Uranium-lead isotopic ages of the Samail ophiolite, Oman, with applications to Tethyan ocean ridge tectonics.

Abstract

Plagiogranites are a minor but widespread component of the Samail ophiolite plutonic member. They crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the Techyan spreading ocean ridge, and their are thought to mark the time of ocean crust formation. Isotopic U-Pb ages of zircons from 13 plagiogranites collected along a 270km segment of the Samail ophiolite subparallel to the regional trend of the sheeted dike complex (the former spreading ridge axis direction) define a narrow time interval of 93.5-97.9Ma, with a pronounced clustering about 95Ma. The zircon ages of the plagiogranites agree remarkably well with the early Cenomanian to early Turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them (Tippit et al, 1981). The agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite U-Pb ages closely date the time span of ocean crust formation.-Authors