Geochemistry and petrogenesis of Proterozoic mafic rocks from East Khasi Hills, Shillong Plateau, Northeastern India

Abstract

The Khasi mafic rocks of Shillong Plateau represent the Proterozoic mafic magmatism of Shillong Cratonic Block (SCB) in Northeastern India. The Khasi mafics are best exposed at Laitlynkot, East Khasi Hills, Shillong Plateau, where they occur as lensoidal bodies that are intrusive into the Shillong group of rocks (represented by sediments or low-grade metasediments). Relict high-grade basement rocks (∼Archean) and younger porphyritic granite intrusive also occur in this area. The investigated mafic magmatic rocks petrographically belong to hornblende-gabbro clan and bear imprints of metamorphism. Geochemical characters and variation trends of major and trace elements as well as selected trace element ratios suggest that the Khasi mafic rocks have a tholeeitic lineage of arc affinity controlled by fractional crystallization with a prominent iron enrichment trend. Incompatible trace element abundance patterns of the investigated rocks marked by LILE and LREE enrichments, relative depletion of HFSE, and pronounced negative Nb anomalies conform to a subduction-related arc signature and indicate the generation of the parent melt by slab dehydration and wedge melting processes. Chondrite-normalized REE profiles with flat HREE patterns indicate melting above the stability field of garnet. Low Nb/Ta (7.67–13.7) and high Zr/Hf (44–95.2) and Zr/Sm (8.92–55.9) ratios, with respect to primitive mantle values (Nb/Ta: 17, Zr/Hf: 36, Zr/Sm: 25) are consistent with parent magma derivation from a metasomatized, enriched mantle source. Critical trace element ratios corroborate an enriched mantle source (EMI) for the magma that produced the Khasi mafics and indicate a prominent role of crustal contamination. The isotopic fingerprint of an enriched mantle reservoir (EMI) has been documented in terms of high 87Sr/86Sr ratios ranging from 0.703769 to 0.711712, while 143Nd/144Nd ratio varies between 0.511536 and 0.511596. The Khasi mafic are characterized by high Al2O3, Rb and Th enrichment and Nb/Th<8 which altogether reflect crustal assimilation and fraction crystallization (AFC) of the magma. Geochemical data suggest that partial melting of a subduction-modified, metasomatized, enriched lithospheric mantle wedge of spinel-peridotite composition produced the parental melt to the Khasi mafics, which evolved and attained a continental arc signature through assimilation of crustal materials. Metasomatism of mantle wedge by slab-dehydrated, LILE-rich fluids and incorporation of subduction-derived sediments account for the enrichment of the source. The role of the subducted sediments for mantle enrichment has also been attested by Ce/Ta vs. Ce relations and Th/Ce ratios. Tectonic affiliation and petrogenetic characters of these mafic rocks point towards their generation and emplacement in a continental margin arc setting involving subduction of oceanic plates beneath continental lithosphere, slab dehydration and mantle wedge melting associated with assimilation of crustal components and fractional crystallization (AFC) processes.