Abstract

Mr H. Roper writes: Lyon et al. 1973, in their paper on the geochronology of the Lewisian of Rona, have outlined a sequence of thirteen events which they have established for the area. They state that the dominant gneissose foliation was formed during the first deformational phase (their event 2) under amphibolite

facies conditions and that only very minor axial-planar recrystallization accompanied fold phases 2 to 5 (their events 4 to 9). Their event 5, however, consists of “abundant foliated (my italics) basic minor intrusions.” Furthermore their statement that “the field relationships of the various amphibolites with the fold phases indicate at least five phases of basic igneous activity” implies that virtually all the basic intrusions are now amphibolites.

Since the authors do not recognize any important metamorphic event after their event 2, do they imply that the basic rocks were emplaced as amphibolites rather than as dolerites? If so, how was their foliation acquired? And if the foliation too dates from the time of their emplacement why is there no trace of these foliation-producing amphibolite facies conditions in the country rocks? At least some of the basic intrusions of Rona would seem to belong to the 'Scourie dyke suite' as suggested by Park (in discussion on Lyon et al. 1973). It has been demonstrated by Sutton & Watson (1951) that where the Scourie dolerites were affected by Laxfordian amphibolite facies metamorphism, they were converted to foliated amphibolites. Since Holland & Lambert (1972) have demonstrated the occurrence of a Laxfordian

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