Abstract

Thermochronologic, petrologic, and geochemical analyses of epidote-amphibolite-, amphibolite-, eclogite-, and blueschist-facies blocks in serpentinite-matrix mélanges from East San Benito Island and Cedros Island, west-central Baja California, provide constraints on the P-T-t history of this disrupted terrane. Results of 40Ar/39Ar step heating experiments on minerals separated from these blocks vary according to metamorphic grade and indicate different P-T-t histories. Mid-Jurassic (160-170 Ma) epidote-amphibolite-and amphibolite-facies blocks were probably derived from oceanic crust and associated sediments that were metamorphosed during initiation of subduction. Coarse-grained blueschist blocks were likely metamorphosed during continued subduction in late Early Cretaceous time (that is, 115-100 Ma). Some epidote-amphibolite blocks are partially over-printed by blueschist-facies mineral assemblages and apparently record both metamorphic events. Fission-track analyses of apatites indicate that the blocks underwent significantly different post-metamorphic cooling histories. Epidote-amphibolite and amphibolite blocks cooled below ∼100 °C from mid-Jurassic to Paleocene time; blueschist blocks cooled below ∼100 °C in Oligocene-Miocene time. In general, mafic blocks have trace-element concentrations and REE patterns characteristic of ocean-floor basalts. This study demonstrates that samples with very different P-T-t histories can evidently occur over relatively small length scales (< <1 km) in serpentinite-matrix mélanges.

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