ND-ISOTOPIC COMPOSITION OF JURASSIC BIMODAL METAIGNEOUS SUITES AT THE NORTHERNMOST SEAFLOOR REMNANT OF THE ROCAS VERDES BASIN (PATAGONIAN ANDES)

New Nd-isotopic composition, in concert with fieldwork and detailed petrography, revealed novel features about the petrogenesis of the northernmost metaigneous floor remnant of the Rocas Verdes basin in southern Patagonia (51-52°S). The extrusive and intrusive bimodal metaigneous belts of the Sarmiento Complex, in which mixed mafic-felsic terrains occur with contrasting crosscutting relations (mafic rocks intruding felsic ones and vice versa) and the metafelsic rocks of the Tobífera Formation are considered. Nd isotope analysis was also conducted in metapelites of the Paleozoic metamorphic basement rocks and local hornblende-bearing lithologies within the Sarmiento Complex. Typical єNd150 mean values are strongly negative in Paleozoic metasedimentary rocks (-7), progressively higher in granophyres and metatuffs (-5), least negatives values in dykes of dacite intruding pillow basalt successions (-4) and positive values in metamafic rocks (+2; sills and dykes of amphibolitized gabbro, pillow basalts and plagiogranites). The Nd isotopic of metamafic rocks and enrichment in LILE and Nb-Ta depletion in metabasalts are attributed to geochemical processes of mantle preconditioning in a supra-subduction setting. Metafelsic rocks probably represent the crystallization products derived from intermediate magmas formed by partial melting of fertile lower crust. Former intermediate magmas may have interacted with upper crustal rocks and differences in isotopic compositions are considered to reflect different degrees of assimilation, mainly controlled by the composition and initial temperature of country rocks. The Nd isotopic composition of ganophyres and metatuffs indicate that assimilation processes were favored by anatexis and assimilation of crustal derived partial melts. Partial melting in the roof of axial magma chambers is the invoked process for the generation of hornblende-bearing plutonic and subvolcanic rocks (with єNd140 values ca. +2) during periods of enhanced magmatic flux along spreading centers. The bimodal metaigneous complexes are interpreted as fragments of a transitional oceanic-continental crust formed in a volcanic rifted margin along the southwestern margin of Gondwana in Late Jurassic times.