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

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 XMg content; Idappadi (Ida)–Tiruchengodu (Tir) granite biotites with low XMg, Sankari (San), Sirumugai (Sir) and Punjaipuliyampatti (Pun) granite biotites with high XMg and Karamadai (Kar) granite biotites with moderate XMg. 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.