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

Abstract

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.

Mafic volcanic rocks of western Iron Ore Group, Singhbhum Craton, eastern India: Geochemical evidence for ocean–continent convergence

“…This method is found to be effective and reveals precise result similar to that obtained while using radiogenic isotope ratio data. In this view, these incompatible trace element ratios were used elsewhere for bracketing mantle sources (Chaudhuri et al, 2014; Ray et al, 2013). Certain incompatible trace element ratios for the studied mafic volcanic rocks (Table 3) invoke an enriched (EM1‐EM2 type) mantle component in source and almost all trace element ratios unequivocally suggest effects of continental crustal assimilation of parent magma.…”

Early Cretaceous alkaline/ultra-alkaline silicate and carbonatite magmatism in the Indian Shield – a review: implications for a possible remnant of the Greater Kerguelen Large Igneous Province

“…Nambiar and Golani (1985) have reported a carbonatite and alkaline dykes from the Swangkre area of the Shillong Plateau but no detail information is available except on a potassic lamprophyre dyke, mentioned above. The Mawpyut complex is consisting of cumulate (broadly derivatives of clinopyroxenite, gabbronorite, and gabbro) and non-cumulate (gabbro, monzonite, monzodiorite, and quartzsyenite) units of ultramafic/mafic rocks and minor younger syenitic veins (Chaudhuri et al, 2014). A melanite garnet-bearing nepheline syenite intrusion is also reported from this complex (Maitra et al, 2011).…”

“…A melanite garnet-bearing nepheline syenite intrusion is also reported from this complex (Maitra et al, 2011). Based on whole rock geochemistry and isotope systematic, Chaudhuri et al (2014) have suggested that melts generated through partial melting of an enriched mantle source, which later modified due to assimilation and fractional crystallization, have produced the variety of cumulate-noncumulate lithologies of the Mawpyut complex, however more detailed work is required to confirm this assumption.…”

“…A fair amount of the bulk-rock Sr and Nd isotopic data for lamproitic intrusions (ultrapotassic rocks), carbonatites and associated silicate rocks of the CGC and the SPMH is available (e.g. Middlemost et al, 1988;Rock et al, 1992;Veena et al, 1998;Kumar et al, 2003;Srivastava et al, 2005Srivastava et al, , 2019Srivastava and Sinha, 2007;Ghatak and Basu, 2013;Chaudhuri et al, 2014;Saha et al, 2017). Chalapati have evaluated the available Sr and Nd isotopic data on lamproites and suggested (i) their derivation from source regions with long term incompatible element enrichment relative to that of Bulk Earth, and (ii) 87 Sr/ 86 Sr i and εNd i values of these ultrapotassic rocks are different from the primitive Kerguelen plume component but similar to those of the pristine Kerguelen mantle plume derived basalts.…”