PROVENANCE OF MESOPROTEROZOIC SEDIMENTARY ROCKS IN THE SOUTHWESTERN UNITED STATES: EVIDENCE FROM DETRITAL MICA (40AR/39AR) AND ZIRCON (U-PB) GEOCHRONOLOGY FOR THE NATURE OF THE GRENVILLIAN FORELAND BASIN AND SUPERCONTIENT CONNECTIONS

This study demonstrates the power of using both U-Pb zircon and Ar-Ar muscovite analyses for detrital grain studies of sedimentary basins. Zircon dominantly records crystallization ages of felsic plutons; muscovite gives cooling ages (about 350° C) of unroofed crystalline terranes. Together, they faithfully reflect unroofing of felsic crytalline basement provenance, but they also record differing hydraulic behavior in the sediment transport system. We apply these techniques to Mesoproterozoic successions (1.3-1.1 Ga) from southwestern Laurentia. Detrital data suggest that Franklin Mountain, Debacca, Apache, Unkar, and Crystal Spring Groups are all correlative parts of a 1.2-1.1 Ga flexural foreland basin to the Grenville orogen. Published 1340 zircons in the Pioneer and 1250 zircons in the Castner and Bass limestones are potentially xenocrystic grains as suggested by youngest grain analysis. The foreland basin had the following history: Ar-Ar K-spar MDD modeling suggest widespread unroofing of SW Laurientia basement from 1.3 to 1.2 Ga. Following initial 1.2 Ga carbonate deposition, N-flowing continental-scale river systems deposited sediments across the foreland basin from 1.2-1.1 Ga. Detrital muscovites indicate unroofing of 1.4 to 1.1 Ga middle crustal metamorphic rocks in Grenville thrust sheets. To explain both the 2.0-1.9 and 1.6-1.5 detrital zircons and muscovite, ages that are essentially unknown for southern Laurentia, we postulate exotic southern source terranes, probably Australia and/or Amazonia within an assembling Rodinia. Diabase intrusions and basalt flows spanned approximately 30 million years (1.1-1.07 Ga); this large igneous province magmatic event was a continuation of foreland contraction as shown by paleostress analysis (NW sigma 1; vertical sigma 3) that apparently shut off foreland basin deposition at about 1.1 Ga.