Geochronological constraints on Cretaceous‐Paleocene volcanism in South Westland, New Zealand

Abstract

Cretaceous and Paleocene sedimentation in South Westland, New Zealand, is recorded in the Otumotu Formation, Tauperikaka Coal Measures, Whakapohai Sandstone, Arnott Basalt, Buttress Conglomerate, and Tokakoriri Formation, originally named and mapped by Nathan in 1977. Within this stratigraphic sequence, the name Buttress Conglomerate was used to describe volcanic conglomerates at Porphyry and Buttress Points that contained rounded clasts of plagioclasephyric intermediate volcanic rocks. Stratigraphically, the volcanic conglomerate at Porphyry Point forms sharp contacts with the underlying Arnott Basalt (Haumurian) and overlying Tokakoriri Formation (Teurian). The volcanic conglomerate at Buttress Point, however, is entirely fault-bounded. Clasts from each unit were collected and U-Pb zircon dated using the TIMS and ELA-ICP-MS methods. A trachyandesite clast collected at Buttress Point gives an age of 96.9 ±1.6 Ma, whereas a rhyolite clast collected at Porphyry Point gives an age of 61.4 ± 0.8 Ma. Petrological, geochemical, and stratigraphic data suggest that erosion of the clasts closely followed volcanism, and that these ages accurately reflect the depositional ages of the conglomerates. Conglomerates at Porphyry and Buttress Points have been formally renamed the Porphyry Point Member of the Tokakoriri Formation and the Buttress Point Conglomerate, respectively.

Temporal and geochemical evolution of the Cenozoic intraplate volcanism of Zealandia

“…The Arnott Basalt may have been part of a largely submarine shield volcano. An U/Pb zircon age of 61.4 Ma was obtained from a rhyolitic clast at Porphyry Point, from a deposit that stratigraphically overlies the Arnott Basalt (Phillips et al, 2005), providing a minimum age for the basaltic rocks. The lamprophyric Alpine Dike Swarm extends 110 km southeast from the Alpine Fault.…”

Magmatism during Gondwana break-up: new geochronological data from Westland, New Zealand

“…Additionally, post-110 Ma felsic (e.g. Phillips et al 2005;Tulloch et al 2009) andmafic (e.g. Nathan 1978;Randall 2004;Timm et al 2010;Ryland 2011) magmatic rocks occur sporadically throughout south Westland.…”

“…Lamprophyric dikes are also reported between Bald Hill and the southernmost parts of Westland (south Westland: Nathan 1977Nathan , 1978Bald Hill: Ryland 2011;Haast area: Randall 2004). In south Westland, the 64.8 ± 0.8 Ma Arnott Basalt (Timm et al 2010) and a 61.4 ± 0.8 Ma rhyolite clast (Phillips et al 2005) remain the only accurately dated magmatic events during Tasman Sea spreading. Mafic dikes have also been found in the deformed Fraser Complex and Granite Hill Complex (Rattenbury 1991;Waight et al 1997).…”

A tectonically uplifted marine shoreline deposit, Knights Point, Westland, New Zealand

“…The basement is overlain unconformably by a thick sequence of breccias-the Otumotu Formation of probable Cretaceous age (Nathan 1977)-and then by a terrestrial (Tauperikaka Coal Measures) to marine, transgressive sequence with the Whakapohai Sandstone extending upwards into the Tititira Formation. Extensive volcanism occurs throughout the sequence, initially with the Arnott Basalt (the top of which was dated at 61 Ma by Phillips et al 2005) and later with the Otitia Basalt. The volcano-sedimentary succession has been extensively deformed and now occurs in sequences comprising often steeply dipping or overturned strata bounded by the Arnott, Grave, and Mistake Fault zones (Young 1968).…”

“…The geology of South Westland follows work by Wellman (1955) and Nathan (1975Nathan ( , 1977, with modifications by Phillips et al (2005). The basement rocks of South Westland comprise the Greenland Group, a sequence of greywackes and their low grade regional and contact metamorphosed equivalents of probable early Paleozoic age (R. Cooper 1975;Ireland 1992).…”