The Zircon Age of Plagiogranites and Gabbros Based on SHRIMP Data: Pleistocene Intrusion in the MAR Rift Valley, 5°30.6′-5°32.4′ N

摘要

According to the plate tectonic theory, oceanic spreading centers are considered as areas of oceanic crust generation. Therefore, the intrusive rocks in rift zones are supposed to be young formations (no more than a few million years old). At the same time, zircon isotope datings of gabbroids from the Mid-Atlantic ridge (MAR) rift valley widely range from Proterozoic to Mesozoic values. In particular, the zircon age of gabbroids from ODP Hole 153 drilled in the ridge axis, south of the Kane Fracture Zone, was determined as 330 ± 17 and 1623 ± 12 Ma. It was proposed that the ancient age of zircons in the modern spreading zone is caused by features of the magmatic process, which retained only relict zircon, or by specific tectonic conditions at the spreading center near the transform faults. The zircon age (97.42 ± 0.15 Ma) of amphibolized Ti-Fe gabbro dredged in the MAR rift valley (Markov Deep, 6° S) was considered to be the crystallization age of the rocks. The zircon age of MAR gabbroids (13° and 16° N) varies from 299 ± 0.1 to 2231 ± 0.075 Ma. This variation is attributed to the heterogeneity of the mantle source and to the contribution of Precam-brian material.