Journal q/Southeast Asiatt Earth Scimw, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ Vol. 13, No. 2, pp. 145-158, 1996 Cmvrieht ST, 1996 Elsevier Science Ltd Pergamon PII: s0743-!2547(%)ooo16-5 Printed’; &ea;Britain. Ail rights reserved 0743-9547196 515.00 + 0.00 Carbonatite magmatism in northeast India D. Kumar,* R. Mamallan* and K. K. Dwivedyt *Atomic Minerals Division, Department of Atomic Energy, Shillong 793 011, India and tAtomic Minerals Division, Department of Atomic Energy, Hyderabad 500 016, India Abstract-The Shillong Plateau of northeast India is identified as an alkaline province in view of the development of several carbonatite complexes e.g. the Sung Valley (Jaintia Hills), Jasra (Karbi-Anglong), Samchampi and Barpung (Mikir Hills) and lamprophyre dyke swarms (Swangkre, Garo-Khasi Hills). On the basis of limited K-Ar data, magmatic activity appears to have taken place over a protracted period, ranging from the Late Jurassic to the Early Cretaceous. The carbonatite complexes of the Shillong Plateau share several common traits: they are emplaced along rift zones, either within Archaean gneisses or Proterozoic metasediments and granites, and exhibit enrichment in the light rare-earth elements, U, Th, Nb, Zr, Ti, K and Na. The enrichment in incompatible trace elements can best be accounted for if the parental magmas were of alkali basaltic type (e.g. mela-nephelinite or carbonate-rich alkali picrite). Copyright 0 1996 Elsevier Science Ltd Introduction The Shillong-Plateau-Mikir-Hills massif forms an outlier of the metamorphic basement of the eastern Indian shield, situated in the states of Meghalaya and Assam. Based on available K-Ar ages, carbonatite magmatism in this region belongs to two distinct phases (Table 1): Late Jurassic (c. 150 Ma; Veena Krishna zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC et al. 1991; Sarkar et al. 1992) and Early Cretaceous (c. 105 Ma; Acharyya et al. 1986). These periods were marked by the rifting and fragmentation of Gondwana and the northward drift of the Indian plate, following the separation of India from Australia-Antarctica. Recent studies have identified at least four carbonatite complexes in the Shillong-Plateau-Mikir-Hills area (Fig. 1; vi,-. Sung Valley, Jasra, Samchampi and Swangkre). In addition, lamprophyre dykes are abun- dant in the Swangkre region. The carbonatite complexes and dyke swarms are associated with major E-W, NE-SW and N-S trending faults, such as the Dawki, Barapani-Tyrsad, Raiba, Kalyani and Nongcharam lineaments (Fig. 1). In the field, each complex has a distinct morphology, forming a circular or elliptical depression. The objectives of this paper are to discuss the salient geological and geochemical features of the carbonatite complexes and attempt to constrain their origin. Geological setting The Shillong Plateau covers an area of c. 40,000 km* and includes the Garo, Khasi, Jaintia and Mikir Hills (Fig. 1). Geologically, it forms a northeastern outlier of the Indian peninsular shield. The oldest rocks of the Plateau are Archaean gneiss and schist, overlain unconformably by the Proterozoic Shillong Group (quartzite, phyllite, amphibolite). These rocks have been intruded by granite and ultramafic rocks. A small outcrop of Early Cretaceous basalts, known as the Sylhet Traps, overlie the Precambrian rocks of the southern part of the plateau. The Shillong Plateau carbonatite complexes, though separated by considerable distances, show many similarities in petrology and chemistry. A brief account of each of the complexes is given below. Samchampi The Samchampi complex is one of the smaller carbonatite complexes in the Shillong Plateau. It covers an area of 16.5 km? and is a roughly circular, stock-like body emplaced in Archaean gneiss (Fig. 2; Kumar et al. 1989). Fission track dating of apatite crystals from the Samchampi complex indicates a cooling event in the Early Cretaceous (c. 105 Ma; Acharyya et al. 1986). The E-W trending Kalyani lineament appears to have played an important role in the siting of the complex. In order of volume (greatest to least), the rock types consituting the complex are syenitic fenites, magnetite- hematite rock, pyroxenite, ijolite-melteigite, syenite, carbonatite and associated fenites. In addition, there are minor quantities of phosphatic breccia, volcanic tuff, phonolite and silicified rock. Three types of carbonatite have been identified on the basis of the presence of accessory minerals, such as olivine, phlogopite, biotite and magnetite (Fig. 2). The syenites show considerable petrographical variation, ranging from alkali syenite to foyaite, quartz syenite and nepheline syenite. The mineralogy of the carbonatites and syenites is outlined in Table 2. Economic minerals present in the complex include pyrochlore, zircon, monazite, fluorapatite, rutile and thorite. Enrichment in the rare-earth elements (REE) has also been noted in the metasomatised (fenitised) zones of country rock. Sung Valley The Sung Valley carbonatite complex is the first to have been reported from the Shillong Plateau (Yusuf and Saraswat 1977). The complex lies in the Wah-Sung Valley of Jaintia Hills district, Meghalaya, at the intersection of two major faults (Krishnamurthy 1985; Fig. 3). The complex is a roughly oval-shaped stock, intruding Shillong Group metasediments. The K-Ar age of a phlogopite separated from a Sung Valley siivitic 145