Abstract

Two views on the origin of the Laxfordian gneiss complex have been proposed (1) that it was formed by the repeated modification of gneisses originally produced during early Scourian ( = Badcallian) metamorphism at about 2700 Ma and (2) that it was derived from metamorphism of a post-Badcallian supracrustal series. The paper describes, from the type Laxfordian complex, remnants of metamorphosed layered basic-ultrabasic bodies which can be matched in terms of individual rock-types and of the total rock-assemblage with pre-Badcallian layered intrusives in the adjacent Scourian complex. The wide distribution and structural relationships of these remnants show that much of the Laxfordian complex is derived from pre-Badcallian parent rocks. Field evidence supports the view that the distinctive geochemical features of the Laxfordian complex (high K, Rb, U, Th relative to the depleted granulites of the Scourian complex) were established in pre-Laxfordian times and suggests that differences in the history of granitic intrusion during Scourian events may have been a more important factor in establishing these features than differences in the crustal level at which metamorphism took place.

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