The origin of Brazilian Gondwana: using isotopic proxies for subduction to reconstruct ancient oceans

Abstract

Ancient reconstructions of the geography of the planet are essential to understand the tectonic setting of petroleum and mineral systems, the controls on past climates, and the distribution of ancient ecosystems. Yet, the available reconstructions do not incorporate oceanic crust - despite this making up two thirds of the planet. This is because no in situ oceanic crust exists from before ~200 million years ago; however, proxies for it lie in the products of ancient subduction-related volcanic arcs. These products can be interrogated in the following systematic ways using existing and emerging techniques: 1) The age range of subduction can be determined by the U-Pb age of zircons in the coherent remains of volcanic arcs and in detrital sand grains within sedimentary rocks derived from the erosion of these arcs; 2) The fraction of continental crust involved in the subduction zone can be estimated from the whole-rock 143Nd/144Nd isotopic ratio of the coherent arc remains or from the 176Hf/177Hf isotope ratio preserved in previously dated detrital zircons; and 3) The nature of this continental crust can be determined by examining the 18O/16O isotopic ratio preserved in the zircons. Oxygen isotopes are only readily fractionated by low temperature, surficial processes. So this ratio preserves a record of whether surface-derived sediments were involved in the subduction magma. These data provide, respectively: 1) the timing and location of oceanic-crust subduction; 2) whether the subduction zones involved the consumption of old continental crust or the formation of new continental crust; and 3) whether the subduction-zone magmas incorporated surface-derived rocks. This research will be used to address the geography of the central Brazilian region, in the South Paragay and Araguaia belts, from 750 to 500 million years ago - a 250 million year period of time in which multi-cellular life was rapidly evolving, the climate was experiencing the most extreme fluctuations known, and the oldest economic petroleum source rocks were being deposited. (AU)