Geology of Bluff Hill, Southland, New Zealand.
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The mineralogy and structure of two gabbronorite intrusions forming Bluff Hill, is described. They intrude volcanogenic metasediments of the Greenhills Group and together comprise a segment of Brook Street terrane (Coombs et al. , 1976) , the inferred products of a Permian island arc environment.
The Bluff Hill Gabbronorite is of arc-tholeiitic affinity and appears to represent the middle zones of a small south-west dipping pluton, the upper levels now removed by erosion. Mineralogical differences and a fractionation trend revealed by microprobe analysis of co-existing pyroxenes provide information on the structure and orientation of the pluton. The original magma was rather hydrous, as evidenced by the abundance of hornblende and the low temperatures (850° ~ 100° C) obtained from co-existing pyroxene geothermometry. A convection current origin is put forward to explain -several··strongly flow-banded, thin xenoli thbearing zones within the main mass.
A segment of upper level Ocean Beach Gabbronorite, also of tholeiitic affinity,is described from Bluff Hill's western coast, which appears to post-date the Bluff Hill Gabbronorite.
Later intrusives include several thick basaltic andesite (?) dykes, innumerable thin hornblende microgabbro veins and dykes, occasional hornblende pegmatite veins and diorite dykes.
A Tertiary strandline deposit, the Foveaux Formation (new formation) is described. Its rich brachiopod and warm-water benthic foraminifera fauna would suggest a Waitakian to Otaian age. The cement consists largely of the zeolites thomsonite, gonnardite, and chabazite together with minor well-crystallized smectite and gibbsite.
The Bluff Hill Gabbronorite is of arc-tholeiitic affinity and appears to represent the middle zones of a small south-west dipping pluton, the upper levels now removed by erosion. Mineralogical differences and a fractionation trend revealed by microprobe analysis of co-existing pyroxenes provide information on the structure and orientation of the pluton. The original magma was rather hydrous, as evidenced by the abundance of hornblende and the low temperatures (850° ~ 100° C) obtained from co-existing pyroxene geothermometry. A convection current origin is put forward to explain -several··strongly flow-banded, thin xenoli thbearing zones within the main mass.
A segment of upper level Ocean Beach Gabbronorite, also of tholeiitic affinity,is described from Bluff Hill's western coast, which appears to post-date the Bluff Hill Gabbronorite.
Later intrusives include several thick basaltic andesite (?) dykes, innumerable thin hornblende microgabbro veins and dykes, occasional hornblende pegmatite veins and diorite dykes.
A Tertiary strandline deposit, the Foveaux Formation (new formation) is described. Its rich brachiopod and warm-water benthic foraminifera fauna would suggest a Waitakian to Otaian age. The cement consists largely of the zeolites thomsonite, gonnardite, and chabazite together with minor well-crystallized smectite and gibbsite.
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90 p. : ill. (some col.), maps ; 30 cm.
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1981Bosel
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Bosel, Craig A. (Craig Alan), “Geology of Bluff Hill, Southland, New Zealand.,” Otago Geology Theses, accessed May 15, 2025, https://theses.otagogeology.org.nz/items/show/143.