Mantle source characterization of Sylhet Traps, northeastern India: A petrological and geochemical study

Abstract

In this study, mineralogical, geochemical, and isotopic data are presented for the Sylhet Trap at the southern flank of the eastern Shillong Plateau, northeastern India, to determine the magma genesis in relation to the Kerguelen plume mantle source. Sylhet Trap rocks are porphyritic tholeiite and have diverse chemical compositions from picro-basalt, basalt, andesite to dacite, but mostly are within the subalkaline field. Major and trace element data were used to identify two distinct magma fractionation trends, a low and medium K series, characterized by relatively flat MORB-like (analogous to Rajmahal Traps (II)) and enriched OIB chondrite-normalized Rare Earth Element (REE) patterns. Initial 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, and 206 Pb/ 204 Pb isotope compositions were widely varied, ranging from 0.70435-0.71357, 0.51196-0.51266, and 17.92-19.72, respectively, when compared with basalts from the West Bengal, the Rajmahal Traps and the Kerguelen plume. Correlations among isotopic and trace element ratios of the Sylhet Traps provide evidence for the involvement of (1) HIMU-like mantle component, (2) the Kerguelen plume-like component, and (3) EMII-like crustal component. Magma from the Sylhet Traps was originated from a melting that derived directly from the heterogeneous Kerguelen mantle plume (components 1 and 2), which strongly suggests the presence of the Kerguelen plume-head in the Bengal basin.

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 concentration of Pb ranges from 0 to 1.3 ppm, which is lower compared to Ocean Island Basalt (OIB), with a concentration of 3.2 ppm (Sun & McDonough, 1989). Additionally, published geochemical data proposed a range of Pb concentrations from 0.64 to 18 ppm (Ghatak & Basu, 2011; Islam et al, 2014; Sen et al, 2019). The Light REEs La and Ce are also almost three times higher than the E‐MORB trend.…”

Petrological and geochemical study of the Sylhet trap basalts, Shillong plateau, N.E. India: Implications for petrogenesis

“…ST basalt is exposed only southern flank of the Shillong Plateau along the E-W Dauki Fault because of thinning of the continental crust of this region. The decompression melting further supported by the young magmatic event of ~84 ma (Islam et al, 2014). Emplacement of the ST basalt along the Dauki fault inferred that structural discontinuities, viz., the Dauki Fault was active simultaneously during Gondwana break up which have acted as conduit for magma ascent (Olierook et al, 2016).…”

“…All the previous geochemical studies on STB such as Ghatak & Basu (2011;2013), Islam et al (2014), Kent et al (1997;, Ray et al (1999), Srivastava & Sinha (2004; and Storey et al (1992) have advocated for the Plume related origin of the rock type. Many other geochemical studies on Indian Ocean plateau basalts (e.g Doucet et al, 2004Doucet et al, , 2005;Escrig et al, 2004;Frey & Coffin 2000a;Frey et al, 2002;Graham et al, 2006;Hanan et al, 2004;Ingle et al, 2003;Mahoney et al, 2002;Weis et al, 2002;Nauret et al, 2006;Neal et al, 2002;Rehkämper & Hofmann, 1997;Storey et al, 1991;Weis & Frey, 1991; and on basalts occurring at the paleo-Gondwana margins (Courtillot & Cisowski 1987;Jordan et al 2004;Saunders et al, 1992;White & McKengie, 1989) have also implicated for a plume hypothesis for the basalts on the Eastern margin of the Paleo-Gondwana that includes the STBs.…”

“…Only a few workers inferred partial involvement of the plume (only heat derived and not substance) (see, e.g., (Frey et al, 1996;Kent et al, 1997;Mahoney et al, 1983). Recently Ghatak & Basu (2011) has advocated lower crustal contamination for the STB magmas, while Islam et al (2014) have suggested the involvement of two mantle components viz. (i) the Kerguelen plume-like component and (ii) HIMU-type component in the generation of magma for STB.…”