Ž . Journal of Volcanology and Geothermal Research 93 1999 31–52 www.elsevier.comrlocaterjvolgeores The Recent Isla San Luis volcanic centre: petrology of a rift-related volcanic suite in the northern Gulf of California, Mexico Francisco A. Paz Moreno a, ) , Alain Demant b,1 a UniÕersidad de Sonora, Departamento de Geologıa, Apdo. Postal 847, 83000 Hermosillo, Sonora, Mexico ´ ´ b Laboratoire de Petrologie Magmatique, UniÕersite Aix-Marseille III, Case 441, 13397 Marseille Cedex 20, France ´ ´ Received 28 January 1998; accepted 9 July 1998 Abstract Isla San Luis, one of the most recent eruptive centres in the Gulf of California, presents a complete magma evolution trend from basaltic andesites to rhyolites. The less-evolved lavas are palagonite tuffs, related to Surtseyan-type activity which characterized the emergent stage of the island. Subaerial lava flows and later high-energy hydromagmatic eruptions are dacites which make up the tuff rings of the southeastern corner of the island. Rhyolites are the latest erupted products and the development of the younger dome in the centre of the island was preceded by ash and pumice fallout containing Ž . quenched bombs with an obsidian crust and a pumiceous core. Basaltic andesites contain olivine Fo , calcium-rich 87 – 80 plagioclase and sparse clinopyroxene microphenocrysts; the typical mineralogy of dacitic and rhyolitic lavas is plagioclase q two-pyroxenes. Major and trace elements vary regularly with MgO taken as a differentiation index. Isla San Luis lavas are enriched in LILE and depleted in Nb, Ta and Ti. Their characteristics are intermediate between tholeiitic and calc-alkaline wŽ . x lavas. They are also LREE enriched and the LarYb ratios steepen slightly with differentiation. Such an evolution is n better explained by combined fractional crystallization and assimilation processes. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Gulf of California; volcanic island; volcanology; hydromagmatism; petrology; AFC process 1. Introduction Ž X Y X Y Isla San Luis 29858 20 N and 114824 14 W at . the summit point , also named Salvatierra or Encan- tada mayor, is located in the northern part of the ) Corresponding author. Fax: q52-62-59-21-11; E-mail: paz@geologia.uson.mx 1 Fax: q33-04-91-28-86-27; E-mail: alain.demant@lithos.u-3mrs.fr. Ž . Gulf of California Fig. 1 . It covers an area of about 4.5 km 2 and has an overall maximum elevation of 180 m. The well-preserved morphology of the vol- canic structures clearly support a late Quaternary age for all the eruptive activity. A striking feature of Isla San Luis is the presence of a complete evolution trend, from basaltic andesite to rhyolite, erupted in a short time interval. Isla San Luis along with Isla Tortuga, a tholeiitic basaltic shield volcano located Ž . near Santa Rosalıa Batiza, 1978; Batiza et al., 1979 , ´ 0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0377-0273 99 00083-9