RECYCLED CRUSTAL MATERIALS AS A SOURCE OF MAGMA AND CRUST

RAÚL BECCHIO; ALFONSO SOLA; NESTOR SUZAÑO; MARCIO PIMENTEL

Heidelberg

Encuentro; 22nd Colloquium on Latin American Earth Sciences; 2011

Germany Research Foundation

Lower Paleozoic intermediate to felsic magmas, has been emplaced at different structural levels (middle to upper) in the Southern Puna?Eastern Cordillera border. These magmas are represented by batholithic granites (monzogranite, granodiorite, syenogranite) leucogranite and trondhjemites stocks, quartz diorites ? granodiorite bodies with magma mixing-mingling evidences and rhyodacitic subvolcanics and volcanics units interbeded into the paleozoic sediments.             The aim of this study is to understand the relationship between the genesis of magmas, migmatites, anatexis and the thermal anomalies in middle crust during the Paleozoic. These thermal anomalies were active over a long period of time, 500-440 Ma, generating a hot type orogen with a HT/LP type metamorphism (Lucassen and Becchio, 2003). The ages of the intrusive and volcanics units, range from ca 500 to 460 Ma in agreement with the age of metamorphism. Mafic to intermediate magmas, in comparison with experimentally derived melts (Patiño Douce, 1999), displays low Al2O3/(FeO+MgO+TiO2) (<5) and high Al2O3+FeO+MgO+TiO (20-50), may be derived by dehydration melting of a mafic amphibole and plagioclase bearing lower crustal source (Jung, et al. 2009). Granodiorites probably represent a fractional crystallization from the quartz diorites, or may represent partial melting products from a mafic to intermediate lower crustal source if we consider low degrees of melting in water present conditions. The leucogranites-syenogranites display higher Al2O3/(FeO+MgO+TiO2) ratios (3-25) and narrow range of Al2O3+FeO+MgO+TiO2 (15-20) and are generated by dehydration melting of metasedimetary sources.             Geochemical and isotope data suggest homogenization and recycling of pre-existing heterogeneities in the crust by anatexis at different levels. On the other hand, Lower Paleozoic felsic intrusive and volcanic units, upper tertiary ignimbrites and felsic xenoliths from the lower and middle crust show a similar isotopic composition with little involvement of mantle magmas. Therefore it is recognized the ability of the crust itself to produce a wide varieties of magmas (quartz diorite, granodiorite, and leucogranite) without significant contribution to cortical growth.