Geology and chemical composition of the Jaralito and Aconchi batholiths in east-central Sonora, Mexico

The Aconchi-El Jaralito area is 100 km northeast of Hermosillo, and about 200 km southwest of Douglas, Arizona. The intrusive rocks of the region are divided into three units on the basis of their field relations, morphology, mineralogy, chemical composition, and isotopic ages. These units are two granitic batholiths and a group of porphyritic rhyolite stocks. The oldest batholith is the granitic-granodioritic El Jaralito batholith, which has the largest exposure area. Lithologically it varies from a true granite to quartz monzonite, quartz diorite, and granodiorite. The rocks of this batholith are considered I-type or Cordilleran granites. The K/Ar ages reported for EI Jaralito batholith range from 51.8 to 69.6 Ma. The second batholith is the Aconchi batholith, which has a smaller exposure area. It is lithologically simple, consisting almost exclusively of alkali granites, including abundant pegmatites. This is considered an S-type granitoid, with primary muscovite and red garnet. One K/Ar date for this unit is 35.96 + or - 0.70 Ma. Chemical analyses show no clear separation between the two batholiths. There is an overlap in the values of some of the oxides. The youngest intrusive rocks in the area are two porphyritic rhyolite stocks, which have quartz, feldspar, and biotite phenocrysts set in a quartz-feldspar matrix. The chemical composition of the stocks is similar to the alkali granites of the Aconchi batholith. Cataclastic deformation was observed within both batholiths in isolated localities; however, the structure of these deformed rocks was not studied in detail. Chemical and isotopic data for these batholiths remain incomplete; it is highly recommended that future chemical data include trace-element concentrations.