The origin of garnets in andesitic rocks from the Northland arc, New Zealand, and their implication for sub-arc processes

Abstract

An unusual andesitic suite from the Miocene volcanic arc in Northland, New Zealand, comprises pyroxene andesite and garnet-bearing hornblende-pyroxene, hornblende and biotite-hornblende andesites. Garnet crystals occur as 1-10 mm single crystals or more commonly as two or more annealed crystals and as garnetite lenses. The andesitic rocks also contain enclaves of high-MgO pyroxenite, hornblendite, and pyroxene-hornblende gabbro as well as high-Al 2O 3 hornblende gabbro, garnet-hornblende gabbro, and anorthosite. Garnet crystals in the andesitic volcanic rocks and in the enclaves show comparable compositional ranges, zoning patterns and inclusions, which indicate that they share a common petrogenetic history. They can be grouped into four distinct types on the basis of mode of occurrence, chemical composition and zoning patterns, which leads to their interpretation as antecrysts rather than orthocrysts. The compositions of the garnets, as well as their included mineral assemblages, reflect a petrogenetic trend from high-temperature pyroxene-bearing high-Mg garnet to low-temperature Fe-rich garnet at relatively constant pressure. Well-preserved zoning patterns, in particular those of the Ca- and Mg-rich garnets, reflect processes within a deep crustal arc environment. Later assimilation is suggested by some zoning patterns that show decreasing Ca and increasing Fe and Mn contents. The garnets are interpreted as being derived by disintegration of discrete but closely related cumulate material that formed at pressures of 8-10 kbar. The host volcanic rocks and their garnet crystals together with the enclaves thus represent a consanguineous mixture of liquid and solid components that developed where subduction-related magmas ponded and interacted at or near the base of the crust. Together they represent a rare snapshot of the processes and components that produce arc-type rocks. © The Author 2012. Published by Oxford University Press. All rights reserved.