Petrogenesis of strongly peraluminous volcanic rocks from northern Puna. Unraveling the crustal end member of the APVC dacite mix.

CAFFE, P.J., TRUMBULL, R.B., SIEBEL, W.

Melbourne

Congreso; IUGG General Assembly 2011; 2011

IUGG

The relative contribution of mantle and crustal components to magmas in continental arc settings is relevant to understand magmatic accretion, crustal recycling, delamination and tectonic erosion processes. In the central Andean Plateau, the voluminous dacitic ignimbrites from the Altiplano Puna Volcanic complex (APVC) represent 1:3 to 1:1 mixtures of mantle:crustal magmas. Rocks derived from crustal melting alone are very rare in the APVC. Thus, to define their compositional spectrum, nature of protholiths, as well as inferred melting mechanism is very important for estimating the proportion of crust in hybrid APVC magmas and material flux through time/space. Strongly peraluminous volcanic rocks (SPVR) are thought as the main products of melting of felsic protholiths, the latter predominantly forming the crust under the APVC. In the southeastern APVC, a few SPVR that potentially represent pure crustal melts were identified. These are separated into 2 types based on chemical and isotopic compositions. The first type comprises relatively enriched (87Sr/86Sr 0.713-0.717) high-SiO2 rocks (Ramadas, Tocomar and Coyaguayma ignimbrites) similar to SP leucogranites, with chemical compositions consistent with origins by disequilibrium dehydration melting of metapelites in upper/middle crustal settings. The composition of hosted metamorphic xenoliths reveals that rhyolite sources are either less radiogenic than the latter, or their isotopic signatures were lowered prior to melting. The second type includes low-SiO2 cordierite rhyolites (Corral Negro) that are isotopically more enriched, and whose chemistry resembles typical SPVR from the Eastern Cordillera of Bolivia-Peru. This type most probably represents equilibrium dehydration melting of greywacke-like sources in the middle/lower crust.