The ocean floor of Rocas Verdes Basin in Tierra del Fuego, Argentina.

CERREDO, M. E.; TASSONE, A.; REMESAL, M.; VILAS, J. F.

Mendoza. Argentina.

Simposio; Gondwana 12; 2005

Gondwana

The Rocas Verdes back-arc basin (RVB) evolved along the SW border of Gondwana from Late Jurassic to Early Cretaceous (Dalziel, 1981; Stern et al., 1992; Mukasa and Dalziel, 1996) after a widespread event of continental rifting and associated, mostly silicic, magmatic activity. The latter encompasses Middle to Late Jurassic volcanic rocks of Patagonia and the Antarctic Peninsula, (Pankhurst et al., 2000; Rilley et al., 2001) along the palaeo-Pacific margin of Gondwana. Remnants of the oceanic floor of the RVB are represented by the ophiolite complexes of Sarmiento at the NW tip of the basin, Tortuga in southern Chilean archipelago and the Larsen Harbour Complex of South Georgia. These complexes display a fairly complete oceanic upper crustal section including gabbros, a sheeted dyke complex and pillow-lavas and breccias (Stern and de Wit, 2003). Restricted outcrops of basaltic rocks in Tierra del Fuego have been considered as representing a phase of the same igneous event that formed the Rocas Verdes ophiolite complexes (Stern and de Wit, 2003; Stern, 1980; Bruhn et al., 1978). The oceanic remnants occur as tectonic slices interlayered with black shales and deformed rhyolites of the Lemaire Formation, and correspond to porphyritic lavas and lesser gabbros. As elsewhere in the RVB, the remnants of oceanic crust in Tierra del Fuego record a complex evolution which involved the magmatic stage, followed by the overprinting of sea-floor metamorphism, later variably modified by Andean compression. The preserved primary igneous associations consist of clinopyroxene and plagioclase phenocrysts (±sphene and opaque minerals) set in a subophitic groundmass. Mineral transformations related to hydrothermal sea-floor metamorphism include unorientated amphibolite (actinolite-zoisite-albite) and greenschist facies (chlorite±pumpellyte±calcite) assemblages. Andean compression produced a dramatic microstructural reorganization -spanning slightly foliated to mylonitic textures- and very low-grade mineral associations resulting both from infiltration-driven assemblages and from closed-system reactions (Rapalini et al., 2005). New geochemical data from the less modified rocks indicate their basaltic nature (including basalts, basaltic andesites and rare trachybasalt types) and an extensive tholeiitic differentiation trend, with increasing FeO*/MgO (from 0.98 to 4.17) without significant increase in SiO2 (46 53%). Low transition metal abundances (Cr 30–400 and Ni 60–200 ppm) preclude direct derivation from the mantle. Ti/Zr, Ti/V ratios as well as HFSE contents indicate MORB affinities, whereas the variable LILE enrichment and Nb-Ta troughs on MORB-normalized multi-element plots point to the influence of a subduction-related component in the petrogenesis of RVB oceanic crust.