Low-temperature recrystallization of Franciscan greywackes from Pacheco Pass, California.

Low-T metamorphism of the Franciscan complex at Pacheco Pass occurred at P-T conditions near the so-called jadeite isograd. In a W-E traverse, four distinct prograde metamorphic zones are defined: 1) albite-quartz zone, 2) albite-rich, albite-quartz-clinopyroxene assemblage, 3) clinopyroxene-rich, albite-quartz-clinopyroxene assemblage and 4) clinopyroxene zone. Metamorphic pyroxenes are restricted to zones I-IV in metagreywackes, but metabasites contain augite-rich pyroxenes even in zone I. With increasing grade, the compositions of pyroxenes in metagreywackes change from virtually Jd₁₀₀ in zone II to approx Jd₆₀ in zone III. The metamorphic T is estimated to have been approx 170oC in zones I and II and approx 230oC in zones II and III; P estimates range 4.5-8 kbar. A gently folded thermal structure for the Cretaceous subduction metamorphism is postulated for this area.-R.A.H.