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Metamorphic to magmatic ortho/clinopyroxene bearing, massive to foliated charnockites sensu lato are exposed in the central part of the Shillong-Meghalaya Gneissic Complex (SMGC), Northeast India. The two pyroxene bearing metamorphic (group-1) charnockites are tonalitic to granodioritic in composition and together with associated high grade rocks pre-date all deformational episodes and successive metamorphic events. On the contrary, the orthopyroxene bearing (group-2) and fayalite + quartz bearing (group-3) magmatic charnockites post-date all deformational and metamorphic episodes. Both group-2 and-3 charnockites are granitic in composition. Thermobarometric estimations and quantitative P-T pseudosection modeling constrain the peak metamorphism of Group-1 charnockites at 900°C/6–6.5 kbar indicating a clockwise P-T trajectory. The ferroan to magnesian charnockites are calc-alkalic to calcic, and weakly peraluminous/metaluminous. The A2-type charnockites with strongly fractionated REE patterns [(La/Lu)N : 3.95‒27.87] and negative Eu anomalies (Eu/Eu*: 0.28‒0.46) are depleted in Nb, Ta, Sr and Ti abundances that correspond with a post-collisional setting. By comparison, in the associated mafic granulites as enclaves/bands, the REEs are 10-60 times enriched relative to chondrite, have flat patterns, and with no negative Eu anomalies. The charnockite suite is derived by the partial melting of varied protoliths including garnet-bearing amphibolites and crustal sources. Tightly-constrained chemical dates in chemically-zoned monazites, hosted within recrystallized grains of pyroxenes and plagioclase in three charnockites samples vary between 463±40 and 526±37 Ma (mean: 503±4 Ma). The magmatic to metamorphic charnockites attest to the prevalence of high-T during the period of post-collisional East Gondwana assembly related accretion at a late stage of Pan-African orogenic cycle, arguably continuous with the Pan-African Prydz Bay suture (East Antarctica) within the East Gondwanaland. |
