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%o.

Magmatic (87Sr/86Sr)i values from Criffell lie between 0.7052 and 0.7069, δ18O values ranging from 8.54 to 11.84%o. 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. δ18 O 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.

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