INVESTIGADORES
COIRA beatriz lidia luisa
artículos
Título:
Petrogenesis of early Neogene Magmatism in the Northern Puna. Implications for magma genesis and crustal processes in the Central Andean Plateau.
Autor/es:
CAFFE, P.,TRUMBULL, R., COIRA, B., ROMER, R.
Revista:
JOURNAL OF PETROLOGY
Editorial:
OXFORD UNIV PRESS
Referencias:
Lugar: Oxford; Año: 2001 vol. 43 p. 907 - 942
ISSN:
0022-3530
Resumen:
New compositional data and petrogenetic models are presented for pre-Upper Miocene volcanism in the northern Puna of Argentina (22ºS-24ºS). 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/86Sr T -0,706; End T -3). The UOLM-2 group is more silicic and peralominous, and has isotopic compositions indicating a substancial crustal contribution (87Sr/86Sr T -0,713; End T -8). The Mid-Miocene phase (MM: 15-12 Ma) produced rocks similar in composition to those of the UOLM-2 group (87Sr/86Sr T -0,710; End T -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 suggest that the westward shift in the arc began in the Northern Puna in the Mid-Miocene. Neither the exposed Paleozoic felsic basement nor the lower-crustal granulites known from xenolithe suites and 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 modeling and thermal arguments rule out magma mixing as the process of hybridization. Successful assimilitation-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.