LEAD ISOTOPE CONSTRAINTS ON A TECTONIC RECONSTRUCTION OF SOUTHERN NEVADA

New Pb isotope data from clasts of 1.45 Ga rapakivi granite in Miocene avalanche breccia at Frenchman Mountain in southern Nevada are remarkably similar to data obtained from their long-accepted source at Gold Butte, 60 km to the east. Samples were crushed and sieved, and feldspar picked from the non-magnetic fraction (210-750 μm). Pb isotope composition was measured by MC-ICP-MS (Multi-Collector Inductively Coupled Plasma Mass Spectrometry), with precisions of 0.03% for both ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb. Average Pb concentrations in feldspar are high (50 ppm), while U and Th are excluded from the crystal lattice. Thus feldspar records the initial Pb isotope composition of the host. Published studies of regional Pb isotope variability were based on whole rock analysis by TIMS (Thermal Ionization Mass Spectrometry), with precisions of ~ 0.15% for ²⁰⁷Pb/²⁰⁴Pb and 0.1% for ²⁰⁶Pb/²⁰⁴Pb. In order to make comparisons with those data, which are distributed along a 1.45 Ga isochron on a Pb-Pb plot, we calculated delta-207, following the procedure of Wooden and DeWitt (1991, Arizona Geol. Soc. Digest, 19, 27-50).

Our results are surprising for three reasons. 1) Extension-related tilting at Frenchman Mountain (from 12-9 Ma) postdates rapid exhumation of Gold Butte (the assumed footwall; 17-14 Ma). 2) New ⁴⁰Ar/³⁹Ar dating of clasts of two-mica granite from the Miocene at Frenchman Mountain indicates exhumation prior to 1.1 Ga (muscovite and biotite ages). The previously assumed source of the clasts at Gold Butte is of Cretaceous age (68-66 Ma). 3) A recent re-evaluation of the crustal structure in a nearby transect from the Meadow Valley Mountains, Nevada to the Beaver Dam Mountains, Utah implies only ~ 20 km of extension (C.D. Walker, Ph.D., 2008), compared with the generally accepted figure of 54 ± 10 km (McQuarrie and Wernicke, 2005, Geosphere, 1, 147-172). Two interpretations of the new data are suggested. 1) The rapakivi granite clasts were derived from Gold Butte, consistent with 65 ± 15 km of west-southwest tectonic transport of Frenchman Mountain (McQuarrie and Wernicke, 2005). 2) Regional Pb isotope variations in southern Nevada are sufficiently small that sources other than Gold Butte are possible. Additional analyses are being undertaken.