The Farallon-Aluk active ridge collision with South America and the slab window magmas geochemical changes from fore to back arc

EUGENIO ARAGÓN; LUCIO PINOTTI; FERNANDO D'ERAMO; ANTONIO CASTRO; OSVALDO RABBIA; JORGE CONIGLIO; MANUEL DEMARTIS; IRENE HERNANDO; CLAUDIA CAVAROZZI; YOLANDA AGUILERA

Geoscience Frontiers

Elsevier

Lugar: Beijing; Año: 2013 vol. 4 p. 377 - 388

1674-9871

The collision of a divergent ocean ridge may evolve into two end cases: in the continuity of ocean-floor subduction, or in the detachment of the subducted plate. The northern Patagonia active plate margin hasthe unique situation that in Cenozoic time it has been subjected to two divergent ridge collisions, each onerepresenting one of the end members. The Neogene Antarctica-Nazca divergent ridge collision evolved asa continuous ocean-floor subduction system, promoting a magmatic hiatus at the arc axis, the obduction of part of the ridge ocean-floor in the fore-arc, and basaltic volcanism in the back-arc. In contrast, the  Paleogene Farallon-Aluk divergent ridge collision evolved into a transform margin, with the detachment and sinking of the Aluk plate and the development of a large slab window. As in the previous case, thiscollision promoted a magmatic hiatus at the arc axis, but the tectono-magmatic scenario changed topostorogenic synextensional volcanism that spread to the former fore-arc (basalt, andesite, rhyolite) andformer back-arc (bimodal ignimbrite flare-up, basalt). Geochemistry of this slab window synextensional volcanism shows more MORB-like basalts towards the former fore-arc, and MORB-OIB-like basaltstowards the former back-arc. Instead, an isolated undeformable crustal block in the former back-arc, withan “epeirogenic” response to the slab window and extensional regime, was covered by OIB-type basalts   after uplift. Major elements show that slab window basalts reach TiO2 values up to 3 wt%, as compared  with the top value of 1.5 wt% of arc magmas. Besides, the MgO with respect to (FeOt þ Al2O3) ratio helps to  distinguish slab window magma changes from the former fore-arc to the former back-arc and also with respect to the “epeirogenic” block. Higher contents of HFS elements such as Nb and Ta also help to    distinguish this slab window from arc magmas and also, to distinguish slab window magma changes from   the former fore-arc to the former back-arc and “epeirogenic” block settings. The isotope compositions of      slab window magmatism show a disparate coeval array from MORB to crustal sources, interpreted as     a consequence of the lack of protracted storage and homogenization due to the extensional setting.