AbstractThe issue of when and how continental crust first formed on Earth is one of primary importance for a whole range of scientific disciplines as it is intimately linked to the formation of oceans, atmosphere and life. Equally, the rates at which the continental crust has grown through time and been reworked (eroded and also recycled back into the mantle) are a major criterion governing the Earth system. These factors must have changed significantly as the Earth’s main system of heat loss and movement of crust evolved to being controlled by plate tectonics. This study explores the geological record during a gestational period for the onset of modern plate tectonics and the role of Archaean terranes during recycling and crustal growth in the Palaeoproterozoic.
The Mineiro Belt, Brazil records a delayed transition from TTG to sanukitoid (high Ba-Sr) magmatism to shortly after the Archaean-Palaeoproterozoic boundary, compared with other cratons. This transition occurred partially during the so-called ‘magmatic lull’ between 2.4 and 2.2 Ga, when few evolved juvenile magmas were added to the continental crust. Superchondritic whole-rock εNd(t) and zircon εHf(t) and a narrow range of zircon δ18O values characterize the mantle source for the earliest magmas. In progressively younger rocks, some crustal contamination is identified by a broader spectrum of δ18O above mantle values and a concomitant decrease in εHf(t). The belt also contains a group of 2.17 – 2.18 Ga granitoids derived from partial melting of an Archaean mafic crust with long crustal residence time, marked by low εHf(t) and εNd(t) values and mantle derived oxygen signature.
The docking of the Mineiro Belt against the Archaean terrane is marked by the absence of a magmatic arc upon the continental margins, suggesting a soft transpressional collision. The Archaean foreland, nonetheless, endured partial melting of sediments in the upper crust, which generated small-scale felsic magmatism within the Archaean and Palaeoproterozoic supracrustal sequences. U-Pb ages from detrital zircons of the surrounding sedimentary rocks show that most of the sediments come from the Archaean blocks to the north of the belt. An extensive passive margin is inferred for the early stages of the Mineiro Belt geodynamic evolution, which involved recycling of a short-lived oceanic crust and underplating melt and/or contamination of the mantle wedge from a Palaeoproterozoic subduction zone.
|Date of Award||Aug 2019|
|Supervisor||Craig Storey (Supervisor) & Mike Fowler (Supervisor)|