Inherited isotope systems and the source region pre-history of early Caledonian granites in the Dalradian series of Scotland

Rb-Sr and U-Pb isotope analyses are reported for two pre-metamorphic Caledonian granites which intrude Dalradian rocks in the Central Highlands of Scotland. These data indicate that the origin of the granitic magmas involved partial fusion of old crustal material. U-Pb systems of zircon size and magnetic fractions from the Ben Vuirich granite are strongly discordant. However, U/Pb isotopic ratios precisely define a chord which intersects concordia at 514(+6-7)m.y. and 1316(+26-25)m.y. Geological constraints suggest that the lower intersection records the post-F2, pre-M3 emplacement age of the granite. Rb-Sr whole-rock systems of the Ben Vuirich granite are also strongly discordant, although 8 out of 13 data points scatter about an 'errorchron' of 564+-24m.y. with an initial 87Sr/86Sr ratio of about 0.716. This is interpreted as a spurious result due to incomplete homogenization of Sr isotopes in the source region during partial fusion. Initial 87Sr/86Sr ratios at the time of emplacement indicated by the zircon data ranged from 0.7173 to 0.7191. (Auth.)