PROVENANCE AND GEOCHRONOLOGY OF MESOPROTEROZOIC SEDIMENTARY ROCKS FROM ACROSS THE SOUTHWEST UNITED STATES REVEALED BY 40AR/39AR DATING OF DETRITAL MUSCOVITES

⁴⁰Ar/³⁹Ar geochronology of detrital muscovites from Mesoproterozoic sedimentary rocks of the Pahrump Group (Death Valley), the Unkar Group (Grand Canyon), the Apache Group (southeast Arizona) and the De Baca Group (New Mexico) are presented. Muscovites from the Crystal Spring Formation, Pahrump Group have a major age population between 1.6 and 1.7 Ga, which is typical of Yavapai-aged basement. The Dox Formation, Unkar Group is dominated by 1.13 to 1.25 Ga muscovites which indicate a Grenville crustal provenance. Because the Dox depositional age must be older than the overlying 1.1 Ga Cardenas Basalt, Grenville source material was exhumed, eroded and transported several hundred kilometers to the Unkar basin within ~30 Ma, and indicates the entire Dox section was deposited in a fairly short interval. Muscovites from the Pioneer Shale of the Apache Group have a large 1.4 Ga age population, which indicates that their parent rocks were 1.4 Ga granites, or older crust that was elevated above muscovite argon closure at ca. 1.4 Ga. Pioneer Shale detrital muscovites are consistent with a depositional age of 1.33 Ga, previously proposed by Stewart et al. (2001). Muscovites from the Dripping Springs Quartzite, which overlies the Pioneer Shale, shows a significant age population at about 1.2 Ga, indicating that the age of deposition of this unit must be younger than 1.2 Ga. Assuming no post-depositional argon loss, the overlying Mescal Limestone is younger than 1.2 Ga, and questions correlations with the 1.255 Ga Bass Formation, Grand Canyon. The De Baca Group muscovites, have age populations at 1.4 Ga and 1.6 Ga, which are indicative of southwest USA source areas but also contains a few crystals as young as 1.1 Ga. These critical data reveal temporally distinct patterns of changing provenance and allow general models that constrain regional correlations, regional drainage patterns and source exhumation histories for the Mesoproterozoic.