The lithospheric Earth Mantle beneath Patagonia

The South American platform was amalgamated and consolidated as a craton by the end of the Proterozoic whereas Patagonia consolidated during the Paleozoic. Basement outcrops in the Patagonia geological province comprise two massifs, the North Patagonian (or Somn Cura) in the north bounded by the Colorado and Neuquen basins in the north and San Jorge and Caadon Asfalto in the south. The Deseado massif is located in the east of Santa Cruz province, south of the two previously mentioned basins. These two massifs were remobilized by Paleozoic orogenies. As to their origin, two main contrasting hypotheses occur: the first hypothesis considers this region as authochthonous and the second hypothesis as an allochthonous part of the Gondwana supercontinent. Although the allochtonous hypothesis has been widely accepted, the real boundaries of Patagonia, the role of possible sudduction zones, its accretion and the time of its final amalgamation to the Gondwana supercontinent remain the subject of discussions. Considering the ongoing debate, the precise geochemical and petrological characterization and the time of stabilization of the sub-lithospheric mantle in this region would provide information on the origin of the overlaying crust and contribute towards understanding the evolution of the southernmost part of the S. American subcontinent. We are planning a comprehensive detailed geochemical, petrological and isotopic approach that will be applied on carefully selected fertile and residual metasomatized and none metasomatized mantle xenoliths from Patagonia. The proposed study will be focused on three major units that will serve: (1) to understand the geochemical processes taking place within the Lithospheric Mantle beneath Patagonia in order to explain the unique geodynamic situation of Patagonia i.e. where the different pieces of Patagonia are coming from, when the Lithospheric Mantle has been stabilized after crust formation and what degree affected the subducted Nasca and Antarctic Plates this back-arc region from east to West. (2) to decipher the origin and nature of the metasomatic processes that could trigger partial melting in the Lithospheric Mantle beneath Patagonia and to study the melt (metasomatic and/or host) -xenolith interaction in order to understand the reaction mechanisms and the development of mineral zones due to chemical potential variations and the element fractionation across the reaction rim(s).

Mantle xenoliths-fragments of the subcontinental lithospheric mantle (SCLM)-from beneath Patagonia were petrographically and geochemically characterized and analyzed for their Os-Hf-Nd-Sr isotopic composition. Ten different mantle xenolith outcrops, spread all across Patagonia, were chosen and samples were carefully selected to obtain new insights into the origin and evolution of the lithospheric mantle underneath the southern part of Argentina. These SCLM fragments provide the opportunity to determine estimates on the formation age of continental terranes. An evolutionary link of crust and its underlying mantle makes dating continental fragments possible, even in areas where crustal basement rocks are scarce or completely inaccessible, as is the case for the southern part of Patagonia. The Re-Os isotopic system represents a useful and reliable tool to determine lithospheric mantle stabilization ages and subsequently provides information on the timing of formation of continental terranes.Our study shows that Patagonia is made up of multiple continental fragments with Neo- to early Paleoproterozoic formation ages. The southernmost part, known as the Pali Aike area, represents the oldest part of southern Argentina and its early Paleoproterozoic (2.5 Ga) formation age indicates an evolutionary connection to parts of western Antarctica. The Deseado Massif area yields distinctly younger lithospheric mantle stabilization ages (1.3 Ga) and suggests a common origin with parts of South Africa. These new age determinations clearly show that the southern part of Patagonia (the Santa Cruz Province) is made up of at least two lithospheric fragments. A third fragment could be represented by the region of Tres Lagos, located southwest of the Deseado Massif. Re-Os SCLM formation ages indicate a mid Paleoproterozoic (1.9 Ga) origin. Age determinations from northern Patagonia, an area known as the North Patagonian Massif (NPM), are more difficult to interpret as the SCLM in this region has undergone extensive metasomatic overprinting. Minimum mantle stabilization ages suggest a Mesoproterozoic (1.0 to 1.3 Ga) evolution of the SCLM beneath the western part of the NPM. The lithospheric mantle underneath Prahuaniyeu, located roughly in the center of the NPM yields older, late Paleoproterozoic (1.7 Ga) formation ages. This suggests at least two variably old continental fragments beneath the northern part of Patagonia.