GENESIS OF PERIDOTITES FROM THE OPHIOLITE COMPLEX OF MACQUARIE ISLAND, SOUTHWESTERN PACIFIC-OCEAN

Characteristic features of the mineral composition of peridotites from the ophiolite complex of Macquarie Island were studied for the first time. Four genetic rock types were distinguished on the basis of petro graphic and petrologic investigations: (1) spinel harzburgites; (2) pl agioclase-spinel harzburgites; (3) cumulative rocks of the dunite-wehr lite-troctolite series; and (4) cumulus olivine orthopyroxenites. The first three types are regarded as a complex formed in an oceanic sprea ding center with the plagioclase-spinel harzburgites being probably pr oducts of contact metasomatic alteration of spinel harzburgites under the influence of melt or related magmatic fluid. Rocks with similar mi neralogical characteristics are widespread in mid-ocean ridges. The sp ecific features of the mineral composition of the olivine orthopyroxen ites preclude their formation in an oceanic spreading center and their provenance is related to the intraplate basaltic magmatism of oceanic plateaus, which is common in the western Pacific Ocean. The character of subsolidus metamorphism in the peridotites and the occurrence of h igh-temperature amphibolite metamorphism in some of the spinel harzbur gites and olivine orthopyroxenites are comparable to the metamorphic p arameters of oceanic peridotites in large transform faults.