Middle Miocene calc-alkaline volcanism in Central Patagonia (47·S): petrogenesis and implications for slab dynamics

Abstract

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.