Geology, geochronology and thermobarometry constraints of the extensional segmentation of the Los Cabos batholithic complex, southern Baja California, Mexico.

The Los Cabos Block, at the southern end of Baja California, exposes the southernmost part of the Late Cretaceous Peninsular Range Batholith over an area of 2700 km2. In the Los Cabos block, the Los Cabos Batholitic Complex (LCBC) is exposed at Sierra La Laguna massif to the west and Sierra La Trinidad massif to the east. The two massifs are separated by the ~75 km long, N-S striking, San José del Cabo extensional fault system, and the homonymous basin. Here we present field observations, new U-Pb and Ar-Ar ages, and thermobarometric determinations that constrain the magmatic history of this composite batholith and the structural relations between Sierra La Laguna and Sierra La Trinidad. Field observations show that the batholith at Sierra La Laguna comprises several granodiorite bodies, felsic sills, and dikes with complex magma mingling and mixing features typical of an upper-middle crust system within a mush zone of intermediate to silicic composition. U-Pb zircon crystallization ages for La Laguna massif define two main pulses of plutonism at 100-88 Ma and 81-74 Ma, with progressively younger ages from west to east. Zircon chemistry and published whole-rock isotopic data point to primitive arc magmatism with significant crustal thickening with time. Ar-Ar hornblende, biotite, plagioclase, and feldspar ages from Sierra La Laguna reveal a relatively slow cooling rate of 13 to 51 °C/m.y. in the Late Cretaceous. Al-in-Hb thermobarometry for six samples suggests a wide range interval of crystallization depths from 12 to 20 km and temperatures between 670 and 740 °C. Sierra La Trinidad is lithologically less complex, with crystallization ages similar to the eastern, younger part of Sierra La Laguna, and is locally covered by slightly younger felsic volcanic rocks (~81 to 72 Ma), indicating a shallower emplacement. Detrital zircons ages from the sedimentary units filling the San José del Cabo basin, assigned to mid-late Miocene on a paleontological basis, fall entirely within the range of the LCBC (~99 to 73 Ma), with a median age of the three dated formations being progressively younger upsection. The integration of our study with previous works suggests that Sierra La Trinidad is the hanging wall block of the San José del Cabo fault system.