A tale of two margins: Thermobarometry and chronology of eclogites from North Carolina and California.

摘要

This dissertation is a contribution to the petrology, mineralogy and geochronology of eclogite from the Paleozoic eastern and Mesozoic western margins of Laurentia. Quantitative thermobarometry of inclusions in eclogite garnet from Bakersville, NC defines a shallow P/T path, implying subduction of young oceanic crust. Granulite-facies and amphibolite-facies retrogression generate a clockwise P-T loop with no evidence of metamorphism in the coesite field. Clinopyroxene cores contain oriented quartz needles intergrown with amphibole tablets, reintegration of which yields a precursor clinopyroxene with no Ca-Eskola (Ca0.5[]0.5AlSi 2O6) solid solution, used as evidence for ultrahigh pressure conditions. A reexamination of the experimental literature shows that 10--20 wt% CaEs is stable in the quartz field. Oriented quartz needles in clinopyroxene do not require a CaEs-bearing precursor or ultrahigh pressures. The Junction School eclogite (Healdsburg, CA) is a tectonic block hosted by the Franciscan formation, and contains inclusions in garnet that buffer multiple quantitative thermobarometers. Epidote-sphene-rutile-grossular-quartz-H2O barometry confirms older calibrations of the garnet-phengite-clinopyroxene barometer yielding a steep P/T prograde path in garnet cores and mantles. The ∼10 kbar discrepancy between different calibrations of the latter barometer must be addressed in future experimental studies. Some euhedral garnets preserve evidence of resorption and renewed garnet growth recording shifting fluid composition. Blueschist-facies retrograde conditions are recorded by inclusions in garnet rims and texturally late matrix garnet-phengite-clinopyroxene. Blueschist-facies prograde temperatures are in contrast with higher prograde temperatures of eclogitized garnet amphibolite, suggesting blocks were subducted during various thermal stages of Franciscan subduction. Conventional ion probe U-Pb ages (130--145 Ma) of texturally late zircon from the Junction School eclogite are similar to existing ages of blueschist-facies minerals in Franciscan blocks. This may be the first documentation of blueschist-facies zircon growth. Zircons analyzed by ion probe in thin section yield anomalously old ages, possibly as an analytical artifact. The Lu-Hf isotopic analyses of these rocks did not yield reproducible results for garnet, but suggest the presence of zircon out of isotopic equilibrium with the peak minerals. Inclusion thermobarometry and geochronometers are used to refine tectonic models of eclogite formation and uplift.