Jing‑Sui Yang

The well developed Troodos ophiolite, Cyprus, is preserved within a regular anticline structure, allowing the study of constructional features of the Troodos-type ocean crust and the examination of relationships between constructional features of ophiolite and in-situ oceanic crust. Field observations and measurements were carried out along 18 profiles spaced at about 2.5 km, and oriented approximately normal to the average spreading direction in the Extrusive Sequence, i.e., between the sediment-extrusive interface (SEI) and the Sheeted Dikes Complex (SC), on the northeastern part of the ophiolite.

A number of important features of the upper part of this type of oceanic crust have been recognized in this study: (1) Within the 45 km of spreading distance covered by the 18 lines, the proportion of pillowed lava flows at different stratigraphic levels is remarkably constant and a high proportion of pillows and thick pillowed sequences are generally concentrated at the top of extrusive sequence. At least two volcanic cycles are present in vertical section of the central part of the area. (2) Dikes occur throughout the extrusive sequence but increase in abundance with depth at different rates from place to place. (3) Three regional hydrothermal alteration zones can be recognized within the extrusive sequence. They are, from top to bottom, the brownstone (approximating to the zeolite facies), transition, and greenstone (approximating to the greenschist facies) alteration zones. A transition in alteration, from a clay-carbonate brownstone facies to a chlorite-quartz greenschist facies takes place at between dike abundance of 25% and 50%. (4) The 13 sulfide deposits in the study area, comparable with that observed on the present oceanic spreading ridges, occur just above the 25% dike abundance contour on the flanks of major areas of sheet flows.

Features are closely linked on a regional scale to dike abundance, suggesting that dike abundance is the most important factor in controlling many features of the uppermost 1-2 km of Troodos and perhaps in-situ oceanic crust. An average of 41% of pillowed flows in the extrusives, combined with some other characteristics in the ophiolite, suggests that the Troodos oceanic crust was formed at a relatively intermediate-fast spreading rate (4-7.5 cm/yr half rate). It is likely that differential preservation has played a role in present distribution of sulfide deposits and dikes were possibly a major heat source responsible for the hydrothermal alteration and sulfide mineralization accompanying crustal formation in the ancient ocean represent by the Troodos ophiolite.

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Supervisor: James Hall