AN INVENTORY OF MAFIC AND ULTRAMAFIC ROCKS OF CORDILLERAN PROTEROZOIC BASEMENT(?) IN NORTHERN SONORA, MEXICO: PRELIMINARY RESULTS

Initial exploration mapping for platinum metal group on the San Pablo maffic and ultramaffic complex of Cenomanian age (96 Ma) located north to the city of Mazatlan, Sinaloa gave the idea to shift exploration to Sonora to find targets associated mainly to the late Proterozoic basement in northern Sonora, Mexico. This new exploration reconnaissance geology at a first stage propose an inventory or catalogue of proven, probable and possible maffic and ultramaffic rocks (M+UMR). The preliminary results of this catalogue show at least, three genetic groups, as follows. 1) A well known Proterozoic basement west of Banámichi region, where M+UMR are associated, includes the Jacalon granodiorite (1700 Ma) and a micrographic granite of 1100 Ma. 2a) In east Sonora in Sierra La Campaneria, probable Proterozoic basement including M+UMR is associated with Early and Late Paleozoic sequences that carry slivers of M+UMR which are interpreted to have been probably derived from an oceanic source. 2b) In central Sonora, La Lamina ranch region, the M+UMR are including within the Precambrian rocks of the La Lamina thrust sheet. Metaleucogranite, metagranite, leucopegmatites together with metamorphic rocks of intermediate composition are some of the rocks exposed in that area. 3) The M+UMR associated with possible Proterozoic basement and Mesozoic volcano-sedimentary rocks in Sierra the Santa Rosa, north-central Sonora. The Santa Rosa Formation of Early Jurassic age contains the Santa Rosa and Las Cruces pyroxenitic layered intrusions of unknown age. To the east, in Sierra Tres de Mayo, the Caracahui Formation or Santa Rosa Formation both of Early Jurassic age contains the El Remolino/La Fatima pyroxenitic complex of unkown age. To the northeast of Estación Llano the El Aguaje M+UMR complex is associated with the El Represo Formation of Early Cretaceous age. La Lamina and La Campaneria M+UMR distribution and associated rocks suggests a compressional thrust front of Jurassic age while Santa Rosa-Las Cruces-Tres de Mayo-Aguaje suggest either: a) the bottom of a rift basin (The San Antonio basin) or b) a proterozoic lineament. Quemical analysis from the Tres de mayo location reports presence of metals of the platinum group.