Abstract

The origin of jadeite-bearing metagray-wackes from the Franciscan Formation of California and their relationships to the blue amphibole-rich “blueschists” are discussed. Owing to their unfavorable chemical composition the regionally prevalent graywackes seldom develop into blueschists, and most blueschists appear to have originated from mafic igneous rocks and retrograde eclogites. The formation of blueschists from graywackes near igneous intiusive rocks has been examined, and metasomatism preceding the metamorphism is considered to be a significant factor in their origin.

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