Description
Cenozoic volcanic rocks are exposed in a 20 kilometer-long belt along the eastern flank of the Laguna Juarez adjacent to Laguna Salada in northern Baja California, Mexico. The volcanic rocks occur as sequences of flows that unconformably overlie Cretaceous batholith rocks and locally occur as dikes cutting basement. On the Geological Society of America 1:250 000 geologic map of northern Baja California, these rocks are mapped as ‘Tmb – Miocene basalt and basaltic andesite’. The occurrence of these rocks is not far from middle Miocene volcanic rocks of the Jacumba Volcanics and Alverson Formation in eastern San Diego and western Imperial counties on the US side of the border, which occur in a similar geologic setting along the western edge of the Salton Trough. However, no petrologic investigation has ever been undertaken on the Baja California volcanic rocks and there are no radiometric ages on the Baja California rocks. The purpose of this study was to conduct a reconnaissance petrologic study of the Baja California volcanics to investigate their possible relationship to the Jacumba Volcanics and Alverson Formation. Sixteen samples were collected for analysis. One of these samples is a 10 meter wide dike intruded across plutonic and metamorphic crystalline basement rock on the Baja highway 2 La Rumorosa grade that is distinct from the rest of the samples with very high strontium contents. The remaining 15 samples are all from the southern part of the Tmb exposures ~25 km south of the border. Whole rock chemistry reveals two main groups of volcanic rock types: 1) high magnesium andesites with Mg numbers from 67-69, and 2) higher silica andesite-dacite flows. These are all subalkaline rocks with potassium contents similar to calc-alkaline arc volcanics. These rocks, however, have high strontium concentrations averaging around 1000 ppm, which is high compared to arc volcanic rocks. They also have relatively low yttrium contents with an average around 10 ppm, which is quite low compared to normal calc-alkaline arc volcanics. These trace element features are similar to volcanic rocks of the Alverson and Jacumba volcanics as well as high magnesium post-subduction volcanics in central and southern Baja California.
