Melt‐producing and melt‐consuming reactions in the Achankovil cordierite gneisses, South India

Abstract

Migmatitic cordierite gneisses within the Achankovil Zone (AZ) of southern Pan-African India record melt-producing and subsequent melt-consuming mineral reactions. Early mineral assemblages Bt-Sil-Qtz and Bt-Sil-Spl, deduced from inclusion textures in garnet prophyroblasts, break down via successive dehydration melting reactions to high-T phase assemblages (e.g. Grt-Crd-Liq, Opx-Liq, Spl-Crd-Liq). Later back reactions between the restite and the in situ crystallizing melt resulted in thin cordierite coronas separating garnet from the leucosome, and partial resorption of garnet to Opx-Crd or CrdBt-Qtz symplectites. Leucosomes generally display a moderate (low-strain gneisses) to strong (highstrain gneisses) depletion of alkali feldspar attributed to mineral-melt back reactions partly controlled by the degree of melt segregation.Using a KFMASH partial petrogenetic grid that includes a melt phase, and qualitative pseudosections for microdomains of high and low Al ⁄ Si ratios, the successive phase assemblages and reaction textures are interpreted in terms of a clockwise P-T path culminating at about 6-7 kbar and 900-950°C. This P-T path is consistent with, but more detailed than published results, which suggests that taking a melt phase into account is not only a valid, but also a useful approach. Comparing P-T data and lithological and isotopic data for the AZ with adjacent East Gondwana fragments, suggests the presence of a coherent metasedimentary unit exposed from southern Madagascar via South India (AZ) and Sri Lanka (Wanni Complex) to the Lu¨tzow-Holm Bay in Eastern Antarctica.

Spinel+Quartz association from the Kerala khondalites, southern India: evidence for ultrahigh-temperature metamorphism

“…The biotite analyses show X Mg in the range of 0.54 0.76, and are similar to the range reported by Santosh (1987, up to 0.74) and Cenki et al (2002;up …”

“…Such migmatitic textures and presence of leucosomes are common in granulite grade pelitic rocks in various terrains and some workers correlate these textures to dehydration melting reactions during prograde metamorphism which generate in situ melts that subsequently crystallize as leucosomes (e.g. Fitzsimons, 1996;Cenki et al, 2002).…”

“…The TGB has been the focus of various geological, petrological, isotopic and geochronological investigations for nearly two decades (e.g., Srikantappa et al, 1985;Chacko et al, 1987;Santosh, 1987, Santosh et al, 1990Jackson and Santosh, 1992;Raith and Srikantappa, 1993;Harley and Santosh, 1995;Braun et al, 1996;Nandakumar and Harley, 2000;Cenki et al, 2002;Fonarev et al, 2003). Among these, some studies have also proposed the possible high temperature history of the terrane (e.g., Braun et al, 1996;Chacko et al, 1996).…”

“…Particular interest has been focused on cordierite, sillimanite and spinel bearing assemblages commonly found in this terrain which provide important clues on pressure temperature conditions and exhumation history of the rocks (e.g. Santosh, 1987;Santosh et al, 1993;Nandakumar and Harley, 2000;Shabeer et al, 2002;Cenki et al, 2002).…”

Pan–African granulites of Madagascar and southern India: Gondwana assembly and parallels with modern Tibet

“…There are spatial variations in metamorphic grade (Nandakumar and Harley, 2000;Pattison et al, 2003), but typical peak conditions of 0.7-1.3 GPa and 850-1050°C are followed by decompression as part of a clockwise P-T path (Raith et al, 1997;Nandakumar and Harley, 2000;Cenki et al, 2002;Sajeev et al, 2004Sajeev et al, , 2006Tadokoro et al, 2008;Tsunogae and Santosh, 2010;Brandt et al, 2011;Johnson et al, 2015;Taylor et al, 2015). Monazite and zircon ages of 0.63-0.47 Ga imply a long metamorphic history (Braun and Bröker, 2004;Ghosh et al, 2004;Braun and Appel, 2006;Taylor et al, 2014).…”