HIGH-TEMPERATURE THERMOCHRONOLOGY OF LOS CABOS BLOCK BASEMENT ROCKS

The Los Cabos Block is a massif of high-standing Mesozoic crystalline basement that underlies the southernmost tip of the 1200 km-long Baja California Peninsula. Voluminous and widespread plutonic rocks consist of an earlier suite of calc-alkaline mafic to intermediate pre- to syntectonic intrusions and a later post-tectonic high-potassium granitoids. Previous low-temperature thermochronology has indicated strong Neogene exhumation of the Los Cabos block synchronous with early rifting of the Gulf. Unfortunately, higher temperature thermochronology needed to constrain the overall magnitude of denudation has been lacking. We have undertaken a regional analysis (N > 100 samples) of hornblende and biotite K-Ar ages to provide this information and support our efforts to evaluate the source region(s) of the Magdelena fan. Our results are consistent with those of previous studies that detected medial to Late Cretaceous cooling broadly synchronous with pluton emplacement. The new detailed data set reveal significant regional differences in cooling patterns, however. In the northeast, basement exposed in the Sierra Las Cruces, Sierra La Gata, and Isla Cerralvo cooled through biotite Ar closure between 88-102 Ma with a maxima at 93 Ma. Hornblende and biotite yield generally concordant ages. Most basement rocks exposed in ranges located further south cooled later, between 75-90 Ma. Comparisons with available U-Pb zircon data from the crystalline rocks indicate that intrusion and biotite K-Ar closure were closely spaced in most areas. Overall, our thermochronologic measurements and previous thermobarometry lead us to conclude that most exposed basement rocks of the Los Cabos block crystallized at depths less than 10 km. Additional analyses of hornblende, biotite, and K-feldspar sampled along the main range front of the Sierra de la Laguna are in progress.