Paper No. 11
Presentation Time: 3:55 PM
HIGH-PRECISION U-PB BADDELEYITE DATES FROM DIABASE IN THE SOUTHWEST U.S.: IMPLICATIONS OF COEVAL SILICIC AND MAFIC MAGMATISM IN A MESOPROTEROZOIC LARGE IGNEOUS PROVINCE
Voluminous Mesoproterozoic (~1.1 Ga) magmatism in the SW U.S. and Mexico includes diabase sills, dikes, and sheets, gabbro intrusions, and silicic plutons. The region extends from SE California to the Texas Panhandle and likely includes the Pikes Peak batholith in Colorado. Samples have SiO2 from 47–50 wt.%, Mg# (molar) from 69–83, and εNd from +4.7 to -1.4. Previous efforts at dating the mafic components have been hampered by difficulties in obtaining baddeleyite from diabase and the slope of concordia in this age range, which makes evaluating Pb-loss challenging. We have obtained four high-precision U-Pb baddeleyite dates on diabase in Arizona using TIMS, and two low-precision zircon dates (SIMS and LAICPMS) on diabase from New Mexico, with inherited zircons indicating crustal contamination. Baddeleyite analyses yield a narrow spread of concordant ages. We use the weighted mean of the oldest overlapping analyses on concordia but some Pb-loss may have occurred. These ages (±2σ) are: 1094 ± 2 Ma (Sierra Ancha), 1088 ± 3 Ma (Hualapai Mtns.), 1080 ± 3 Ma (Pinaleño Mtns.), and 1080 ± 2 Ma (Salt River). Zircon dates from southern New Mexico localities are ~1.1 Ga and place this magmatism in the same event. Most previously published ages that are as old as 1163 Ma and as young as 1040 Ma do not accurately date this event. Previous zircon ages from silicic rocks range from 1104–1070 Ma. We suggest that the silicic and mafic rocks are part of a large igneous province (LIP) that covers an area of at least 750,000 km2 or 1,500,000 km2 if Pikes Peak magmatism is included. We evaluate three models for LIP formation that all involve mantle-derived mafic magma causing partial melting of the crust to form silicic magma. These include (1) post-Grenville lithospheric delamination occurring in response to right-lateral shearing; (2) a mantle plume occurring coeval with the Keweenawan plume; or (3) a single plume splitting into two plume heads in response to topography at the base of the lithosphere, one creating Mid-Continent Rift magmatism and another creating the SW U.S. province magmatism. The ages do not support formation of the SW U.S. LIP from extension inboard of a continental collision during the Grenville orogeny, as magmatism largely post-dates the Grenville collision in the southwest U.S.