The evolution of Troodos ophiolite dikes and their relationship with the spreading ridge-transform fault system on Cyprus

Abstract

As windows into fossil oceanic crust, ophiolites provide unique information on processes such as crust formation, seafloor spreading and hydrothermal alteration. The Troodos ophiolite is located on Cyprus, an island on the southern Anatolian plate in the eastern Mediterranean. The ophiolite formed during the Late Cretaceous (90-92 Ma) and contains the full ophiolitic sequence: ultramafic cumulates, gabbros, mafic and felsic sheeted dikes, pillow lavas and a sedimentary cover. The southeast of the ophiolite is cut by the Arakapas fossil spreading ridge-transform fault system. It connects two parts of a spreading ridge, which in turn is the origin of the Troodos dikes. The influence of the Arakapas Transform Fault activity on the intrusive sequence is not yet known. In addition, more information on both evolution within the dike complex and between dike types is needed. This thesis aims to fill these knowledge gaps by conducting fieldwork and microscopic research. Dikes are classified into groups, sampled, and studied with optical microscopy, Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray Spectroscopy (EDS). Two important alteration trends are identified from the geochemistry: alteration from pyroxene to amphibole and from Ca-rich plagioclase to Na-rich plagioclase is observed, indicating a decrease in calcium and an increase in sodium. Additionally, bulk rock data show an increase in SiO2 content. An increase in scale allows for determining the degree of alteration per dike type at set distances from the Arakapas Fault. No correlation is found; no influence of the Arakapas Fault activity on the dikes can be identified from this study. Next, the degree of alteration per dike type, irrespective of the distance to the transform fault, was established. The degree of alteration varied between groups, possibly caused by compositional diversity. The presence or absence of certain minerals may lead to differences in susceptibility to alteration or differences in permeability. Finally, the possibility of black smoker hydrothermal system activity in these rocks is explored, which explains K and Ca depletion and can induce an increase in permeability.