Petrographic and geochemical constraints on mantle xenoliths from northern Patagonia, Argentina

MALLMANN GUILHERME; CONCEIÇÃO, ROMMULO VIEIRA; KOESTER EDINEI; BERTOTTO GUSTAVO WALTER; SCHILLING MANUEL; CHEMALE JR. FARID; CINGOLANI CARLOS ALBERTO

San Francisco (CA) EEUU

Congreso; AGU Fall Meeting 2003; 2003

American Geophysical Union

Mantle xenoliths brought to the Earth’s surface by alkaline basic magmatism in the back-arc setting of the northern Patagonian Andes, Argentina, provide insights on processes like mantle depletion and metasomatism near subduction zones. Petrographic and geochemical features of two sets of xenoliths collected in distinct Miocene-Pleistocene volcanic events are here presented. Cerro El Mojon xenoliths (41º06’S;70º13’W) are mainly depleted (Mg#84-85), anhydrous, spinel-bearing lherzolites to harzburgites with some websterites and olivine websterites. They are unveined and have protogranular textures. Some samples have clinopyroxene-rich bands and fertile characteristics. Estancia Alvarez xenoliths (40º46’S;68º46’W) are all veined (metasomatized?), anhydrous, spinel-bearing lherzolites with porphyroclastic to protogranular textures. We recognized depleted (Mg#85) and fertile (Mg#73-79) xenoliths. Kink bands and wave extinction in olivine and pyroxenes, and Ca exsolution and symplectic texture of spinel in orthopyroxene were observed. The REE and multi-element spidergrams are similar to both sets of xenoliths. They have relative flat REE pattern (close to the chondritic values) with slightly enrichment in LREE. On the multi-element spidergrams, both show values similar to the chondrites without negative Ti-Nb-Ta anomalies, which could be an evidence of absence of subducted-slab derived sources. Mg# = [100MgO/(MgO+FeOt)].