Crustal basement terranes under the Taupō Volcanic Zone, New Zealand: Context for hydrothermal and magmatic processes

Abstract

Continental magmatic arcs commonly have heterogeneous and anisotropic metamorphic basement foundations hosting their plutonic roots and associated geothermal systems. Quaternary volcanic and volcaniclastic rocks of New Zealand's Taupō Volcanic Zone (TVZ) lie above and between the Torlesse and Waipapa Composite Terranes, two of New Zealand's major basement geological units. Deep geothermal and magma systems beneath TVZ interact with and/or are hosted in these Mesozoic metasedimentary crustal basement terranes rather than in Quaternary volcanic and volcaniclastic fill. New mineralogical, whole rock geochemical, O, Sr, Nd and Pb isotopic, and U-Pb zircon age data from lava-hosted xenoliths and ignimbrite-hosted lithics imply that the terrane boundary is not sub-vertical as previously thought, but instead dips gently west. Most of the non-igneous crust under the TVZ is thus composed of Torlesse Composite Terrane metasedimentary rocks, which have higher quartz, mica and SiO2 abundances, higher 87Sr/86Sr, 206Pb/204Pb and δ18O ratios, lower chlorite and MgO contents and lower 143Nd/144Nd ratios than their Waipapa Composite Terrane counterparts. This new interpretation provides important new constraints to refocus geodynamic understanding, petrological modelling, and future deep geothermal drilling.