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

Abstract

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 −7 +6 m.y. and 1316 −25 +26 m.y. Geological constraints suggest that the lower intersection records the post-F 2, pre-M 3 emplacement age of the granite. The upper intersection reflects the presence of old zircon xenocrysts incorporated into the granite magma without complete isotopic resetting. The ultimate source of these xenocrysts is probably a metamorphic basement complex which formed about 1320 m.y. ago, but the immediate source region of the granites could have been Dalradian sediments derived therefrom. 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 ± 24 m.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. Whole-rock samples from the Dunfallandy Hill granite have Rb/Sr ratios 2–3 times higher than those from Ben Vuirich and define a reasonably good isochron age of 491 ± 15 m.y. with an initial 87Sr/ 86Sr of 0.7185 ± 0.0008. This may date granite emplacement or subsequent resetting of the high Rb/Sr rocks during Caledonian metamorphism. Rb Sr systematics indicate that the crustal source regions of these and other Caledonian granites separated from the upper mantle at least ca. 800 m.y. ago and probably ca. 1300 m.y. ago, thus confirming the interpretation of the upper intersection age of the zircon U Pb data.