New Mexico Geological Society Annual Spring Meeting — Abstracts

Exposures of Grenville-age (1.3–1.1 Ga) sedimentary rocks in the southwest U.S. include outcrops in the Llano Uplift, Van Horn area, and Franklin Mountains of Texas, the Burro Mountains in southwest New Mexico, the Apache and Unkar Groups in Arizona, and the Pahrump Group in Death Valley. Igneous activity includes arc magmatism from ~1325–1150 Ma and 1150–1120 Ma (Mosher et al., 2008). The Grenville orogeny may have ended around 1120 Ma. Widespread diabase magmatism and coeval silicic intrusions range from 1100–1075 Ma and may reflect an episode of plume magmatism or lithospheric delamination with associated partial melting of the crust to form granitoids (Bright et al., 2014).

We have obtained U-Pb zircon ages from sedimentary rocks and interbedded tuffs from the Transmountain Road area of the Franklin Mountains, west Texas. These include, from top to bottom: (1) Thunderbird Rhyolite, 1143 ± 13 Ma (previously dated at 1111 ± 43 Ma; Roths, 1993); (2) Tuff in the Castner Marble, 1232 ± 6 Ma (previously 1251 ± 46 Ma and 1260 ± 20 Ma, Pittenger et al., 1994); (3) a carbonate sandstone with a youngest population at 1251 ± 16 Ma and other peaks at 1478 Ma, 1630–1717 Ma, and one grain at 2330 Ma. This sequence was intruded by the Red Bluff granite at 1124 ± 4 Ma (Howard, 2013; previously dated at 1120 ± 35 Ma, Shannon et al., 1997). Detrital zircon U-Pb ages were obtained from Mesoproterozoic sedimentary rocks in central Arizona. The Dripping Spring Quartzite is part of the Apache Group and underlies the Mescal Limestone. Maximum depositional age (MDA) for this unit is 1258 ± 19 Ma and the MDA in the overlying Troy Quartzite is 1252 ± 8 Ma. 

Williams and Amato (this meeting) dated Grenville-age sedimentary rocks that include marble and calcite-bearing metasedimentary rocks. These have MDAs of 1230 ± 11 Ma and are thus coeval with the Castner Marble. Thus we correlate the carbonates in the Franklin Mountains of Texas and Burro Mountains of New Mexico, and although they may be slightly younger than the Mescal Limestone of the Apache Group of Arizona, they probably are part of the same depositional system (possibly also including the Bass Limestone of the Grand Canyon and the Crystal Spring Formation of Death Valley interpreted to have been deposited in the back-arc of a subduction zone associated with Grenville convergence. These correlations expand on Shride (1967) who originally correlated some of these carbonate units.

References:

1. Bickford, M. et al., 2000, GSA Bulletin, 112:1134-1148.
   
2. Bright, R. et al., 2014, Lithosphere, in press.
3. Howard, A., 2013, [M.S. thesis]: U. Colorado.
   
4. Mosher, S., et al., 2008, Geology, 36: 55-58.
   
5. Pittenger, et al., 1994, J. Sedimentary Res., B64: 282-297.
   
6. Roths, P. J., 1993, GSA South Central Section Field Guide, 11-35.
   
7. Shannon, W., et al., 1997, J. Petrology, 38: 1279-1305.
   
8. Shride, A.G., 1967, U.S.G.S. Prof. Paper 566.