Glimmeritic Enclave in a Lamprophyre from the Settupalle Alkaline Pluton, Eastern Ghats Mobile Belt

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.