Journal of the Geological Society; August 1983; v. 140; no. 4;
p. 574-576; DOI: 10.1144/gsjgs.140.4.0574
© 1983 Geological Society of London
Discussion
DR C. M. GRAHAM commented that on the basis of the observed extraction of K and Na from carbonate lithologies of the Vassalboro Formation, Dr Ferry has speculated that infiltration may provide an alternative model to partial melting to account for the depletion of these and other incompatible elements in the lower crust. Carbonate rocks comprise only a small proportion of most metamorphic terrains, and deductions based on data obtained from them may give a biased impression of the general importance of infiltration as a process capable of producing incompatible element depletion on a regional scale. The question, therefore, is whether there is any evidence from non-carbonate lithologies in the area studied by Dr Ferry, particularly from pelitic lithologies, either for the operation of infiltration mechanism or for accompanying incompatible element depletions?
DR FERRY responded: I have studied twother lithologies in the Waterville Formation (the stratigraphic unit adjacent to the Vassalboro Formation) for evidence of incompatible element depletions. Sulphide-rich schists in the Waterville Formation show evidence for infiltration by 1–5 rock volumes C–0–H fluid during metamorphism. The high-grade sulphide-rich schists are depleted in S and Na but not in K relative to their low-grade equivalents (Ferry 1981). Normal pelitic schists from the high-grade portion of the Waterville Formation are depleted in neither Na nor relative to normal low-grade phyllites (Ferry 1982). The normal pelitic schists, however, show no evidence for infiltration (at least none has been detected yet). There is no question that infiltration and element depletion has yet to
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