OXYGEN AND HYDROGEN ISOTOPE GEOCHEMISTRY OF THE MOURNE MOUNTAINS TERTIARY GRANITES, NORTHERN-IRELAND

An oxygen and hydrogen isotopic study of minerals and whole rocks from the granites of the Mourne Mountains Tertiary complex, and related ro cks, shows that whereas a significant circulation of meteoric water wa s associated with the complex, it had only minor and localized effects on the granites themselves. The Silurian slate and greywacke country rocks, which would have had deltaO-18(SMOW) values of + 10 to + 20 par ts per thousand before the Tertiary igneous events, have been depleted in O-18 to values of - 4.0 to - 0.5 parts per thousand; Tertiary acid minor intrusions outside the main granite masses are also O-18 deplet ed. DeltaO-18 whole-rock data on the granites show a range of +6.0 to +9.5 parts per thousand, and include values significantly higher than most of those obtained for the granites of the Tertiary central comple xes of Skye, Mull, and Ardnamurchan. Many of the lowest whole-rock del ta-O-18 values are found in samples where the minerals are not in isot opic equilibrium. The mineral oxygen isotopic data can be explained in terms of localized interaction with meteoric water, resulting in pref erential O-18 depletion in feldspar(s) and biotite, with quartz being much less affected. The granites all show low values of deltaD(SMOW) f or biotite and amphibole separates (- 137 to - 104 parts per thousand) . The lowest values occur close to the margins of the plutons, near in ternal contacts or near greisen localities, and these probably reflect limited interaction with meteoric water. The higher deltaD values are from samples which show evidence of chloritization. This process appe ars to have occurred both during interaction with meteoric water, and also during autometasomatism by an exsolved magmatic fluid in other pa rts of the plutons, including central locations where there is little or no evidence for the penetration of meteoric water. Granite samples which exhibit near-equilibrium oxygen isotope fractionations for const ituent minerals are characterized by magmatic O-isotopic compositions. The G2 granite, the largest pluton of the eastern centre, has a magma tic deltaO-18(SMOW) value of approximately + 9.5 parts per thousand; i ntrusions G3 (eastern centre) and G4 (western centre) both have deltaO -18(SMOW) values of approximately + 9.0 parts per thousand. The other two main intrusive phases have distinctly lower deltaO-18(SMOW) values : approximately + 7.5 parts per thousand for G1 (the least fractionate d granite of the Mourne Mountains central complex), and from + 7.5 to + 8.5 parts per thousand for G5. The oxygen isotopic data rule out sim ple partial melting of the country rocks as the origin of the granites and also preclude an origin by closed-system fractional crystallizati on of basaltic magma typical of that represented by Tertiary basic ign eous rocks of the region.