Ordovician evolution and structure of the Famatinian Arc, Argentina

OTAMENDI, J.; ROBERTO DONATO MARTINO; VUJOVICH, G.I

Buzios

Congreso; Gondwana 14. Reuniting Gondwana: East meets West; 2011

Universidade Federal do 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 the Early Cambrian Pampean orogenic velt to be followed by the outbboard 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 accetionary orogens, which extended along the margin of Gondwana from Australia through the Pacific border of Antarctica 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 shallow-emplaced plutonic and eruptive igneous rocks interbedded with sedimentary rocks, where they are broadly grouped as Faja Eruptiva Oriental and Occidental. Regional geological dat helped to further refine the Ordovician crustal section, which extends from surficial volcanic rokcs and associate shallow plutons in the north to deep-shallowest igneous rocks 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 seguiences. Widespread partial to almost complete melting of pelite and greywacke host rocks took place at more than 15 km paleo-depths (> 5 kbar). Integrating observatios 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 oustanding features shown by the Famatinian arc is that it was fragmented when the southern segment (28º-39º S) 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.