Patterns and processes of alteration in the lavas and dykes of the Troodos Ophiolite, Cyprus

Abstract

Alteration patterns in the lavas and dykes of the Troodos Ophiolite, Cyprus, record a complex history of axial hydrothermal alteration, crustal aging, and subsequent uplift and emplacement of the ophiolite. Field mapping shows that distribution of five alteration zones, each with distinct mineralogical, geochemical, and hydrologie characteristics, is influenced by igneous stratigraphy, structure, and the nature and thickness of the overlying sediments. Paragenetic sequences of secondary minerals indicate that alteration conditions changed progressively as the crust cooled and moved off‐axis. Along spreading axes, low temperatures (≤50°C) were maintained by the rapid flow of seawater in and out of the lavas, and only minimal alteration took place. In contrast, lower water/rock ratios and higher temperatures (>200°C) in the dykes promoted extensive seawater‐rock interaction. Although the sharp rise in temperature between the two regimes generally coincides with the lava‐dyke transition, late‐stage intrusions or hydrothermal upwelling zones locally cause high‐temperature alteration to extend upward into the lavas. As a segment of crust moved off‐axis, temperatures remained low in the lavas and progressively decreased, from >250° to <80°C, in the dykes. High permeability in the uppermost lavas led to the downward migration of an oxidative alteration front whose thickness and spatial distribution was dependent upon the rate and nature of sedimentation and, thus, the original seafloor morphology. Although field relations show that alteration has a consistent vertical pattern in Troodos, the alteration zones are not laterally continuous, and the stratigraphie depth of their boundaries varies considerably.