Some aspects of the geology of the Northern coast of Stewart Island, New Zealand

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Waddell, Stewart John.

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Rocks exposed around the north coast of Stewart Island are mapped on a scale of 1 inch:0.5 ml and are documented in petrographic detail. The area of coast mapped runs from just south of North Red Head around to Saddle Point, a distance of 35km. The rocks have been previously mapped as Rakeahua Granite, Paterson Group and Anglem Complex. These three broad divisions are followed herein.
The Rakeahua Granite around North Red Head is found to be an alkali leucogranite or alaskite, as distinct from biotite granite previously known from the Tin Range and elsewhere. The alaskite is considered co-eval with this biotite granite. A moderately sheared and crushed equivalent of the alaskite is mapped as Crushed Rakeahua Granite; apart from mechanical effects, this rock is virtually mineralogically identical to the alaskite. It is found within the Freshwater Graben associated with Paterson Group rocks. Partial analyses of Rakeahua Granite and Crushed Rakeahua Granite rocks revealed 3.9 to 4.5% Na2O, 4.5 to 5.0% K2O (with two lower values from the Crushed Rakeahua Granite), and 0.3 to o.6% CaO. From phase diagram and mineralogical considerations the Rakeahua Granite is considered to have been intruded at around 700°C at a depth approximating 4km. (1.3 kb PH20). The Rakeahua Granite is correlated with Fiordland granites (?Kakapo Granite, ?Pomona Granite) and is considered to be most likely Late Mesozoic in age.
Paterson Group rocks are exposed on West Ruggedy Beach as "interbedded" rhyolite, meta-andesite, laminated meta-siltstone (unfossiliferous) and shear foliated metaconglomerate. This distinctive group of rocks is here named the West Ruggedy Formation. Incipient to moderately well developed greenschist facies assemblages are present in all rocks of the West Ruggedy Formation.
The most common assemblage is quartz-albite-biotite-epidote-sphene. The laminated siltstone is unusually enriched in boron, containing up to 5% (authigenic) tourmaline. The West Ruggedy Formation occurs as an infaulted sliver in Crushed Rakeahua Granite in the Freshwater Graben, and probably assumed its present position as a result of the tectonic activity associated with the formation of this graben. Suggested correlatives of the West Ruggedy Formation are the Permian sedimentary and tuffaceous rocks east of the Longwood Range and in the Riverton-Bluff areas.
Anglem Complex rocks are mapped as two suites viz. migmatitic and intrusive (or plutonic). The migmatitic suite comprises intimately associated well foliated amphibolite-diorite-tonalite-granodiorite-adamellite and associated ptygmatic quartz-feldspar veins. The intrusive suite is distinguished by its near lack of foliation (more massive "plutonic" appearance) and its greater meso- and macroscopic homogeneity. It comprises hornblende gabbro, noritic leucogabbro and plutonid granodiorite-adamellite. The migmatitic nature of the migmatitic suite is deduced from microtextures, inferred mineralogical reactions and field relations. In the intrusive suite, hornblende in the hornblende gabbro is shown to be secondary, and origin by uralitisation of pyroxenes in noritic leucogabbro is postulated. Granitic and rare andesitic dyke rocks intruding the Anglem Complex rocks are also described. The Anglem Complex rocks were found to be strikingly similar to those at Pahia at the southern end of the Longwood Range and also to those between Pahia and Bluff. Strong correlation with these rocks is emphasised. Upper Permian age for the Anglem-PahiaBluff rocks is deduced from radiometric ages and fossil evidence from the coastal Southland rocks.
The dominant structural trend of northern Stewart Island swings from near east-west on the east coast to northwest-southeast on the north and northwest coast. This is the trend of the Freshwater Graben, which is more or less parallel to near vertical sometimes gneissic foliation in the migmatitic suite of the Anglem Complex. The gneissic foliation becomes more obvious to the southwest towards the northeast bounding fault of the graben.
A magnetic survey of the Freshwater Graben with a vertical force balance magnetometer successfully delineated the extent of the West Ruggedy Formation. Overall correlation between magnetic vertical force profiles, magnetic susceptibilities and geology proved feasible. Deduced magnetic anomalies in the area of The Neck (Paterson Inlet) are compatible with Southland Syncline geomagnetics, and are correlated with the eastern end of the Stewart Island Negative Anomaly. It is suggested that this anomaly passes through the northwest coast in the Ruggedy Beaches area, several miles further north than previously thought.
In a regional sense, two contrasting terranes are present in Stewart Island. The Anglem Complex is broadly correlative with other plutonic-migmatitic suites along the western margin of the New Zealand Geosyncline and the Wakatipu Metamorphic Belt and the Rakeahua Granite is correlative with other granitic bodies emplaced within the Tasman Metamorphic Belt. The Median Tectonic Line of New Zealand probably passes through Stewart Island along the margin of the Freshwater Graben between Paterson Group rocks and the Rakeahua Granite. Two appendices discuss staining techniques used in distinguishing plagioclase and k-feldspar, and problems encountered in compositional determinations of alkali feldspar.

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xiii, 143 leaves : illus.(1 col.) fold. maps ; 26 1/2 cm.

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1971Waddell

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http://download.otagogeology.org.nz/temp/Abstracts/1971Waddell.pdf

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Waddell, Stewart John., “Some aspects of the geology of the Northern coast of Stewart Island, New Zealand ,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/57.

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