THE CABORCA BLOCK: AN INHOMOGENEOUS PIECE OF PALEOPROTEROZOIC CRUST IN SONORA

Paleoproterozoic crystalline basement of the Caborca block in Sonora, northern Mexico, has been proposed to lie southwest of a hypothetical NW-trending, left lateral, Jurassic Mojave-Sonora megashear. Restoration of the proposed 700 to 800 km of displacement would place the Caborca block against the westernmost part of the Mojave province. This interpretation suggested that basement rocks of the Caborca block should have geochemical and isotopic affinities similar to those of the Mojave province.

At present, a total of 43 whole-rock samples from Paleoproterozoic granitic plutons at several key localities within the Caborca block have been analyzed for their geochemical and isotopic characteristics. The northwesternmost Caborca block (north of the Pinacate volcanic field) is composed of rock units with highly depleted crustal signatures (initial eNd between +3.4 and +4.5), similar to reported values for plutons of the Mazatzal province, and indicating a possible correlation. In contrast, the vast majority of our Caborca plutonic samples (including ones from El Creston, Estacion Llano, Cerro Rajon, Quitovac, Sierra San Francisco, Quitobaquito and Andrade Quarry) exhibit initial eNd values between +0.5 and +2.7 that are still somewhat depleted, and similar to values reported from samples of the Yavapai province or the Mojave-Yavapai transition zone. Samples from the southwesternmost Caborca block (Bamori area, Sierra Verruga, and south of Sierra Santa Rosa) are the only Paleoproterozoic granitic rocks that have yielded negative initial eNd values (-0.5 to -2.7). These values indicate that significant proportions of crustal material were involved in the production of these plutonic rocks, and could also identify the presence of a small portion of Mojave crust (characterized by near zero to –5.5 eNd values) in Sonora.