The Origin of Plagiogranites: Coupled SIMS O Isotope Ratios, U-Pb Dating and Trace Element Composition of Zircon from the Troodos Ophiolite, Cyprus

U-Pb ages, trace element content and oxygen isotope ratios of single zircons from five plagiogranite intrusions of the Troodos ophiolite were measured to determine their crystallization age and assess the importance of fractional crystallization versus crustal anatexis in their petrogenesis. The results indicate that oceanic magmatism in Troodos took place at 94.3 +/- 0.5 Ma, about 3 Myr earlier than previously recognized. Later hydrothermal alteration has affected most of the Troodos plagiogranitic rocks, resulting in growth of new zircon and/or partial alteration of zircon domains, causing slightly younger apparent crystallization ages. The new age inferred for seafloor spreading and ocean crust accretion in Troodos nearly overlaps that of the Semail ophiolite in Oman (95-96 Ma), strengthening previous indications for simultaneous evolution of both ophiolites in similar tectonic settings. Average delta O-18(Zrn) values in the Troodos plagiogranites range between 4.2 and 4.8 parts per thousand. The lower values in this range are lower than those expected in equilibrium with mantle-derived melt (5.3 +/- 0.6 parts per thousand), indicating variable contribution from hydrothermally altered, deep-seated oceanic crust in most of the Troodos plagiogranite intrusions. The inferred substantial involvement of crustal component is consistent with the existence of a shallow axial magma chamber, typical of fast-spreading mid-ocean ridge settings, within the Troodos slow-spreading ridge environment. This apparent contradiction may be reconciled by episodically intense magmatism within an otherwise slow, magmatically deprived spreading axis.