U-PB ZIRCON GEOCHRONOLOGY OF PALEOPROTEROZOIC BASEMENT IN NORTHWESTERN SONORA, MEXICO: EVIDENCE FOR AFFINITY TO SW US PROVINCES

Nineteen new U-Pb zircon ages, determined using both conventional ID-TIMS and SHRIMP methods, were obtained from granitoid samples of Paleoproterozoic basement in NW Sonora and SW-most Arizona. These results, plus other recently-published zircon ages, indicate granitic magmatism occurred over a similar age range as rocks in previously-defined Paleoproterozoic provinces of the SW US. These new results are mainly from three regions of NW Sonora; the Pinacate-Cabeza Prieta (PCP) region that overlaps the US-Mexican border, the Quitovac-San Francisco (QSF) region just south of Sonoyta, and the Caborca-Bamori (C-B) region, south of Caborca.

In the PCP region, two samples of foliated granodiorite and granite are ~1728 Ma, a monzogranite is ~1695 Ma, and three augen gneisses are between 1655 and 1640 Ma. In the QSF region, ages of four foliated granites range between 1755 and 1740 Ma; ten other granitic samples, variably deformed and metamorphosed, range between 1716 and 1690 Ma; and a least-deformed granite, is 1657 ± 4.4 Ma. In the C-B region, a foliated granite, NW of Cerro Calavaras, is 1737 ± 17 Ma, a granite from Cerro Rajo is 1703 ± 12 Ma; and three other granitic samples are between about 1780 and 1760 Ma. Zircon age results from four other samples not within these specific regions include two samples north of the PCP area and two samples between the PCP and QSF areas. Ages include: [1] augen gneiss from Pilot Knob (1672 ± 11 Ma), [2] granodioritic gneiss from the Gila Mtns (1702 ± 14 Ma), [3] augen gneiss from NW of Sonoyta (1749 ± 23 Ma), and [4] granite from Cerro el Aguila (1697 ± 11.5 Ma).

With the exception of three samples, these crystallization ages indicate pulses of magmatism between 1780-1740 Ma, 1715-1690 Ma, and 1660-1640 Ma, similar to age ranges reported for magmatism within the Yavapai and Mazatzal provinces of central Arizona. Of specifc note: (1) There are no ages greater than 1780 Ma, (2) both the C-B and the QSF regions have plutons in the 1755 - 1740 Ma range that apparently are absent from the PCP area; (3) all three regions contain plutons coeval with the Yavapai Orogeny at about 1700 to 1690 Ga; (4) the C-B region is conspicuously devoid of ca. 1660 - 1640 Ma plutons (Mazatzal Orogeny), and (5) no Paleoproterozoic ages younger than 1640 Ma have been found.