Nepheline syenite intrusions from the Rengali Province, eastern India: Integrating geological setting, microstructures, and geochronological observations on their syntectonic emplacement

Abstract

Ten nepheline syenite intrusions have been concordantly emplaced within or close to a polydeformed, regionally metamorphosed sedimentary sequence in the Rengali Province, eastern India during the Mesoproterozoic. The intrusions and the country rocks share a common deformation history and record three phases of folding. The nepheline syenites preserve deformation structures and textures indicative of continuous deformation that started in the presence of a melt and ended under sub-solidus conditions. This deformation in nepheline syenites is divided on the basis of mechanical behavior into three stages: an early one with the development of varied types of magmatic layering and flattened magmatic enclaves, a middle one with the development of asymmetrical magmatic folds and intramagmatic shear zones, and a late stage with melt-enhanced embrittlement. Igneous zircons from two of the intrusions, namely Chhatabar and Gunjighara are dated at 1356 ± 5 Ma and 1358 ± 4 Ma (U–Pb concordant age, LA–ICP–MS), respectively. Electron microprobe U–Th–total Pb geochronology of monazites from two samples of sillimanite grade mica schists in the host metasedimentary sequence of the Chhatabar nepheline syenite yields metamorphic ages of 1362 ± 12 Ma and 1376 ± 23 Ma, which indicate broad synchronism of regional metamorphism of the country rocks and the nepheline syenite magmatism. Contrary to the common proposition that nepheline syenite intrusions in eastern India and in some other regions of Precambrian crust were emplaced anorogenically in intracontinental rifts and subsequently deformed by continental collision, we develop a structural interpretation in which the nepheline syenites were emplaced syntectonically in the sense that magma transport and cooling occurred during regional metamorphism and deformation. The study shows that an integration of structural, textural and geochronological data from both nepheline syenite and country rock is crucial for an understanding of the spatial and temporal relationships between contractional deformation and alkaline magmatism. Our results imply that deformed nepheline syenites may not always be useful in constraining the initiation of a Wilson Cycle heralded by intracontinental rifting.