New Mexico Geological Society Annual Spring Meeting — Abstracts

Detrital zircons from sandstones from southern New Mexico were analyzed for U-Pb ages using LA-ICP-MS. These include: (1) Proterozoic quartzite from the San Andres Mts.; (2) Cambrian-Ordovician Bliss Sandstone from the San Andres Mts., San Diego Mt., Mud Springs, Florida Mts., San Lorenzo, and the Burro Mts.; (3) A quartzite within the Pennsylvanian La Tuna formation at Bishop Cap southeast of Las Cruces; and (4) Cretaceous Beartooth Quartzite from the Burro Mts. An average of 75 grains/sample were analyzed. "Peaks" refers to peaks on a relative probability distribution diagram. The results have implications for provenance and maximum depositional age of the units.

Two samples of foliated micaceous quartzite from the San Andres Mts. were collected from below the unconformity with overlying Bliss Sandstone. They both yielded a single population at 1680 Ma. These ages have been reported from basement rocks locally and throughout the Mazatzal province.

The Bliss Sandstone has some variation in ages among the 12 samples analyzed, but most have the following main peaks: 1250 Ma, 1470 Ma, 1715 Ma, and 1850 Ma. The sample of Bliss Sandstone from the Florida Mts. lies unconformably over the Cambrian Florida granite. The dates from the granite range from 501 ± 6 Ma to 512 ± 8 Ma. Samples had only one population of zircons at ~500 Ma. The 500 Ma zircons were found in Bliss from the Burro Mts., and a single 500 Ma grain was found at San Lorenzo. The Bliss differs from all of the other units with Proterozoic grains in that the main peak is at 1250 Ma instead of ~1650 Ma suggesting a larger proportion of 1.2 Ga igneous rocks exposed at the time.

The sandstone at Bishop Cap is a thin member within the Pennsylvanian La Tuna formation. The main peaks are at 1650 Ma, 1440 Ma, and 1100 Ma. Two grains have ages of ~320 Ma, or Mississipian, but the youngest cluster of 3 or more ages is 493 Ma; this is much older than the depositional age as determined by fossil ages. The Cretaceous Beartooth Quartzite in the Burro Mountains has several Proterozoic peaks including 1050 Ma, 1450 Ma, and 1680 Ma, a strong peak at 214 Ma, and a young peak of 102 ± 4 Ma that includes seven grains and provides a maximum depositional age near the Early/Late Cretaceous boundary. This is the first known absolute age constraint on this non-fossiliferous unit.