Heavy minerals in the Torlesse Terrane of the Wellington area, New Zealand

Smale, David

doi:10.1080/00288306.1997.9514779

“…Classical stratigraphic nomenclature was widely abandoned during the early 1980s in favour of the tectonostratigraphic nomenclature of terranes and superterranes described above MacKinnon 1983;Bishop et al 1985). The provenance and age of the terranes that make up the Torlesse Superterrane have been described in detail (MacKinnon 1983; Crampton & Landis 1988;Frost & Coombs 1989;Barnes 1990;Bradshaw et al 1993;Dean 1993;Mortimer 1995;Smale 1997;Roser et al 2000;Wandres 2002;Wandres et al 2004), however sedimentological descriptions of these units have lagged behind (Andrews 1974;Howell 1981;Retallack & Ryburn 1982;Barnes 1988).…”

Section: Nri'ementioning

confidence: 99%

Pahau Terrane type locality: Fan delta in an accretionary prism trench‐slope basin

Bassett¹,

Orlowski

²

2004

New Zealand Journal of Geology and Geophysics

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The sediments of the Pahau Terrane, part of the Torlesse Superterrane, have never been adequately described or formally denned, although an unofficial type area in the Pahau River exists. This research formalises the type locality and interprets the sedimentology and tectonic setting of the deposits. Four well-defined lithofacies are able to be distinguished in the area: mudstone, interbedded mudstonesandstone, sandstone, and conglomerate. We propose the Lochiel Formation to encompass these lithofacies and the Mt Saul Member for the conglomeratic bodies found within the Lochiel Formation. The Pahau River Group is to encompass the Lochiel Formation and future formations defined within the Cretaceous Torlesse Supergroup and is envisioned to extend over a large part of the Pahau Terrane. A shallow-water fan delta model is proposed for the Lochiel Formation in the type area, based primarily on coarsening and thickening upwards sequences, mixed terrestrial and marine depositional setting indicators such as rootlets and dinoflagellates, and conglomeratic bodies with deposits indicative of both bedload and debris flow transport. A fan-delta model is chosen over a coarse-grained delta as local relief had to be high enough to produce debris flows in the delta plain. The presence of rootlets directly under a debris flow deposit indicates a subaerial environment. Paleocurrent indicators from 14 locations suggest a paleoflow direction towards the east-northeast corrected for regional rotation to east-southeast. The rare occurrence of bi-directional and wave ripples suggests that these deposits were not greatly influenced by tidal currents or wave reworking, but were dominated by fluvial unidirectional currents. The tectonic setting for the Pahau Terrane is interpreted as distal forearc to accretionary prism. QFL analysis for the Lochiel Formation suggests a transitional arc provenance, similar to other analyses in the Pahau Terrane. Conglomerate clast rounding and the wide variety of clast compositions indicate a large catchment area, suggesting that the relief needed to produce the debris flows was provided locally by a large fault scarp. This was likely to have been a thrust fault with the Pahau River fan delta deposited in a trench-slope basin on top of the accretionary prism.

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“…High MnO stage 0 cores from samples DMNR-80 and 90 do neither plot near any detrital garnet compositions, or Torlesse terrane source garnet compositions (Yokoyama, 1994;Smale, 1997;Grapes, et al, 2001), so it is unlikely that these cores are detrital. However, these cores do have similar compositions to high MnO garnet from the Otago Schist (Brown, 1967;White, 1996) and the most MnO rich garnets from the McArthur Range (Williams, 2006), therefore the potential sources of these stage 0 cores are the Otago Schist or earliest Alpine Schist events.…”

Section: 15a and Bmentioning

confidence: 94%

“…As stages 1, 2 and 3 are interpreted to have grown during the Alpine Schist event, a gradual transition in composition may be expected from stage 0 to stage 1 (or 2) if garnet growth is during the same event. However, the sharp transition from stage 0 to stage 1 (or 2) may suggest these garnet cores grew during the earlier Otago Schist event and not (Grapes, et al, 2001;Smale, 1997;Yokoyama, 1994). b) Possible source rocks for detrital garnets in Torlesse sediments.…”

Section: 15a and Bmentioning

confidence: 99%

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Metamorphism and the P-T History of Alpine Schist from the Newton Range, Southern Alps, New Zealand

Murphy¹

0

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<p>Metamorphic rocks have the potential to record in their mineral assemblages, mineral compositional zoning, and textures, information about geological changes and processes that occur during tectonic events. Interpretations of metamorphic pressure-temperature (P-T) records have traditionally relied on results of geothermobarometry studies, but that approach is not suitable in every case. Metamorphosed greywacke, which makes up ~95% of the New Zealand Southern Alps, has long proven problematic for traditional geothermobarometry because it develops intractable mineral compositions and/or assemblages, especially at relatively low temperature (greenschist facies) conditions. An alternative forward modelling approach using the computer program THERMOCALC was recently used to extract the first detailed P-T history (P-T path) from such previously intractably difficult "greyschist" rocks from a single site in the New Zealand Southern Alps. The present study is the first attempt to apply those new methods to rocks from another study area, and is the first detailed geological study of the Newton Range in the New Zealand Southern Alps. The Newton Range is a ~15 km-long, east-west trending range located ~30 km southeast of the town of Hokitika, ~110 km northeast of the Franz Josef-Fox Glacier region, and immediately to the east of the Alpine Fault in the Southern Alps, South Island, New Zealand. The rocks in the Newton Range are mainly derived from Torlesse Terrane accretionary prism greywacke and argillite (Alpine Schist, greyschist), together with a large pods of ultramafic rock (part of the Pounamu Ultramafic Belt (PUB)) and minor associated metabasic layers (greenschist), all metamorphosed to greenschist facies conditions. The dominant mineral assemblage in the greyschist (Qtz + Ms+ Bt ± Chl ± Ep ± Pl ± Ilm ± Ttn ± Grt ± Zrn ± Tur ± Ap ± Cal), much like that found elsewhere in the Southern Alps. As elsewhere in the Southern Alps, the dominant high-grade metamorphic mineral assemblages in the Alpine Schist in the Newton Range are inherited. The mineral assemblages, compositions, and some textures thus record evidence of processes that took place during tectonic events, presumably mainly in Cretaceous time, prior to the formation of the modern Southern Alps, which are forming today by the ongoing oblique continent-continent collision of the Pacific Plate against the Australian Plate at the Alpine Fault. Compositional zoning in garnet from the greyschist is an important record of the metamorphic P-T path traversed by the host rock as the garnet grew. Occasionally, garnet from the study area contains an inmost core (stage 0) of unusual (anomalously high- or low-MnO) composition. The cores with extremely low MnO are possibly detrital in origin, and those with extremely high MnO may perhaps have grown in the early tectonic episode that formed the Otago Schist. Typically, garnet shows the following core- to rim zoning sequence. Stages 1 & 2 show a progressive decrease in MnO and increase in FeO from core to rim, with higher MnO cores present in rocks with higher whole-rock MnO compositions. Stage 3 is characterised by a gradual decrease in CaO and signifies the growth of Ca-bearing oligoclase late in the garnet growth history. Stage 4 is a discontinuous overgrowth characterised by an abrupt increase in CaO. Such overgrowths have in the past been attributed to garnet growth accompanying the development of the Alpine Fault mylonite zone in the late Cenozoic. In the Newton Range they were only observed on garnet adjacent to the main outcrop of the PUB at ~4.5km from the Alpine Fault, far from the mylonite zone, so local element availability during decompression (and possibly fluid flow and/or metasomatism) may have played a part in the growth of these rims. A P-T path for Alpine Schist from the Newton Range has been estimated using detailed garnet composition data measured along core-to-rim transects across individual garnets, together with predicted garnet compositions and P-T pseudosection results calculated using THERMOCALC. The P-T path starts at ~3.5kbar/400°C, where both garnet and albite coexist, and increases in pressure and temperature to ~6.5bar/500°C where garnet coexists with both albite and oligoclase. The estimated peak metamorphic conditions probably correspond to peak metamorphic pressures, unlike in the Franz Josef-Fox Glacier region where peak conditions (~9.2kbar and 620°C) probably coincided with peak metamorphic temperatures.</p>

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“…The most likely source is a detrital garnet grain which has been preferentially nucleated upon during Alpine Schist metamorphism. Some detrital garnets grains in Torlesse terrane greywackes have similar compositions (Yokoyama, 1994;Smale, 1997). P-T estimates of this core cannot be obtained, as if it is detrital, it grew in a rock with different mineral assemblages and a different bulk-rock composition.…”

Section: Low Mno Coresmentioning

confidence: 99%

Metamorphism and the P-T History of Alpine Schist from the Newton Range, Southern Alps, New Zealand

Murphy¹

0

View full text Add to dashboard Cite

<p>Metamorphic rocks have the potential to record in their mineral assemblages, mineral compositional zoning, and textures, information about geological changes and processes that occur during tectonic events. Interpretations of metamorphic pressure-temperature (P-T) records have traditionally relied on results of geothermobarometry studies, but that approach is not suitable in every case. Metamorphosed greywacke, which makes up ~95% of the New Zealand Southern Alps, has long proven problematic for traditional geothermobarometry because it develops intractable mineral compositions and/or assemblages, especially at relatively low temperature (greenschist facies) conditions. An alternative forward modelling approach using the computer program THERMOCALC was recently used to extract the first detailed P-T history (P-T path) from such previously intractably difficult "greyschist" rocks from a single site in the New Zealand Southern Alps. The present study is the first attempt to apply those new methods to rocks from another study area, and is the first detailed geological study of the Newton Range in the New Zealand Southern Alps. The Newton Range is a ~15 km-long, east-west trending range located ~30 km southeast of the town of Hokitika, ~110 km northeast of the Franz Josef-Fox Glacier region, and immediately to the east of the Alpine Fault in the Southern Alps, South Island, New Zealand. The rocks in the Newton Range are mainly derived from Torlesse Terrane accretionary prism greywacke and argillite (Alpine Schist, greyschist), together with a large pods of ultramafic rock (part of the Pounamu Ultramafic Belt (PUB)) and minor associated metabasic layers (greenschist), all metamorphosed to greenschist facies conditions. The dominant mineral assemblage in the greyschist (Qtz + Ms+ Bt ± Chl ± Ep ± Pl ± Ilm ± Ttn ± Grt ± Zrn ± Tur ± Ap ± Cal), much like that found elsewhere in the Southern Alps. As elsewhere in the Southern Alps, the dominant high-grade metamorphic mineral assemblages in the Alpine Schist in the Newton Range are inherited. The mineral assemblages, compositions, and some textures thus record evidence of processes that took place during tectonic events, presumably mainly in Cretaceous time, prior to the formation of the modern Southern Alps, which are forming today by the ongoing oblique continent-continent collision of the Pacific Plate against the Australian Plate at the Alpine Fault. Compositional zoning in garnet from the greyschist is an important record of the metamorphic P-T path traversed by the host rock as the garnet grew. Occasionally, garnet from the study area contains an inmost core (stage 0) of unusual (anomalously high- or low-MnO) composition. The cores with extremely low MnO are possibly detrital in origin, and those with extremely high MnO may perhaps have grown in the early tectonic episode that formed the Otago Schist. Typically, garnet shows the following core- to rim zoning sequence. Stages 1 & 2 show a progressive decrease in MnO and increase in FeO from core to rim, with higher MnO cores present in rocks with higher whole-rock MnO compositions. Stage 3 is characterised by a gradual decrease in CaO and signifies the growth of Ca-bearing oligoclase late in the garnet growth history. Stage 4 is a discontinuous overgrowth characterised by an abrupt increase in CaO. Such overgrowths have in the past been attributed to garnet growth accompanying the development of the Alpine Fault mylonite zone in the late Cenozoic. In the Newton Range they were only observed on garnet adjacent to the main outcrop of the PUB at ~4.5km from the Alpine Fault, far from the mylonite zone, so local element availability during decompression (and possibly fluid flow and/or metasomatism) may have played a part in the growth of these rims. A P-T path for Alpine Schist from the Newton Range has been estimated using detailed garnet composition data measured along core-to-rim transects across individual garnets, together with predicted garnet compositions and P-T pseudosection results calculated using THERMOCALC. The P-T path starts at ~3.5kbar/400°C, where both garnet and albite coexist, and increases in pressure and temperature to ~6.5bar/500°C where garnet coexists with both albite and oligoclase. The estimated peak metamorphic conditions probably correspond to peak metamorphic pressures, unlike in the Franz Josef-Fox Glacier region where peak conditions (~9.2kbar and 620°C) probably coincided with peak metamorphic temperatures.</p>

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Heavy minerals in the Torlesse Terrane of the Wellington area, New Zealand

Cited by 11 publications

References 12 publications

Pahau Terrane type locality: Fan delta in an accretionary prism trench‐slope basin

Pahau Terrane type locality: Fan delta in an accretionary prism trench‐slope basin

Metamorphism and the P-T History of Alpine Schist from the Newton Range, Southern Alps, New Zealand

Metamorphism and the P-T History of Alpine Schist from the Newton Range, Southern Alps, New Zealand

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