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Abstract

The Clear Lake Volcanics (~2.1 to 0.01 Ma) are the youngest of a series of volcanic fields in the nothern Coast Ranges of California whose ages roughly parallel the timing of northerly migration of the Mendocino triple junction (Dickinson and Snyder, 1979; Johnson and O’Neil, 1984; Fox et al., 1985). There is compelling chemical, isotopic, and textural evidence that partial melting of crustal rocks played a major role in magmatism at Clear Lake (Futa et al.,1981; Hearn et al., 1981; Johnson and O’Neil, 1984). Physical evidence includes the presence of high-grade metamorphic xenoliths in many basaltic to andesitic lavas of the area (Brice, 1953; Hearn et al., 1981). The xenolith suite is dominated by high-T, low-P granulite facies schist and gneiss, but also includes mafic igneous inclusions interpreted as cognate with the Clear Lake Volcanics (Stimac, 1991). Together, this suite is thought to represent fragments of mid- to lower-crustal mafic intrusions and their contact aureoles (Stimac et al., 1992a).

Exposed basement rocks in the Clear Lake area include the Late Jurassic to Late Cretaceous Franciscan Complex (FC), the Middle Jurassic Coast Range ophiolite (CRO), and the Late Jurassic to Early Cretaceous Great Valley sequence (GVS) (McLaughlin, 1981; McLaughlin and Ohlin, 1984). The metamorphic xenoliths do not resemble exposed graywackes of the FC and GVS, although they could represent the residuum of partial melting and recrystallization of graywacke and intermediate to mafic igneous rocks of the FC or GVS. This indicates that either the exposed country rocks are metamorphosed to higher

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