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

摘要

Understanding the petrogenesis of crustal-scale silicic magmatic systems is a major research frontier. The similar to 2600-2300 Ma granitoid covering >90% of the exposed part of the >26,000 km(2) 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 degrees C) for the granitoid magma that crystallized over similar to 200 degrees C (similar to >= 900 degrees-706 degrees 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 (similar to 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.