Metamorphism of Franciscan tectonostratigraphic assemblage, Pacheco Pass area, east-central Diablo Range, California Coast Ranges

Abstract

The Pacheco Pass area, well known for the relatively high-pressure, low-temperature mineral assemblages developed in a well-ordered section of chiefly metaclastic strata, was restudied in order to elucidate the geologic, paragenetic, and textural relationships among the various lithologic units. The mapped 7½ minute quadrangle extends from 4 km west of the range crest eastward to the Ortigalita fault, tectonic contact of the Franciscan assemblage with coeval Great Valley strata. Semicontinuous metachert beds, underlain by scraps of Na-cpx- and/or Na-amphibole-bearing metabasaltic rock, mark the base of terrigenous tectonostratigraphic units, evidently decoupled from the downgoing, oceanic-crust-capped plate. Underplating along newly recognized bedding-plane faults has juxtaposed members of an apparently conformable Franciscan sedimentary sequence. Four depositional units, each consisting of monotonous metagraywacke and interlayered metashale, are stacked within the accretionary section: from top down, these are tectonostratigraphic members A, B, C, and D. Unit D differs from overlying members in possessing a greater abundance of bluestone/greenstone pods, minor serpentinite bodies, and rare metaconglomerate lenses; varying degrees of stratal disruption indicate that it has been arrested in the transformation to broken formation. The Ortigalita fault is nearly vertical, strikes north-northwest, and exhibits apparent dextral offset; it truncates the east-west-striking Gonzaga fault and other Franciscan structures. The topographic low occupied by San Luis Reservoir is a Cenozoic tectonic depression sited at a releasing bend on the Ortigalita fault. Newly acquired textural, chemical, mineralogic, and areal relations of albitic and jadeitic pyroxene-bearing Franciscan metagraywacke near Pacheco Pass demonstrate that (1) Na-cpx and Na-amphibole are metamorphic minerals, not clastic materials; (2) pumpellyite is a minor neoblastic phase in albite-bearing metagraywacke west of Pacheco Pass, and in the north-western corner of the quadrangle, but it does not occur in jadeitic pyroxene-bearing metaclastic rocks; (3) higher textural grade metagraywacke specimens generally contain higher modal proportions of jd; (4) abundance, textural grade, and chemistry of Na-cpx are not related to tectonostratigraphic units; and (5) because of sluggish reaction rates, albite persisted metastably into the higher pressure, jadeitic pyroxene + quartz P-T field. Production of Na-cpx resulted from a reaction of the sort ab + chl = jd + Iws + qtz ± gln ± H 2 O. Irregular, oscillatory zoning within single jadeitic pyroxene prisms and variable jd/ab modes for rocks of the same bulk composition reflect growth governed by diffusion rather than recrystallization within a P-T transition zone; high-pressure overstepping of the phase boundary explains variable modal proportions of ab and jd, as well as chemical heterogeneity of the jd. Elevated P attended metamorphism of Na-cpx-bearing rocks exposed in the Pacheco Pass area, as indicated by the wide-spread coexistence of quartz and NaAlSi 2 O 6 - rich pyroxene. Inferred physical conditions of prograde metamorphism were 150 ± 50 °C at 7-8 kbar or more. Franciscan rocks in the east-central Diablo Range underwent subduction-zone metamorphism accompanying paleo-Pacific lithospheric plate descent during mid-Cretaceous convergence. Underplating, contraction, and sequestering of the tectonostratigraphic assembly at depths of 25-30 km during underflow, followed by gradual rise, took place under conditions of refrigeration due to protracted descent of the oceanic slab. Paleogene diapiric uplift and extension followed, indicating more complete decoupling of the buoyant trench complex from the downgoing plate. Finally, passage of the East Pacific Rise triple junction in Neogene time initiated the present dextral-slip regime in the California Coast Ranges.