Geochemistry and petrogenesis of early Cretaceous sub-alkaline mafic dykes from Swangkre-Rongmil, East Garo Hills, Shillong plateau, northeast India

Abstract

Numerous early Cretaceous mafic and alkaline dykes, mostly trending in N-S direction, are emplaced in the Archaean gneissic complex of the Shillong plateau, northeastern India. These dykes are spatially associated with the N-S trending deep-seated Nongchram fault and well exposed around the Swangkre-Rongmil region. The petrological and geochemical characteristics of mafic dykes from this area are presented. These mafic dykes show very sharp contact with the host rocks and do not show any signature of assimilation with them. Petrographically these mafic dykes vary from fine-grained basalt (samples from the dyke margin) to medium-grained dolerite (samples from the middle of the dyke) having very similar chemical compositions, which may be classified as basaltic-andesite/andesite. The geochemical characteristics of these mafic dykes suggest that these are genetically related to each other and probably derived from the same parental magma. Although, the high-field strength element (+rare-earth elements) compositions disallow the possibility of any crustal involvement in the genesis of these rocks, but Nb/La, La/Ta, and Ba/Ta ratios, and similarities of geochemical characteristics of present samples with the Elan Bank basalts and Rajmahal (Group II) mafic dyke samples, suggest minor contamination by assimilation with a small amount of upper crustal material. Chemistry, particularly REE, hints at an alkaline basaltic nature of melt. Trace element modelling suggests that the melt responsible for these mafic dykes had undergone extreme differentiation (∼ 50%) before its emplacement. The basaltic-andesite nature of these rocks may be attributed to this differentiation. Chemistry of these rocks also indicates ∼ 10-15% melting of the mantle source. The mafic dyke samples of the present investigation show very close geochemical similarities with the mafic rocks derived from the Kerguelen mantle plume. Perhaps the Swangkre-Rongmil mafic dykes are also derived from the Kerguelen mantle plume.

The Kerguelen mantle plume activity in Sylhet Trap mafic rocks of Southern Shillong Plateau,NEIndia: Implications for its role in magmatism of eastern India

“…The Sylhet Trap basalt is associated with alkaline-carbonatite differentiated complexes such as Sung Valley, Swangkre-Rongmill, Jasra, Samchampi-Samteran (Srivastava, 2021;Srivastava & Sinha, 2004a, 2004b and Mawpyut (Chaudhuri et al, 2009). The Tertiary shelf sediments are the youngest unit occupying the southern fringe of the Shillong Plateau.…”

“…The palaeomagnetic pole position of the tuff and presence of mafic dikes of the Cretaceous period all over the Basement Gneissic Complex (BGC) of Shillong Plateau suggest that widespread continental flood volcanism occurred in the vast area of southern SP (Kapawar & Mamilla, 2021). The interaction between the lithosphere of the north-eastern margin of Greater India and the Kerguelen hotspot during the Lower Cretaceous period may represent the initial stage of the split of Greater India from F I G U R E 1 (a) Regional geological and tectonic framework of Shillong Plateau and Mikir Hills (after Srivastava & Sinha, 2004a, 2004b. (b) Lithological map of Sylhet Traps (Tyngngar-Mawlong-Sohbar) showing eastern and central inliers and studied section around Sohbar Village in the southern part of Shillong Plateau modified after Talukdar and Murthy (1971).…”

“…(a) Regional geological and tectonic framework of Shillong Plateau and Mikir Hills (after Srivastava & Sinha, 2004a, 2004b). (b) Lithological map of Sylhet Traps (Tyngngar‐Mawlong‐Sohbar) showing eastern and central inliers and studied section around Sohbar Village in the southern part of Shillong Plateau modified after Talukdar and Murthy (1971).…”

Geochemical constraints on the petrogenesis of mafic rocks (metadolerites) from the Proterozoic Shillong Basin, Northeast India: implications for growth of the Greater Indian Landmass

“…Lithologically, the AMGC comprises basement gneissic complex, amphibolites, migmatites, calc-silicate rocks, granulites, Shillong Group of metasedimentary rocks and Mesozoic-Tertiary igneous and sedimentary rocks (Nandy, 2001;Srivastava & Sinha, 2004). The Shillong Group of rocks is divided into two formations, viz.…”

“…the Lower Metapelitic Formation and the Upper Quartzitic Formation. The Shillong Group of rocks unconformably overlies the basement gneissic complex (Nandy, 2001;Srivastava & Sinha, 2004). Several discordant granitoid plutons (500-700 Ma) are found intruded within the basement gneissic complex as well as the Shillong Group of rocks (Ghosh et al 1991;Kumar, 1990).…”