Deformation, Fluid-Rock Interaction, and REE Equilibration in Eclogite and Garnet Amphibolite

The intensity of fluid-rock interaction and deformation within an accretionary wedge vary over time. Both could affect trace element equilibration between slab protoliths and exotic components. We studied REE in garnet, epidote, apatite and titanite. These minerals, and 7 host samples of relatively dry, coronitic and mylonitized eclogite from the Monviso (M) and Voltri (GdV) massifs in the Alps, and of fluid-rich, less- and more-mylonitized eclogite from the Franciscan Complex (FC), California, were analyzed by LA-ICP-MS. INAA mineral and rock data for clinopyroxene-bearing and migmatitic garnet amphibolites from the Catalina Schist (CS), southern California, compare an extremely fluid-rich setting. Whole-rock-normalized plots and budgets show that these 4 minerals contain most of the REE in the rocks. Chondrite-and whole-rock-normalized REE patterns show garnet's depletion of LREE relative to HREE. The REE patterns of other minerals vary more. Most complement garnet, with LREE-richer, HREE-poorer patterns, but LREE abundances and fractionations also reflect the assemblage. For example, retrograde epidote from one dry Alpine eclogite shows the LREE of former apatite; epidote in another sample shows garnet-like LREE. Deformed samples yield similar results to coronitic ones. These features suggest closed system partitioning of REE among garnet, epidote, and apatite on the thin section scale, and preservation of protolith REE contents. Fluid-rich FC samples yield similar conclusions. In contrast, migmatitic CS garnet amphibolites, which manifest intense fluid-rock interaction, show trace element disequilibrium among garnet, titanite, and apatite. This probably reflects lack of equilibration with very REE-rich hydrothermal epidote. During high P/T metamorphism, in both fluid-poor and fluid-rich terranes (M, GdV, F), deformation neither creates nor enhances disequilibrium REE distributions amongst garnet, epidote, apatite, and titanite in eclogite. However, extremely fluid-rich migmatitic garnet amphibolites (CS) testify to both disequilibrium and bulk REE addition. Only extreme fluid-rock interaction greatly disturbed REE systematics of slab rocks from these paleosubduction zones.