Chemical alteration trends, fluid inclusion patterns and stable isotope compositions in the plutonic sequence of the Troodos ophiolite, Cyprus

Drill hole CY-4 in the Cyprus Crustal Study Project of the International Crustal Research Drilling Group is the deepest hole at present in the intrusive sequence of the Troodos opiolhite. It intersects diabase dykes, isotopic and layered gabbros and cumulate ultramafic rocks before terminating in serpentinized periodotites at a depth of 2263 m. Studies of the alteration of the sequence shows that the diabase dykes have been transformed to greenschist-facies grade spilites characterized by albite, actinolite and chlorite. Amphibolite facies grade metamorphic rocks are encountered in the plutonic complex, while the cumulate ultramafic rocks have been extensively serpentinized. Alteration throughout the complex appears to be strongly influenced by fracture porosity. Fluid inclusions in quartz from veins and alteration zones indicates the presence of two distinct fluid types. In the sheeted dykes, fluids of seawater and near seawater salinity dominate, while in the plutonic sequence hypersaline fluid inclusions with daughter crystals of halite and calcium chloride are dominant. Mineralogical and spatial evidence suggests that the hypersaline fluids are a by-product of the extensive serpentinization in the ultramafics of the ophiolites and depletions of ¹³C suggestive of the influence of biogenic carbon. The study suggests that the ophiolite cooled by the convective circulation of seawater through it, with the circulation cells eventually evolving off- axis into two distinct systems, an upper system dominated by the circulation of seawater, and a lower cell strongly influenced by the serpentinization of the deep level peridotites.