‘Older’ granites as time-markers in Dalradian evolution

The ‘older’ granites of Perthshire were emplaced in the Dalradian between the second and third deformation episodes of a Caledonian D1–D4 sequence. Pankhurst & Pidgeon (1976) have furnished a zircon U-Pb and Rb/Sr whole-rock age (514 +6/–7 Ma) for emplacement of the Ben Vuirich granite which provides a minimum age for the D2 event. A somewhat younger Rb/Sr whole-rock isochron age (491 ± 15 Ma) obtained from the Dunfallandy Hill granite may date D3. Migmatisation which occurred in the D2–D3 interval is, by inference, also dated. Dl and D2, which deform Arenig rocks of the Southern Highlands Group are pre-514 Ma events implying a pre-514 Ma time base for the Ordovician.