Kathryn Gillis

Several stages of alteration documented in the upper 1-2 km of the Troodos ophiolite have been related to the evolution of hydrothermal systems, crustal ageing, and the subsequent uplift and emplacement of the ophiolite. Six alteration zones, defined by their mineralogical signatures, have been mapped along the northern flank of the ophiolite and outlined in the ICRDG drillcores, CY-1 and CY-1a. The depositional sequence of the groundmass-replacing and void-filling secondary minerals in each of these zones reflects localized change in physical and chemical conditions through time.

The rocks in the upper tens of metres of the extrusive sequence are pervasively altered, oxidized, enriched in K₂O, CO₂, H₂O, Rb, and Ba, and depleted in SiO₂ and Sr. Where syn-volcanic umbers overlie the volcanic pile, the rocks are not oxidized and fresh glassy rinds are commonly preserved, indicating that the rocks were sealed from penetrating seawater. Below these zones, the rocks are variably altered to assemblages of smectite, celadonite, zeolites, and carbonate and locally enriched in alkalis and MgO and depleted in SiO₂, CaO, and Sr. The low temperature minerals are replaced at the base of this zone by laumonite, mixtures of smectite/chlorite, and quartz. Over a 100- to 150-m-thick interval, the rocks become progressively more recrystallized and assemblages of chlorite + quartz + albite + pyrite + epidote dominate. Rock-dominated conditions resulted in the enrichment of Na₂O and depletion of CaO, K₂O, Rb, and Sr. Local enrichment of MnO, Cu, and Zn are attributed to localized zones of discharge.

Outside the localized zones of hydrothermal discharge, the rocks have not bee pervasively metamorphosed. The intensity of alteration is not directly related to depth but rather depends on permeability, temperature, and the rate of sedimentation. A sharp transition occurs between the low temperature zones and the higher temperature zones with greenschist facies mineral assemblages at different levels within the extrusive sequence and sheeted dyke complex. This indicates that the geothermal gradient was stepped and responded to localized conditions, such as changes in permeability that restricted the penetration of unreacted, cold seawater into the lower zone.

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Pages: 403
Supervisor: Paul Robinson