Upper Tertiary and Quaternary volcanism and subduction zone regression, North Island, New Zealand

Abstract

Radiometric ages for Cenozoic calc-alkaline igneous rocks in the North Island indicate the presence of two contrasted areas of eruption. An older group of volcanics (27–23 Ma) is spread NW-SE along the east coast of Northland between North Cape and Whangarei; these lavas, including garnet andesites, were generated during an isolated igneous event accompanying obduction of ophiolite massifs during the Late Oligocene. A younger group of Miocene to Holocene age (21–0 Ma) is distributed between Tokatoka-Whangarei and Egmont-Taupo Volcanic Zone across a series of NE-SW time-lines (volcanic arcs) which record a southeasterly younging direction for sequential initiation of volcanism; the erupted rocks lie along two major belts trending NW-SE (East Belt and West Belt) parallel to old basement fractures. Depths to magma sources are estimated from K₂O-SiO₂ indices, with deeper levels indicated for the Tokatoka-Egmont West Belt (114–220 km) than for the East Belt (72–148 km) between Whangarei and Taupo Volcanic Zone. The shallow-sourced magmas of the East Belt have higher ⁸⁷Sr/⁸⁶Sr isotope ratios than those of the West Belt, and have developed a metallogenic province with andesitic epithermal gold-silver and porphyry-copper stocks. A subduction system responsible for the Miocene and later volcanism appears to have had the same trend and polarity as the modern Benioff zone beneath the North Island, with NE-SW strike and NW dip. The first stage of this convergence system, preceding the Miocene igneous activity, was a shallow, non-volcanic underplating of the North Island originating from the Hikurangi Trench. By the Early Miocene this lower lithospheric plate had reached as far north as the latitude of Whangarei-Tokatok a where it descended steeply, perhaps due to decrease in velocity. At this stage the mode of convergence for the lower plate changed from progressive underflow to subduction regression, and volcanism commenced along the 21 Ma time-line; this volcanic front subsequently migrated southeastward at rates controlled by oceanward retreat of the subduction zone. In regions traversed by rapid subduction regression (23–27.5 km/myr) the Mesozoic greywacke basement foundered and was extensively depressed by tensional collapse, but in areas of slower migration rates (≈ 10 km/myr) the basement has remained high-standing.