The age, geology and geochemistry of the Tapuaenuku Igneous Complex, Marlborough, New Zealand

The Tapuaenuku Igneous Complex (TIC) is a mid-Cretaceous layered intrusion and dike swarm cropping out at the axis of the Inland Kaikoura Ranges, South Island, New Zealand. The TIC is part of an extensive, but poorly preserved, igneous province that formed during or after cessation of subduction along the margin of Gondwana c. 100 Ma ago.

The TIC is a complex, multiphase intrusion that is subdivided into eight units (in order of relative emplacement): (1) radial dike swarm—basanite, trachybasalt, and shoshonite/latite dikes; (2) Layered Series (LS)—basin-shaped mafic cumulate rocks; (3) Staircase Intrusives— noncumulate gabbro and monzogabbro enclosed by a discontinuous sheet of cumulate gabbro; (4) Lower Hodder Gabbro and minor gabbroic intrusives—small stocks of noncumulate gabbro, monzogabbro, and monzonite; (5) Red Hills Breccia Pipe—heterolithic breccia emplaced in a pipe or diatreme; (6) Hodder Intrusives—a monzonite and sodalite syenite laccolith intruding the LS; (7) monzonite, orthoclase syenite and quartz syenite sills and dikes, and; (8) highly alkaline lamprophyre, phonotephrite, tephriphonolite, and phonolite dikes.

The plethora of rock types in the TIC were produced by fractionation of different mineral assemblages from a trachybasalt or basanite parent magma. The parental magma had incompatible trace element and radiogenic isotopic characteristics similar to HIMU-type ocean island basalt and akin to that of the numerous Cenozoic intraplate volcanic provinces situated in southern New Zealand. However, elevated initial Sr isotope ratios of some of the intrusive units requires them to have assimilated Torlesse grey wacke country rock.