Abstract

Field relationships, K/Ar dates, and metamorphosed fossils define two periods of high-pressure, low-temperature metamorphism in the Leech Lake Mountain-Black Butte region of the northern California Coast Ranges. For the older metamorphism, coarse white mica from glaucophane-lawsonite metabasalts, interbedded with jadeite+quartz metagraywacke, yields K/Ar dates of 150 m.y. These dates are older than the time of deposition of Tithonian through Valanginian (approximately 145 to 130 m.y.) Buchia fossils found at a number of localities in this region. These fossiliferous rocks have been metamorphosed to the lawsonite-albite-chlorite-aragonite metagraywacke facies and yield K/Ar whole-rock dates that spread from 127 to 104 m.y. The age of the younger metamorphism is thus fixed by the fossils and K/Ar dates to within a short period of time in the Lower Cretaceous. In addition, there are still-younger periods of sedimentation, metamorphism, and deformation in the Franciscan terrain.

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