HR: 1330h
AN: V32C-1035    [PDF]
TI: Lu-Hf and U-Pb Chronology of a Possible Triassic Franciscan High-Grade Block, Healdsburg, California
AU: * Page, F Z
EM: fpage@umich.edu
AF: Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, MI 48109.1063 United States
AU: Mukasa, S B
EM: mukasa@umich.edu
AF: Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, MI 48109.1063 United States
AU: Essene, E J
EM: essene@umich.edu
AF: Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, MI 48109.1063 United States
AU: Carrigan, C W
EM: cwcarrig@umich.edu
AF: Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, MI 48109.1063 United States
AB: The chronology of high-grade metamorphic blocks hosted by the Franciscan Formation of California has been defined in previous studies largely by the K-Ar and Ar-Ar systems, which consistently yield 150-160 Ma on hornblende and phengite. A U-Pb age on sphene from a high-grade blueschist gives 162 $\pm$ 3 Ma. The Sm-Nd system has had mixed results for chronology in the Franciscan, but has yielded one 190 $\pm$ 10 Ma, garnet-hornblende isochron, which is significantly older than other ages. It is possible that Ar results are, in some cases, cooling ages, and that the older Sm-Nd age represents earlier, possibly pre-Franciscan, crystallization. Eclogite from Healdsburg, California contains abundant small ($<$ 25 $\mu$m) zircons that can be separated into two different textural populations. Anhedral zircon contains abundant inclusions of quartz and epidote and usually occurs as an inclusion phase in garnet or omphacite. Euhedral zircon contains no inclusions and occurs along the margins of matrix minerals. Little or no zoning in either BSE or CL imaging is visible in either population. Preliminary results of in-situ ion probe U-Pb dating of these zircons suggest that the two morphological groups have different ages. Euhedral zircons (n=5) yield a concordant age of 178.2 $\pm$ 7.2 Ma. Anhedral zircons (n=4) are reversely discordant and yield a $^{206}$Pb$^{*}$/$^{238}$U age of 218.9 $\pm$ 6.0 Ma, significantly older than most of the previously published ages for similar blocks. If these data, now being verified with additional analyses, are correct, they have significant implications regarding the initiation of the Franciscan subduction zone and the provenance of Franciscan high-grade blocks. They may also provide insight in the retentive behavior of zircon during HP metamorphism. Lu-Hf dating of the peak metamorphic assemblage of garnet, omphacite, and rutile produces isochrons that may have a higher closure temperature, and may be more resistant to resetting than the Sm-Nd system. Zircon results will be compared with Lu-Hf mineral isochron data for the Healdsburg eclogite, and the effects of a late phengite-glaucophane overprint will be evaluated. If the high-grade blocks were metamorphosed exclusively during the Franciscan event, then subduction must have begun 60-70 My before current estimates and well before the crystallization ages of Coast Range Ophiolite rocks. Alternatively, the older zircons may represent a crystallization age of a basaltic protolith that crystallized at an oceanic spreading zone in the Triassic; however, older zircons do not possess typical igneous morphology. The Healdsburg eclogite and possibly other blocks may have undergone peak metamorphism in the late Triassic, well before the onset of subduction at 150-160 Ma in the late Jurassic, and only acquired an overprint at that time.
DE: 1035 Geochronology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting