Petrological and Geochemical Constraints on the Pelitic Hornfels of the Champaner Group, Western India: Implications for Its Origin and Evolution

Abstract

The granitic intrusions emplaced into a terrane causes several textural, chemical and compositional changes in the host rock by wall-rock alteration and forms contact metamorphic rocks that is, hornfels. The low-grade metasedimentary rocks of the Champaner Group exposed as horse-shoe-shaped map pattern have envisaged several pulses of granitic intrusion with granitoid ages ranging from 2.5 to 0.95–0.93 Ga enclosing its most peripheries. However, it has been observed that the contact metamorphic signatures are most prominent on the eastern periphery of Champaner fading away towards the west slicing up the Champaner Group into outer, middle, and inner zone rocks. The present work is focused on the detailed study of inner zone pelitic hornfels by providing new constraints on field characteristics, reaction textures, mineral chemical compositional analyses, geochemical signatures, and P–T conditions. Three distinguished varieties of pelitic hornfels that is, biotite hornfels, andalusite-cordierite hornfels, and andalusite-sillimanite hornfels with increasing proximity to granitic intrusion respectively were identified. Based on the mesoscopic features, textural and phase relations, and mineral compositions, biotite, andalusite, cordierite, and sillimanite isograds were demarcated. The development of distinct mineral assemblages in different pelitic hornfels is correlated to the varying Al₂O₃, FeO, and MgO content in the bulk rock composition of the rocks and the geometry of the intruding felsic pluton (Godhra granite) beneath the Champaner supracrustals. The geochemical constraints suggest K-rich clay dominant shale protolith of post archean age comparable to Post-Archean Australian Shales (PAAS) deposited in an active continental margin tectonic setting. The increase in temperature gradient rather than pressure induced by 0.95–0.93 Ga “Godhra granitic” intrusion is considered to be a significant factor controlling the development of the observed progressive mineral assemblages. The P–T conditions constrained through P–T pseudosections built in a KFMASHTi model system suggest LP/HT metamorphism with P–T range from 200 to 350 MPa and 530°C–600°C respectively.