Metamorphism and structure of the schists of the Upper Pomahaka River Valley, West Otago, New Zealand.


Clough, MC


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The Upper Pomahaka River Valley consists dominantly of Caples terrane - derived greywackes and argillites and their metamorphosed equivalents (psammites and pelites). These rocks are poorly sorted and consist mainly of detrital volcanic lithics and lesser sedimentary Iithics with minor feldspar, quartz, monomineralic grains and metamorphic lithics. A basic to intermediate provenance is suggested by the petrography of the sediments. Rudites are very rare.
Four metabasite units crop out in the area. The least deformed of these display pillow lava textures and contain relict igneous clinopyroxenes (mainly augites). Augite geochemistry suggests the original magma that formed these rocks was tholeiitic in character and formed most likely near a spreading centre. The metabasites were probably incorporated into the surrounding sediments at a subduction zone.
Two metachert units were found. A manganiferous metachert is considered to be genetically related to the extrusion of metabasite unit 1. The hematitic metachert, although close to the metabasite is not considered to be intimately related to its extrusion.
Greyschist lithologies display metamorphic assemblages ranging from prehnite - pumpellyite in the south to chlorite zone of the greenschist facies in the north of the area. Sporadic occurrences of sodic amphibole and biotite indicate two main metamorphic events. The first event involved high pressure metamorphism and the formation of sodic amphiboles probably in response to deep burial and tectonic thickening at a subduction zone. Geobarometry suggests the pressures involved were approximately 5.5 ± 0.5 kb and may even have been higher (approx. 7 kb). The second event occurred at higher temperatures as the rocks thermally equilibrated to their deep environment and involved the formation of biotite. Temperatures must have been in the region of 400 - 440°C since these figures have been determined for the formation of biotite. The two events probably form part of a continuum.
Three textural zones have been mapped - IIa, IIb and IIIa. These are separated by two isotects. The development of segregation lamellae at the IIb /IIIa isotect is inhibited by the quartz-poor nature of the greyschists. Segregation lamellae formation seems to depend heavily on the amount of quartz and to a lesser extent feldspar in the rock.
Two early ductile phases and one more recent brittle phase of deformation have been recognized. The first two involved isoclinal and more open folding of bedding and then schistosity. An axial plane cleavage (Sl) was developed during the first phase while a crenulation cleavage (S2) formed during the second. The third deformation phase involved kink and chevron folding associated with faulting. A fracture cleavage is sometimes developed along F3 axial planes. The first two phases appear to form part of a continuum, with D1 being syn-metamorphic and D2 being syn- to post-metamorphic.
Three generations of faulting have occurred: thrust faulting associated with D3; smaller scale normal and lesser reverse dip-slip faulting and large scale normal faulting. Joints are considered to be genetically related to the latter.

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x., 125 p., ill., maps folded in pocket., 30cm.


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Clough, MC, “Metamorphism and structure of the schists of the Upper Pomahaka River Valley, West Otago, New Zealand.,” Otago Geology Theses, accessed May 22, 2024,

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