Geology of the Anglem complex, northeastern Stewart Island, New Zealand


Frewin, Michael Owen, 1962-


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This thesis integrates the results of field, petrographic, mineralogical, and geochemical studies of plutonic rocks of the Anglem Complex, northern Stewart Island.
The rocks are broadly divisible into two major suites, both intruded by a series of minor intrusions. The older Saddle Suite. comprises a spectrum of rock types ranging from dunites through to pyroxene diorites. Mineral and cryptic layering are evident, although the internal structure of the suite is complex, due to disruption by synmagmatitic deformation and the mingling and mixing of multiply intruded magma batches. Rock compositions are compatible with formation as cumulates from a high-Al tholeiitic to calc-alkaline melt. Two-pyroxene geothermometry suggests temperatures of crystallisation between 870-950°C for gabbroic rocks.
The younger Bungaree Suite comprises a series of rock types from pyroxene diorite to granite, although a compositional frequency gap in the high Si02 range could be interpreted as indicating the granites are unrelated to the mafic components of the suite by simple crystal fractionation. Intrusive relationships are complex, a consequence of magma mixing and mingling, multiple injection and remobilisation. Geochemical trends are consistent with formation by crystal fractionation from a calc-alkaline magma, possibly the same, or similar magma which produced the Saddle Suite cumulates.
Enclaves occur in both suites. Metamorphic textured amphibolites could represent restite or remobilized early dikes. Igneous textured enclaves are indicative of magma mingling and chilling. Lineated and foliated amphibolites occur on a large scale in a major E-W shear zone, formed by ductile deformation and greenshist to amphibolite facies recystallisation of mafic parent rocks under local high Pfluid. Corona and symplectite formation is common in the vicinity of the shear zone, although sporadic development elsewhere probably results from re-equilibration during prolonged post-magmatic cooling.
Three minor intrusions have been identified. The Rollers Pluton is a circular, lkm diameter, two-pyroxene diorite intrusive into the Bungaree Suite. The iron enrichment of constituent pyroxenes and hornblende suggest a fractionated melt. The Golden Beach Porphyry is a plagioclase-phyric microdiorite exhibiting sharp chilled contacts against diorite of the Bungaree Suite. The Smoky Granite is a foliated to massive, coarse grained granite of atypical S-type affinity, intrusive into the Saddle Suite.
The available data are consistent with the Anglem Complex representing the root zone of part of a Cretaceous volcanic island arc, part of the Brook Street terrane. Melt generation in the lithosphere was a consequence of subduction beneath the leading edge of Gondwanaland, a land mass to which the Anglem Complex was ultimately accreted.
Sediments of Quaternary age occur sporadically along the Stewart Island coastline and comprise boulder-cobble and sand raised beach deposits, together with carbonaceous silts containing large tree roots. Ice scoured boulders on the slopes of Mt. Anglem imply the presence of a once extensive ice field or glacier.
Surface precipitates or encrustations of whewellite, a calcium oxalate mineral, occur widely on coastal rocks, associated with lichen. Organic-rich solutions and seawater ions appear necessary catalysts for the surface weathering of ea-bearing minerals in the rocks, forming whewellite.

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1 v. (various pagings) : col ill., maps ; 30 cm.


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Frewin, Michael Owen, 1962-, “Geology of the Anglem complex, northeastern Stewart Island, New Zealand,” Otago Geology Theses, accessed May 22, 2024,

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