RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 2, NO. 3, PAGES 269–278, doi:10.2205/2000ES000044, 2000

The age of magmatic and metamorphic events in the Mid-Atlantic Ridge: interpretation of K-Ar isotope dating data

¹GEOKHI RAS, Moscow, Russia
²Institute of geology and geochronology RAS, Sankt-Peterburg, Russia
³Institute of geology of deposits, petrography and mineralogy RAS, Moscow, Russia
⁴IFREMER, Brest, France
⁵Universite Pierre et Marie Curie (Paris 6), Paris, France

Abstract

[1]  The object of this study was gneissose gabbro from the plutonic rocks composing the opposite sides of the rift valley of the MAR segment located directly north of the 15^o 20 fracture zone. The results of our K-Ar dating suggested the potential age asymmetry of the similar rocks composing the opposite side of the MAR rift valley between the 15^o and 16^o of the north latitude. The age value obtained for the gabbro sample collected from the western side of the rift valley seems to reflect the time of the magmatic interaction of this gabbro with the injections of tronghjemite melt and apparently corresponds to the crystallization stase of tronghjemite vein complex. On the other hand the age defined for the gabbro sample collected from the eastern side of the rift valley probably marking the time of its crystallization in the magma system, or the time of its high-temperature crystallization during the metamorphism. The proved age asymmetry of the opposite sides of the rift valley is believed to have been unassociated with any differences in the metamorphic evolution of the gabbro, its samples showing the same degree of metamorphism. The results of this study and the synthesis of all data available for the isotopic dating of the various MAR plutonic rocks suggest the presence of two types of igneous rock complexes in the crest zone of the ridge, which do not correspond to the modern igneous activity of the rift valley: (1) the rocks localized in the segments with the Hess-type crust, associated with very low spreading rates and with the repeated melting under the mantle-like conditions of the lenses and interlayers of cumulative rocks often of abnormal geochemistry; (2) The rocks represented by the lithospheric relicts of the Gondwana supercontinent lithosphere, characterized by the isotopic ages of hundreds of million years.

Received 16 November 2000; published 5 January 2001.

Keywords: isotope dating, magmatic, metamorphic events, Mid-Atlantic Ridge, gneissose gabbro, plutonic rocks.