Metamorphism in the Mid-Atlantic Ridge near 24° and 30° N

Abstract

Metabasites (metabasalts and metagabbros) occur abundantly in association with serpentinites in transverse fracture zones and on walls of the median valley. These metabasites were formed by burial metamorphism probably in deeper parts of the crust and the upper mantle beneath the Ridge crest, and were brought up to the surface of the crust probably by serpentinites rising along fracture zones and by normal faulting along the median valley. The metabasalts are in the zeolite and greenschist facies and a transitional state from the greenschist to the amphibolite facies, whereas metagabbros tend to have been recrystallized at higher temperatures, being in the greenschist and amphibolite facies. Compositionally the metabasites are divided into two groups, I and II. Group I comprises those which retain the approximate composition of the original rocks except for water content, whereas group II comprises those which underwent marked chemical migration, as regards sodium in zeolite-facies rocks and calcium and silicon in greenschist-facies rocks. In rocks of group I, calcic igneous plagioclase remains unaltered, and albite and epidote did not form. This fact, along with the absence of epidote-amphibolite facies rocks, would be due to the low rockpressure during metamorphism. In some rocks of group II, albite and epidote occur. Burial metamorphism takes place probably mainly beneath the Ridge crest where the geothermal gradient is great. The resultant metamorphic rocks are probably of the low-pressure type, and move laterally by ocean-floor spreading to form the main part of the oceanic crust. Contact metamorphic gneisses, probably derived from gabbros, have been found. Some metagabbros were subjected to cataclasis by fault movements along fracture zones and the median valley.