Excavation of buried Dun Mountain–Maitai terrane ophiolite by volcanoes of the Auckland Volcanic field, New Zealand

Abstract

We present a description of the crustal rocks that underlie the Auckland Volcanic Field (AVF) based on a diverse suite of country rock-derived lithic clasts in the phreatomagmatic tuff of Glover Park and Taylors Hill volcanoes. The clasts are dominated by mafic schistose and non-schistose, amphibolite grade, meta-igneous (metabasite) rocks. Structural and mineralogical studies of these rocks reveal a complex structural and metamorphic history, including metasomatism and retrogressive metamorphism, which can be linked to their deformation history. These metabasites have no genetic relationship with associated AVF basalts. Location, composition, deformation, metamorphism and metasomatism indicate that the metabasite clasts come from a melange along the eastern boundary of the ophiolite that causes the regional Junction Magnetic Anomaly (JMA), which passes beneath the Auckland region and connects up with exposures of the Dun Mountain–Maitai terrane of the South Island. This conclusion is supported by one specimen from a suite of lower-grade metamorphic, greywacke-type clastic ejecta that contains shell fragments which we interpret as equivalents of the Atomodesma fragments in the Dun Mountain–Maitai terrane. Other lithic ejecta are sandstones from the underlying Miocene Waitemata Group, Mesozoic greywacke with prehnite veins, and chert. Pervasive cataclasite networks in the lithic clasts indicate that the Glover Park and Taylors Hill volcanoes mined a crustal fault zone within the Mesozoic basement several hundred metres deep, at an unconformity between basement rocks and overlying Cenozoic sediments.