U-PB AGES OF DETRITAL ZIRCONS FROM SANDSTONE OF THE OLIGOCENE CHUSKA FORMATION, NORTHWESTERN NEW MEXICO

We sampled semiconsolidated crossbedded eolianite of the Narbona Pass Member of the Oligocene Chuska Formation from an outcrop on the north side of NM Highway 134 (at mile 75.5 of the 2003 NMGS 54th Annual Field Conference Guide). The sands that make up the formation accumulated between 33.5 and 27 Ma. A point count of the sample yielded 55% quartz, 38% feldspar (with K-feldspar including microcline and perthite twice as abundant as plagioclase), and 7% lithic fragments including 2% chert grains and subequal proportions of volcanic and sedimentary rock fragments. The grains themselves are well-rounded of medium sand size. A total of 95 detrital zircons analyzed using the LA-MC-ICP-MS with a beam diameter of 35 microns have concordant or nearly concordant U-Pb ages. The largest fraction (55%) of the detrital zircons were derived from Yavapai-Mazatzal basement (1600-1800 Ma) of southwest Laurentia. Other significant fractions are Paleozoic to Neoproterozoic (500-650 Ma) and Grenville (1000-1200 Ma) in age (6% each), Mesoproterozoic (12% at 1200-1400 Ma and 10% at 1400-1600 Ma), and Paleoproterozoic to Archean (7%). We found no detrital zircons of Oligocene age even though derivation of significant proportions of Chuska sand from Oligocene volcanic fields has been inferred from regional sedimentological analysis. Their lack in our sample may reflect a paucity of zircons of sand size in potential volcanic source rocks, although the low content of lithic fragments and the low P/F ratio (0.34) of the sand suggests derivation dominantly from plutonic basement sources.