[1] The transformation of subducted and tectonically buried crustal rocks to denser eclogite plays a fundamental role in the dynamics of mountain building and crustal recycling. However, a complex of partially eclogitized granulites on the island of Holsnøy in the western part of the Norwegian Caledonides reveals that such densification depends in part on the antecedent history of rock masses and that large bodies of untransformed rock can persist metastably even after long residence times in the lower crust. The rocks on Holsnøy suggest that conversion to eclogite was delayed until lower or subcrustal seismic events allowed aqueous fluids to enter the previously dry and unusually strong granulite complex. Metamorphism then occurred in a spatially heterogeneous manner, with several distinct physical processes involving feedbacks between deformation, fluid infiltration, and chemical reactions operating simultaneously in different parts of the rock mass. Field relations show that the introduction of fluids and conversion to eclogite significantly weakened the rocks. A reservoir-flux systems model is used to represent the various processes contributing to eclogitization of the granulites, which operated at vastly different rates over disparate intervals of time. The model suggests that the metamorphic process may have been brief ((1 Myr), and ultimately self-limiting, arrested by the change in rheology associated with the conversion of brittle granulite to ductile eclogite.

An Upper Proterozoic unconformity in northern Wedel Jarlsberg Land, southwest Spitsbergen: Lithostratigraphy and tectonic implications M. BJORNERUD BjOrnerud, M. 1990: An Upper Proterozoic unconformity in northern Wedel Jarlsberg Land, southwest Spitsbergen: Lithostratigraphy and tectonic implications. Polar Resenrch 8 , 127-139.Recent field studies of Upper Proterozoic rocks in northern Wcdel Jarlshcrg Land, southwest Spitshergen. have shed new light on thc prc-Caledonian evolution of the region. A regional angular unconformity divides the greenschist-facies metasedimentary rocks into two distinct tectono-slratigraphic sequenees. Thc sub-unconformity (Nordbukta) sequence, exposed in the southwestern part of the study area, con5ists mainly of quartzites, phyllitcs and dolomites, and may be correlative with Proterozoic rocks exposed ca\t of Recherchebreen (Magnethegda scqucnce) and south of Torellbreeii (Deikggd scqucncc). The Nordhukta sequence was affected by l a r g e -~l e recumbent folding during late (?) Proterozoic tectonism. Strata above the unconformity (Dundcrbukta-Recherchefjorden sequence) include conglomerates, dolomites. green and black phyllites, meta-basalts and Vendian (?) diamictites, with laterally complex dcpiisitional relationships. The continuation of this sequence south of Torellbreen is the Sofiebogen Group in the Hornsund arca. The apparent continuity of both sub-and supra-unconformity Proterozoic rocks across Rcchcrchehreen and Torellbreen is not compatiblc with the hypothesis that a major late Dcvonian strikcslip terranc boundary lies beneath these glaciers. Unioersity, Oxford. Ohio 45056, U.S.A.; Mu.y 1990 (reoiserf August 1990. M . Bj@merud, Geology Depnrtment, Miami

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Subduction is the principal mechanism by which the hydrosphere and interior of Earth interact. Today, subduction involves the dehydration of ocean crust at depths of 60-120 km depending on the age of the slab. Release of the water leads to generation of arc magmas (future continental crust), and the slab is then transformed into denser eclogite that helps to pull more of the slab into the trench. However, it is unlikely that the first continental crust formed this way. Growing geochemical evidence indicates that large volumes of continental crust were produced over a short period of time in the Archean, when the planet was probably too hot for modern plate tectonics to operate. A significant increase in the kinetics of eclogite-forming reactions may have been the key to the transition from Archean to modern tectonics. Under the higher geothermal gradients of the Archean, tectonically buried ocean crust would have been severely dehydrated before reaching eclogite facies pressures. Because rapid eclogitization is dependent on water as a medium for advective ion transport, the very shallow dehydration in the Archean may have inhibited the formation of eclogite facies minerals. The importance of water in eclogite metamorphism is illustrated by a complex of partly eclogitized mafic granulites in Holsnøy, western Norway, in which reaction progress was limited by the availability of water. When water is scarce or absent, metastable granulite facies mineral assemblages can persist at eclogite facies depths owing to the extremely slow reaction kinetics when diffusion is the only chemical transport mechanism. Such dehydrated but uneclogitized mafic crust would have been very strong and too buoyant to sink into the mantle, and it may have formed the substrate for the first continental lithosphere.

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