Petrogenesis of Early Neogene Magmatism in the Northern Puna; Implications for Magma Genesis and Crustal Processes in the Central Andean Plateau

Abstract

New compositional data and petrogenetic models are presented for pre-Upper Miocene volcanism in the northern Puna of Argentina (22°S–24°S). Two phases of volcanism produced small dome complexes of mainly silicic andesite to low-SiO2 rhyolite. The Upper Oligocene–Lower Miocene phase (UOLM, 20–17 Ma), produced two distinct groups of rocks. The UOLM-1 group is metaluminous and mainly andesitic, with isotopic compositions like those of the recent arc (87Sr/86SrT ∼0·706; εNdT −3). The UOLM-2 group is more silicic and peraluminous, and has isotopic compositions indicating a substantial crustal contribution (87Sr/86SrT ∼0·713; εNdT −8). The Mid-Miocene phase (MM: 15–12 Ma) produced rocks similar in composition to those of the UOLM-2 group (87Sr/86SrT ∼0·710; εNdT −7) but with higher incompatible element contents. Ratios of Ba/Nb and Zr/Nb in the UOLM group rocks are uniform and similar to those of the current arc, whereas the ratios in MM centres show a mixed arc and back-arc affinity. This suggests that the westward shift in the arc began in the northern Puna in the mid-Miocene. Neither the exposed Palaeozoic felsic basement nor the lower-crustal granulites known from xenolith suites are compositionally suitable as protoliths for the UOLM and MM magmas. The preferred petrogenetic model for the magmas involves hybridization of a depleted arc basalt with partial melts of the felsic basement. Geochemical modelling and thermal arguments rule out magma mixing as the process of hybridization. Successful assimilation–fractional crystallization (AFC) solutions indicate an increase in crustal assimilation from 15–25% in UOLM-1, to 40–60% in the case of UOLM-2 and MM group rocks. Assuming the same end-member compositions, the modelling suggests genesis of the MM magmas at higher pressure than the UOLM-2 centres (≥10 kbar vs 7 kbar), which may reflect the influence of crustal thickening in the plateau region by the mid-Miocene. The felsic dome complexes of this study are compositionally similar to the large-volume, caldera-sourced felsic ignimbrites that dominated volcanism in the region from 10 to 2 Ma and our results suggest that there is no fundamental difference in magma genesis between them. The differences in the volumes and the mode of eruption reflect changes in the stress and thermal regime with time.