JOUR.GEOL.SOC.INDIA, VOL.74, JULY 2009 108 ADITI MANDAL AND ARIJIT RAY Petrology of Mafic-Ultramafic Rocks Along North Puruliya Shear Zone, West Bengal ADITI MANDAL and ARIJIT RAY Department of Geology, Presidency College, Kolkata - 700 073 Email: rayarijit22@redifffmail.com Abstract: The North Puruliya Shear zone (NPSZ) is characterized by occurrence of mafic-ultramafic rocks aligned parallel to the shear zone, intruding the high grade Proterozoic rocks of Chhotanagpur Gneissic Complex. The ultramafic rocks occur as small lenses, pockets, veins, thin dykes and are intimately associated with mafic (gabbro, norite) rocks. Pyroxenites (viz. olivine websterite, websterite, plagioclase websterite) and hornblendite are the two important members of the ultramafic rocks containing clinopyroxene, orthopyroxene, olivine, plagioclase, amphibole, phlogopite and ilmenite. The mafic-ultramafic rocks show evidence of shearing and retrogressive metamorphism. Linear correlation of chemical attributes suggests fractionation-controlled magmatic differentiation. Enrichment of LILE and LREE in the mafic- ultramafic suite suggests an enriched mantle source and pronounced negative Eu-anomalies in all the rock types except hornblendite suggest fractionation of plagioclase under low f O 2 condition. Progressive iron enrichment trend in rocks of the mafic-ultramafic suite also indicate magmatic differentiation under low f O 2 condition. Early fractionation and accumulation of clinopyroxene and plagioclase from a basaltic magma may have given rise to the ultramafic rocks of the area. Little change in the Nb/Zr and Ce/Zr ratios of ultramafic and mafic rocks (except alkali norite) strongly support low crustal contamination. A few samples of norite and gabbro-norites appeared to be variably contaminated by a crustal component or affected by late granitic intrusion resulting in enrichment of alkali in the former. Keywords: Ultramafic and mafic suite, Eu anomaly, Assimilation, Fractional crystallization, North Puruliya shear zone, West Bengal. either sides of the Gondwana basin and NPSZ in the Proterozoic terrain of Chhotanagpur Granite Gneiss Complex (CGC) (Ghose, 1983, 1992; Ghose and Chatterjee, 2008). They are associated with mafic, intermediate and felsic members. In the northern part of CGC, ultramafic rocks are rare and post-tectonic intrusion of ultramafic rocks have been reported from southern margin of Gondwana Basin at Richughuta (Ghose, 1970) and 1km west of Bagaliya railway station, 1.6 km south of Gaurangdih, both from the northeastern part of Puruliya district (Mahadevan, 1992). The aim of the paper is to report the mode of occurrence, mineralogy, geochemistry of a few mafic and ultramafic rocks occurring along the NPSZ and to discuss their petrogenesis. GEOLOGICAL SETTING The study area forms a part of the CGC characterized by medium- to high-grade rocks viz. khondalite, granulite, gneisses and amphibolites. Several intrusive rocks like INTRODUCTION Northern part of Puruliya district is characterized by a megalineament corresponding to North Puruliya shear zone (NPSZ) (GSI, 1977; Mahadevan, 1992; Dasgupta et al. 2000). The NPSZ has a roughly E-W trend near Jaipur (western sector) which changes to NE-SW near Panrkidih (in the central sector), and subsequently to E-W near Hariharpur (in the eastern sector, Fig.1). The shear zone is characterized by the occurrence of brecciated quartzite, apatite-magnetite- bearing chert, U-Th mineral-bearing pegmatite, alkali granite and mafic-ultramafic rocks (at Jaipur, Bara Irga, Hariharpur, Asanbani and Bagaliya; Mahadevan, 1992, Fig.1).The shearing is pervasive and has affected all rock types including the country rocks, e.g., porphyritic granite gneiss, khondalite, amphibolite and mafic granulite. Ultramafic rocks occur as small lenses, pockets, thin dykes and are always associated with mafic rocks. Extensive occurrence of ultramafic rocks has been reported from the Auranga-Koel valley in the western, on JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.74, July 2009, pp.108-118 0016-7622/2009-74-1-108/$ 1.00 © GEOL. SOC. INDIA