Petrology, geochemistry of hornblende gabbro and associated dolerite dyke of Paharpur, Puruliya, West Bengal: Implication for petrogenetic process and tectonic setting

Abstract

Paharpur gabbroic intrusive is an arcuate body running east-west paralleling the foliation of Chhotanagpur Granite Gneiss which acts as country rock. The main gabbroic body is intruded by a number of dolerite dykes running north-south. It is composed of clinopyroxene (Wo 48 En 40 Fs 12-Wo 51 En 40 Fs 09 , mg no. 72-82), plagioclase (An 52-An 90), hornblende (magnesian hornblende to ferro-tschermackite), orthopyroxene (En 76-En 79) and ilmenite. Hornblende occurs as large poikilitic grain and constitutes around 60% of the rock. Both gabbro and associated dolerite dykes, show relatively primitive character (mg no. 65-73). Primitive mantle-normalized and MORB-normalized spider diagrams indicate enrichment in Rb, Ba, Th, La, Sr and depletion in Nb, Zr, Y, Ti and Nd. The LILE enrichment and Nb, Ti, Zr, Y depletion suggest arc like geochemical signature for the gabbroic and doleritic rocks of Paharpur. Flat to slightly LREE fractionated pattern and variable degree of REE enrichment is observed. An early stage fractionation of clinopyroxene, plagioclase, orthopyroxene, ilmenite and late stage reaction of cumulate pile and evolved melt/hydrous fluid is suggested for magmatic evolution of gabbro. Associated dolerite dykes, which are geochemically similar to the gabbro, have tholeiitic with boninitic character. The mineralogical and chemical compositions of intrusive rocks also have some similarity with mafic rocks of ophiolite complex of subduction zone.

Magneto-Mineralogical Characterization and Manifestations of Magnetic Fabrics from the Gneissic Rocks and Associated Intrusive Bodies in and around Bankura and Purulia Districts, West Bengal, India

“…But in the present case such possibilities can be ruled out based on field evidences which suggests that the mesoscopic field foliation (gneissic banding) is not horizontal. Besides, studies by Baidya 2 and Mandal et al 4 provide reports of the presence of migmatites near the studied rocks Paharpur and Gopalpur region, which is also reflected in the distribution of susceptibility axes on the equal area diagram. Such inferences can be drawn in analogy with results obtained by Ferre et al 27 7 c).…”

“…This feature proves that magnetic foliation is near horizontal. This type of near-horizontal magnetic foliation is indicative of the presence of isoclinal folds within the field 2,4 . Possibilities of near-horizontal magnetic foliation can also result from the presence of homoclinal planar structures.…”

“…The rocks in the studied part of CGC (Figure 1) are medium to high grade metamorphosed with concordant and discordant intrusive bodies like dolerite and pegmatite 4 . Geochronologically, these rocks are assigned to two age groups, 870  40 Ma and 810  40 Ma, from the K-Ar dating of biotite from the porphyritic granitoid and muscovite from the leuco-granitoid 5,6 .…”

Petrology and geochemistry of late Carboniferous hornblende gabbro from the Awulale Mountains, western Tianshan (NW China): Implication for an arc–nascent back-arc environment

“…Hornblende-bearing gabbros are typical of subduction-related magmatic rocks and have been considered to represent magmatic differentiation process in arc magmas (Arculus and Wills, 1980;Beard, 1986;Debari, 1994;Hickeyvargas et al, 1995;Mandal et al, 2012). The presence of abundant hornblende in the subduction-related gabbroic rocks has been considered either as an early stage mineral crystallized directly from the water-rich basaltic magma (Conrad and Kay, 1984;Beard and Borgia, 1989;Wan et al, 2013) or as a product of reaction between early stage minerals (olivine, pyroxene and plagioclase) and water-rich evolved melt (Dong et al, 1997;Prouteau et al, 2001;Costa et al, 2002;Yan and Niu, 2014).…”

“…Petrographically, the former type of hornblende usually occurs as smaller crystals in gabbroic rocks and anorthite exists as the most extreme end product in the late crystallization stage (e.g., Ulmer et al, 1983;Sisson et al, 1996;Wan et al, 2013) as a high water content in a basaltic magma often suppresses the crystallization of plagioclase (Sisson and Grove, 1993). Whereas, the later type usually occurs as poikilitic crystals enclosing earlier-formed plagioclase grains and showing reaction relation with them (e.g., Smith et al, 1983;Ulmer et al, 1983;Dong et al, 1997;Costa et al, 2002;Bonev and Stampfli, 2009;Mandal et al, 2012).…”