Time-evolution of magma sources in a continental back-arc setting: the Cenozoic basalts from Sierra de San Bernardo (Patagonia, Chubut, Argentina)

BRUNI S; D'ORAZIO M; HALLER MJ; INNOCENTI F; MANETTI P; PÉCSKAY Z; TONARINI S

GEOLOGICAL MAGAZINE

Cambridge Journals

Lugar: Londres; Año: 2008 vol. 154 p. 714 - 732

0016-7568

East of the Patagonian Andes, mafic volcanic rocks (mainly lava flows and scoriae) are  exposed in the Sierra de San Bernardo fold belt and neighbouring areas (central Patagonia; 44.5–  46◦ S, 69–71◦ W). They were erupted over a wide interval of time (late Eocene–Pleistocene; 14 new  K–Ar ages), and show systematic chemical and Sr–Nd–Pb isotopic variations in time. The alkaline  lavas (Mg number 57–66) erupted during the late Eocene and early Miocene, have an intraplate  geochemical affinity, and have the highest 143Nd/144Nd and 206Pb/204Pb and the lowest 87Sr/86Sr ratios  of the dataset. Their compositions indicate that their depth of equilibration in the mantle was greater  than that of subsequent lavas. In contrast, the Plio-Pleistocene alkaline lavas (Mg number 58–71) are  the most enriched in incompatible elements, still showing an intra-plate signature, and have the lowest  143Nd/144Nd and 206Pb/204Pb and the highest 87Sr/86Sr ratios. A distinctive group of early Miocene  subalkaline lavas is characterized by slightly more evolved compositions (Mg number 56–59), coupled  with very low incompatible element contents, flat LREE and fractionated HREE patterns (‘kinked’  pattern), and intermediate Sr–Nd–Pb isotope compositions. The Pleistocene basanites (Mg number  71–72) from the Cerro Ante monogenetic cone, on the easternmost slopes of the Patagonian Andes,  have a marked orogenic geochemical signature and Sr–Nd–Pb isotope ratios that overlap with those  of volcanic rocks from the adjacent active Andean arc. They originated in a mantle source extensively  modified by the addition of materials from the subducting Pacific oceanic plates. We suggest that  the wide chemical and isotopic variability of the Sierra de San Bernardo lavas reflects the upwelling  of asthenospheric mantle beneath the study area, which induced lithospheric erosion and progressive  involvement of enriched mantle domains in the genesis of magmas. In this context, late Eocene and  early Miocene alkaline magmatism was dominantly sourced from the asthenospheric mantle, whereas  Plio-Pleistocene alkalinemagmas contain the largest proportion of an enriched lithospheric component.  The peculiar compositional features of the early Miocene subalkaline lavas are interpreted in terms of  high-degree mantle melting followed bymelt–lithosphericmantle reaction processes. Based on current  knowledge about the relative movement and decoupling between lithosphere and asthenosphere, we  propose that the asthenosphere below the study area rose up to compensate for the westward drift of  the mantle wedge coupled with the South American lithosphere.