The geology and geochemistry of the Cambrian Devil River volcanics, Anatoki Range, Northwest Nelson
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Middle Cambrian Volcanics of the Takaka Terrane, the Devil River Volcanics, crop out in the Anatoki Range area of Northwest Nelson, New Zealand. Three units are recognised within the Devil River Volcanics. The oldest unit is the Christmas Conglomerate Member, which is overlain by two volcanic suites, the Circular Bush suite and the Paradise suite. All have greenschist facies metamorphic mineral assemblages. Overlying these is a post arc sedimentary sequence which progressively grades into Tasman Formation-like lithologies. The Christmas Conglomerate Member is composed of polymict conglomerate containing chert, sandstone, siltstone, and volcanic clasts. Basaltic andesite clasts show geochemical affinities to low-K tholeiitic rocks. The Circular Bush suite is at least 1200 metres in thickness, and contains volcanic derived debris flows, reworked tuffs and lapilli-tuffs, volcanic conglomerate and rare chert. Volcanic clasts are predominantly basaltic-andesitic to andesitic in composition, with subordinate basalt and rhyolite. Volcanic clasts have immobile element abundances similar to medium-K calc-alkaline series rocks. The Paradise suite is composed of basaltic pillow lavas, massive flows and rare hyaloclastite pillow breccias, which have the geochemistry of high alumina basalts, but have a tholeiitic trend of iron enrichment. The suite ranges from 50 metres to in excess of 400 metres in thickness. The Paradise suite also has immobile element abundances similar to medium-K calc-alkaline rocks. The Circular Bush and Paradise suites both show a subduction-related geochemical signature, however, it is not clear whether the arc was on a continental margin or in an intra-oceanic setting. The succession is folded into a series of north-south striking, tight to isoclinal folds (F1), which have been refolded by two subsequent phases of folding of unknown orientation (F2 and F3). Basic alkaline dykes intrude the sequence, crosscutting F1 fabrics. The dykes have a geochemistry broadly similar to continental flood basalts, and are inferred to have formed in an intraplate setting. Similar intrusives have been documented elsewhere throughout the Takaka Terrane.