NEW HIGH-ACCURACY ZIRCON U-PB DATING METHOD: EXAMPLES FROM THE COAST RANGE OPHIOLITE (CRO) OF CALIFORNIA

Recognition that ophiolites represent oceanic crustal assemblages spurred a burst of geochronological studies (K-Ar starting in the early 1970's, zircon U-Pb in the mid 1970's). U-Pb dating of zircon from plagiogranites quickly became a favored method, and many of these early U-Pb ages are still quoted. However, most of this early work was done by methods we would regard as primitive today. A new method, CA-TIMS (e.g., Mattinson, 2003, 2005), is particularly well suited for ophiolite dating. CA-TIMS pre-treats zircons by high-temperature annealing of natural radiation damage, followed by multi-step partial dissolution analysis. The annealing of radiation damage is a crucial step, and eliminates leaching effects that have hampered earlier multi-step zircon analyses. Early dissolution steps typically remove zircon domains that have lost Pb via natural discordance processes. Later steps sample “closed-system” zircon, yielding highly precise and accurate 206Pb*/238U “plateau ages” of igneous crystallization for zircons that lack any older inheritance (as is the case for most ophiolite zircons). For example, non-magnetic and slightly magnetic zircon fractions from Point Sal ophiolite plagiogranite yield ages of 165.28 ± 0.11 Ma and 165.25 ± 0.12 Ma, respectively. Plagiogranite zircons from the San Simeon ophiolite remnant yield a very similar age at 165.46 ± 0.12 Ma, supporting a correlation between the Pt. Sal and San Simeon remnants (Hall, 1975). In contrast, other CRO remnants are younger, e.g., Del Puerto/Red Mountain at 160.85 ± 0.09 Ma, or older, e.g., Paskenta at 167.80 ± 0.13 Ma. These new, high-accuracy ages, along with an improved Jurassic timescale (e.g., Palfy et al., 2000), allow a more detailed evaluation of CRO “plate stratigraphy” following Cliff Hopson (e.g., Hopson et al., 1996). For example, the new data require a depositional hiatus of 9±5 Ma between ophiolite formation and deposition of the immediately overlying volcano-pelagic sequence at Point Sal.