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

• Felipe Espinoza ^[1] ; Diego Morata ^[1] ; Mireille Polvé ^[2] ; Yves Lagabrielle ^[3] ; René C. Maury ^[5] ; Aude de la Rupelle ^[5] ; Cristèle Guivel ^[4] ; Joseph Cotten ^[5] ; Hervé Bellon ^[5] ; Manuel Suárez Dittus
  1. [1] Universidad de Chile

     Universidad de Chile

     Santiago, Chile

     
  2. [2] Paul Sabatier University

     Paul Sabatier University

     Arrondissement de Toulouse, Francia

     
  3. [3] University of Montpellier

     University of Montpellier

     Arrondissement of Montpellier, Francia

     
  4. [4] University of Nantes

     University of Nantes

     Arrondissement de Nantes, Francia

     
  5. [5] Université de Bretagne Occidental

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• Localización: Andean geology: Formerly Revista geológica de Chile, ISSN-e 0718-7106, ISSN 0718-7092, Vol. 37, Nº. 2, 2010, págs. 300-328
• Idioma: español
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• Resumen

  • 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.