Ordovician evolution and structure of the Famatinian arc, Argentina

OTAMENDI, J.E., MARTINO, R.D., VUJOVICH, G.I.,

Buzios

Congreso; Gondwana 14; 2011

Universidad Federal de Rio de Janeiro

Lower Paleozoic magmatism along South America developed as part of the Cambrian-Late Ordovician evolution of the western margin of Gondwana. The first-order driving tectonic force is linked to progressive switching from passive to active margins initiated after Gondwana amalgamation. The first stage in the evolution of this continental destructive margin created to Early Cambrian Pampean orogenic belt to be followed by the outboard Famatinian Ordovician magmatic system. The Ordovician magmatic arc is exposed from northern Patagonia to northern Peru along the strike of the modern central Andes. Also,this arc was part of one of the largest and longest-lived accretionary orogens, which extended along the margin of Gondwana from Australia through the Pacific border of Antartica to the northern Andes. Significantly, the Early Ordovician magmatic arc is the earliest subduction-related plutonic-volcanic belt reflecting the oceanic boundary of the West Gondwana. One striking feature of the Famatinian arc is that the transition from plutonic to volcanic rocks is tracked over large regions in the Sistema de Famatina, argentina. The transition from middle to upper crustal paleo-depths of the Ordovician arc is found in Sistema de Famatina (27º-31ºS present). In Cordillera Oriental and Puna there are Early Ordovician shallow-emplaced plutonic and eruptive igneous rocks interbedded with sedimentary rocks, where they are broadly grouped as Faja Eruptiva Oriental and Occidental. Regional geological data helped to further refine the Ordovician crustal section, which extends from surficial volcanic rocks and associated shallow plutons in the north to deep-seated plutonic root in the central section. In the northwestern, Early Ordovician volcanic complex represent the shallowest igneous rocjks constructed over coeval sedimentary sequences. From southern Puna to northern Sierras Pampeanas and at paleo-depths upper than ca. 15 km, the wall-rocks of the Ordovician plutonic batholiths are metamorphosed siliciclastic with subordinate carbonates sedimentary and volcanic sequences. Widespread partial to almost complete melting of pelite and greywacke host rocks took place at more than 15 km paleo- depths (>5 kbar). Integrating observations at all paleo-depths, we hypothesized that at an earliest stage the Famatinian arc experienced rollback of the paleo-Pacific plate, which kept marginal and intra-arc basins actively forming. Extension caused the opening of back arc basins, perhaps also contributing to attenuate the Pampean orogen that was partly buried beneath Early-Middle Ordovician basins. Another outstanding features shown by the Famatinian arc is that it was fragmented when the southern segment (28º-39ºS) was closed by a terminal collision that formed a mountain from Middle Ordovician to the Early Devonian. This mountain-building orogeny first closed back arc basins and then caused extensive exhumation of deep crystalline sections along convergent shear.