Isotopic and trace element composition of volcanic glasses from the Akaki Canyon, Cyprus: implications for the origin of the Troodos ophiolite

Abstract

We report major elements, K, Rb, Cs, Sr, Ba, Sc, Cr, Ni, Hf, Ta, Th, and REE concentrations and isotopic compositions of Sr, Nd, Pb and O of carefully handpicked volcanic glasses from the Akaki River section of the Troodos ophiolite complex. On the basis of Sr and O isotopic composition and Fe 2O 3/FeO ratios, nine of the ten glasses analyzed are considered to be fresh, even with respect to those elements that are easily affected by alteration. The glasses range in composition from basaltic andesite to dacite. Incompatible element concentrations in the mafic compositions are low (10 × chondritic) and the light REE are strongly depleted. However, the ratios of low field strength elements (K, Rb, Cs, Sr, Th) to high field strength elements (Ta, Hf, Ti) and rare earth elements are higher than those in N-MORB. Nd isotopic compositions show a very small range with ε Nd(T = 80Ma varying from +6.5 to +7.5. Initial 87Sr/ 86Sr ratios range from 0.7033 to 0.7040. Pb isotope ratios have a restricted range with 206Pb/ 204Pb= 18.60–18.70, 207Pb/ 204Pb= 15.55–15.59 , and 208Pb/ 204Pb= 38.08–38.51 . The oxygen isotopic composition for fresh glasses falls within the range reported for fresh, mafic mantle derived volcanics, with δ 18O varying from 5.4 to 6.5. All geochemical characteristics of the Troodos volcanic glasses are similar to those observed in supra-subduction zone volcanics, but we cannot distinguish between an early stage island arc or back-arc origin for the Troodos ophiolite on a geochemical basis only. However, considering our data and previously published geologic and geochemical data, we suggest that the Troodos ophiolite formed in the earliest stages of island arc development.