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Isotope geochemistry and petrogenesis of peralkaline Middle Miocene ignimbrites from central Sonora: relationship with continental break-up and the birth of the Gulf of California

Jesús R. Vidal Solano^1,2, Henriette Lapierre^3,, Joann M. Stock⁴, Alain Demant², Francisco A. Paz Moreno¹, Delphine Bosch⁵, Pierre Brunet⁶ and Andrea Amortegui³

¹ Departamento de Geología, Universidad de Sonora, Apdo Postal 847, 83000 Hermosillo, Sonora, Mexique. jrvidal{at}ciencias.uson.mx
² Pétrologie magmatique, Université Paul Cézanne (Aix-Marseille 3), CC 441, 13397 Marseille cedex 20, France
³ Laboratoire de Géologie des Chaînes Alpines, UMR 5025, Université Joseph Fourier (Grenoble 1), BP 53, 38041 Grenoble cedex, France.
⁴ California Institute of Technology, Seismo Lab 252-21, 1200 E. California Blvd., Pasadena CA 91125 USA
⁵ Laboratoire de Tectonophysique, UMR-CNRS 5568, ISTEEM, Université de Montpellier 2, CC049, 34095 Montpellier cedex 05, France
⁶ Laboratoire Mécanismes de Transfert en Géologie, UMR 5563, Université Paul Sabatier (Toulouse 2), 31400 Toulouse, France

Middle Miocene peralkaline ignimbrites constitute a specific geodynamic marker of the early stage of opening of the Gulf of California, preserved either in central Sonora or the Puertecitos area, in Baja California. Very uniform ages (12-12.5 Ma) obtained on these rocks show that this volcanic episode corresponds to a specific stage in the tectonic evolution of the proto-gulf area. Field observations and slightly different Sr and Nd isotopic signatures support eruptions from several small volume magma batches rather than from a large-volume caldera forming event. Isotopic ratios help to constrain the petrogenesis of the peralkaline liquids by fractional crystallization of transitional basalts in a shallow reservoir, with slight contamination by Precambrian upper crustal material. Less differentiated glomeroporphyritic icelandites erupted at about 11 Ma, mark an increase in the magma production rate and highlight an easier access to the surface, illustrating an advanced stage in the weakening of the continental crust. The tilting of the Middle Tertiary sequences results from a major change in the tectonic regime, from E-W extension giving rise to N-S grabens, to NNW-SSE strike-slip motion that can be related to the transfer of Baja California from North America to the Pacific plate. The location of peralkaline volcanism coincides with the southern edge of the Precambrian crust and the southernmost extension of the California slab window at 12.5 Ma.

Key Words: Peralkaline ignimbrites • Middle Miocene • Isotope geochemistry • Gulf of California