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Petrology and geochemistry of the ultramafic–mafic Mawpyut complex, Meghalaya: a Sylhet trap differentiation centre in northeastern India



The present article describes, for the first time, petrological and geochemical details of the Mawpyut differentiated complex which is related to the Sylhet trap located at Jaintia Hills district, Meghalaya, northeastern India. The Mawpyut complex occurs as an arcuate body that intrudes into the surrounding Shillong Group rocks. The complex in general contains ‘ultramafic’ and ‘mafic’ rocks, as well as minor syenitic veins that postdate the main units. The lithotypes correspond to cumulate and noncumulate units. The cumulate unit is represented by olivine clinopyroxenite, clinopyroxenite, plagioclase-bearing ultramafic, olivine gabbronorite, mela-gabbronorite, melagabbro, orthopyroxene gabbro, and gabbro, all with a pronounced cumulus texture. The noncumulate unit is marked by gabbro, monzonite, monzodiorite, and quartzsyenite.

The use of several major and trace element variation diagrams suggests that magmatic differentiation led to the formation of cumulate and noncumulate units. In chondrite-normalized REE diagrams the cumulate rocks show flat LREE and MREE patterns and a moderate positive Eu anomaly (in plagioclase-bearing ultramafics) due to plagioclase cumulation. The rocks of the noncumulate unit show a strongly fractionated REE pattern and no Eu anomaly. The noncumulate mafic rocks are geochemically comparable to high-phosphorous/high-titanium basalts (HPT) indicative of low pressure fractional crystallization. In a primitive mantle-normalized multielement diagram some of the cumulate rocks show pronounced negative anomalies for K and P, indicating anorogenic mafic magmatism in a within-plate setting. The rocks of the noncumulate unit show a slight negative anomaly for Yb and a Nb–Ta trough, indicating a subduction-related signature that perhaps is inherited from subducted sedimentary rocks incorporated during crustal contamination of the derived magma (left after crystal cumulation) with country rocks. Various trace element ratios for the cumulate mafic rocks indicate parent EMI/EMII/HIMU sources with a very limited crustal signature. The noncumulate mafic rocks (corresponding to the derived evolved magma) indicate EMI/EMII/HIMU sources with a pronounced crustal contamination. The Sr–Nd isotopic compositions of the Mawpyut samples typically plot in the continental flood basalt field, with an affinity to the EMII source. The isotopic compositions of the noncumulate rocks also clearly indicate crustal contamination. We suggest that partial melting (involving garnet in the residue) of the enriched mantle source EMI/EMII/HIMU could have derived the parental melt; this melt, in turn, underwent assimilation and fractional crystallization to produce the variety of cumulate-noncumulate lithologies of the Mawpyut complex. Copyright © 2013 John Wiley & Sons, Ltd.