Metasedimentary, granitoid, and gabbroic rocks from central Stewart Island, New Zealand

Abstract

A NNE—NE trending strip, 3–8 km wide, extending from the Freshwater valley across Mt Rakeahua, Table Hill, and Mt Allen to the northern end of the Tin Range was mapped at a scale of 1:12 500 to locate and investigate the boundary between the Median Tectonic Zone (MTZ) and Western Province on Stewart Island. A NNE‐trending fault, herein termed the Escarpment Fault, separates predominantly ductily deformed rocks on its south side from essentially undeformed rocks to the north. North of the Escarpment Fault, a small (2–3 km2) pluton of alkali‐feldspar granite (Freds Camp) intrudes gabbroic rocks tentatively considered to be associated with gabbro/ anorthosite/diorite of the Rakeahua pluton, centred on Mt Rakeahua. Both units were subsequently intruded by I‐type biotite granite of the South West Arm pluton. South of the Escarpment Fault the oldest intrusions are biotite tonalite‐granite orthogneisses (Ridge and Table Hill plutons) intercalated with the sillimanite‐cordierite‐bearing Pegasus Group metasedimentary rocks, considered to represent the Western Province. They contain titanite, allanite, and magmatic epidote‐bearing assemblages, implying affinities with I‐type granitoids. These older granitoids have been affected by at least three phases of ductile deformation. Immediately south of the Escarpment Fault, the Escarpment pluton (hornblende, biotite quartz monzonite‐quartz monzodiorite) only exhibits effects of the third phase of deformation. Minor gabbroic intrusives concordant with with the S3 fabric intrude the Pegasus Group and intercalated orthogneisses. Plutons of two‐mica, garnet ± cordierite granite (Blaikies and Knob) and younger biotite‐titanite‐magmatic epidote granite (Campsite) cut fabrics associated with the third phase of ductile deformation. Preliminary U‐Pb dating indicates Devonian‐Carboniferous, Carboniferous, Jurassic, and Early Cretaceous emplacement ages for Ridge Orthogneiss, Freds Camp pluton, South West Arm pluton, and Blaikies pluton, respectively. South West Arm and Rakeahua plutons are provisionally correlated with the MTZ, which extends the (south) western boundary of the MTZ at least 12 km further southwest from the Freshwater faults. Correlation of gabbroic rocks south of the Escarpment Fault with the MTZ would imply an autochthonous rather than allochthonous relationship of the latter to the Western Province.