Abstract

The Tomatin (Findhorn) Granite in the NW Grampian Highlands has been separated into two distinct complexes: a late-tectonic Glen Kyllachy Granite (tectonically foliated) and a post-tectonic Findhorn Granite (flow foliated). For the Glen Kyllachy complex, Rb-Sr analyses of muscovites from the granite and from an associated suite of cross-foliated pegmatites yield an emplacement age of 443⁺⁵₋₁₅ Ma. Whole-rock Rb-Sr data support field and textural evidence that the pegmatite and granite emplacement was late-tectonic to the last (F3) phase of Caledonian folding. Initial granite ⁸⁷Sr/⁸⁶Sr of 0.7176 supports field and geochemical evidence of derivation from upper crustal metasediments first metamorphosed during the Grenville event. For the Findhorn Granite, concordant Rb-Sr and K-Ar mineral data establish an age of 413± 5 Ma, and an initial ⁸⁷Sr/⁸⁶Sr of c. 0.706 indicates a lower crustal and/or mantle source. This age and isotopic contrast between these granites is characteristic of the whole Grampian region, in which a plutonic hiatus between c. 440 and 415 Ma coincides with the peak of sedimentary accretion in the Southern Uplands, and may be explained in terms of the lack of hydrous materials passing down the associated subduction zone.

Structural, metamorphic and radiometric evidence suggests (a) that the late-F3 Glen Kyllachy pegmatites are comparable with the 442 ± 7 Ma old, syn-F3 pegmatites in the N Highlands—both pegmatite suites are situated in the axial zone of the metamorphic Caledonides displaced by the Great Glen Fault; and (b) that the Caledonian, c. 454 Ma old ‘peak’ tectonic-metamorphic event (D2) previously documented in the N Highlands coincides with a Caradocian basin closure along the Highland Boundary Fault.

For Silurian events in the N Highlands we propose a separate subduction history; in this region, the peak of calc-alkaline plutonism was accompanied by continuous folding and westward thrusting at the same time as a plutonic hiatus in the Grampian Highlands. Current evidence suggests that the c. 160 km or greater sinistral movements on the Great Glen Fault were not initiated until late Silurian–early Devonian time.