Early Paleozoic metasedimentary rocks of Southwest Fiordland, New Zealand : petrology, geochemistry, and detrital zircon geochronology

Simpson, Chloe Charlotte.

Lower Paleozoic rocks crop out along the length of the western South Island, forming New Zealand's oldest crust. They represent a sliver of the immense pile of sediments and volcanics that accumulated along the Paleo-Pacific margin of Gondwana. In NW Nelson, these rocks comprise two distinct tectonostratigraphic terranes, the Ordovician to Devonian Buller Terrane and Cambrian to Devonian Takaka Terrane. Low- to medium-grade metasedimentary and metavolcanic rocks of the Fanny Bay, Edgecumbe and Cameron Groups in Southwest Fiordland, have been proposed to represent a southern extension of these terranes on the basis of rare fossils and lithology. A thorough understanding of their tectonostratigraphic relations have, however, been impeded by the intensity of multiple episodes of metamorphism and deformation. This study, impelled by the GNS Science QMap Fiordland programme, provides geochemical and geochronological constraints upon the stratigraphy, depositional ages, provenance and correlations of these rocks. Whole-rock major and trace element compositions of metasedimentary samples are consistent with those of continent-derived passive margin sediments. REE patterns are uniform, with ubiquitous negative Chondrite-normalised Eu-anomalies (EuN/EuN* = 0.4 to 0. 7) signifying an upper continental crust source. Initial N d isotope values ( ENct = -7.2 to - 9.6 at 460 Ma) indicate that this crust was Proterozoic in age and chemical index of alteration values (CIA= 48-83) and oxygen isotope values (8 180 = +10.6 to +19.1) suggest it was moderately weathered. Subtle variations between the lithostratigraphic groups are also evident. Low CIA values (<58) and Th/Sc ratios(< 1) suggest a less mature, arc source of sediment in samples of the Edgecumbe Group. Elevated Ba (:::=: 1300) and V/Sc (20 to 86) in Cameron Group and correlative samples are indicative of a pelagic sediment component. Mafic metavolcanic rocks of the False Edgecumbe Formation exhibit high V/Ti ratios and REE patterns closely resembling those of arc volcanics in the Takaka Terrane. Distributions of concordant to slightly discordant U-Pb ages of detrital zircons (418 of 514 grains analysed) from two Fanny Bay and five Edgecumbe Group psammites are strikingly similar with two major peaks at 500-700 Ma (Ross-Delamerian and Pan-African) and 950- 1150 Ma (Grenvillian), with minor peaks at ea. 1400 and 1700 Ma, and scattered Archean ages. Similar detrital zircon age spectra have been reported for lower Paleozoic metasedimentary rocks from throughout eastern Australia and Antarctica and appear to be a characteristic of sediment deposited along the Paleo-Pacific margin of Gondwana. Zircons with trace elements diagnostic of alkaline rock parentage (Y <400 ppm, Lu<20 ppm) in the 500-700 Ma age populations suggest a sediment source in either East Africa or ice-covered East Antarctica. Upon the basis of their youngest zircons, Fanny Bay Group samples give maximum depositional ages in the earliest Ordovician (480-490 Ma), in agreement with previous fossil evidence, while the five Edgecumbe Group samples have slightly older maximum depositional ages ranging from early to mid-Cambrian (507-540 Ma). These and other geochemical characteristics support previous correlations of the Fanny Bay Group with the Buller Terrane. Definitive identification of an extension of the Takaka Terrane into Southwest Fiordland is more difficult due to similarities in the geochemistry and detrital zircon geochronology of the sedimentary successions in both terranes. These similarities extend throughout the Western Province indicating that the early Paleozoic terranes were in close proximity. That such similarities reach as far as the Australian and Antarctic fragments of the Paleo-Pacific Gondwana margin suggests that an extensive highland source provided detritus to a great marginal basin where efficient mixing allowed for the homogeneity of geochemical characteristics and detrital zircon signatures.

xiii, 186 p. : ill., maps ; 30 cm.

2006Simpson