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Himalaya gneiss dome formation, focused radiogenic heating in southern Madagascar, and fertilization of the Neoproterozoic ocean by mantle-derived phosphorus

  • Author(s): Horton, Forrest Miller
  • Advisor(s): Hacker, Bradley
  • et al.
Abstract

(I) Geochronology, thermochronology, and structural observations across Gianbul gneiss dome provide insight about the exhumation of middle crust in the India-Asia collision zone: Doming (1) initiated during the early stages of extension; (2) was driven by a positive feedback among dehydration melting, buoyancy, and decompression; and (3) culminated with the injection of anatectic melts into the upper levels of the dome. The dome was subsequently exhumed as part of a footwall block beneath a brittle normal fault.

(II) Focused internal heating led to melting, metamorphism, and crustal weakening during the Neoproterozoic continent-continent collision between East and West Gondwana. Numerical models based on chronologic and thermal constraints across southern Madagascar indicate that radioactive decay of thorium was the principal heat source responsible for regional metamorphism at temperatures >900° C in the middle to lower crust.

(III) The Neoproterozoic era was punctuated by profound tectonic, evolutionary, and environmental change. Biologic and climatic conditions may have been especially sensitive to fluxes of phosphorus (P) from the weathering of continental crust. Large igneous provinces—containing abundant P and highly susceptible to chemical weathering—occurred regularly during the breakup of the Rodinia supercontinent. An estimated bioavailable P flux to the ocean from the weathering of basalt peaked at ~720 Ma, immediately prior to rapid biologic diversification and the Sturtian glaciation; I postulate that the burial of organic carbon that resulted from this unprecedented P flux helped facilitate glaciation and triggered the oxidation of the ocean-atmosphere system.

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