April 2008 LIP of the Month

Petrology, geochemistry and paleomagnetism of earliest magmatic rocks of Deccan Volcanic Province, Kutch, North Western India

Dalim K Paul^a,*, Arijit Ray^a, Brindaban Das^a, Shiva K Patil^b and Sanjib K Biswas^a

a- Department of Geology, Presidency College, Kolkata -700073, India
b- Indian Institute of Geomagnetism, Allahabad, India
* Corresponding author, e mail : dalimpaul@yahoo.co.in

Extracted from Paul et al. (2008) Download PDF (2 Mb)

The Deccan Volcanic Province (DVP) occupies an area of about 500,000 sq km in western and central India. Jay and Widdowson (2008) have estimated an eruptive volume of 1.3x10⁶ km³. Deccan magmatism is believed to have initiated from northwest part of Indian (Basu et al., 1993). Tholeiitic basalts and alkali basalts occurring in southern coastal belt and the central part of the Kutch basin, respectively, are the northernmost on- land exposures of Deccan Traps (Mahoney, 1988) in the peripheral region of the Réunion plume. Our work is confined to the study of magmatic rocks of the Kutch basin (Fig.1).

The Kutch basin is an east-west trending peri-cratonic rift basin at the western margin of Indian Craton (Biswas, 2002). The Nagar Parkar Fault (NPF) bounds the rift on the north and the North Kathiawar Fault (NKF) limits it to the south. The rift basin features intra-basin tilted blocks and intervening half grabens Magmatic rocks of Kutch include extensive tholeiitic basalt flows, alkali basalt plugs, gabbroic dykes, alkaline mafic dykes and sills and a differentiated alkaline complex with units such as pyroxenite, theralite, teschenite, nepheline syenite and lamprophyre. Our studies from North Kutch indicate the occurrence of alkaline mafic dykes, sills and a differentiated alkaline complex. These alkaline rocks are contemporaneous with Deccan Traps (65 Ma) (perhaps slightly older) and thus should be considered as integral component of the DVP. Alkaline magmatic rocks of northern Kutch have been emplaced along deep seated rift-related faults which provided the channel-ways for upward movement of magmas to shallow crustal depths (Fig.2).

Magmatic rocks of Kutch basin are classified into three groups on the basis of mode of occurrence, petrology, geochemistry and magnetic properties. All the alkaline rocks of northern Kutch are characterized by presence of kaersutite either as phenocrysts or as a constituent of the groundmass (Fig 3a, 3b, 3c). Such kaersutite -bearing rocks are not reported from the eastern and southern parts of the DVP. This suggests that the sub -continental lithospheric mantle beneath Kutch was rich in volatiles and alkalis (metasomatised and fertile) prior to melting. Moreover these kaersutite phenocrysts contain carbonatite-alkaline silicate melt inclusions preserved as an immiscible liquid (Fig.3d). The presence of such melt inclusions in phenocrysts confirms the role of CO₂ and other volatile phases during magma generation. Spinel lherzolite, wehrlite, and dunite xenoliths are found in the alkali basalts of Kutch (Fig.3e) and this is the only region in the DVP where such xenoliths have been reported (De, 1964; Krishnamurty et al, 1989; Karmalkar et al., 2005). Mineralogical and textural studies suggest the presence of phlogopite, apatite and calcite in xenoliths and there is evidence of transformation of orthopyroxene to an assemblage of clinopyroxene and olivine by reaction with carbonatitic fluid (wehrlitisation cf. Yaxley et al., 1991). P-T estimates on these xenoliths also suggest shallow level (around 40 km) re-equilibration during metasomatism (Mukherjee and Biswas, 1988; Krishnamurty et al 1989; Karmalkar et al 2005).

On a TAS diagram the tholeiitic basalts of southern coastal belt and gabbroic dykes of central Kutch occupy fields of basalt. Alkali basalt plugs of the central part of the basin occupy fields of tephrite-basanite and sometimes show transitional character between basalt and basanite. The alkaline mafic sills and dykes of northern Kutch occupy fields of picrobasalt and tephrite-basanite. Rocks of differentiated complexes fall in the fields of picrobasalt, basalt, basaltic andesite and tephrite-basanite. Alkali basalt plugs of central Kutch, and alkaline rocks of northern Kutch show LILE- and LREE- enriched character compared to the tholeiites of southern Kutch and Deccan tholeiites. This enrichment was inherited from a fertile mantle source. Trace element abundances and ratios like Nb/Zr, La/Zr, Ce/Zr, as well as Sr, Nd isotopic composition indicate that crustal contamination did not play significant role in the evolution of these magmatic rocks. Fractionation of  the early formed olivine, clinopyroxene, plagioclase, kaersutite was responsible for the  differentiation of magma yielding a diversity of rock types.

Paleomagnetic investigation of thirty magmatic bodies from Kutch yield a Virtual Geomagnetic Pole (VGP) at 33.7^o N and 81.2^oW (dp/dm = 5.81/9.18). This pole is statistically concordant with that of Deccan Super Pole (36.9^oN:78.7^oW). The magmatic rocks of Kutch basin are broadly contemporaneous straddling the 30N-29R-29N chrons. Available geophysical and geological data suggest the presence of a subcrustal magma chamber (Fig.2) which is spatially (and probably genetically) linked to Kutch basin rifting.

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