DEPOSITIONAL AGE AND PROVENANCE OF JADEITE-GRADE METAGRAYWACKE FROM THE FRANCISCAN ACCRETIONARY PRISM, DIABLO RANGE, CENTRAL CALIFORNIA - SHRIMP PB-ISOTOPE DATING OF DETRITAL ZIRCON

Rocks of the Sierran-Klamath magmatic arc, Great Valley forearc basin (GVG), and Franciscan accretionary prism record Mesozoic plate convergence and accretion along the western margin of North America. Monotonous stratigraphy, extreme brittle deformation, lack of fossils, and high-P, low-T metamorphism make it difficult to determine the age and spatial relationship of the Franciscan depositional basin relative to that of the adjacent GVG, juxtaposed along a complex of faults and ophiolite bodies. Zircons separated from (1) a metagraywacke-chert mélange with the assemblage, Jd + Lws + Arg + Gln + Rt (tectonite TZ-2b fabric), and (2) a coherent turbidite (broken formation), with Jd + Ab + Pl + Lws + Cal + Gln + Ttn, (immature litharenite TZ-1 texture), were dated with SHRIMP-RG. Zircons do not record the high P (7-8Kb), low T (150±50°C) metamorphism.

Detrital zircon age spectra, dominated by first cycle plutonic zircons, are bimodal with (1) 23% of mélange ages in range 118-98 Ma and 66% in range 181-134 Ma, and (2) 45% of broken formation ages in range 105-85 Ma and 29% in range 165-143 Ma. Maximum depositional ages, 98 Ma for the mélange, 85 Ma for the broken formation, are younger than adjacent fossiliferous GVG strata (Albian 112-96 Ma). Franciscan age spectra are distinguished from those of GVG rocks from the Diablo Range (Surpless et al, 2002 GSAB) by (1) peaks at 92 and 102 Ma in the broken formation and 102 and 115 Ma in the mélange that are recorded only in the youngest (Campanian) GVG sample, (2) absence of the 120 Ma peak that dominates spectra of Cenomanian and older GVG samples, and (3) a far greater fraction of ages >140 Ma than in any GVG sample from the Diablo Range.

Franciscan age spectra are broadly similar to the pattern of apparent magmatic flux for the Sierra Nevada batholith (Ducea, 2001 GSA Today) characterized by two brief episodes of voluminous magmatism at 100-85 Ma and 160-150 Ma. Both spectra have a gap with no ages in range 134-118 Ma (mélange), and 143-105 Ma (broken formation) corresponding to the minimum in Sierran magmatic flux at 150-115 Ma. While Franciscan cherts and pillow basalts are exotic to the clastic sediments in which they now occur, provenance of Franciscan zircons in the Sierra Nevada Batholith precludes long distance lateral transport of the accretionary complex.