Granite intrusions & Greenland Group metasediments from the Haast region, South Westland

Author:

Randall, Scott Bruce.

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Abstract:

This report summarises findings from the first detailed study of the basement rock west of the Alpine Fault in the Haast region, South Westland, New Zealand. Long held assumptions about the hills rising out of the coastal plain being granite plutons was proven to be incorrect. Observed outcrops were dominated by Greenland Group metagreywackes with granite, observed only as minor sills. Quaternary associated glacial erosion is considered the best explanation for the current morphology of the conical knobs. Greenland Group metasediments are dominated by resistant, granofelsic psammite with thin subordinate interbanded pelites and minor calc-silicate bands. Sillimanite was observed in some pelite samples, allowing classification in the amphibolite grade for the metasediments. Quartz-albite-biotite-muscovite assemblages are common in psammite (quartz-rich) and pelites(biotite-rich) with expected variations in ratios between the two end members. This is a reflection of the homogeneous polycyclic continental source region. Geochemical analysis showed a minor increase in CaO and Sr when compared with samples from North Westland (Nathan, 1976). Two feldspar, subsolvus granites contained quartz, biotite and m1ca reflecting high K20/Na20. ASI and normative corundum values are typical of S-type crusta! melts (after Chappell & White, 2001) formed by local anatexis. Zircon saturation temperatures (Miller et al, 2003) determined that these low volumes partial melts were formed at low temperatures consistent with a crusta! source (650-800°C). These melting temperatures required significant fluid input into the crust (> 5wt %) for melt generation (Acosta-Vigil, 2003). Field relationships suggest that the granite melts were expelled during active deformation along cleavage planes in mechanically weaker pelite layers. Hornfelsic porphyroblasts developed in small contact zones ( <5m), are associated with granite intrusions and overprint earlier regional metamorphic rock fabrics. The folding of some granite sills appears to have been coeval with emplacement and no furthur metamorphic events are seen in the metasediments. Cross-cutting lamprophyre dikes can be correlated with late Cretaceous opening of the Tasman Sea.

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iii, 94 leaves : col. ill., col. maps (1 folded) ; 30 cm.

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2004Randall

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http://download.otagogeology.org.nz/temp/Abstracts/2004Randall.pdf

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Citation

Randall, Scott Bruce., “Granite intrusions & Greenland Group metasediments from the Haast region, South Westland ,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/436.

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