Geology of the Upper Pyke region, north west Otago


Johnstone, Roy D.


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A sequence of Permain Eglinton Subgroup (Englinton Volcanics), of metasedimentary and metavolcanic rocks exposed in the Upper Pyke region, is younging to the east with volcanic metasediments at the base and basaltic and andesite flow rocks and breccias at the top.
Alkali anlysis of Eglinton Volcanics support the general trend, shown by Challis (1968), of high K2O/Na2O ratios for the western belt (Eglinton-Brook Street terrain) compared with the eastern belt (Livingstone-Lee River terrain). The validity of plotting variation diagrams using mobile elements (e.g. NaO2, K2O, CaO) for rocks that have been metamorphosed may be in doubt.
Coarse grained rocks of quartz diorite, diorite, and hornblende gabbro compositions intruding the Eglinton Volcanics are correlated with the Mackay Intrusives. In the light of the new radiometric dates, their age of emplacement may be younger than previously thought.
Rocks of the Bryneira Group (Upper Permain) and Barrier Formation (Upper Jurassic) are exposed. The name Wilmot Formation is proposed for rocks of similar age to those of the Barrier Formation with outcrop 2km west of the Barrier Formation.
The contact aureole in the Eglinton Volcanics caused by intrusion of the Mackay Intrusives contains rocks metamorphosed to hornblende hornfels facies.
Textures indicate that there may have been a retrogressive metamorphism from pyroxene hornfels facies. An irregular block (Xenolith) within the Mackay Intrusives has been metamorphosed to pyroxene hornfels facies.
The regional metamorphism of the Alabaster Group shows an increase in metamorphic grade from east to west, from prehnite-actinolite facies possibly to lower greenschist facies.
Metamorphism in the Bryneira Group (lawsonite-albite-chlorite facies), Barrier Formation (prehnite-pumpellyite facies) and Wilmot Formation (Metamorphic grade unknown) is apparently less systematic.
Structures in the area are correlated with other regional features, (Hollyford Fault, Hollyford Graben, Key Summit Syncline). The structure in the Wilmot Formation is synclinal as is that for the Bryneira Group.

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85 leaves : illus. maps (fold) ; 29 cm.


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Location (WKT, WGS84):

POLYGON ((168.191685063076676 -44.41627920084531,168.201984789150742 -44.290475453757061,168.319705985507539 -44.29566832594589,168.307124558108683 -44.421984698380932,168.191685063076676 -44.41627920084531))




Johnstone, Roy D., “Geology of the Upper Pyke region, north west Otago ,” Otago Geology Theses, accessed May 22, 2024,

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