Eruptive chronology and tectonic context of the late Pleistocene Tres Vírgenes volcanic complex, Baja California Sur (México)

Abstract

"Based on a detailed geological map and stratigraphy aided by new Th-230/U-238 and P-206/U-238 age determinations we established the eruptive chronology of the Tres Virgenes Volcanic Complex (TVVC). The complex consists of three northeast-southwest aligned stratovolcanoes, which from older to younger are El Viejo, El Azufre, and La Virgen and associated cinder cones and domes. The alignment was fed from fissures that mark the southern trace of the left-lateral Cimarron Fault. The TVVC was built upon the basement rocks of the Peninsular Ranges Batholith (similar to 99 Ma), the Santa Lucia Formation (21.6 Ma), the Esperanza basalt (7.6 Ma), and the 1.1 Ma ignimbrite of El Aguajito caldera. The TVVC began its formation at ca. 300 ka mainly with dacitic lava flows and domes that formed El Viejo volcano (4.2 km(3)). At around 173 ka, activity migrated to the southwest to build El Azufre volcano (3.8 km(3)) with the emission of dacitic lavas and several phases of dome construction and destruction with the generation of pyroclastic density currents. El Azufre's activity ended similar to 128 ka with the extrusion of the summit dome. At about 112 ka, volcanism migrated 42 km to the southwest to begin the construction of La Virgen Volcano through the emission of andesitic and dacitic lavas that built the largest cone of the complex until similar to 22 ka (31.2 km(3)). This volcano was built upon several stages of cone construction with the emission of lava flows and peripheral dacitic domes and cinder cones. Between 128 and 112 ka six cinder cones vented along the Cimarron Fault between the La Virgen and El Azufre volcanoes. By assuming a constant emission of magma the total volume of the TVVC (39 km(3)) was erupted at an average rate of 0.13 km(3)/kyr. Of this volume, effusive eruptions dominated the evolution of the complex (89%) of which 62% are dacites and 38% andesites. The TVVC rocks have compositions of 50-67 wt% in silica, and low-medium K (0.5-2.5 wt% K2O). Trace element signatures of the TVVC rocks (Sr/Y <40, La/Yb <10, Nb-Ti negative anomalies and relative enrichments of LILE and LREE) suggest a post-subduction mixture between adakitic and calc-alkaline magmas. Xenoliths of granodiorites hosted in lavas and La Virgen tephra suggest that magma chambers feeding the complex have stagnated at depths >2 km within the Peninsular Ranges Batholith as confirmed by geobarometry, aeromagnetic data, and the drill-holes of the geothermal field. Today, the Tres Virgenes geothermal field generates 10 MW of electricity although future exploration of several nearby areas may increase such capacity."