Enigmatic association of the carbonatite and alkali-pyroxenite along the Northern Shear Zone, Purulia, West Bengal: A saga of primary magmatic carbonatite

Abstract

The Purulia carbonatite, 'carbonatite'-'alkali-pyroxenite'-'apatite-magnetite rock' association, is located at Beldih area of Purulia district, West Bengal and falls within the 100 km long Northern Shear Zone (NSZ). Published literature suggests that the Purulia carbonatite was formed by the process of liquid immiscibility from under-saturated silicate parent magma. However, no silica under-saturated rocks like ijolite, nepheline-syenite etc. is known from the area. The trace element geochemistry (Ba/La, Nb/Th, Nb/Pb and Y/Ce ratios in the present study) also does not support this view. Present study indicates that the Purulia carbonatite is enriched in 6REE and incompatible elements but the carbonatite is also poorer in Nb, Th and Pb compared to the world average of calicocarbonatites. The lower value of Nb is characteristics of carbo(hydro)thermal carbonatite where carbonatite is associated with alkali-pyroxenite and suggests probable origin of the carbonatite as carbothermal residua evolved from an unknown parentage. However, the field, petrographic and geochemical data indicate the genesis of this carbonatite from a primary carbonatitic magma of mantle decent. The 87 Sr/ 86 Sr ratio of the carbonatite and apatite separated from the carbonatite (~0.703) implies primary magmatic derivation of the Purulia carbonatite. Close similarity of the apatite of the apatite-magnetite rock with the mantle apatite (of type Apatite B) indicates that they are also of primary magmatic origin. The present work portrays a unique example where primary magmatic carbonatite is associated with the alkali-pyroxenite.

Proterozoic Alkaline rocks and Carbonatites of Peninsular India: A review

“…Carbonatite occurs as oblong shaped intrusive body and comprise predominantly magnesian rauhaugite (dolomitic carbonatite), ankeritic carbonatite, and minor calciocarbonatite as dykelets within dolomite and ankeritic carbonatite (Viladkar and Wimmenauer, 1986). The Purulia complex contains multiple isolated exposures of alkaline bodies in form of sigmoidal outcrops and dykes (Mitchell and Chakrabarty, 2012;Basu and Bhattacharyya, 2014), which are exposed in the Santuri-Kankarkiari area (Das et al, 2018), Sushina Hill (Chakrabarty et al, 2016), and Beldih (Chakrabarty and Sen, 2010). Beside these three exposures, alkaline rock complexes of Proterozoic eon are mainly concentrated in the EGMB and SGT (Fig.…”

“…4) These results show that crystal fraction has played an important role in the formation of Proterozoic alkaline rocks. Bulk major oxides composition of Proterozoic carbonatite namely calciocarbonatite (n = 98), dolomitic carbonatite (n = 32), ferrocarbonatite (n = 19), silicocarbonatite (n = 30), and benstonite carbonatite (n = 4), compiled in study from various sources (Phadke and Jhingran, 1968;Subramanian et al, 1978;Sukheswala and Viladkar, 1978;Nair et al, 1984;Viladkar and Wimmenauer, 1986;Natarajan et al, 1994;Viladkar and Subramanian, 1995;Kumar et al, 1998;Pandit et al, 1998Pandit et al, , 2002Schleicher et al, 1998;Viladkar, 1998; Catlos et al, 2008;Vladykin et al, 2008;Chakrabarty and Sen, 2010;Ray et al, 2013;Renjith et al, 2016b;Ackerman et al, 2017), are presented in Fig. 5…”

An Overview of the Carbonatites from the Indian Subcontinent

“…The Purulia carbonatites are enriched in P 2 O 5 as generally observed for other provinces, however, one of the samples show up to 5% SiO 2 concentration. They are enriched in ΣREE and incompatible elements but also poorer in Nb, Th and Pb compared to the world average of calicocarbonatites [56]. The Primitive Mantle normalized spider diagrams show depletion peaks for Rb and Nb for these carbonatites.…”

“…In Sarnu-Dandali-Barmer area strong sodic fenitization has been reported [76,126]; similarly in Mundwara area the host granite is fenitized by sodarich fluids [101]. In Purulia area alkali pyroxenite shows sodic fenitization [56] and so is the case with Loe Shilman, Silai Patti, Jawar and Jambil areas of Pakistan where phyllites and gneisses show development of sodic amphiboles and soda feldspars indicating prominent sodic fenitization ( Figure 1). However, increased percentage and grain size of K-feldspar, surrounded by clusters of small globules of albite, increased concentration of biotite near Fe-oxides indicate presence of strong potassic fenitization [15,124,127,128].…”

“…Sev-eral alkaline intrusive bodies, including the Sung Valley carbonatite complex are genetically related to Kerguelen hotspot, which produced basaltic lava for about 130 Ma and extended upto the Ninety-East Ridge in the Indian Ocean. The Purulia carbonatites and nepheline-syenites of West Bengal are intruded within the Chandil formation of 1500-1600 Ma age and lies in the close proximity of the Chotanagpur Granite Gneissic Complex (CGGC) [56]. The ages reported from variants of syenites is 1510 Ma with poly-phase metamorphic imprints ranging from 1300-960 Ma age.…”