THE P-T-T HISTORY OF BLOCKS IN SERPENTINITE-MATRIX MELANGE, WEST-CENTRAL BAJA-CALIFORNIA

Thermochronologic, petrologic, and geochemical analyses of epidote-amphibolite-, amphibolite-, eclogite-, and blueschist-facies blocks in serpentinite-matrix melanges 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 Ar-40/Ar-39 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 overprinted 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 approximately 100-degrees-C from mid-Jurassic to Paleocene time; blueschist blocks cooled below approximately 100-degrees-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 (< < than 1 km) in serpentinite-matrix melanges.