ALLOCHEMICAL METAMORPHISM OF MANTLE PERIDOTITES IN THE HAYES FRACTURE-ZONE OF THE NORTH-ATLANTIC

Data on the first peridotite samples dredged from the southern flank o f the Hayes Fracture Zone are presented. Judging from the composition of the primary spinel (Cr# = 0.32-0.36) and the composition of the roc ks in terms of elements immobile during metamorphic recrystallization (Cr, V, and Ti), the protolith of the metaperidotites was clinopyroxen e-bearing mantle harzburgite, which is similar to those dredged south of the area (within the Atlantis Fracture Zone) and differs from the e xtremely depleted mantle harzburgites that are widespread to the north of the area. This makes it possible to regard the Hayes Fracture Zone as the southern boundary of the influence area of ''a mantle source o f the Azorean type'' which is consistent with data available on the ge ochemistry of basalts. The compositions of some of the metamorphic min erals (amphibole, talc, chlorite, and ferrite-chromite) led us to conc lude that the rocks underwent a few stages of retrograde metamorphism: the earliest stage (550-650 degrees C) proceeded in a homogeneous thi ckness of peridotite, while the second stage (450-500 degrees C) occur red after the peridotites had been tectonically juxtaposed with gabbro ic rocks. During the youngest stage of metamorphic recrystallization, the rocks were locally serpentinized. It was in the second stage that the peridotites were recrystallized most fully and were simultaneously partly replaced by talc and amphibole. The strongly allochemical natu re of this process (enrichment in Si, Fe, and Zn) makes it different f rom the typical oceanic metamorphism of the peridotites and somewhat s imilar to the contact-metasomatic recrystallization of peridotites in ophiolite massifs. Distinctive features of the recrystallization of th e rocks allow us to regard them as the products of specific oceanic me tamorphism: contact metasomatism along the tectonic contact between pe ridotites and basites under the effect of circulating crustal fluids. Metamorphism of the Hayes type differs from the other type of oceanic contact metasomatism-rodingitization-by the development of different m ineral assemblages at higher temperatures.