| Summary: | The rupturing of the continental !ithosphere comprises early stages of continental rifting the transition from late-stage continental extension to the earliest stages of oceanic crust generation and seafloor spreading. Geophysical investigation in modcrn extensional settings (e.g. Bransfield Strait, Gulf of Califonia, Red Sea and Japan Sea) revealed novel features about thc therrnal and compositional architecture of the lithosphere during extension. Nevertheless, because sampling of the igneous rocks that under!ie thick succession of sedirnents in thc hasins is difficult, most studies of magmatism and metamorphisrn during rifting to sea-spreading fbCUSCS in "ophiolite" complexes that represent fragments of oceanic or quasi-oceanic lithospherc tectonical!y cmp!aced over continental crust. In the southern Patagonian Andes an orogen parallel string of Late Jurassic to Ear!y Cretaceous ophiolite bodies (e.g. Stern and de Wit 2003 and references therein) has been tectonically thrust upon South America during ensuing Andean orogenic processes. Ophiolite comp!exes are interpreted as remnants of the Rocas Verdes hasin. The northern seafloor rernnant, the Sarmiento Comp!ex (52°S), bears hirnodal metaigneous suites interpreted as part of the !ateral lithological transition of continental rifting to initial scafloor spreading. The Sarmiento Complex represents a particu!arly appropriate geosite whcre a comprehensive investigation of magma genesis during the intcraction of rnafic magmas aud crustal rocks is addressed. !n addition, a belt of highly-crust contarninated Jurassic granite in ihe South Patagonian batholith (Hervé et al submitted) was generated in tlie continental crust. Assirnilation processes in magma chambers are though to be dorninated by the incorporatiun ni silicic melts derived from the anatexis of surrounding rocks. Processes of high temperature and !ow pressure metamorphism and anatexis are well documented in the Puerto Edén Igneous and Mctamorpliic Complex (49°S; Calderón 2005). Therefore, the study of gneisses, migmatites and granites at Puerto Edén aim to address and solve sorne petro!ogical problerns about thc involvemcnt of anatexis of metape!ites in granitoid magma genesis in the continental crust contcmporaneous with the Rocas Verdes basin formation. The geothermobarometry of foliated metarhyolites that formed part of the igneous substrate of the Rocas Verdes hasin allow tu constrain the geodynarnic scenario for part of the history of basin closure.
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