On the Origin of Sodic Granitoids during the Precambrian

The discussion on the origin of Archean sodic granitoids, tonalite-trondhjemite-granodiorites (TTG), is connected to many important topics in Earth Science, such as the differentiation of the silicate Earth, the tectonic style of the Early Earth and its secular evolution towards plate tectonics as we know it today, the origin of arc magmas in general. Slab-melting processes are known to result in a variety of magma types. However that is still debatable whether it can be applicable for the specific TTG suite. Nowadays it seems clear that several tectonic scenarios might be responsible for the formation of the whole range of sodic granitoids in the Archean continental crust. Throughout the Archean these magmatites show intriguing chemical trends, of which it is suspected that they reflect changes in tectonic processes over time. In this project we will study the dynamics and melt sources for this type of magmatic rocks using a 2D petrologicalthermomechanical numerical model. The main point of interest will be to design contrasting models for TTG magma formation and to analyze them based on the variety of chemical compositions of TTGs. The causes for the evolution of chemical composition of TTG magmas throughout the Archean will be explored by analyzing numerically two plausible geodynamic situations (oceanic-continental subduction and continental collision) and testing a number of model parameters such as upper-mantle temperature, lithospheric and crustal structure, heat production and mantle and crustal rheology. The project will introduce one of the most modern plate tectonics modeling tools (in cooperation with Prof. T. Gerya, ETH Zurich) to the Austrian research landscape and will be carried out at Graz University where there is a strong research group on mountain building processes. The numerical experiments will be tested against voluminous structural and petrological datasets for TTGs available through international cooperation with Prof. M. Brown (University of Maryland), as well as Prof. C. Hauzenberger (Graz University), the two worldwide-recognized experts in petrology, geodynamics, and origin of the Precambrian complexes.

The tectonic style of the Earth in the Archean and its evolution towards plate tectonics as we know it today remains one of the most important subjects of debate for the earth scientists. Although the signature of plate tectonics is recognized with some confidence in the Phanerozoic geological record of the continents, evidence for plate tectonics becomes less certain further back in time. Thus, the best way to improve our understanding of the early Earth evolution is to combine knowledge derived from the geological record with results from well-constrained numerical modeling. Within the project we took this challenge, and numerically tested possible geodynamic scenarios that could characterize the Earth before the plate tectonic regime became stable. We used a 2D coupled petrological-thermomechanical tectono- magmatic numerical model with initial conditions appropriate to the early Archean. The results of the numerical experiments allowed us to confirm a set of simultaneously occurring tectono-magmatic settings including crustal overturns in which a prevailing rock type for the Archean crust (sodic granitoids) could be produced. Moreover, we identified the sequential development of two distinct types of continental-like crust widely recognized in Archean cratons. The early type of continental crust formed from the original thick mafic crust at dominantly vertical tectonics. With time these blocks of the crust could undergo tectono-magmatic reworking due to locally occurring horizontal tectonics (with some plate tectonics signatures) producing another type of the continental crust (reworked). Thus, we have provided a conceptual framework for separating different types of continental crust according to the likely tectonic regime. The second part of the project was devoted to stability of these regimes. The large parameter study produced showed that the intensity of the crustal overturns, most probably responsible for the formation of the majority of sodic granitoids, is strongly controlled by the amount and distribution of the underplated mantle-derived melts as well as by the compositional/rheological structure of the crust. The wide spread occurrence of sodic granitoids is correlated with the high mantle temperatures typical for the early Archean and the ability of mafic crust to weaken due to ascending melts. According to our experiments the disappearance of extensive sodic granitoids formation roughly coincides with the appearing of the first evidences of the modern style of plate tectonic regime on the Earth.