Tectonic evolution of the Eastern Fiordland Gondwana margin

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Scott, James Morfey.

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Eastern Fiordland is an eroded Carboniferous to Cretaceous arc assemblage juxtaposed against the Western Fiordland Gondwana continental margin along the Grebe Shear Zone. In the Manapouri region, Eastern Fiordland is composed of scattered metasedimentary and plutonic rocks of Carboniferous, Jurassic and Jurassic-Early Cretaceous age. Quantitative P-T estimates on rare paragneiss assemblages, coupled with LA-ICP-MS analyses of metamorphic overgrowths on detrital zircon grains, demonstrate metamorphism at low to middle amphibolite facies (lt;6 kbar, c. 600 C) at 145.0 2.8 Ma (all quoted errors at 2[sigma]). The Manapouri-Lake Te Anau area of Eastern Fiordland also exposes scattered fragments of the Mesozoic volcano-sedimentary Loch Burn Formation. Relict sedimentary features within this long-lived Early Jurassic to Early Cretaceous unit indicate deposition in a mostly terrestrial or shallow water environment that was fed by debris flows from proximal granitic and volcanic topographic high points. Deposition of the Loch Burn Formation in the Murchison Mountains is bracketed between a 342.3 1.5 Ma basal granite and an intrusive 157.6 1.4 Ma quartz diorite. Metamorphism throughout the unit achieved greenschist and amphibolite facies temperatures (P unconstrained) in the Early Cretaceous (post c. 148 Ma and prior to c. 121 Ma).

Although metasedimentary rocks provide insights into the tectonic evolution of Eastern Fiordland, a range of compositionally heterogeneous plutonic rocks dominates the geology. At Lake Manapouri, these comprise four principal associations: (1) the composite Pomona Island Granite (Carboniferous-Permian and Jurassic), (2) the Beehive Diorite (148.6 2.3 Ma), (3) the heterogeneous Hunter Intrusives (Carboniferous, Jurassic and Early Cretaceous) of the Darran/Median Suite and (4) HiSY granitoid dikes of the Separation Point Suite (123.5 l.2Ma). The latter suite also occurs in immediately adjacent parts of Western Fiordland, forming the Refrigerator Orthogneiss (120.7 1.1 Ma), the Puteketeke Granite (120.9 0.8 Ma) and the West Arm Leucogranite (116.3 1.2 Ma). Geobarometry indicates the Jurassic portions of the Darran/Median Suite were emplaced between 4 - 6 kbar and Western Fiordland Early Cretaceous Separation Point Suite between 5 - 7 kbar. Zircon initial ¹⁷⁷Hf/¹⁷⁶Hf isotopic ratios suggest that Separation Point Suite magma could be derived from the same Paleozoic - Late Neoproterozoic mantle source as the Jurassic portion of the Hunter Intrusives member of the Darran/Median Suite. However, Early Cretaceous plutons west of the Early Cretaceous active margin (and study area) have significantly more evolved source regions, reflecting the influence of continental Gondwana on lithosphere composition. Initial ¹⁷⁷Hf/¹⁷⁶Hf ratios from the Loch Burn Formation Carboniferous basal granite zircon are slightly less primitive than either Darran/Median or Separation Point Suite but nowhere near as evolved as similar-aged zircon in the Eastern Fiordland Mt Crescent Paragneiss unit in the Hunter Mountains.

The Cambrian/Early Ordovician Russet Paragneiss, which lies just west of the Grebe Mylonite Zone in Western Fiordland and has been intruded by a range of Early Paleozoic to Mesozoic plutons, was metamorphosed at 7.5 1.2 kbar, 633 25 C at 348.6 12 Ma and exhibits no evidence for Jurassic re-equilibration. Zircon U-Pb isotopes from a pelitic schist enclave within the Western Fiordland Mt Murrell Amphibolite are interpreted to show that these and associated intrusive rocks were also metamorphosed at kyanite-grade in the Carboniferous. This event, 'M1', generated a pervasive lineation and distinctive pargasite-anorthite-kyanite/corundum-bearing assemblages in layered aluminous components to the Mt Murrell Amphibolite, garnet-amphibole-biotite-kyanite-gedrite-plagioclase-quartz in metasomatised tonalite at the Mt Murrell Amphibolite margins, and low CaO-garnet in pelitic schist enclaves within the amphibolite. P-T estimates suggest M1 took place at 6.6 0.8 kbar, 618 25 C. Both the timing and P-T conditions of M1 overlap with metamorphism of the Russet Paragneiss. However, the layered amphibolites and pelitic schist enclaves partially re-equilibrated in the Early Cretaceous (c. 115 Ma) at higher pressure (8.8 0.9 kbar). This event, 'M2', generated static assemblages of margarite, epidote, chlorite, oligoclase-andesine and second-generation kyanite in the layered amphibolites and relict olivine gabbronorite, and high-CaO garnet rims, biotite, plagioclase, quartz, kyanite and staurolite in the pelitic schist enclaves. Trace element chemistries of c. 340 Ma zircon grains in the schist have unusual smoothed Ce/Ce* anomalies and high Th/U ratios. These properties may be result of fluid flow and metasomatism from the enveloping amphibolite during imposition of the penetrative M1 lineation. Early Cretaceous (c. 115 Ma) zircon overgrowths and chemistries (low heavy rare earth elements, low Th/U ratios, large Eu/Eu* anomalies) are compatible with formation in the presence of local M2 garnet and plagioclase. M2 was coeval with amphibolite to garnet-granulite facies metamorphism of the regionally extensive Western Fiordland Orthogneiss and Arthur River Complex, thus demonstrating that high-pressure metamorphism was not restricted to the Western Fiordland Early Cretaceous components and their marginal metasedimentary rocks.

The Grebe Mylonite Zone forms a lithologic, metamorphic, isotopic and structural boundary between Eastern and Western Fiordland. This 200 to 300 metre-wide and gt; 50 km long north-striking mylonitic zone is the prominent manifestation of deformation associated with the wider (c. 30 km) Grebe Shear Zone, which extends into Eastern and Western Fiordland. Qualitative and quantitative P-T estimates indicate the currently exposed level of the Grebe Mylonite Zone was active at amphibolite facies conditions (c. 600 C and c. 6 kbar). Coupled U-Pb and Ar-Ar data indicate the mylonite zone was active at, or between, c. 128 and 116 Ma. Temperature-time profiles constructed along a transect perpendicular to the shear zone, used in conjunction with fabric data and the orientation of nearby Tertiary unconformities, suggest that the currently sub-vertical shear zone was rotated during the Cenozoic from an initially steeply east-dipping geometry with a reverse sense of shear. This style of deformation is consistent with an inclined continuously partitioned transpressional structure. Synkinematic emplacement and deformation of the Refrigerator Orthogneiss implies that Grebe Shear Zone provided a crustal anisotropy that facilitated the movement and emplacement of some Separation Point Suite magmas through the crust.

Data collected here are interpreted to show that the Grebe Shear Zone is a terrane-bounding suture. Differences in metasedimentary rock composition, age, provenance and metamorphism across the zone suggest that the crustal framework to Eastern Fiordland did not forth in its current tectonic position. Instead, the Mesozoic portion of Eastern Fiordland is inferred to have developed allochthonously with respect to Western Fiordland, with components internally dismembered and rearranged during Jurassic metamorphism and juxtaposition in the Early Cretaceous. However, the Jurassic portion of the arc may have developed near the Gondwana margin because the Jurassic Borland Paragneiss contains detritus that can be partly matched to sources in the Western and Eastern Provinces of New Zealand, as well as early parts of the Darran/Median Suite and Loch Burn Formation. Recognition that the Eastern Fiordland arc was faulted against and then over Western Fiordland in the Early Cretaceous provides a possible driving mechanism for coeval transpressive shortening, rapid burial and high-pressure metamorphism (e.g., as seen in the Mt Murrell Amphibolite) of the lower Western Fiordland crust.

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viii, 192, 124 p. : ill. (chiefly col.), maps ; 30 cm.

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2008Scott

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POLYGON ((167.334260115256114 -45.517353412529786,167.63231431661319 -45.533507263887245,167.597404249597133 -45.916935790203695,167.103689544118708 -45.893877154797543,167.135230861244935 -45.65518325819049,167.315974776531732 -45.664647643178853,167.334260115256114 -45.517353412529786))

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http://download.otagogeology.org.nz/temp/Abstracts/2008Scott.pdf

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Scott, James Morfey., “Tectonic evolution of the Eastern Fiordland Gondwana margin,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/500.

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