New Mexico Geological Society Annual Spring Meeting — Abstracts

The Proterozoic history of the San Andres Mountains has been investigated through detailed field mapping, geochronology and geochemistry. SHRIMP U/Pb zircon geochronology was performed on a quartzite and several granitic plutons. The quartzite was collected from below the Bliss unconformity. Zircons (n=98, 2σ) from the quartzite yield a large population at 1668 ± 7 Ma and two smaller populations at 1729 ± 8 Ma and 1793 ± 15 Ma. The ~1.67 Ga zircons probably were locally derived from basement rocks as rocks of this age have been dated in other Mazatzal Province crust in southern New Mexico. The older populations were derived from Yavapai Province rocks from the north. In the southern part of the range, Proterozoic crystalline rocks include a deformed granitic orthogneiss that experienced east-west shortening. Zircons from this unit were dated at 1649 ± 9 Ma (n=8, 2σ). An unfoliated granitic pluton that cuts the orthogneiss has yielded ages of 1632 ± 17 Ma (n=5, 2σ) and 1626 ± 14 Ma (n=10, 2σ). These plutons were intruded by a series of amphibolite dikes at ~1600 Ma (n=8). Boudinage of these dikes in the southern part of the range suggests subsequent east-west extension. Though abundant in the southwestern U. S., ~1.4 Ga plutons have not been dated in this study. However, zircons from the overlying Cambrian Bliss sandstone yielded dates of 1455 Ma (n=87), suggesting these rocks may have once been locally present.