Abstract

Zircon populations from two plagiogranites in the plutonic section of the Troodos ophiolite yield U-Pb dates between 90.3 ±0.7 and 92.4 ±0.7 Ma. Zircons from one of the two samples yield U-Pb dates of 90.3 ±0.7 and 92.1 ±0.7 Ma. This difference is greater than our analytical error, and the zircons are therefore considered to be slightly discordant. Two fractions from another sample are internally and externally concordant and have an average date of 91.6 ±1.4 Ma. This date is adopted as the age of plagiogranite crystallization. The plutonic section of the Troodos ophiolite is complex in that several generations of intrusion can be identified. The plagiogranites occur at the top of the intrusive section and represent fractionated residua of magma chambers related to the lower pillow lavas. They are possibly slightly older than small ultramafic layered complexes that represent the final intrusive episode within the Troodos ophiolite. There is good agreement between our results and the ages of radiolaria which occur in umbers intercalated with the upper pillow lavas of Troodos (Blome and Irwin, 1985). This shows that the 75 to 85 Ma K-Ar dates on the lavas and sheeted dikes (Desmet et al., 1978; Delaloye and Desmet, 1979; Delaloye et al., 1980; Staudigel et al., 1986) represent age resetting unrelated to ophiolite formation.

The Troodos U-Pb zircon ages are slightly lower than U-Pb zircon ages from the Samail ophiolite in Oman, which vary between 93.5 and 97.9 Ma and average 95 Ma (Tilton et al., 1981). The small difference in age between the Troodos and the Samail ophiolites and the geochemical similarities of basalts from both complexes to island-arc volcanic rocks imply that a 3000-km axis of rift-related island arcs formed between 91 and 98 Ma. Emplacement of the ophiolites followed during the early Maastrichtian.

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