The Petrology of the Lower Old Red Sandstone Lavas of the Eastern Sidlaw Hills, Perthshire, Scotland

The calc-alkaline lava sequence of the eastern Sidlaw Hillsforms a small part of an extensive volcanic province of LowerOld Red Sandstone (Devonian) age in Scotland and N. England.The Sidlaw lavas ranging from olivine basalt to dacite are allporphyritic with combinations of olivine, plagioclase, clinopyroxene,orthopyroxene, and opaque oxide pheno-crysts. Chemically, thelavas are slightly more alkalic than modern calc-alkaline lavas.There is considerable variation in the ‘incompatible elements’.The differentiation of the lavas can be accounted for by fractionationof olivine+plagioclase+minor ore from a chemically variable,immediately parental magma at low pressure (c. 1 kb P_H2O). Itis suggested that fractionation of variable amounts of olivineand clinopyroxene from an olivine tholeiite at moderate P_H2Ocould give rise to this chemically variable, high alumina, immediatelyparental magma.