A revised stratigraphy and petrogenetic interpretation of the Cordilleran basement in the Mazatlan, Sinaloa region (NW Mexico): Evidence from new geochemical and geochronological data

The age, composition, and thermal history of the cordilleran basement in NW Mexico are still poorly known. We focus here on the Mazatlan, Sinaloa region where previous works identified the following sequences: 158 Ma orthogneisses(U-Pb) are covered by metamorphosed clastic sediments of supposed Jurassic age which are intruded by a mafic complex mainly represented by 134-139 Ma gabbros (K-Ar hornblende ages). Both rock series are considered as parts of the basement in this region. The remaining stratigraphy is characterized by a batholitic complex formed by two major magmatic events. The first one, around 100 Ma (U-Pb and K-Ar), is tonalitic in composition with some signs of regional deformation associated to a syntectonic emplacement. The second one, between 46 and 83 Ma (K-Ar), varies from diorite to granodiorite. The latter is the most widely distributed event, hosts the important ore deposits in the region and is interpreted to be emplaced postectonically, and. Our new U-Pb geochronological data include the identification of two additional plutonic events. The first occurred at 136 Ma and was obtained from the Concordia granodiorite 30 km E of Mazatlan. This unit was formerly dated at 54 Ma (hornblend, biotite; K-Ar). A fourth plutonic unit (Copala granodiorite) was dated at 28 Ma and can be correlated to the Upper Sierra Madre Supergroup. Additionally we obtained detrital zircon ages from a paragneiss covering the 158 Ma orthogneisses with peaks at 256 Ma, 492 Ma, and 1014 Ma. Our field observations indicate a reset of the 134-139 Ma gabbro ages by the close Concordia granodiorite, whose biotites were in turn rehomogenized by the 46 to 83 Ma plutonic event. Until now very few geochemical and Sr-Nd isotopic studies with petrogenetic interpretation of the units mentioned above are published. The results of our new investigations will be presented at the meeting as well as a new dataset for the coastal Quaternary Piaxtla basalt and its peridotitic xenoliths. These rocks are related to the opening of the Gulf of California and give evidence for the local upper mantle composition. In the light of the new geochronological information an updated stratigraphy and chronology of the magmatic and thermal events in the area is under work. The discrepancy between some of the previously reported ages and our new data can be explained by a complex thermal history which resetted some of the earlier published K-Ar hornblende and mica ages.