Regionally metamorphosed rocks of Triassic and Jurassic age together with granitic plutons and metamorphosed ultramafic-mafic rocks form an elongate belt of approximately 300 sq mi in the northern Klamath Mountains of Oregon. The crystalline complex is named the May Creek Schist belt. Detailed mapping and petrographic studies in the northernmost part have established the pattern of isograds; metamorphism is progressive, and grade increases toward an axis which includes the locus of granitic plutons and ultramafic-mafic rocks. The pattern of isograds indicates steep thermal gradients and suggests that crystallizing granitic plutons supplied the energy for metamorphic reactions. Based on assemblages in pelitic rocks, metamorphism is interpreted as intermediate between low-pressure, regional contact metamorphism and higher pressure Barrovian-like metamorphism.

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