High-P/T metamorphic belts were classified into types A and B according to their protoliths. The A-type (collision-type) blueschists possess passive-margin protoliths characterized by platform-type carbonates, bimodal volcanics, and peraluminous sediments. B-type (Cordilleran-type) blueschists consist of active continental-margin protoliths in an accretionary complex characterized by bedded chert, MORB and ocean-island basalts, reef limestones, and graywackes. The spatiotemporal distribution of blueschists and eclogites of the world was compiled; among 250 recognized high-P/T belts, about 20% belong to the A type and the rest to the B type. Most A-type zones lie in Europe and the Tethyan domain, include ultrahigh-pressure metamorphic terranes, and have metamorphic pressure up to 45 kbar. B-type zones occur mainly in the circum-Pacific orogenic belts and intracontinental orogens in Asia, and were recrystallized at P <12 kbar. Associated peridotites include garnet peridotite in the A type and strongly serpe...

Blueschists and Eclogites of the World and Their Exhumation

Exhumation of oceanic blueschists and eclogites in subduction zones :Timing and mechanisms

A Late Cretaceous and Cenozoic reconstruction of the Southwest Pacific region: Tectonics controlled by subduction and slab rollback processes

Phase relations of epidote-blueschists

The transfer of fluid and elements from subducting crust to the overlying mantle wedge is a fundamental process affecting arc magmatism and the chemical differentiation of the Earth. While the production of fluid by breakdown of hydrous minerals is well understood, the liberation of trace elements remains generally unconstrained. In this paper, we evaluate the behaviour of trace elements during prograde metamorphism and dehydration using samples of high-pressure, low-temperature metamorphic rocks from New Caledonia. Samples examined include mafic and pelitic rock-types that range in grade from lawsonite blueschist to eclogite facies, and represent typical lithologies of subducting crust. Under lawsonite blueschist facies conditions, the low temperatures of metamorphism inhibit equilibrium partitioning between metamorphic minerals and allow for the persistence of igneous and detrital minerals. Despite this, the most important hosts for trace-elements include lawsonite, (REE, Pb, Sr), titanite (REE, Nb, Ta), allanite (LREE, U, Th), phengite (LILE) and zircon (Zr, Hf). At epidote blueschist to eclogite facies conditions, trace-element equilibrium may be attained and epidote (REE, Sr, Th, U, Pb), garnet (HREE), rutile (Nb, Ta), phengite (LILE) and zircon (Zr, Hf) are the major trace-element hosts. Chlorite, albite, amphibole and omphacite contain very low concentrations of the investigated trace elements. The comparison of mineral trace-element data and bulk-rock data at different metamorphic grades indicates that trace elements are not liberated in significant quantities by prograde metamorphism up to eclogite facies. Combining our mineral trace-element data with established phase equilibria, we show that the trace elements considered are retained by newly-formed major and accessory minerals during mineral breakdown reactions to depths of up to 150 km. In contrast, significant volumes of fluid are released by dehydration reactions. Therefore, there is a decoupling of fluid release and trace element release in subducting slabs. We suggest that the flux of trace elements from the slab is not simply linked to mineral breakdown, but results from complex fluid-rock interactions and fluid-assisted partial melting in the slab.

Redistribution of trace elements during prograde metamorphism from lawsonite blueschist to eclogite facies; implications for deep subduction-zone processes

P. M. Black

Papers

Regional eclogite facies in the high-pressure metamorphic belt of New Caledonia