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%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.