The geological interpretation of potassium-argon ages of metamorphic rocks from the Scottish Caledonides

Abstract

SynopsisThe K-Ar age pattern in the Scottish Caledonides is interpreted in terms of a cooling history. Eighty-four new K-Ar ages are reported and these are systematically related to be retentivities of the materials dated and to the structural position of the rocks sampled. Dalradian slates near the Highland Border with a flow-cleavage associated with the initial (F1) deformation yield whole-rock ages averaging 505±6 m.y. This is a minimum age for F1 in the Scottish Dalradian.Post-500 m.y. Dalradian mica ages reflect cooling times following the main (post-F2, pre-F3) metamorphism, but a lower limit of 471±18 m.y. for this metamorphism is obtained from whole-rock analyses of high-level Dalradian slates, among the first rocks to cool after the metamorphic peak. The F3 deformation is dated from peripheral slates with F3 strain-slip cleavage of the Middle Dalradian and Ballapel successions, which yield whole-rock ages averaging 448±13 m.y. and 443±14 m.y. respectively. Uplift associated with F3 accelerated cooling of the main mass of Dalradian and Moine schists and gneisses and these yield mica averages of 451±17 m.y. (muscovite) and 440±13 m.y. (biotite). Intrusion of Newer Granites occurred between 425 and 390 m.y. ago, and localised high temperatures associated with this activity are evidenced in the age pattern obtained from the metamorphic rocks.