LA PURISIMA VOLCANIC FIELD, BAJA CALIFORNIA SUR, MEXICO: MAGMATIC EVOLUTION FROM MIOCENE TO QUATERNARY

La Purísima Volcanic Field (PVF) displays a wide geochemical and petrologic variability for Middle Miocene to Quaternary volcanic rocks. It is limited to the east by the Main Gulf Escarpment and to the west by the Pacific shoreline. Baja California has been the locus of many events of volcanism including the emplacement of typical calc-alkaline lavas, magnesian andesites (adakite affinity), tholeiitic basalts and adakites. In that area, the calc-alkaline serie of the Comondú volcanic arc ranges in age from 16.4 ± 0.9 Ma to 11.0 ± 0.3 Ma. In the northwestern part, large mesas of tholeiitic basalts overlie tilted Tertiary sediments. They correspond to the southernmost tip of Esperanza Basalt. The low-K tholeiitic lavas are are silica-oversaturated. Their concentrations in most incompatible elements are low, and the corresponding multielement patterns normalized to primitive mantle present very flat shapes. Their whole-rock isotopic ages range between 10.6 ± 0.8 Ma to 9.33 ± 0.32 Ma obtained respectively from the well-crystallized cores of pillow lavas and their glassy margins. The Late Miocene to Quaternary volcanism belongs to the magnesian andesitic series. Their main geochemical characteristics are SiO2 contents ranging from 51.5 to 55.5 wt%, high values of MgO and K2O, together with high contents of incompatible elements such as Ba and Sr. In addition, these lavas are LREE-enriched and HREE-depleted. The oldest magnesian andesites are dated between 9.7 and 8.8 Ma. The second magnesian andesitic event, is dated at 5.48 Ma. A large magnesian andesitic mesa was emplaced at ca. 2.72 Ma. Younger mesas as well as rather small flows associated with spatter and strombolian cones were emplaced inside a N-NW trending elongated graben until very recent times, suggesting that Quaternary volcanic activity was discontinuous. The magnesian andesitic volcanism of La Purísima area is contemporaneous with adakitic volcanism, which occurred also in Baja California Sur well after the supposed end of the subduction of the Farallon plate. The geochemical characteristics of these magmas are still typical of subduction-modified sources and suggest partial melting of hot slab related to the formation of an asthenospheric window.