Deciphering metamorphic timescales in a Neoproterozoic continental collision zone using coupled petrochronology and geospeedometry.

Abstract

The emerging paradigm of rapid metamorphic timescales (<10 Mry) estimated by geospeedometry and high-precision geochronology in modern continental collision zones, challenges the traditional notion that metamorphism faithfully records the steady-state nature of tectonothermal processes. The exceeding fast rates of metamorphic processes started to appear in the geological record at the end of the Cambrian period and seem to be a common feature of almost all modern continental collision zones. However, the inherent uncertainties of geochronology and preservation biases currently hinder our understanding of whether ancient orogens experienced comparable metamorphic timescales. This project aims to explore the rates of metamorphic processes associated with burial, heating, exhumation, and cooling within a typical small-cold to transitional Neoproterozoic orogen located in NE Brazil - the South Borborema Orogen. This orogen was formed in the core of the West Gondwana paleocontinent and displays a well-preserved inverted metamorphic sequence that outcrops along the orogenic belt. This geological setting will be used as a natural laboratory for investigating the timescales of disturbances in the steady-state geotherms recorded by exhumed metamorphic rocks. Using an integrated approach involving radiometric-based petrochronology and diffusion chronometry considering the effect of intracrystalline compositional stress, we expect to comprehend better the metamorphic timescales implicated in the thermal budget during orogenesis. This will shed light on the mechanisms governing metamorphic inversion and has the potential to critically advance the main scientific problems surrounding the question of when collisional orogens started to behave in a modern way in the Earth's geological history.