Deposition, Metamorphism and Mineralization of the Onekaka Schist in northwest Nelson

Abstract

The Onekaka Schist, a constituent of the eastboard Takaka Terrane in northwest Nelson, is a north-trending fault slice of stratigraphically intact Ordovician-Silurian quartzofeldspathic, amphibolite and quartzite metasediments. The pressure, temperature, timing and deformation (P-T-t-D) history and depositional setting of the Onekaka Schist is examined through petrology, geothermobarometry, whole rock chemistry and zircon geochronology. Quartzofeldspathic metasediments (metaconglomerates, sandstones, mudstones, and amphibolites) were deposited into an active continental margin (convergent/collisional) environment, whereas quartzites were deposited into a passive margin environment.

The Onekaka Schist contains a typical peak equilibrium biotite-andesine-garnet-quartz-calcite (± hornblende-staurolite-kyanite) mineral assemblage. Four structural and mineralogical fabrics occur in aluminous schists; S0) sedimentary contacts and composition variation, S1) primary foliation observed as relict quartz inclusion trails in garnet, S2) development of a pervasive lepidoblastic biotite foliation, S3) crenulation of the earlier foliation accompanied by garnet growth, and S4) a chlorite, epidote and muscovite lineation. S3 represents peak metamorphic conditions. All fabrics are rotated by north trending kilometre-scale folding. Two generations of mineralized veins cross-cut the foliation; K-feldspar veins containing Cu-sulfides and chlorite-carbonate veins containing Fe-sulfides. In both cases, mineralization occurred by retrogressive alteration of the peak mineral assemblage. Kyanite-quartz veins contain a peak kyanite-staurolite-garnet-andesine-biotite-quartz assemblage in alterations zones reflecting S3 conditions, which has been pervasively altered into a muscovite-margarite-chlorite-ilmenite assemblage. Peak equilibrium pressure-temperature conditions were calculated by major and minor element geothermobarometry on garnet-biotite-plagioclase-quartz assemblages. Conditions increase southeast across the Onekaka Schist, from 576 ± 12 °C at the northern end of the fault slice to 610 ± 12 °C and 7.5 ± 0.5 kbar conditions at the southern end (Anatoki River). Ti-in-quartz geothermometry of quartz in hydrothermal veins indicates that quartz crystallized between 460 – 550 ± 20 °C. Ti-in-quartz geothermometry of quartzites develop a S1 – S2 foliation at c. 400 ± 20 °C. Detrital zircon age populations indicate Ross-Delamerian, Pan-African and Grenville Orogeny provenance. Variation in age population abundance between quartzofeldspathic and micaceous schists to quartzites may indicate a changing sediment source. A c. 390 – 400 Ma zircon population is likely to reflect S3 metamorphism of the Onekaka Schist. The Onekaka Schist represents a slither of Paleozoic metasediments deposited in a predominantly convergent-collisional continental setting. Amphibolite facies metamorphism across the Onekaka Schist correlates to the Pikikiruna Schist, and indicates that barrovian metamorphism was occurring on a regional scale during terrane amalgamation of the Buller and Takaka terranes prior to regional deformation during the Rangitata Orogeny.