Petrology, geochemistry & tectonic affiliation of metabasite in mesozoic tectonostratigraphic terranes, Otago schist New Zealand

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Cambridge, Katherine

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The Mesozoic Caples, Torlesse and Aspiring (inferred) Terranes of the Otago Schist, South Island, New Zealand, are thought to be sedimentilogically distinct, despite having experienced a common moderate P-T Harrovian metamorphic history, The origin and amalgamation of the Otago Schist constituents has been a subject of considerable research and debate. Minor, but widespread metabasite horizons within three tectonostratigraphic terranes provide additional data to address these issues. Metabasite bands often occur in association with metachert and are either intercalated or surrounded by metapelite and metapsammitic schist. In this study, metabasite horizons from the Caples Terrane (Harris Saddle Formation) and the Rakia (older Torlesse) Terrane (Ophir) were examined in detail and compared with data obtained previously from metabasites of the Aspiring Lithological Association (Aspiring Terrane) (Matukituki Valley). Sampled Caples metabasites are pumpellyite-actinolite facies rocks with two associated ductile deformation phases. Torlesse metabasite samples are chlorite zone, greenschist facies rocks with a complex D1-D4 structural history. Aspiring Terrane metabasites samples are gamet-biotite-albite zone, greenschist facies rocks which have also undergone polyphase deformation. Whole-rock major (XRF) and trace element (LA-ICP-MS) data were obtained for samples taken along and across strike of the metabasite horizons. Mobile major and trace elements (NA20, Si02, and LILE) show some scatter as expected for low-grade metamorphism under water-rich conditions (seafloor ± accretionary wedge). Concentrations ofless mobile trace elements (REE, HFSE) are relatively uniform within and between analysed horizons. Normalised trace element plots indicate that the majority of analysed metabasites from all three terranes have OIB-like characteristics. The Caples and Torlesse metabasites have very similar trends. Aspiring metabasites do not share the relative depletion in Ta and Nb or emichment of La exhibited in the other two terranes. Aspiring metabasites are depleted in HREE in contrast to Torlesse and Caples samples. i Abstract The geochemical data obtained for the metabasites place important constraints on their igneous protoliths. The Aspiring metabasites appear to represent either potentially far-travelled ocean islands/seamounts accreted from a subducting place or near-trench alkalic basalt lavas/sills produced in response to subduction of a midocean ridge or localised flexure of a subducting plate. The Caples and Torlesse metabasites probably also originated as ocean island or near-trench alkalic basalts, in regions where contamination from crustal components with relict arc signatures could occur. Alternatively, remarkable similarities in trace element patterns occur with the Karoo (South Africa)-Ferrari (Antarctica) large igneous province (LIP). This may imply that the Otago Schist metabasites have a similar mantle source as the KarooFerrar LIP.

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ii, 103, [20] leaves : ill., maps, ; 30 cm

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2008Cambridge

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

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Cambridge, Katherine, “Petrology, geochemistry & tectonic affiliation of metabasite in mesozoic tectonostratigraphic terranes, Otago schist New Zealand,” Otago Geology Theses, accessed February 7, 2025, https://theses.otagogeology.org.nz/items/show/493.

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