THE AIBO-TYPE (~1.1 GA) GRANITIC MAGMATISM IN NW SONORA, MEXICO: FAILED CONTINENTAL RIFTING OF RODINIA?

The Late Mesoproterozoic (~1.1 Ga) Aibo-type granitic magmatism is named after granite outcrops near the Aibo Ranch, 25 km south of the town of Caborca in NW Sonora, Mexico. This enigmatic pulse of granitic magmatism intrudes Paleoproterozoic plutonic and metamorphic basement rocks of the Caborca block. We present new SHRIMP U-Pb zircon ages, and geochemical and isotopic results from ~1.1 Ga granites of the type location in the Aibo Ranch as well as from the Campo Bustamante area in the Quitovac region, 140 km NW of the town of Caborca.

Concordant zircon analyses for the Aibo granite yielded a weighted mean 206Pb/238U age of 1079±18 Ma and corresponding upper-intercept age of 1080±20 Ma that we interpret as the crystallization age for the intrusive body. Two samples from different granitic bodies from the Campo Bustamante area yielded concordia intercept ages at 1102±11 Ma and 1055±16 Ma, although the latter may be somewhat older if several slightly discordant spot analyses are deleted from the regression. In that case, a weighted mean 207Pb/206Pb age of 1063±20 Ma or a weighted mean 206Pb/238U age of 1083±21 Ma is obtained.

This suite of granites is highly enriched in K2O, Y, Rb, Ba, Th and REE relative to other granitic rocks of the Caborca basement. eNdt values between -1.4 to -4.6 indicate that these granites could have been formed by mixing of depleted upper mantle (eNdt ~+5.5) with a significant amount of Paleoproterozoic crust of the Caborca block (eNdt ~-7). Scarcity of Aibo-type granites in NW Sonora complicates our understanding of their tectonic setting. However, these new age and geochemical constraints indicate a correlation with the potassic granites of the ~1.08-Ga Pikes Peak batholith in central Colorado. As has been proposed in Colorado, this magmatism in Sonora could have been associated with a period of crustal extension associated with the early stages of failed continental rifting of the supercontinent Rodinia.