Origin of graphite in glimmerite and spinellite in Achankovil Shear Zone, southern India

Abstract

We report occurrence and origin of graphite in glimmerite and spinellite from an unusual ultramafic complex in Achankovil Shear Zone (ACSZ) in southern India. Raman analyses of primary graphite flakes in close association with spinel and phlogopite confirm the highly crystalline nature. The carbon isotope composition ranges from -7‰ to -9.8‰ typical of mantle -derived graphite. A genetic model is proposed in which carbon was derived from a reduced juvenile mantle source linked to the genesis of volatile rich ultramafic rocks in ACSZ.

Desiccation drives the growth of crystalline graphite in the cold mantle wedge: Evidence from the Achankovil Suture Zone, southern India

“…The ACSZ, also known as Achankovil lineament (Figure 1a), marks the boundary between the Trivandrum Block in the south and the Madurai Block in the north, with a NW-SE trend, 15 km width and lateral extent of 120 km (Rajesh et al, 2004;Rajesh et al, 2009;Rajesh & Arai, 2006). Available petrological and geochronological data and recent models based on geophysical studies suggest that the ACSZ marks a Neoproterozoic subduction-accretion-collision zone (e.g., Santosh et al, 2017;Santosh, Maruyama, & Sato, 2009;Tsunogae, Santosh, & Dubessy, 2008).…”

“…Southern Kerala region at the southern tip of Peninsular India (Figure 1a) forms part of the Southern Granulite Terrane (SGT) and incorporates Palaeo-to Neoproterozoic basement rocks belonging to the Trivandrum and Nagercoil blocks amalgamated to the Madurai Block along the Achankovil Suture Zone (ACSZ) (Clark et al, 2015;Collins et al, , b, 2014Liu et al, 2016;Rajesh et al, 2004;Rajesh & Arai, 2006;Rajesh, Arai, & Satish-Kumar, 2009;Santosh et al, 2006,b;Santosh et al, ,b, 2015Santosh et al, , 2017Taylor et al, 2014;Zhang & Santosh, 2019). The SGT is a collage of crustal blocks ranging in age from Mesoarchean to Neoproterozoic (Santosh, 2020;Yu, Santosh, Amaldev, & Palin, 2020), and the latest granulite-facies metamorphism ranged up to ultrahigh temperatures at P-T conditions of 6-8 kbar and up to 950 C during Late Neoproterozoic-Cambrian, associated with the final assembly of the Gondwana supercontinent (Geological Survey of India, 1995;Harley & Nandakumar, 2014;Harley & Santosh, 1995;Rajesh et al, 2004;Rajesh & Arai, 2006, Rajesh et al, 2009Santosh, Yokoyama, Biju-Sekhar, & Rogers, 2003;Santosh, Collins, Morimoto, & Yokoyama, 2005;Santosh et al, ,b, 2015Santosh et al, , 2017.…”

“…The dominant rock types in the ACSZ are intensely deformed and migmatized garnet-biotite gneiss, granulite facies metapelites (khondalite), orthopyroxene and cordierite gneiss, charnockite, quartzo-feldspathic gneisses, and minor metacarbonates (Rajesh et al, 2004;Rajesh & Arai, 2006, Rajesh et al, 2009. The high-grade metamorphic rocks in this area (Figure 1b) are variously intruded by felsic plutons, mafic dykes, and mafic to ultramafic bodies (Rajesh et al, 2004;Rajesh et al, 2009). The Kakkaponnu ultramafic body investigated in this study (Figure 1b) is composed of spinel-dunite, phlogopite-dunite, glimmerite, graphite-spinel-glimmerite, and phlogopite-graphite-spinellite.…”

“…Moreover, flaky graphitic materials display carbon isotope zoning suggestive of a fractionation effect associated with an abiotic hydrothermal growth process (Santosh & Wada, 1993a). Later on, Rajesh et al (2009) noted that the graphite-bearing spinellite and spinel in the glimmerite contain abundant fluids (CO 2 -rich, graphite, magnesite and H 2 O-rich fluids), as well as mineral inclusions.…”

Crustal Density and Susceptibility Structure beneath Achankovil Shear Zone, India

“…The Achankovil Zone consists of a variety of magmatic and metamorphic lithologies including garnetbiotite gneiss, garnet-cordierite gneiss, khondalites, charnockite, two-pyroxene mafic granulite, calc-silicate, and quartzite [2,23,29,39]. The high-grade metamorphic rocks in AKSZ are intruded by felsic plutons, mafic dykes, and mafic to ultramafic bodies [42][43][44].…”