Gondwanide continental collision and the origin of Patagonia

PANKHURST, R.J; RAPELA, C.W.; FANNING, C.M; MÁRQUEZ, M.

Mendoza, Argentina

Congreso; GONDWANA 12; 2005

Academia Nacional de Ciencias

We present a new integrated model of continental collision as the cause of the Late Palaeozoic Gondwanide fold belts of South America and Africa, mainly based on the igneous, structural and metamorphic history of  Patagonia revealed by a long-term programme of  U-Po zireon dating (SHR1MP). The kernel of  this model is a Carboniferous collision of  southern Patagonia (including the Deseado Massif) with an autochthonous Gondwana margin that included the North Patagonian Massif, following a period of  northeast-directed subduction. This can explain two long-standing geologceal enigmas - the origin of  the fold belts and the origin of  Patagonia. It both demonstrates and explains widespread Permian magmatism in the region. Our data lead to the following conlusions: (a) most of the North Patagonian Massif is autochtonous to Gondwana and represents continental crust thinned during a Cambrian rifting identified in the crystalline basement of  the Sierra de la Ventana. It supported an Ordovician continental-margin calealkaline arc in the same way as the Sierras Pampeanas, as is exemplified in the northeastern North Patagonian Massif by the Famatinian granitoids of  Pichi Mahuida, Sierra Grande and Arroyo Salado, as well as the high-grade metamorphism of  Mina Gonzalito. (b) a narrow ocean between Gondwana and the rifted terrane was consumed by northeastward subduction during the Early Carboniferous (c. 335-320 Ma), as revealed by deformed calcalkaline metaluminous hornblende-biotite granodiorites of this age in athe westernmost part of  northern Patagonia (Cordon del Serrucho, El Platero). ENdt values (+0.1 to +2.8) and low initial 87 Sr/6Sr (0.7034-0.7046) indicate an upper mantle contribution to this magmatism, which could be an extension of  the magmatic are of  the Coastal Cordillera of  Chile to the northwest. (e) collision occurred  shortly after 320 Ma, resulting in high-temperature metamorphism of  the overriding Gondwana plate (gneisses of  the Cushamen Formation, El Maitén, La Potranca), and anatexis generating peraluminous  S-lype garnet-bearing leucogranites in the southwestern margin of  the North Patagonian Massif (Paso del Sapo and Sierra de Pichiñanes). These granites have strongly depleted heavy-REE patterns, high K20/Na20 ratios, unradiogenic ENdt values (-5.0  to -6.0 , four samples) and relatively high initial87 Srl6Sr (0.7078­0.7098) indicative of  generation by crustal melting. TDM -Nd model ages of 1500 Ma could  represent the mean age of their crustal source. (d) penetrative deformation within adjacent thinned Gondwana crust and large-scale folding in the Gondwanide zones founded on normal cratonic crust (Sierra de la Ventana, the Cape region of South  Africa) continued until  Early Permian times, as the Gondwana margin was driven against the back-stop of the Rio de La Plata craton. In the lower plate foreland basin deposits developed in the Tecka-Tepuel basin. (e) intense and widespread Permian acid magmatism occurred in the Gondwana plate following collision, induced by break-off of  the subducted oceanic slab. This event is expressed by granites of  widely ranging composition (e.g., 87Sr/86Sr 0.704-0.712) and age (290-260 Ma). It is manifested throughout the North Patagonian Massif (including many assigned to the Mamil Choique and La Esperanza complexes, as well as the Navarrette granite), but also extends to Lihue Calel (La Pampa), Lopez Lecube (Sierra de la Ventana) and Dolavon, near the Atlantic coast. This is the main source of Permo-Triassie detritus in Late Palaeozoic sedimentary rocks along the SW Gondwana margin. The southern colliding block is that which rifted off from the margin in Cambrian times, and which was formerly part of  the Neoproterozoic  precursor of  Gondwana - i.e., it is parautochthonous rather than allochthonous. Thus Cambrian granites similar to those of1 the Sierra de la Ventana and Cape Fold Belt basements occur  beneath the Jurassic volcanie cover of Tierra del Fuego. Separation may not have been great during the Cambrian­-Carboniferous interval, which would explain the very short period of subduction (no more than 20 m.y.) preceding collision, and this could account for some similarities in the Palaeozoic history of  the North Patagonian and Deseado massifs (e.g., both have a record of Devonian granite emplacement in the west). If the Deseado block was separate from SW Gondwana from Cambrian until Carboniferous times, the Palaeozoic flora and fauna of these areas could also have followed significantly different evolutionary paths.