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

CERREDO, M. E.; REMESAL, M. B.; TASSONE, A.; MENICHETTI, M.

Ciudad de Mendoza. Argentina

Simposio; Tercer Simposio Argentino del Jurásico; 2007

IANIGLA y CRICYT-CONICET

The ensialic back-arc Rocas Verdes Basin (RVB) evolved along the SW border of Gondwana from Late Jurassic to Early Cretaceous, after a widespread event of continental rifting and associated silicic magmatism along the palaeo-Pacific margin of Gondwana. Restricted outcrops of basaltic/andesitic rocks in Argentine Tierra del Fuego are considered as a phase of the igneous event that formed the Rocas Verdes ophiolite complexes elsewhere. The oceanic remnants outcrop as discontinuous belts, of roughly E-W orientation in the central and SW areas of Tierra del Fuego. The rock association is dominated by porphyric lavas with minor gabbros. Both facies display magmatic assemblages (clinopyroxene and plagioclase phenocrysts set in a subophitic groundmass) overprinted by unoriented assemblages related to sea-floor metamorphism of amphibolite (actinolite-zoisite-albite) and greenschist facies (chlorite±pumpellyte±calcite). Late Cretaceous RVB inversion resulted in variable microstructural reorganization -spanning from slightly foliated to mylonitic textures- and very low-grade mineral associations. Representative chemical data indicate their subalkalic basaltic nature with a tholeiitic differentiation trend, with increasing FeO*/MgO (from 0.98 to 4.17) without significant increase in SiO2 (46-53%). Low abundances of transition metal elements (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 plots suggest a subduction-related component in the petrogenesis of RVB oceanic crust. All samples display a flat REE pattern, a slight LREE enrichment as well as a marked Eu anomaly only in more evolved terms.