Thermobaric structure of the Franciscan complex in the Pacheco Pass Region, Diablo Range, California

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 ± albite + lawsonite ± sodic amphibole ± 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 between the zones are disrupted by minor secondary high-angle normal faults. Cinopyroxene of the buffered assemblage (clinopyroxene + quartz + albite) has nearly constant X_Jd (0.75-1.0) except for a few crystals with higher FeO content; those from albite-free rocks have a less jadeitic composition (X_Jd = 0.6-0.95). This difference in compositional ranges indicates that rocks may have reached equilibrium, although irregular zoning and nonequilibrium texture such as sector-zoning also have been reported. The modal ratio of Cpx/(Cpx + albite) in the transition zones increases from 0 to 0.95 toward the structural middle but remains nearly constant (0.95 to 1) in the Jd zone. Modal ratio is the most sensitive indicator of increasing metamorphic grade for the Franciscan metagraywackes. The observed thermobaric structure does not record the original metamorphic structure of the subduction zone but, rather, formed during the exhumation process of the Franciscan Complex.