A CASE STUDY OF A ~1.66 GA QUARTZITE-RHYOLITE SUCCESSION IN CENTRAL NEW MEXICO: SIGNIFICANCE OF TWO QUARTZITE-RHYOLITE “EVENTS” DURING ASSEMBLY OF LAURENTIA

Deposition of quartzite-rhyolite assemblages throughout the southwestern United States apparently took place at two times during Paleoproterozoic accretion of the Yavapai and Mazatzal provinces to Laurentia. Thick, quartz arenite successions with >95% quartz were deposited in fluvial to shallow marine settings (Soegaard and Eriksson, 1985) during regional contractional deformation (Karlstrom and Bowring, 1988; 1993; Karlstrom et al., 2004). This hypothesis for syntectonic basins (Jessup et al., in prep) requires that the nature, timing, and tectonic significance of the basins be further tested. U-Pb geochronology available so far suggests an older, ~1.70 Ga suite and a younger suite, ~1.66 Ga and therefore a repetition of the same tectonic styles and processes at two times in the orogenic evolution. This study focuses on one of the best exposed areas of the younger of the quartzite-rhyolite assemblages, located in the Manzano Mountains of central New Mexico. In this locality, rhyolitic ash flow tuffs and associated flows occur stratigraphically above and below the Manzano Group quartzites, allowing a unique opportunity to precisely date these quartzites and help resolve the previous wide range of dates found for the Manzano area rhyolites, 1662 ± 1 to 1680 ± 20 Ma (Shastri, 1993), 1680 ± 20 Ma (Bowring et. al, 1983), and 1700 ± 20 Ma (Unruh, unpubl). Contact relationships suggest a continuous depositional sequence that involved bimodal volcanics overlain by immature volcanogenic arenites, clean quartz arenites, and finer grained siltstones interbedded with rhyolitic ash flows. New field, structural, and geochronologic data, as well as comparative studies between age correlative quartzite-rhyolite successions in Arizona and elsewhere in the Southwest, will help test the hypothesis that the Manzano Group succession records a back arc or intra arc volcanic/sedimentary syntectonic basin where deposition was broadly synchronous with calc-alkaline granitoid plutonism (1662-1643 Ma) and with thrusting in the Manzano thrust belt of the Mazatzal orogeny.