GEOCHEMISTRY AND TECTONIC SIGNIFICANCE OF MIOCENE GRANITOIDS IN NORTHWESTERN MEXICO

The tectonic transition from an Andean-style convergent plate boundary to extension along the margin of northwestern Mexico during the Tertiary produced volcanic rocks in three distinctive magma pulses. First, calc-alkali rocks were emplaced from 23.5 to 15 Ma during the subduction of the Farallon plate and its derivatives. Second, tholeiitic basalts were emplaced between 10.3 to 8 Ma during orthogonal rifting of the Gulf of California. The emplacement of 14.4 Ma felsic granitoids, including El Papalote granitoid, and 12 to 11 Ma voluminous dacites between the two distinctive pulses represents a significant change in style and magma chemistry. The character and origin of El Papalote granitoid within the volcanic masses in central Sonora are controversial.

Field relations, petrographic, geochemical and isotopic analyses suggest that El Papalote granitoid is a shallow complex of multiple intrusives associated with the final stages of Guadalupe microplate subduction. Its age of emplacement is indicated by stratigraphic evidence and by a whole rock 40Ar/39Ar age of 14.4 ± 2.5 Ma. Geochemically, El Papalote granitoid is a felsic (>72% SiO2) calc-alkali unit characterized by higher K, Rb, 87Sr/86Sr and light REE abundances than the older rocks. Pronounced negative Nb-Ta, Sr, P and Ti anomalies together with lower 143Nd/144Nd and heavy REE abundances distinguish the granitoid from younger rocks.

Our data suggest that granitoid exposures are controlled by the Tertiary tectonic history of the Guadalupe microplate. In addition our data are compatible with studies from the Gulf of California region where subduction ended ca 12.5 Ma ago. Voluminous 12 Ma synrift volcanism was emplaced along the northern part of the Baja California Peninsula. In contrast, Sonoran granitoids are probably associated to the tectonic transition from subduction to rifting. Local differences in age and chemistry of rocks around the Gulf of California may reflect complex irregularities along the margins of a fragmented Farallon plate.