DETRITAL ZIRCON AND MAGMATIC U-PB GEOCHRONOLOGY OF MESOPROTEROZOIC BASEMENT UNITS, WEST TEXAS, U.S.A.: IMPLICATIONS FOR GRENVILLIAN FORELAND BASIN EVOLUTION ALONG THE GRENVILLE FRONT TECTONIC ZONE

The Grenville orogen lacks a foreland basin along most of its exposed length. Using U-Pb detrital zircon (DZ) geochronology, we explore the exhumation history of the Grenville orogen from its only two foreland basin sequences: the southwest U.S. and the buried midcontinent. Trends of the Grenville Front Tectonic Zone (GFTZ), the cratonward limit of Grenvillian crustal reworking, extends to the Van Horn region, west Texas. The Lanoria and Hazel Fms. are clastic sequences deposited just west of the GFTZ and are interpreted as potential foreland basin deposits. The Thunderbird Group rhyolite (1115 + 2 Ma) overlies, and the Red Bluff Granite (1107 + 2 Ma) intrudes the Lanoria. DZ geochronology of Lanoria quartz arenite and siltstone, and Hazel feldspathic litharenite, provide the following constraints on potential Grenville orogen and foreland basin interaction. (1) The depositional age of the Lanoria is 1115-1107 Ma; the maximum depositional age of Hazel is ~1040 Ma. (2) The Hazel was deposited at least 70 m.y. after the Lanoria and is a completely different lithology. The Hazel and Lanoria are not correlative foreland sequences. (3) Based on depositional age, the Lanoria is probably a Shawinigan foreland basin deposit with a major sediment component derived from the eastern midcontinent, sampling Shawinigan (Geon 11), Granite-Rhyolite (Geon 14), Mazatzal (Geon 16), and Yavapai (Geon 17) Provinces. (4) The Hazel is lithologically and geochronologically correlated with the Middle Run Fm. in the buried midcontinent. (5) Both the Middle Run and Hazel reflect an actively unroofing Grenville orogen as represented by crustal components corresponding to Shawinigan, Elzevirian (Geon 12), and pre-Elzevirian (Geon 13) phases of the Grenville Orogenic Cycle, with significant Granite-Rhyolite Province input. (6) The Thunderbird Group and Red Bluff Granite ages are contemporaneous with the early Midcontinent Rift DZ age mode in the Middle Run (1117-1099 Ma). (7) The presence of only a minor Ottawan (Geon 10) DZ age component in the Hazel and Middle Run indicates that deeper Ottawan crust in the Grenville orogen had not yet been significantly exhumed by ca. 980 Ma. The Van Horn region, like the buried midcontinent, constitutes a section of the Grenvillian clastic wedge that is more proximal to the GFTZ than any other region of Laurentia.