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Clastic lawsonite, glaucophane, and jadeitic pyroxene in Franciscan metagraywackes from the Diablo Range, California

¹ Geology Department, Auckland University, Auckland, New Zealand
² Joint Mineral Sciences Research Laboratory, Victoria University of Wellington, Wellington, New Zealand

Textural relationships and mineral chemistry identify a detrital origin for jadeitic pyroxene and some lawsonite and glaucophane in Franciscan metagraywackes in the Diablo Range. These clasts differ from the neometamorphic (post-depositional) assemblage by having larger grain sizes and deformational features, including bending, cracking, and optical strain polarization. Large strain-polarized jadeitic pyroxenes contain inclusions of strained lawsonite and glaucophane, but none of these minerals has grown across clast/matrix boundaries. By comparison, neometamorphic lawsonite forms small unstrained tablets crossing the margins of large clasts and is associated with quartz + albite + chlorite + white mica in recrystallized metagraywacke matrices. Similarly, authigenic glaucophane forms discrete small prisms or euhedral outgrowths on large clasts of sodic amphibole. Jadeitic clasts in contact with metagraywacke matrices may be deeply embayed and show marginal recrystallization to fine-grained mixtures of albite + chlorite + quartz or internally to pumpellyite + albite.

Backscatter electron imaging and microprobe analyses consistently show that the pyroxenes have extremely variable Jd contents. Within a single clast, compositional heterogeneity can be as much as Jd₅₀Ac₃₉-Aug₁₁ to Jd₉₅Ac₄Aug₁. Because their texture, habit, and chemical variability are similar to those of metasomatic jadeites from tectonic inclusions in serpentinite at New Idria, southern Diablo Range, or to those of some jadeite-quartz associations in extensively recrystallized metabasite knockers, such a provenance is inferred for pyroxene clasts in the Franciscan metasediments. In metaconglomerates, pebbles containing pyroxenes (Jd_59-98) formed original clasts that were overprinted later by the post-depositional lawsonitic metamorphism. Metabasites at Pacheco Pass have compositionally variable Na-pyroxenes (Jd_30-92) which indicate tectonic emplacement of the igneous bodies within enclosing (in some cases, pyroxenefree) metagraywackes.

The Diablo Range metagraywackes are weakly developed blueschist-facies assemblages of lawsonite + white mica + albite + chlorite + titanite ± glaucophane ± aragonite ± pumpellyite, which indicate minimum metamorphic conditions of P = 3.5-5.0 ± 0.5 kb at T = 100-200 °C.

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