EARLY AND MIDDLE PROTEROZOIC CRISTALINE BASEMENT IN SONORA, MEXICO

The first metamorphic event in Sonora is located in Bamori where two pegmatites of regional metamorphism show ages of 1.63 and 1.675 Ga. Of 1.675 Ga in Tuape crops out the San Isidro Gneis. El Alamito Complex in Teguachi and one gneis in Opodepe show a metamorphism of 1.65 Ga. The northeast of Sonora that comprises Sierra de los Ajos, Las Mesteñas, and La Morita hills there are schists with regional metamorphism, at Sierra de los Ajos, of 1.715 Ga ago. In conclution in Sonora an important regional metamorphism event took place between 1.715 and 1.63 Ga ago.

The protolites of metasedimentary and metavolcanic were formed during the base of the Early Proterozoic or even in the final phase of the Arqueozoic and could be part of the Proto Laurentia Continent.

The first magmatic event in Sonora begins with granites exposed in Quitovac with 1.78 and 1.69 Ga; In Bamori diorites and cuarzo latites of 1.75 Ga and cuarzo monzonites and granodiorites with 1.74 Ga; adittionally in Opodepe and Teguachi can be found granodiorites with 1.7 Ga. All the intrusive bodies were cooled in the upper half of the Early Proterozoic and they coincide with the Hudsonian Orogeny.

The metamorphic and intrusive sequences were under an erosive process during about 200 Ma.

A second intrusive event took place in Sonora because granites of the Sierra de Mazatan had ages of 1.48 Ga; further a granitic rock at Sierra de los Ajos, Las Mesteñas hills, and Hachita Hueca Range of 1.44 Ga; also 100 km northward of Hermosillo can be observed the Nochebuena Granite of 1.425 Ga; finally one granite in Opodepe and El Tuti Granite in Tuape yielded 1.4 Ga. All these granites were formed during the base of the Middle Proterozoic at the same time that the Elsonian Orogeny.

The lapse comprise between 1.4 Ga and 1.1 Ga, middle part of the Middle Proterozoic, seems to correspond a long erosive period.

Other magmatic event, the youngest and corresponding to the Grenvilean Orogeny, is characterized by the Aibo Granite of 1.11 Ga; in Quitovac one granite of 1.1 Ga; in Bamori an ultrabasic rock of 1.1 Ga; and in Tuape, Opodepe, and Teguachi micrographic granites of ages of 1.1 Ga. These rocks were formed during the upper third of the Middle Proterozoic, coinciding with the Rodinia Supercontinent.

Other erosive interruption degraded the proterozoic sequences, during probably 400 Ma.

The Later Proterozoic, between 700 and 570 Ma, cuncluded with a voluminous sedimentation.