Melt generation and melt segregation at oceanic spreading centres: constraints from the Troodos ophiolite, Cyprus

Though a great deal is understood about the construction of the upper oceanic crust at spreading centers, the processes accreting the lower oceanic crust remain more enigmatic. The Troodos ophiolite, Cyprus, is one of the best-preserved and best-studied fragments of oceanic lithosphere in the world. It is regarded as having formed above a subduction zone in the Cretaceous, and the range of extrusive liquid compositions observed is much greater than MORB. Current models explaining the accretion of the Troodos lower crust are conflicting, and propose either one, or several small magma chambers. Borehole CY-4 is a unique 2263 m long complete section through sheeted dykes and mafic and ultramafic cumulates of the Troodos lower crust. In the most extensive geochemical study to date, it has been interpreted as intersecting an axial magma chamber that was intruded by an off-axis pluton (Thy et al., 1989; Malpas et al., 1989). This model is based solely on a "stepped" increase in clinopyroxene titanium content at 1331 m deep. The aim of our investigation is to use the broad range in geochemical variations from this SSZ ophiolite to better constrain the number of discrete magma bodies within the CY-4 section and the Troodos lower crust overall, and to link plutonic with extrusive units. We have acquired XRF, solution ICP-MS, electron microprobe and LA-ICP-MS data for over one hundred CY-4 samples. Preliminary results from this, the first comprehensive major element, REE, LILE, HFSE dataset of the Troodos plutonic rocks, combined with the data of Coogan et al. (2003), indicate a broader range of magma compositions than previously recorded. This range is also broader than the extrusive liquid compositions, with implications for magma homogenization processes within spreading ridge magma chambers.