The Origin of New Zealand Ultramafic Intrusions

Abstract

Dun Mountain, Red Hills, and Red Mountain, are three of the largest ultramafic bodies associated with Permian rocks in the South Island of New Zealand. In the three intrusions a central core of relatively unserpentinized dunite, harzburgite, and pyroxene peridotite, is surrounded by a margin of serpentinite. At the western margin of the Red Hills intrusion, the Permian volcanics show high-grade thermal metamorphism with development of pyroxene hornfels at the contact. Layering between dunite and harzburgite is well developed at Red Hills, and on a larger scale at Dun Mountain. Layers at Red Hills are ¼ inch to 20 inch thick and can be traced over an entire outcrop. Harzburgites show poikilitic texture and euhedral crystals. Dimensional orientation of platy olivines, which lie on the broad (010) face in the plane of layering, produces a strong fissility in some of the rocks. Petrofabric analysis shows a strong X maximum normal to the plane of layering and a Z maximum in the plane of layering. Size analysis of crystals demonstrates a size grading in the enstatites from large at the bottom of a layer to small at the top. Olivines are also size-graded, but increased secondary enlargement of crystals at the top of a layer has complicated this. Olivine (Fo93·8—89·4) is the main mineral in all the rocks. Enstatite (En93—88·5) contains numerous exsolution lamellae of clinopyroxene on (100). Chrome diopside is nearly always present in small amounts and picotite is a constant accessory. Comparison of the textures and mineral chemistry of the ultramafic rocks with those of the ultramafic parts of layered intrusions, suggests that the New Zealand rocks have been derived by gravitational differentiation from a tholeiitic magma. It is proposed that the intrusions represent the sub-volcanic differentiates of a chain of Permian volcanoes on the margin of the New Zealand geosyncline, and that the associated olivine-poor tholeiitic Permian basalts are the extrusive differentiates.