ABSTRACT The Coast Range ophiolite (CRO), along with the overlying Great Valley Group sediments and underlying Franciscan complex, form a classic convergent margin assemblage comprising forearc basin crust (CRO) and sedimentary fill (Great Valley Group), underlain by a subduction zone accretionary complex (Franciscan assemblage). The CRO near Stonyford, California, USA, is by a unique forearc volcano underpinned by serpentine mélange and by upper mantle peridotites, which may represent the refractory residue of melting that formed the volcanic complex. This locality allows us to reconstruct a detailed history of its formation and evolution using geochemistry and 39 Ar- 40 Ar dating of the volcanic rocks, U-Pb dating of plutonic diorites associated with the volcanic complex, whole rock and mineral chemistry of the mantle peridotites, and biostratigraphic dating of chert intercalated with the volcanic rocks. This guide summarizes the geologic, age, and geochemical relations of the CRO and adjacent rocks near Stonyford, and provides locations that illustrate these relationships in the field.

ABSTRACT Late Jurassic basaltic sandstones of the basal Great Valley Group near Stonyford, California, USA, are unique to this area and are not found elsewhere in the California Coast Ranges. These basaltic sandstones are dominated by coarse volcanic detritus and are compositionally distinct from plutonic arc sources in the Sierra Nevada and Klamath Mountains, which have been proposed as sources for wackes and arkoses of the Great Valley Group elsewhere. The basaltic sandstones have abundant calcite cement and occasional limestone clasts; when corrected for secondary calcite, they have compositions that are lower in silica than typical Great Valley Group arkoses and higher in Fe, Mg, and Ca, consistent with a significant volcanic component that was generally dacitic in composition. Compositions of relict clinopyroxene are consistent with a calc-alkaline volcanic source. Overall, the chemical and mineral compositions of the basaltic sandstone unit suggest a proximal juvenile arc source. Volcanic (tuffaceous) sandstones are found as blocks in serpentinite mélange adjacent to Stonyford, structurally and stratigraphically below sediments of the Great Valley Group. Despite their proximity, these volcanic sandstones are distinct from the basaltic sandstones petrographically, with abundant lithic fragments and lower silica contents (basaltic andesite to andesite in composition). Mafic component (Fe, Mg, Ca) concentrations are similar to those in the basaltic sandstones. Compositions of relict clinopyroxene are consistent with a mixed calc-alkaline/tholeiitic volcanic source. The volcanic sandstone blocks in serpentine mélange are possibly correlative with the Crowfoot Point breccia, a volcanic conglomerate that overlies the Coast Range ophiolite farther north, which suggests derivation from the underlying ophiolite volcanics. No local source for the basaltic sandstone unit exists in the Stonyford area. The most likely source terrane is the Stonyford volcanic complex, which lies ~10 km west. However, the Stonyford volcanic complex comprises tholeiitic basalt, alkali basalt, and primitive high-Al basalts that are chemically and petrologically distinct from the basaltic sandstones. Based on the composition of the basaltic sandstone sedimentary unit and its coarse-grained juvenile character, the source appears to have been a calc-alkaline island arc that was located west of the forearc basin and proximal to the site of deposition. This is consistent with proposals for large-scale transcurrent movement of the forearc during the Late Jurassic.

This volume contains guides related to the GSA Cordilleran Section Meeting in Sacramento, California, and the Rocky Mountain Section Meeting in Provo, Utah, USA. Explore the classic forearc triad of the Franciscan subduction complex, Coast Range ophiolite, and Great Valley forearc basin, eastern California Coast Ranges, as well as the forearc volcanism and mantle peridotites of the Coast Range ophiolite in Stonyford, California. Next, learn about emergent, experimental, and established geothermal resources in Utah’s Great Basin. Then visit the Cenozoic Marysvale volcanic field, southwest Utah, to examine mega-scale gravity slides resulting from the southward collapse of the field.

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 TiO 2 = 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.