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

摘要

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–($pm$ hornblende $pm$ 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 $imes$ 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 $leftrightarrow$ 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 f O$_{2}$ = ?15.76 to ?16.18, T = 780$^{circ}$–690 $^{circ}$C; San and Sir granites: log f O$_{2}$ = ?14.59 to ?15.05, T = 820$^{circ}$–730$^{circ}$C; Kar granite; log f O$_{2}$ = ?14.66 to ?15.78, T = 790$^{circ}$–710 $^{circ}$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. $f{Highlights}$ $ullet$ 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. $ullet$ Seven plutons were selected for petrogenetic studies during the Gondwana amalgamation. $ullet$ All the granitic variants share a common mineral assemblage (Qz + K-felspar + Na-plagioclase – Biotite $pm$ Amphibole). $ullet$ Magnetic Susceptibility values of these granitic plutons are 3.00 $imes$ 10–3 SI units, indicating ilmenite granite series. $ullet$ 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.