The Island Arc Setting of a New Zealand Cambrian Volcano-Sedimentary Sequence: Implications for the Evolution of the SW Pacific Gondwana Fragments

The Waingaro Fault Slice, one of several structural units of the early Paleozoic Takaka Terrane in Northwest Nelson, South Island, New Zealand, consists of six fault-bounded subslices that represent different sections of a late Middle to early Late Cambrian volcano-sedimentary succession. Three basaltic to basaltic-andesitic suites and their volcaniclastic equivalents (150-500 m thick; previously mapped as Devil River Volcanics) are intercalated in a nonvolcanic sequence of turbiditic sandstones and siltstones (approximately 1800 m thick). Compositional systematics of the relatively immobile high field strength elements (HFSE) and rare earth elements (REE) as well as the depositional facies provide strong evidence for an island arc setting for the three basaltic to basaltic-andesitic suites. The oldest (Benson volcanic suite) has incompatible trace elements similar to arc-tholeiites, whereas the two younger suites (Cobb Flat and Peel volcanic suites) show affinities with calc-alkaline and high-K suites. Enrichment in LREE and, as inferred, in large ion lithophile elements (LILE) increases up stratigraphic section and is interpreted to reflect heterogeneities in the source rocks, possibly due to increasing slab-derived metasomatism of the mantle source. Fractionation of olivine and clinopyroxene from picritic to tholeiitic parental magmas is inferred to have caused variation in abundance of the elements Mg, Ca, Al, Ni, Cr, V, Se within individual suites. Comparable subduction-related Cambrian volcanics in most other fragments of southeast Gondwana (southeast Australia, Tasmania, and West Antarctica) are dominated by more evolved compositions. Volcanics of the Waingaro Fault Slice most closely resemble the predominantly basaltic Glasgow Volcanics of the early Paleozoic Bowers Terrane, Antarctica, consistent with their inferred close relationship in Cambrian time.