EVOLUTION OF THE GRANITIC BATHOLITHIC COMPLEX, SINALOA, MEXICO, AND COMPARISON WITH OTHER CORDILLERAN BATHOLITHS OF MEXICO

Field relations and 98 K-Ar and 4 U-Pb zircon dates within a 120 x 120 km transect show that the Sinaloa batholith formed in several stages. Early layered gabbros have hornblende ages of 139 and 134 Ma, although whether these record emplacement, cooling from metamorphism, or excess Ar is unresolved. Relatively mafic tonalites and granodiorites were emplaced before or during an episode of deformation, and all lie within 50 km of the coast. A U-Pb date on one intrusion is 101 Ma. Hornblende K-Ar ages range between 98 and 90 Ma and probably record cooling soon after emplacement. Posttectonic intrusions consisting mostly of leucocratic granodiorite crop out from within ~20 km of the coast to the Sierra Madre Occidental, where they are covered by mid-Cenozoic tuffs. U-Pb ages of 67, 48, and 20 Ma on three samples agree within analytical uncertainty with concordant biotite-hornblende ages of 64, 46, and 19 Ma. These data and field relations demonstrate that posttectonic intrusions cooled rapidly following shallow emplacement. The combined age data indicate that posttectonic intrusions were emplaced nearly continuously between 90 and 45 Ma. One intrusion is 20 Ma. Based on outcrop area, volumes of intrusions were relatively constant through time. Posttectonic magmatism shifted eastward at 1-1.5 km/Ma, much slower than in the Peninsular Ranges-Sonora or Jalisco-Oaxaca. The Sinaloa batholith is borderline calc-alkalic/calcic. SiO2 contents of syntectonic rocks are mostly between 59 and 62%. SiO2 contents of posttectonic rocks are mostly between 63 and 74%. Combined with their distribution, these data indicate that intrusions become more silicic eastward. The Sinaloa batholith is similar to other Cordilleran batholiths in its wide compositional range, types of intrusions, sequence from early gabbro through syntectonic to posttectonic rocks, and eastward migration of magmatism. REE patterns are like those of the eastern Peninsular Ranges and central Sonora, which are underlain by Proterozoic crust or crust with a large Proterozoic detrital component. However, southern Sinaloa lies within the Guerrero terrane, which is interpreted to be underlain by accreted Mesozoic crust. The greatest differences are in distance and rate of eastward migration. The area of slower eastward migration roughly coincides with the Guerrero terrane.