"The heterogeneous nature of the Patagonian lithospheric mantle as evidenced by garnet- and spinel-peridotites"

BJERG, ERNESTO A.; GARGIULO, M. FLORENCIA; KOLOSOVA-SATLBERGER, O

Viena

Congreso; European Geosciences Union General Assembly - EGU2012; 2012

Copernicus Meetings and the Bundesministerium für Wissenschaft und Forschung

In several areas of Patagonia, Oligocene-Pliocene back-arc alkali basaltic lavas (with OIB chemical signatures) and cinder cones carry spinel and garnet bearing xenoliths. Lack of slab-derived component(s) rules out the participation of a subducted slab in the generation of these basalts. Xenolith textures evidence that the mantle is moderately to strongly tectonized and recrystallized on both local and regional scale, with predominance of deformed textural types. Variably intensive cryptic and modal metasomatism affected the lithospheric mantle of Patagonia. Metasomatized spinel peridotites from Gobernaor Gregores bear unusually large melt pockets that consist of second generation olivine, clinopyroxene and spinel ± relict amphibole immersed in a yellowish vesicular glass matrix. Textural evidences and consistent mass-balance calculations prove that amphibole breakdown was responsible for melt pocket generation. Xenoliths from Tres Lagos, situated inboard of the Volcanic Arc Gap (VAG), are samples of a depleted lithospheric mantle and comprise metasomatized and non-metasomatized anhydrous spinel lherzolites and harzburgites. A two-stage partial melting process could be responsible for the origin of these xenoliths, in the first stage a 2% of batch melting taking place in the garnet peridotite field and subsequently the residue experiencing 2?8% batch melting in the spinel peridotite field. The peridotites have not been affected by subduction-related metasomatic processes and they probably represent an old isolated piece of depleted lithosphere, in which metasomatism was not a significant process.