Role of Thorium-Bearing Granitoids in Thermal Buffering During Granulite Metamorphism in the Central Indian Tectonic Zone

Granulite-facies metamorphism of the continental lower crust reflects not only the attainment of high peak temperatures but also prolonged residence at elevated thermal conditions, which is critical for equilibration and preservation of high-grade mineral assemblages. In granulite terranes of the Central Indian Tectonic Zone, peak metamorphic temperatures are widely attributed to externally driven tectono-thermal processes, such as crustal thickening, magmatic underplating, or mantle-derived heat input. In contrast, the factors governing post-peak thermal evolution remain less well constrained. This study evaluates the potential role of thorium-bearing granitoid lithologies in modifying post-peak thermal evolution during crustal cooling. Whole-rock trace element data from granitoids reveal pronounced heterogeneity in thorium contents, with several samples exhibiting substantial enrichment relative to surrounding granitoids and average lower crustal values. Although radiogenic heat production from thorium is insufficient to independently generate granulite-facies conditions or to act as a primary heat source, such enrichment represents localized zones of enhanced radiogenic heat production. Once high-temperature conditions are established by external heat sources, these Th-rich granitoid domains may modify post-peak thermal gradients by retarding isotherm relaxation during crustal cooling and exhumation.

This spatially limited thermal buffering effect, subordinate to tectonic heat sources, may nevertheless contribute to prolonged high-temperature residence of granulite-facies mineral assemblages. This study focuses on the role of heterogeneous radiogenic heat distribution in shaping the temporal evolution of crustal thermal regimes and suggests that thorium systematics constrain the duration, rather than the origin, of high-grade metamorphism in continental crust.