Geology of the Hellfire area, Western Stewart Island.


Allibone, AH


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The Paterson Group and the Rakeahua Granitoid Complex, (new name) crop out in the Hellfire area of northwest Stewart Island.
A sequence of metamorphosed crystal tuffs, lapilli tuffs, andesites and minor conglomerates, of unknown thickness comprise the Paterson Group in the area mapped. Development of a narrow high strain zone was accompanied by metamorphism to mid greenschist facies conditions. Later metamorphism to transitional greenschist, amphibolite facies is found adjacent to intrusive contact with the Ruggedy Granite (new name). The age of the Paterson Group is unknown. However, deposition predates intrusion of the Rakeahua Granitoid Complex.
The Rakeahua Granitoid Complex consists of three plutons, in the Hellfire area. The oldest of these is the Ruggedy Granite which is intruded by the Mason Bay Granite (new name) and the Richards Point Porphyry (new name). Both the Ruggedy Granite and the Richards Point Porphyry intrude the Paterson Group.
The Ruggedy Granite and Richards Point Porphyry are highly differentiated, containing 72-75 wt% SiO2 . The former is also notable because it contains only accessory amounts of mafic minerals. All three plutons contain biotite as the major mafic mineral. Major and trace element data indicates I-type affinities for both the Mason Bay Granite and Richards Point Porphyry, but suggests S-type affinities for the Ruggedy Granite. Only the Richards Point Porphyry is surrounded by a chilled margin. This, coupled with its porphyritic texture suggests emplacement at a relatively shallow depth. Sub solidus hydrothermal activity is inferred to have produced alteration in all three plutons.
Post-emplacement deformation of the Rakeahua Granitoid Complex is restricted to mylonite zones 10-100m thick and development of faulted contacts with the Paterson Group, in the Ruggedy mountains.

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ix. 106 p. ill. (some col)., maps folded in pocket, 30 cm.


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Allibone, AH, “Geology of the Hellfire area, Western Stewart Island. ,” Otago Geology Theses, accessed May 22, 2024,

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