New Mexico Geological Society Annual Spring Meeting — Abstracts

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We interpret a newly discovered laminated limestone associated with Middle Proterozoic rocks in the Redrock and Brushy Mountain quadrangles of southwestern New Mexico as representing stromatolites, possibly Conophyton. This locality is geographically aligned with other outcrops of ~1.2 Ga stromatolites documented in the southwest U.S., such as those in the Bass Formation (Grand Canyon, AZ), the Mescal Limestone (Salt River Canyon, AZ), the Castner Marble (Franklin Mountains, TX), and the Allamoore Formation (Van Horn, TX). These stromatolites have similar morphologies to those found in Texas and represent the first instance of Proterozoic fossils identified within the state of New Mexico. The stromatolite-bearing unit, termed here the Ash Creek Limestone, is exposed along with a marble unit surrounded by the ~1225 Ma Redrock Granite (Williams, 2015). Mapping at 1:6,000 scale shows that the carbonates are also associated with a unit consisting of serpentinite interbedded with talc (forming a rock informally referred to as ricolite), meaning they share a similar depositional setting. The carbonates are only found surrounded by granite, whereas ricolite outcrops are typically in contact with a metabasalt. The metabasalt yielded a U-Pb zircon weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1229 ± 12 Ma (n= 35; MSWD= 0.8). This represents the first direct dating of ~1.2 Ga mafic volcanism in the area, and these racks are similar in age to anorthosite dated at 1223 ± 6 Ma and 1231 ± 4 Ma (Ramo et al., 2003). Together, the ~1.2 Ga bimodal magmatism and shallow marine limestone units indicate that the tectonic setting of southwest Laurentia at this time involved a NW-SE-trending Mesoproterozoic seaway that formed as a result of coeval regional extension during the early stages of the Grenville orogeny.

References:

1. Mulder, J.A., Karlstrom, K.E., Fletcher, K., Heizler, M.T., Timmons, J.M., Crossey, L.J., Gehrels, G.E., Pecha, M., 2017, The syn-orogenic sedimentary record of the Grenville Orogeny in southwest Laurentia: Precambrian Res. v. 294, p. 33–52, https://doi.org/10.1016/j.precamres.2017.03.006.
2. Rämö O.T., McLemore, V.T., Hamilton, M.A., Kosunen, P.J., Heizler, M., and Haapala, I., 2003, Intermittent 1630-1220 Ma magmatism in central Mazatzal Province; new geochronologic piercing points and some tectonic implications: Geology, v. 31, p. 335–338, https://doi.org/10.1130/0091-7613(2003)031<0335:IMMICM>2.0.CO2.
3. Williams W.A., 2015, Proterozoic sedimentation and magmatism in the Redrock Area, Burro Mountains, southwest New Mexico [M.S. Thesis]: Las Cruces, New Mexico State University, 146 p.