Geochemistry, geochronology, and petrology of the Arnott basalt and Tokakoriri formation, Tokakoriri Creek, South Westland

Phillips, Carolyn J. (Carolyn Jeanne)

A detailed study of a two kilometre coastal section from Tokakoriri Creek to Porphyry Point, South Westland, encompasses a strip of structurally deformed Cretaceous and Tertiary strata. The Cretaceous-Tertiary rocks include the Arnott Basalt (Mt-Dt), the Tokakoriri Formation and Porphyry Point Member (new) (DtDw), and the lower Abbey Limestone Formation (Dw-Dm). The Arnott Basalt erupted in a submarine environment, and comprises lava flows, hyaloclastite breccias, massive sheet flows, columnar flows, and isolated pillows in broken pillow breccias. Geochemical and mineralogical analyses reveal that the Arnott Basalt is mildly alkaline, and was erupted in a within plate tectonic setting. The strata directly overlying the Arnott Basalt are varied, comprising tuffs and conglomerates of polygenetic origin. While lapilli tuffs have basaltic to intermediate compositions, crystal tuffs and an ash deposit reflect active, explosive rhyolitic volcanism during the early Paleocene (Dt). The Porphyry Point Member (new) is proposed to include mass flow deposits intercalated with the Tokakoriri Formation. Petrological, mineralogical, and geochemical analyses show that the Porphyry Point Member (PPM) contains clasts of porphyritic trachyandesite, trachyte and rhyolite with alkaline affinities. Preliminary petrogenetic major element modelling suggests that volcanic clasts form two series: trachyandesite-trachyte C56-66 wt.%Si02) and trachyte-rhyolite (66-71.5 wt.%Si02). The PPM may be related to the Arnott Basalt through complex fractional crystallization, assimilation, or magma mixing processes. A new U-Pb date reveals that the Porphyry Point Member is 61.43 ± 0.76 Ma, placing an upper limit to the age of the underlying Arnott Basalt, and a lower age limit on deposition of Tertiary strata in the Tokakoriri Creek area. The Tokakoriri Formation is divided into 4 distinct lithofacies, based on field observations, previous micropaleontological work, and petrographic descriptions. This subdivision facilitates determination of younging direction in structurally complex areas, and helps to show that overturned strata are juxtaposed against right way up strata on the western limb of the Flaggy Antiform. Mapping and structural analyses recognize at least 2 phases of deformation: (1) sediment deformation including soft sediment, ductile fabric formation, and bedding parallel faulting and (2) oblique and dip-slip normal faulting. Overall, the Tokakoriri Creek coastal section reflects a complex structural history involving sediment deformation of unlithified or weakly lithified sediments overprinted by tectonic faulting and folding episodes.

xiii, 291 leaves : ill. (some col.), maps (some col., some folded) ; 30 cm.

2001Phillips

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