THE ORIGIN OF OCEANIC PLAGIOGRANITES: COUPLED SIMS O AND U-PB ISOTOPE STUDY OF ZIRCON FROM THE TROODOS OPHIOLITE, CYPRUS

Oceanic plagiogranites have been suggested to form either by extreme fractional crystallization of mantle-derived mafic melts or by partial melting of hydrothermally-altered mafic crust. The uniform mantle-like δ¹⁸O(Zrn) of the modern lower oceanic crust (Grimes et al., 2011) could be viewed to favor closed system magma differentiation, however some ophiolites bear lower than mantle δ¹⁸O(Zrn) values, indicating a certain contribution of altered crust. In the Troodos ophiolite, Cyprus, plagiogranites are well exposed at the sheeted dike-gabbro transition zone and their chemical compositions extend along crystal fractionation trends of both tholeiitic and boninitic lava series (Freund et al., 2014). Crustal anatexis is evident, however, by the occurrence of migmatites at the roof of a gabbro intrusion (Gillis and Coogan, 2002). Here we report U-Pb ages, trace element contents and oxygen isotope ratios of individual zircons from five plagiogranite intrusions of the Troodos ophiolite using Secondary Ion Mass-Spectrometry (SIMS). The high spatial resolution of SIMS, 10 and 20 mm-diameter pits for oxygen and U-Pb isotope analysis, respectively, allows targeting pristine magmatic domains in oceanic zircons, thus assuring that the measured isotope ratios represent original magma composition and time of crystallization. Our results show that the main phase of tholeiitic magmatism in Troodos took place at 94±1 Ma, about 3 Myr earlier than previously thought. An earlier phase of oceanic magmatism at 102±1 Ma is suggested by the occurrence of zircon xenocrysts of that age with clear oceanic affinity. Boninitic magmatism shortly followed the main tholeiitic phase, starting at 92±1 Ma. Average δ¹⁸O(Zrn) values in the Troodos plagiogranites range from 4.2 to 4.8‰. The lower values in this range are significantly lower than those expected in equilibrium with mantle-derived melt (4.7-5.9‰), indicating at least 10 – 25% contribution of hydrothermally altered diabase (δ¹⁸O=3‰). Plagiogranites thus form by mixing of fractionated mantle-derived and crustal melts. Remelting of ~10Myr old oceanic crust as indicated by xenocryst bearing low-δ¹⁸O plagiogranite magma suggests that the Troodos ocean spreading center was longer-lived than previously thought, and its history included periods of slow amagmatic spreading.