Caledonian Magma Genesis and Crustal Recycling |
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Authors: | FROST C D; NIONS R K O' |
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Institution: | 1Department of Geology and Geophysics, University of Wyoming Laramie,Wyoming 82071
2Department of Earth Sciences, University of Cambridge Downing Street,Cambridge CB2 3EQ Department of Earth Sciences Contribution No. E. S. 544 |
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Abstract: | The isotopic compositions of Sr, Nd, Pb and O together withabundance data for Rb, Sr, Sm, Nd, U and Pb are reported forsamples from the component parts of the c. 400 Ma old EtiveComplex, temporally and spatially related Lorne and Glencoelavas, and the Dalradian country rocks into which the Complexhas been emplaced. These and published data available for otherCaledonian granites are used to evaluate the petrogenesis ofthe Etive Complex in particular, and the role of crustal recyclingin the generation and evolution of the Caledonian granites ingeneral. Nd-isotope compositions of Etive samples at 400 Ma range from 9.9 to 4.7 compared with8.4 < 3.2 for the associated volcanics investigated here, and an estimatedvalue for depleted mantle 400 Ma ago at approximately ? 7. Dalradiancountry rocks have 23.4 < 7.5 and two partially digested metasedimentaryxenoliths within the granite have values of 9. 3 and -4.0. Initial 87Sr/ 86Sr ratios forthe Etive Complex range from 07043 to 07079, whereasDalradian metasediment in the immediate vicinity of the granitehas an initial ratio of 0726. Oxygen isotopes in theComplex have 7. 6 per mil < 18O < 10.0 per mil, all inexcess of typical values of mantle oxygen and reflect a crustalcomponent. An upper limit of 25 per cent Dalradian assimilationis set by the Nd-Sr isotopic variations with the granites andxenoliths. The Etive complex parent magma prior to Dalradian xenolith assimilationis estimated to have values between 10 and 5. In order to satisfy the Srand Pb isotope composition, additional components from a deepersource within the lithosphere (lower crust or continental lithosphericmantle) with relatively unradiogenic Sr, Nd and Pb are required. The crustal residence ages of the Etive Complex average about1.5 Ga, similar to those of many other late and post-tectonicCaledonian granites. The generation of the Etive Complex andCaledonian granites in general has been dominated by recyclingof the continental lithosphere, rather than the addition ofnew material from asthenospheric sources. |
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