Abstract
Examination of the thermobaric structure of jadeite-glaucophane-bearing Franciscan rocks in the Pacheco Pass region of the Diablo Range show that they are subhorizontal, and the structurally intermediate horizon has the highest metamorphic grade. Mineral parageneses of more than 700 metagraywackes, plus 38 metagraywacke samples from up to 1200 vertical feet of drill core were examined. Modal compositions of about 100 metagraywackes were determined, and more than 2600 microprobe analyses were completed to quantify compositions of jadeitic pyroxene from about 80 metagraywackes. The jadeite (Jd) zone is defined by the assemblage jadeitic clinopyroxene + quartz $\pm$ albite + lawsonite $\pm$ sodic amphibole $\pm$ aragonite. This zone occurs at an intermediate structural position, sandwiched between subhorizontal jadeitic clinopyroxene-free albite (Ab) zones. Transition zones occur both above and below the Jd zone. The Jd zone covers an area >20 km wide and <400 m thick. The nearly flat-lying boundaries betwee...
Published Version
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