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Metavolcanic rocks form an important component of the Franciscan complex in California and preserve evidence for the origin of oceanic crust during the late Mesozoic time. These rocks occur as tectonic inclusions within Franciscan mélange and, more rarely, as thrust klippen that rest on mélange. Four occurrences of Franciscan metavolcanic rock are studied here: Aliso Canyon, Avila Beach, Stonyford (Snow Mountain and Stony Creek complexes), and Paskenta. These rocks include enriched mid-ocean ridge basalt, transitional subalkaline to mildly alkaline basalt, and highly fractionated Fe-Ti basalt. They range in composition from TiO2 = 1.35 to 2.94 wt.%, La = 13 to 37 ppm, Y > 20 ppm, Ti/V = 22 to 66, and chondrite normalized La/Zr = 1.5 to 2.8. The more alkaline basalts are also high in Nb (10 to 47 ppm) and contain pyroxene that ranges from titan augite to aegerine augite. The high La/Zr ratios observed in all rocks, regardless of alkalinity, imply derivation from a similar, light element-enriched source region by varying degrees of partial melting.

Field and geochemical data indicate that these Franciscan metavolcanic rocks formed in a variety of tectonic settings. Alkaline diabase sills in Aliso Canyon that intrude thick sequences of ribbon chert suggest off-axis, intraplate volcanism. Transitional subalkaline to alkaline basalts of the Snow Mountain Complex represent sea-mount volcanism, as shown by MacPherson (1983) and others. Volcanic rocks from Paskenta and possibly Avila Beach occur as knockers in serpentinite matrix mélange and may have formed in a fracture zone setting. The Stony Creek Complex contains Fe-Ti basalts and occurs within the same serpentinite-matrix mélange as the Paskenta volcanics. The Stony Creek Complex may represent a small seamount associated with a fracture zone, or, alternatively, a propagating ridge segment.

The preservation of true oceanic crust is not, in general, favored by the geometry of subduction. Seamounts and other intraplate volcanics that are structurally detached from the underlying ocean crust may be preserved preferentially during subduction. In a similar fashion, fracture zones, aseismic ridges, and other structural discontinuities in ocean crust (e.g., the pseudofaults associated with propagating rifts) create zones of weakness that may fail during subduction, allowing laterally extensive slabs of ocean crust to be preserved within the mélange. It is not possible, however, to establish with any certainty whether these Franciscan metavolcanic rocks formed in a true ocean basin or within a back-arc/marginal basin-type setting.

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