Dehydration-melting phenomena in leptynitic gneisses and the generation of leucogranites: A case study from the Kerala Khondalite Belt, Southern India

Abstract

Pan-African high-grade metamorphism in the Kerala Khondalite Belt (South India) led to the in situ formation of garnet-bearing leucosomes (L1) in sodic quartz-alkali feldspar-biotite gneisses. Microtextures, mineralogy and the geochemical characteristics of in situ leucosomes (L1) and gneiss domains (GnD) indicate that the development of leucosomes was mainly controlled by the growth of garnet at the expense of biotite. This is documented by the selective transfer of FeO, MgO, Y, Sm and the heavy rare earth elements into the L1 domains. P-T constraints (T>800°C, P>6 kbar, aH2O∼0·3) suggest that the leucosomes were formed through complete melting of biotite in fluid-absent conditions, following the model reaction Biotite + Alkali feldspar + Quartz ⇋ Garnet + Ilmenite + Melt. The fraction of melt generated during this process was low (<10 vol.%). The identical size of the leucosomes as well as their homogeneous and isotropic distribution at outcrop scale, which lacks any evidence for melt segregation, suggest that the migmatite remained a closed system. Subsequent to migmatization, the leptynitic gneisses were intruded by garnet-bearing leucogranitic melts (L2), forming veins parallel and subperpendicular to the foliation. The leucogranites are rich in potassium (K2O ∼ 5·5 wt%), (Ba ∼ 400 p.p.m.) and Sr (∼300 p.p.m.), and exhibit low concentrations of Zr (∼40 p.p.m.), Th (<1 p.p.m.) and Y (<10 p.p.m.). The chondrite-normalized REE spectra show low abundances (LaN ∼ 20, LuN ∼ 3) and are moderately fractionated (LaN/LuN ∼ 7). An Eu anomaly is absent or weakly negative. The higher 87Sr/86Sr ratio at 550 Ma (0·7345) compared with the migmatite (0·7164) precludes a direct genetic relationship between leptynitic gneisses and leucogranites at Manali. Nevertheless, the chemical and mineralogical compositions of the leuocogranites strongly favour a derivation through fluid-absent biotite melting of isotopically distinct but chemically comparable Manali-type gneisses. The undersaturation of & Th and REE, a typical feature of leucogranitic melts generated during granulite facies anatexis of psammo-pelitic lithologies and attributed to disequilibrium melting with incomplete dissolution of accessory phases (zircon, monazite), is weakly developed in the leucogranites of Manali. It is concluded that this is mainly due to the sluggish migration of the melts in static conditions, which facilitated equilibration with the restitic gneisses.