Age and status of the Mackay Intrusives in the Eglinton—upper Hollyford area

Abstract

In the upper Eglinton area, the Eglinton Volcanics (Permian) are intruded by plutonic rocks mapped as the Mackay Intrusives. A sharp fault contact separating the Eglinton Volcanics from crystalline rocks of the Fiordland Complex, i.e., separating New Zealand’s eastern and western provinces, is no longer recognised in this area and rocks previously mapped as Darran Diorite are now included within the Mackay Intrusives. The Mackay Intrusives are subdivided into six units. Two of these units, the Mistake Diorite and Gunn Dolerite, comprise discrete and relatively homogeneous bodies, whereas the other four are termed plutonic suites, each comprising a range of mutually overlapping rock types. Five K-Ar ages ranging from 208-180 m.y. have been obtained from the Mistake Diorite and are interpreted as indicating a Triassic age of intrusion. Leucocratic gabbroic to dioritic rocks in the upper Hollyford area yield ages of 136-130 m.y. and a trondhjemite dike cutting these rocks gives concordant biotite and muscovite ages of 113 m.y. The median tectonic line of Landis and Coombs is thought to be a pre-Cretaceous feature now obscured in this area by mid-late Mesozoic plutonism. © 1978 Taylor and Francis Group, LLC.