0016-7622/2010-75-6-783/$ 1.00 © GEOL. SOC. INDIA JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.75, June 2010, pp.783-790 Glimmeritic Enclave in a Lamprophyre from the Settupalle Alkaline Pluton, Eastern Ghats Mobile Belt N. V. CHALAPATHI RAO Centre of Advanced Study in Geology, Banaras Hindu University, Varanasi – 221 005 Email: nvcr100@gmail.com Abstract: A rare occurrence of glimmeritic (mica-rich) enclave – composed of abundant modal biotite, subordinate proportions of clinopyroxene and apatite, minor amounts of feldspar, carbonate and sphene – is reported from the lamprophyre of Settupalle alkaline pluton, Eastern Ghats mobile belt (EGMB), India. The enclave displays very coarse grained equigranular texture (mica laths up to 5 mm and clinopyroxene grains up to 4 mm). In comparison, the host lamprophyre exhibits a marked porphyritic-panidiomorphic texture comprising phenocrysts of clinopyroxene; other phases such as biotite and potash- and plagioclase- feldspar are restricted to the groundmass. A tight closeness in mineral chemistry of the glimmerite and lamprophyre imply a possible genetic relationship between their parent magmas. Glimmeritic enclave is construed to be an autolith of the proto-lamprophyre magma, which failed to reach the surface, and lined the wall-rock along the conduit of the lamprophyric intrusion. Glimmerite enclave provides a direct evidence for the multi-stage modification of the lithospheric mantle due to the infiltration of the potassium-rich hydrous melts such as lamprophyres. Mineralogy of the glimmeritic enclave is also similar to that of a vein component of the hydrous, mafic and potassic-ultrapotassic veined lithosphere in the EGMB. Keywords: Glimmerite, Enclave, Lamprophyre, Settupalle, Eastern Ghats mobile belt. INTRODUCTION It is well-established from seismological and geo- chemical evidences that the Earth’s lithospheric mantle is heterogeneous on a centimetre scale and that mantle metasomatism or mantle enrichment by the magmas and/or fluids is one of the major processes that led to such inhomogeneities (e.g. Hoffmann, 2003; Anderson, 2008). Two main types of mantle metasomatism are recognized from the study of mantle xenoliths or nodules that are brought up by the kimberlites, lamproites, lamprophyres and alkalic basalts (e.g. Dawson, 1984; Erlank et al. 1987; Nixon, 1987; Pearson et al. 2003): (i) Patent (modal) metasomatism – when it is petrographically recognisable due to replacement textures and development of hydrous phases and (ii) Cryptic metasomatism – where only incompatible element enrichment is observed without any petrographic evidence. A variety of deep-seated potassium- rich alkaline continental magmas, such as kimberlites, lamproites and lamprophyres, are now widely inferred to be the products of such a metasomatized mantle (e.g. Mitchell, 2006). At times, these potassic magmas or melts similar to their composition also constitute meta- somatising agents and modify the mineralogical and chemical composition of the lithosphere (e.g. McKenzie, 1989; Foley, 1992; Kinny and Dawson, 1992; Peterson et al. 1994). In India, a wide-variety of lower-crustal as well as mantle-derived xenoliths are recorded from the Proterozoic kimberlites of the Dharwar craton and the Mesozoic alkalic basalts and lamprophyres of Western India (see Karmalkar et al. 2009 for a recent review). Evidences for both modal as well as cryptic metasomatism are recorded in these xenoliths (e.g. Nehru and Reddy, 1989; Karmalkar and Rege, 2002). The purpose of this paper is to (i) report a rare occurrence of glimmeritic (micaceous) enclave from a lamprophyre from the Settupalle alkaline pluton, (ii) discuss its petrography, mineralogy and mineral chemistry, (iii) evaluate the relationship between the enclave and the host lamprophyre and (iv) highlight the significance of glimmeritic enclave vis-à-vis various models invoking mantle metasomatism in the genesis of alkaline magmatism in the Eastern Ghats mobile belt. GEOLOGICAL SET-UP OF THE SETTUPALLE ALKALINE PLUTON The Settupalle alkaline pluton (Fig.1) forms the saturated