Resumo

REMESAL, Marcela B.; SALANI, Flavia María  e  CERREDO, María Elena. Petrology of the Barril Niyeu volcanic complex (Early Miocene), Patagonia Argentina. Rev. mex. cienc. geol [online]. 2012, vol.29, n.2, pp.463-477. ISSN 2007-2902.

The Oligocene Somún Curá plateau is the largest volcanic field in the northern extra–andean Patagonia of Argentina. It is mostly composed of alkaline to transitional aphyric olivine basalts that cover a sub–circular area of ~25,000 km². Several early Miocene post–plateau volcanic events resulted in small monogenetic centres and large bimodal complexes. The Barril Niyeu Volcanic Complex (BNVC) is one of the largest post–plateau bimodal centres built through several stages of eruptive activity outpoured from at leastfive vents (presently cauldrons) of distinct trachyte/rhyolite and basaltic compositions. New whole rock K/Ar dating of early trachyte (20.6±0.4 Ma) and late basaltic (18.7±0.4 Ma) rocks constrain the time span of the BNVC activity within 2–2.5 My. The earliest eruptive stage of the BNVC produced viscous trachyte lava restricted to the volcanic centre, followed by two explosive eruptive episodes of dominant intermediate to acid composition: The first episode produced air–fall, mainly plinian (lesser strombolian) andpyroclasticflow deposits and the second was dominated by ignimbritic pyroclasticflows. Basaltic lava, minor breccia and spatter cone deposits dominate the final stage. The magmatic series of the BNVC involves basaltic/trachybasaltic and trachyte/rhyolite compositions, with a characteristic gap in the 52–67% SiO₂ range. Transitional olivine basalt is the most primitive rock of the series (Mg# = 66; Ni = 160ppm; Cr = 290ppm) and shows a chemical composition compatible with a liquid that evolved through the crystal fractionation of a basaltic magma derivedfrom partial melting (8–10%) ofa lherzolithic source. Major and trace element contents and element ratios suggest differentiation through crystal fractionation from an olivine basaltic magma. Intraplate–like Th/Yb, and Ta/Yb ratios characterize basalts, whereas high LILE/REE (Ba/La) ratios suggest the contribution of a subordinated subduction zone component. In addition, Rb/Nb ratios along with a relative Th enrichment in some basalt suggest assimilation of upper crustal rocks. Isotopic compositions (Sr–Nd) of BNVC basalts suggest a source from a depleted mantle and minor contribution of EM1 melts. Somún Curá magmatism is attributed to a process of lithospheric delamination associated with gravitational disturbances during plate reorganization, with subordinated contributions from detached segments of a foundered subducted slab (Aluk plate?).

Palavras-chave : bimodal volcanism; Miocene; Barril Niyeu; Patagonia; Argentina.