Petrogenesis of high Ba-Sr granites: the Rogart pluton, Sutherland

The Rogart pluton is a typical example of high Ba-Sr granitic magmatism, em placed in northern Scotland towards the end of the Caledonian Orogeny. It c onsists of three granitoid facies (tonalite, granodiorite, granite) that ar e locally associated with lar-ge enclaves of coeval mafic appinite. The ove rall range of compositions is therefore extreme. with MgO from 11.6 to <0.1 wt%, SiO2 from 47.50 to <greater than>73.0 wt%, with relatively high Na2OK2O especially for the mafic rocks (up to 8.4 wt%), associated with K2O/Na2 O approximate to1.5. Trace element abundances vary extensively and coherent ly, and the typical high Ba-Sr elemental signature of the pluton is also ca rried by the appinites. This is consistent with a genetic relationship thro ughout the suite. Sr, Nd and O isotope ratios are sufficiently similar to s upport this contention, but vary systematically with magma evolution. The a ppinites were derived from an enriched mantle source (Nd-143/(144)Nd(400)ap proximate to0.51194, Sr-87/(86)Sr(400)approximate to0.7061) with high delta O-18 (approximate to +8 parts per thousand), probably related to active co ntemporaneous subduction. Quantitative elemental and isotopic modelling sug gests that the granitoid magmas evolved from the appinites by crystal fract ionation accompanied by minor crustal contamination. Early fractionation fr om appinite to tonalite was driven by crystallization of pyroxene plus biot ite with minor plagioclase, replaced by a Feldspar-dominated assemblage to produce granodiorite and granite. The total amount of crust assimilated was less than 25%, highlighting the juvenile nature of the high Ba-Sr granite class.