高X_镁和卡拉马代（嘉）黑云母花岗岩中度X_镁。Ida和Tir花岗岩的黑云母表现出2Al↔3Fe取代，这是具有同碰撞亲和力的高铝（S型）宿主岩浆的典型特征。但是，Kar，San，Sir和Pun花岗岩黑云母从碰撞后构造的金属（I型）和高铝质（S型）之间的过渡宿主岩浆中结晶出来。矿物组合，黑云母化学，钛铁矿（还原）系列性质（Ida花岗岩：log f O ₂ = -15.76至-16.18，T = 780°–690°C; San和Sir花岗岩：log f O ₂ = -14.59至− 15.05，T = 820°–730°C； Kar花岗岩； log f O ₂ = -14.66至-15.78，T= 790°–710°C）的花岗岩表明，这些花岗岩是由混合地壳源（地壳和变质岩）融化而来的。因此，对所讨论的泰米尔纳德邦中部南部花岗岩地块的花岗岩岩体的综合研究表明，它们是在差异还原条件下形成的，并且是在与泛非超大陆融合和生长相关的长期热事件期间发生的。