Geochemistry and petrogenesis of tertiary granitic rocks from the Island of Mull,Northwest Scotland |
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| Authors: | J N Walsh R D Beckinsale R R Skelhorn R S Thorpe |
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| Institution: | (1) Department of Geology, King's College, WC2R 2LS Strand, London, England;(2) Geochemical Division, Institute of Geological Sciences, 64-78 Grays Inn Road, WC1X 8NG London, England;(3) Department of Geology, City of London Polytechnic, Walburgh House, Bigland Street, El 2NG London, England;(4) Department of Earth Sciences, The Open University, MK7 6AA Milton Keynes, England |
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| Abstract: | The anorogenic igneous rocks of Mull consist essentially of a lava pile of predominantly basaltic composition, cut by an intrusive complex. The basement consists of Precambrian metamorphic rocks of the Moine Series underlain by Lewisian gneiss. The intrusive complex contains a significant proportion of granitic intrusions which can be ascribed to three successive centres of activity, Centres 1–3. We report new major and trace element, including rare earth element analyses, 87Sr/86Sr ratios and  18O values for a comprehensive collection of granitic rocks from the 3 centres. The 18O values range from +4 to –6 indicating variable extent of interaction between the rocks and heated meteoric groundwater. However, correlations of 18O with other major and trace element data and 87Sr/86Sr ratios are uniformly low, apart from Fe2O3. It is thus unlikely that the interaction of the rocks with meteoric water has systematically altered the chemical (including Sr isotope) characteristics. The chemical and Sr isotope data reflect magmatic values and can therefore be used to comment on the petrogenesis of the granitic rocks.These data indicate that there are important differences between granitic rocks of the centres with Centre 1 forming one distinct group and Centres 2 and 3 a different group. For a given SiO2 value, the Centre 1 granites have higher Na2O, MgO, P2O5, TiO2, Sr, and V and lower Al2O3, MnO, Zn, Zr, and Y than those of Centres 2 and 3. In addition, the Centre 1 granites have lower REE contents and higher CeN/YbN ratios than those of Centres 2 and 3. Granites from all three centres have Eu anomalies, those of Centre 3 being generally greater (Eu/Eu* = 0.66–0.10). Finally, there are important Sr isotope distinctions between the three Centres; calculated initial 87Sr/86Sr ratios for the Centre 1 granites (using 58.2Ma) range between 0.71366–0.71646 (average 0.71530) and have a general correlation of 87Sr/86Sr with 87Rb/86Sr. The initial 87Sr/86Sr ratios of the Centre 2 granites range from 0.70663 to 0.70868, but the 87Sr/86Sr data do not define an isochron. Finally, data for the Centre 3 granites define a Rb-Sr wholerock isochron with an age of 58.2±2.5 Ma and an initial 87Sr/86Sr ratio of 0.71003 ±36.Both the chemical trends and isotopic data for the Mull granites can be interpreted in terms of contrasted origins for the granitic rocks of the two groups. The relatively  primitive chemical composition and high initial 87Sr/86Sr ratios of the Centre 1 granites indicate a substantial crustal contribution and we consider that these granites formed by a combination of partial melting of Lewisian basement together with some magma derived by fractional crystallization of basaltic magma. In contrast, the chemical and isotope data for the Centre 2 and 3 granites are consistent with formation dominantly by fractional crystallization of basaltic magma, together with a relatively small proportion of crustal contamination. A model is proposed which emphasises that acid magmatism in Mull is a consequence of the rise and crystallization of basic magma into continental crust. Granite magma has formed both by partial melting and by fractional crystallization and both of these events probably occurred under open system conditions.With oxygen isotope analyses by J.J. Durham, Geochemical Division, Institute of Geological Sciences, 64–78 Grays Inn Road, London, WC1X 8NG, England |
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