On the origins of layered calc-silicate rocks from the Buchan Dalradian and their bearing on the diagenetic history of the Southern Highland Group

Abstract

Synopsis Calc-silicate rocks from the Southern Highland Group of the Dalradian at Banff are metamorphosed carbonate concretions. Many original sedimentary features are preserved even though the calc-silicates now have amphibolite facies mineral assemblages. Those occurring in the andalusite and staurolite zones contain layered mineral assemblages that are characterized by the presence of calcite, clinopyroxene, hornblende, cummingtonite, and biotite from centre to margin. It is unlikely that the layering formed principally by synmetamorphic diffusional processes since diffusion modelling demonstrates that the relative values of phenomenological diffusion coefficients required to reproduce this set of layers are inconsistent with known values. The layer order, with pyroxene adjacent to carbonate and amphibole adjacent to pelite, is also inconsistent with their formation by infiltration of an H 2 O-rich fluid into a pure carbonate body. The layers can be best explained as resulting from metamorphism of compositionally zoned, diagenetic carbonate concretions. If the metamorphism was essentially isochemical, then composition profiles present in the calc-silicates may be compared with those in unmetamorphosed concretions and interpreted in terms of known diagenetic processes. A decrease in CaO from centre to margin results from the progressive elimination of the primary porosity during compaction of the sediment, a conclusion supported by the presence of relict sedimentary structures. Such calc-silicates contain important information about the early burial history of the sedimentary basin where their host rocks were deposited prior to the onset of metamorphism. There is some evidence that the alkali profiles have been modified during metamorphism. However it is not appropriate to use such calc-silicates for studies of synmetamorphic diffusion or other synmetamorphic processes without taking pre-metamorphic composition gradients into account.