RB-SR AND O ISOTOPIC RELATIONSHIPS IN 3 ZONED CALEDONIAN GRANITIC PLUTONS, SOUTHERN UPLANDS, SCOTLAND - EVIDENCE FOR VARIED SOURCES AND HYBRIDIZATION OF MAGMAS

The 3 largest plutons in southern Scotland yield Rb-Sr mineral-whole-rock ages of 408±2 Ma for Doon, 397±2 Ma for Criffell and 392±2 Ma for Fleet. This emplacement sequence is accompanied by a progression towards higher average Thornton and Tuttle Indices, 87Sr/86Sr initial ratios and δ18O values. All 3 plutons are composite and normally zoned and display an overall increase in (87Sr/86Sr)i and δ18O inward from the margins. The Doon southern microdiorites are chemically distinct from the later diorites and some have (87Sr/86Sr)i as low as 0.7041. The main diorites in the S have (87Sr/86Sr)i similar to proximal granodiorites and granites (~0.705) suggesting that the chemical variations were chiefly produced by a process like fractional crystallization. Central granites further N have higher (87Sr/86Sr)i (up to 0.7059). δ18O values for the range of rock types vary from 7.83 to 10.32‰. Magmatic (87Sr/86Sr)i values from Criffell lie between 0.7052 and 0.7069, δ18O values ranging from 8.54 to 11.84‰. Fractional crystallization was important in the centre of the pluton. There was greater interaction between the granite and granodiorite components in the S than in the NE. The Fleet granites have high and variable (87Sr/86Sr)i ranging from 0.7060 to 0.7109. δ18O values obtained are relatively constant (11.17-11.33‰). These data, combined with isotopic studies of the surrounding Lower Palaeozoic sediments, common Pb and U-Pb (zircon) data and geophysical constraints, indicate that the magmas were derived by melting of mantle and /or 'new' basic lower crust and metasediment, the isotopic variations being ascribable to incomplete hybridization between magmas derived from these differing sources. © 1980 The Geological Society.