PRE-10 MA TIMS U-PB ZIRCON AGE FOR THE SMITH MOUNTAIN GRANITE, DEATH VALLEY, CALIFORNIA: IMPLICATIONS FOR TIMING OF MAJOR EXTENSION

The Smith Mountain Granite (SMG) underlies an area of approximately 40 km2 in the central Black Mountains of Death Valley and intrudes the ductile shear zone of the Mormon Point turtleback, one of the keys to interpretations of regional tectonics. Similar rocks that underlie much of the area to the north in the Black Mountains are likely a part of the same intrusive suite. Until now, the crystallization age of the SMG has been controversial: Holm et al., 1992 reported an age of 8.69±0.31 Ma (Ar-Ar hbl) while Pavlis (in Holm, 1994) reported an age of 10.85±0.04 Ma (Ar-Ar orthoclase). We report a TIMS U-Pb zircon age of 10.45±0.22 Ma; this concordant U-Pb zircon age reliably records SMG crystallization.

SMG zircons are clear and vivid pink, with euhedral, stubby to elongate morphologies. Three multi-grain zircon fractions of 5-10 grains each were analysed. To avoid inherited cores, only elongate grains, each containing a long, length-parallel tube, were selected for analysis. Results are concordant and overlapping; an age 10.45±0.22 Ma is based on the 206Pb/238U dates for the two most precise analyses. The data also yield a Ludwig concordia age of 10.45±0.11 Ma (2s errors, decay constant errors included, MSWD=0.26, probability of concordance=0.61).

This age is significant because it indicates that the most voluminous magmatism in the Black Mountains pre-dated 10 Ma. It is also suggestive of a protracted period of intrusion and crystallization for the SMG. Co-mingling textures and mutually cross-cutting relations with the 11 Ma Willow Spring pluton reported by Pavlis (in Holm, 1994) indicate that at least part of the SMG is as old as 11 Ma. Because the SMG intrudes the Mormon Point turtleback, this age also supports the suggestion by Miller and Pavlis (2004) that most of the crustal extension recorded by the turtleback shear zones occurred prior to 11 Ma, and that subsequent pluton emplacement recrystallized fabrics and reset Ar-Ar ages in the turtleback shear zones. This age conflicts with the Rolling Hinge model for Death Valley that requires hinge migration at Mormon Point after 8 Ma and NW directed translation of the 8 Ma Shoshone Volcanics.