U-Pb LASER ABLATION DATA FROM THE EASTERNMOST PAMPEAN SECUENCES. EVIDENCES OF A NEOPROTEROZOIC ARC AND POSTCOLLISIONAL EVENTS

LISANDRO GASTÓN RODRIGUEZ; MÓNICA ESCAYOLA; VICTOR A. RAMOS

Puerto Madryn

Congreso; XXI Congreso Geológico Argentino; 2021

AGA

Based on provenance studies, one of the leading models proposed for the geotectonic evolution of the Eastern Pampean Ranges (Ramos et al 2014), proposes a magmatic arc colision against the Rio de La Plata Craton (RPC) during the neoproterozoic, followed by a change of subduction direction (Escayola et al. 2007) and acretion of the Pampia terrane (Ramos and Vujovich 1993). These ideas are strongly supported by the incrisingly abundance of neoproterozoic zircons to the east of the Pamepan Ranges and the association with a larger neoproterozoic arcs accretion episode that took place along the Gondwana margin for ca.100 m.y. (Escayola et al 2007). Beyond all this data, further evidence about the existence of an active magmatic arc that could explain the absence of paleoproterozoic circons from the RPC in the Eastern Pampean Ranges was still remaining. We sampled and dated U-Pb ages from the easternmost marble rich sequences association firstly described by Kraemer et al (1995), which constitute the first outcrops to the west of the RPC. Datings were done by LA-ICPMS techniques in the Centro de Pesquisas Geocronológicas from São Paulo University (CPGeo-USP). Studied rocks are metacarbonatites, amphibolites and gneises never dated before which could be interpreted as: (i) remaining secuences of the RPC passive margin, (ii) part of the neoproterozoic arc proximal deposits, or also (iii) the Pampia terrane passive platform. Outcrops on the south of the Sierra Chica, located 5 km west of San Agustin village, shows interdigited silicate-rich and marbles deposits that could be interpreted as arc related tuffs preserved in the island shelf deposits, linked to phases III and IV of Oceanic Arc Depositional Systems (OADS) of Larué et al. (1991). Probability density plots (PDP) shows neoproterozoic ages, as well as metamorfic events during the cambrian and ordovician. Determinations on zircon cores (n=50) yielded depositioning ages of 569 Ma.