PETROGENESIS OF THE CONTACT ROCKS, BLIND ROCK DYKE, COUNTY DONEGAL

Sediments of pelitic and siliceous, dolomitic limestone compositions have been metasomatically altered, in two stages, in the exogenous contact-zone of the Blind Rock dolerite dyke, Co. Donegal. Application of published experimental data shows that the load pressure in the contact-zone was around 500 bars. The first stage of metamorphism took place at a temperature of between 750 and 820?C in the presence of a binary H20-CO2 fluid phase in which the mole fraction of CO2 was restricted between 0.1 and 0.4, and produced assemblages containing biotite, diopside, grossular, andradite, hydrogrossular, orthoclase, plagioclase, wollastonite, sphene, pyrrhotite and magnetite. The second stage of metamorphism took place at around 2200C in the presence of C02-free fluid phase of alkaline nature and minerals such as analcite, apophyllite, clinoptilolite, hydromica, serpentine and thomsonite were developed in the rocks of the first stage.

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