Shear zone melange in Caples rocks, Eyre Creek.
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Abstract:
An area of 5 square km is mapped in Caples group rocks in the Eyre Mountains, where a Shear Zone Melange (SZM) is enclosed within Caples group sandstones.
The SZM is a linear feature, the trend of which parallels the NW-SE striking and W-dipping enclosing strata. The SZM is composed of four lithologies enclosed in a matrix. The matrix is composed of phyllonites, mylonites, or highly sheared sandstones. The inclusions are spilites, altered microgabbros, sandstones and banded cherts, all of which form individual lensoid bodies wrapped by the matrix rocks.
The width of the SZM varies from 0.5-1 km. The eastern boundary of the SZM is defined by a sharp contact between the SZM phyllonites and coarse enclosing sandstones to the east. The western boundary of the SZM is gradational; the phyllonites pass westward through 1 km of variously well cleaved, uncleaved or protomylonitized sandstones. Some of the sandstones to the west of the SZM are olistostromal in origin. The banded chert inclusions are layered, both megascopically and microscopically, and could originally have been siliceous turbidites. A conodont retrieved from the cherts revealed a probable lower Carboniferous age.
Relict clinopyroxenes from the altered microgabbros show affinities with within-plate alkalic magmas. Metamorphic grade (Pre-Pump and Pump-Act) is basically similar within the SZM and the enclosing sandstones, and is indicative of contemporaneous metamorphism.
The SZM is interpreted to be a localized zone of shearing within a linear belt of volcanics that probably extends as least 10-15 km further to the SE. The SZM could represent the remains of a seamount assemblage that has been highly sheared during subduction accretion.
The SZM is a linear feature, the trend of which parallels the NW-SE striking and W-dipping enclosing strata. The SZM is composed of four lithologies enclosed in a matrix. The matrix is composed of phyllonites, mylonites, or highly sheared sandstones. The inclusions are spilites, altered microgabbros, sandstones and banded cherts, all of which form individual lensoid bodies wrapped by the matrix rocks.
The width of the SZM varies from 0.5-1 km. The eastern boundary of the SZM is defined by a sharp contact between the SZM phyllonites and coarse enclosing sandstones to the east. The western boundary of the SZM is gradational; the phyllonites pass westward through 1 km of variously well cleaved, uncleaved or protomylonitized sandstones. Some of the sandstones to the west of the SZM are olistostromal in origin. The banded chert inclusions are layered, both megascopically and microscopically, and could originally have been siliceous turbidites. A conodont retrieved from the cherts revealed a probable lower Carboniferous age.
Relict clinopyroxenes from the altered microgabbros show affinities with within-plate alkalic magmas. Metamorphic grade (Pre-Pump and Pump-Act) is basically similar within the SZM and the enclosing sandstones, and is indicative of contemporaneous metamorphism.
The SZM is interpreted to be a localized zone of shearing within a linear belt of volcanics that probably extends as least 10-15 km further to the SE. The SZM could represent the remains of a seamount assemblage that has been highly sheared during subduction accretion.
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x. 156 p. : ill., maps ; 30 cm.
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1982Pound
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POLYGON ((168.364956336000091 -45.357266186999937,168.385202804000073 -45.354383746999986,168.413915730000099 -45.378613042999973,168.400785952000092 -45.392079131999935,168.382173339000019 -45.391244091999965,168.384736924000094 -45.378389464999941,168.363881837000122 -45.365335250999976,168.364956336000091 -45.357266186999937))
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Citation
Pound, Katherine S. (Katherine Shakespear), 1958-, “Shear zone melange in Caples rocks, Eyre Creek.,” Otago Geology Theses, accessed March 23, 2025, https://theses.otagogeology.org.nz/items/show/160.