A Rhyolite Compositional Continuum Governed by Lower Crustal Source Conditions in the Taupo Volcanic Zone, New Zealand

Abstract

Rhyolites generated in the modern Taupo Volcanic Zone (TVZ), New Zealand, have previously been interpreted as having evolved by a combination of extensive fractional crystallization of mantle-derived mafic magmas and limited crustal assimilation (up to 25%). Polytopic vector analysis (PVA), a form of multivariate statistical analysis, of the major-element compositions of over 475 basaltic to rhyolitic bulk-rock samples, representing over 600 kyr of volcanism within the TVZ, has provided a robust platform for rhyolite characterization and new insights into rhyolite petrogenesis. There is a continuum of compositions between two rhyolite end-member magma types (EM1 and EM2), which have been identified on the basis of the PVA and which have distinct petrological and geochemical characteristics, as follows. EM1: crystal-rich (up to 45%), hydrous phases (± hornblende ± biotite ± cummingtonite), high Aluminum Saturation Index [ASI; molar Al<inf>2</inf>O<inf>3</inf>/(CaO + Na<inf>2</inf>O + K<inf>2</inf>O)], low FeO*/MgO (calc-alkaline series), depleted abundances of middle rare earth elements (MREE) and Y, and high Sr; EM2: crystal-poor (<10%), anhydrous phases (orthopyroxene ± clinopyroxene), high FeO*/MgO (tholeiitic series), low ASI, less depleted MREE and Y, and low Sr. The range of ASI values, and relative depletion in MREE and Y in the rhyolites is consistent with the results of experiments to constrain the partial melting behaviour of amphibolite at crustal pressures. The major- and trace-element data are also consistent with 50–60% equilibrium crystallization of a crustally contaminated, hornblende-bearing andesite to produce the TVZ rhyolites. Distinct major- and trace-element variations along the continuum between the two rhyolite end-member types can be effectively modelled by simulating changes in the temperature–<it>f</it>O<inf>2</inf>–<it>f</it>H<inf>2</inf>O conditions in the lower crust where mantle-derived mafic magmas are stored and differentiate. Low <it>T</it> and high <it>f</it>O<inf>2</inf> and <it>f</it>H<inf>2</inf>O in the crustal magma storage zone promote abundant hornblende crystallization and suppress plagioclase crystallization, which produces the EM1 type rhyolite. By increasing the temperature and/or lowering <it>f</it>O<inf>2</inf> and <it>f</it>H<inf>2</inf>O in the magma storage region, plagioclase becomes more dominant and hornblende crystallization is suppressed, producing more EM2-like rhyolitic magma types.