Evolution of a crustal-scale silicic to intermediate tectono-magmatic system: The ~2600–2300 Ma Bundelkhand granitoid, India

Abstract

Understanding the petrogenesis of crustal-scale silicic magmatic systems is a major research frontier. The ~2600–2300 Ma granitoid covering >90% of the exposed part of the >26,000 km2 Bundelkhand Craton in northcentral India is considered so far to have resulted from melting of Tonalite-Trondhjemite-Granitoid (TTG) gneiss at the mid-upper crustal level aided by radioactive heat generated from the U-enriched upper crustal rocks within a period of 60–80 Myr. Here we combine new field, petrographic/microstructural evidences, mineralogical and geochemical data, along with published geochemical/mineralogical, geochronological and geophysical results to develop a unified model that highlights the deeper crustal melting and crustal reworking mediated by repetitive mantle-derived mafic recharge over ≥300 Myr in a common specific stress regime that has controlled the evolution of Bundelkhand granitoid. Our main findings include: (a) Ti-in quartz, apatite and zircon saturation, and primary biotite thermometry indicating high liquidus (≥900 °C) for the granitoid magma that crystallized over ~200 °C (~≥900°–706 °C); (b) magmatic epidote, amphibole (Fe-rich hastingsite to magnesiohornblende), and magmatic titanite indicate a polybaric crystallization of the ascending magma from the deeper crust (~9 kb) through the middle (5–6 kb) to shallow crust (1–2 kb); (c) polybaric crystallization (≤5–10 kb) for mafic magma manifested in magmatic enclaves and mafic dyke swarms; (d) large-scale silicic-mafic magma interactions and (e) crystallization and emplacement of both silicic and mafic magma under a common stress regime. Unlike closed-system dehydration melting, the influx of mantle-derived supercritical basaltic magma provided not only heat and material but also supplied fluids in the deeper crust in a sustained tectono-magmatic condition. We conclude that the intermittent H2O-rich mafic recharge is key to large granitoid production in a dynamic crustal-scale magmatic system worldwide irrespective of tectonic setting.