The structural geology of Lawsonite and glaucophane schists of the Ouégoa district, New Caledonia

Abstract

In the north of New Caledonia glaucophane is found in a great variety of layered rocks ranging from metamorphosed shale with undeformed Cretaceous fossils high in the succession, to very coarsely crystalline gneisses of complex mesocopic structure, presumed to be deep in the succession. Lawsonite occurs in the finer-grained rocks of comparatively undeformed aspect. The metamorphism is probably Oligocene. Schists, well exposed, show very distinct foliation which trends very regularly on the larger scale, although it bears abundant small-scale folds which generally plunge down the foliation planes at 45° to 70°; indeed, folds with horizontal axes are uncommon. On a large scale the strike of the foliation appears as a great flexure, produced probably later than the main metamorphic event and almost contemporaneously with faulting, some of which was transcurrent, and which has caused serpentinite to arise along narrow thrust zones. Coarse glaucophanites and gneisses occur among the serpentinites of the fault zones and have presumably also risen as detached lenses by diapir action. The process of serpentinisation may have partly caused the great pressures in which lawsonite- and glaucophane-bearing rocks are produced, and the main mass of rocks probably moved on deep serpentinised surfaces of décollement. There is no clear evidence to support the idea of a vast, overthrust ultrabasic sheet directed towards the west or south-west as a cause for the high-pressure metamorphism.