Magnetic petrofabric and petrographic evidence for late- to post-tectonic emplacement of the Kalpetta Granite, Nilgiri Block, Southern Granulite Terrain, India

Anisotropy of Magnetic Susceptibility (AMS) provides an ideal methodology for identifying subtle internal deformations in rocks that lack visible macro-scale strain markers. This study investigates the petrographic and magnetic petrofabric characteristics of the Neoproterozoic alkaline-subalkaline Kalpetta Granite, an oval-shaped pluton located in the western portion of the Nilgiri Block within the Southern Granulite Terrain. The Kalpetta granite is a medium to coarse-grained body primarily composed of quartz, plagioclase, and alkali feldspar, with significant mafic contributions from biotite and hornblende. Detailed petrographic analysis reveals a complex crystallisation history, notably including chessboard patterns in quartz, indicative of solid-state deformation at high temperatures near the solidus. AMS analysis reveals a high degree of magnetic anisotropy in the Kalpetta granite, predominantly controlled by ferrimagnetic minerals, notably magnetite. The resulting AMS ellipsoids exhibit an oblate shape, corresponding to the weak alignment of these magnetite grains within the rock matrix. Magnetic lineation and foliation trends within the pluton align obliquely to the general trend of the regional Bavali-Moyar shear zone. These findings suggest that the granite experienced significant magmatic flow and high-temperature deformation before its final solidification. Yet, it strikingly lacks the low-temperature deformation features found in surrounding lithologies. The associated host rocks, including hornblende-biotite gneiss, charnockite, and pyroxene granulite, display a range of intense deformation textures, moving from high-temperature grain boundary migration to low-temperature, high-strain conditions. This sharp disparity supports a post-tectonic emplacement model for the Kalpetta granite, distinguishing its relatively stable cooling history from the syntectonic deformation of the surrounding terrain. Thus, this research underscores the need to distinguish between high- and low-temperature deformation textures to accurately reconstruct the timing and tectonic setting of granite emplacement within the Southern Granulite Terrain.