Abstract
The Skippers Range in NW Otago is a structurally isolated block bounded by the Alpine Fault to the northwest, the Glade-Darran Fault to the west and the Hollyford Fault to the east. Within it are five intrusive and fault bounded units: The Mantle Volcanics Formation, Twin Lakes Trondhjemite, Skippers Formation, Slip Hill Intrusives and Mount
Webb Gneiss. A small probably fault bounded conglomerate unit of unknown age occurs along the southwestern boundary of the Twin Lakes Trondhjemite.
The Mantle Volcanics Formation is an undeformed, moderately southwest dipping > 1300 m sequence of Early Permian pyroclastic and epiclastic volcanogenic marine sediments extensively intruded by cogenetic basaltic dikes and sills. The sediments are predominantly coarse breccias and crystal lithic tuffaceous sandstones deposited by debris- flows and high-density turbidites on the flanks of an active and at least partially emergent volcanic edifice. A diverse fossil fauna from a new locality has been collected and includes the first reported occurrence of Eurydesmidae in New Zealand. The intrusive rocks span a continuous range from high MgO to high Al2O3 tholeiitic basalts, are characteristically clinopyroxene phyric often with crystals of quite large size and many can be classed as ankaramites. The intrusive suite is shown to be derived from high MgO, Cr and Ni primary parental melts which are represented in the dikes. These melts were emplaced into high crustal levels and erupted often with high crystal contents but without having undergone significant fractionation. The rocks of the Mantle Volcanics Formation have an incipiently developed greenschist facies mineral assemblage but have not fully equilibrated to these conditions. The Mantle Volcanics Formation represents a portion of the Brook Street Terrane, a north trending discontinuous belt of lower Permian island-arc derived volcanics and volcaniclastic sediments offset by the Alpine Fault, and a discussion of this entity is included.
To the west of and separated from the Mantle Volcanics Formation by the Wilmot Fault and the Twin Lakes Trondhjemite pluton is the Skippers Formation with a structural thickness of approximately 2 km. This is composed of essentially three protolithic types: layered ultramafics, a crustal level basic dike/sill complex and highly deformed basaltic tuffaceous sediments. Taken together, these units represent scraps of dismembered island-arc basement possibly formed in a fore-arc setting. The whole of the Skippers Formation is characterised by well equilibrated greenschist facies and actinolite (after ?clinopyroxene) blastoporphyritic textures similar to clinopyroxene porphyritic textures in the Mantle Volcanics Formation are common in all three protoliths.
Three tabular granitoid plutons, the Twin Lakes Trondhjemite between the Mantle Volcanics and Skippers Formations and two coterminous dioritic bodies, Slip Hill Diorite and Slip Hill Granodiorite, between the Skippers Formation and schists and gneisses of the Mount Webb Gneiss are exposed in the Range. The contacts of the Twin Lakes Trondhjemite are presently faulted but there is evidence of it being intrusive into the Mantle Volcanics Formation and Skippers Formation placing a relative age constraint on their juxtaposition. The Slip Hill Intrusives are fault bounded to the east with the Skippers Formation and the Slip Hill Diorite is shown to be intrusive into the earlier deformed Mount Webb Gneiss. The Slip Hill Diorite is very similar to the Mistake Diorite, which intrudes Brook Street Terrane rocks in the Eglinton Valley, in terms of age (Early Triassic) and petrography.
The Mount Webb Gneiss was previously mapped and correlated with the Thurso Formation occurring along the coast of northern Fiordland. Some questions are raised as to the viability of this correlation.