Blueschists and Eclogites of the World and Their Exhumation

Abstract

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 serpentinized plagioclase- or spinel peridotite in the B type. The A type may or may not be accompanied by a poorly developed calc-alkaline magmatic arc, whereas most B-type belts are associated with well-developed arcs and related low-P/T metamorphic rocks.

Both A- and B-type high-P/T belts have similar modes of occurrence and occur as a <2-km (B type) or <10-km (A type) thin slab, bounded on the top by a normal fault and on the bottom by a reverse fault. Large pressure gaps occur along the paired faults. A “wedge extrusion” model was proposed for exhumation resulting from delamination (slab breakoff) of the subducted slab for the A type and shallowing angle of subduction upon approach to a mid-oceanic ridge for the B type. Modern analogues of A-type blueschists occur at the Timor-Tanimbar-Seram forearc north of Australia, and of the B-type blueschists on the Olympic Peninsula of Washington.

The oldest blueschists, about 700 to 800 Ma in age, have been documented from the Pan-African orogen in Africa, in India, and along the NW margin of the Tarim craton, western China. These occurrences indicate that, by Late Proterozoic time, subduction of lithospheric plates was able to refrigerate the hanging wall of a subduction complex to create and preserve blueschist and eclogite, reflecting a cooling Earth. There may be a drastic change of P/T conditions at 700 to 800 Ma, after which abnormally high-P/T metamorphism began.

The total length of blueschist belts versus geologic time shows three peaked periods of blueschist/eclogite formation, at 80 to 130 Ma, 400 to 500 Ma, and 700 Ma. The peak heights decrease exponentially with increasing age, indicating a cooling trend of the Earth. Such an age-total length relationship correlates well with the time of sea-level change, at least in the Phanerozoic—i.e., worldwide major periods of transgression correspond to extensive blueschist/eclogite facies events and those of worldwide major regression to less active periods of blueschist/eclogite formation. This relationship suggests that more rapid ocean-floor spreading—and hence higher subduction rates (or higher frequency of RTT migration)—tend to favor formation and exhumation of blueschist/eclogite belts. The periodic pattern of blueschist/ eclogite formation is similar to that of ophiolite, and is roughly analogous to that of granite formation in North America. These similar patterns support speculation concerning the control of periodic blueschist formation by the Wilson plate-tectonic cycle.