Resumen de Middle Miocene calc-alkaline volcanism in Central Patagonia (47·S): petrogenesis and implications for slab dynamics
Felipe Espinoza, Diego Antonio Morata Céspedes, Mireille Polvé, Yves Lagabrielle, René C. Maury, Aude de la Rupelle, Cristèle Guivel, Joseph Cotten, Hervé Bellon, Manuel Suárez Dittus
We presenta chronological (K-Ar), petrologic and geochemical study (major and trace elements, SrNd isotopes) ofMiddle Miocene (ca. 16-14 Ma) calc-alkaline rocks (basalts to andesites) extruded in the present-day back-arc region ofCentral Patagonia (Zeballos Volcanic Sequence (ZVS), 47°S). This magmatism started shortly after mafic plutonism ceased in the are region (ca. 16 Ma, 200 km west), and ended ca. 2 My before the onset ofvoluminous slab tear-related back-arc alkaline basaltic magmatism (ca. 12 to Pliocene). The studied calc-alkaline rocks have a typical subduction-related signature (high LILEIHFSE ratios, depletion in Nb, Ta and Ti; BaiLa >20; Ta/Hf <1.5; ( 87Sr/86Sr)0=0.70366-0.70402, ENd=+O.l-+3.8). Major and trace elements contents are consistent with their evolution by closed system fractional crystallization of a presumed parental liquid similar in composition to the most basic rock ofthe suite. Moreover, a strong subducted sediment imprint is recognized (increasing Th!HFSE and decreasing Ce/Pb during differentiation). However, these rocks show striking similarities with volcanic complexes emplaced above areas where a gently dipping slab occurs (high K contents; similar LREEIHREE, Nb/Zr, Ba/Nb; Th/Hf; Th/Ta, Ta/Hf<0.3), particularly the present-day Andean flat-slab region and the Neuquén Basin during the Late Miocene. A comprehensive tectono-magmatic model is here presented to explain the generation and extrusion ofthese calc-alkaline magmas during the Middle Miocene. The development of a transient low-angle subduction and the resulting eastward migration of the volcanic front are then proposed. Mixing between stored remnants of calc-alkaline magmatism and later primitive alkaline melts is envisioned as the most likely process accounting for the transitional signature (i.e., intermediate between calc-alkaline and alkaline, La/Nb > 1; Ti02 <2 wt%) of sorne basalts extruded synchronously with genuine alkaline lavas in the Neogene Patagonian Plateau Lavas province.
