Zoned garnets in metapelites in western Fiordland, southwest New Zealand: Polychronic crystallisation and insight into the nature and extent of Early Cretaceous regional metamorphism

Abstract

Abstract Two distinct types of compositional zoning are recognised in idioblastic to subidioblastic garnets in amphi‐bolite facies metapelites in western Fiordland. One type occurs in kyanite (± sillimanite)‐bearing rocks and is characterised by a core to rim increase in CaO content, and a sympathetic decrease in FeO and MgO, with MnO remaining essentially constant, or decreasing slightly near the rim. This zoning pattern formed during garnet growth, and records adjustment of mineral equilibria under conditions of increasing load pressure. Garnets showing the same zoning pattern occur within metapelitic rocks in rafts within nearby Western Fiordland Orthogneiss (equivalent to western Fiordland granulites), and within a contamination zone overprinting this orthogneiss at the margin of a highly aluminous raft. Garnet zoning thus provides evidence that these rocks experienced the same Early Cretaceous tectono‐metamorphic event. The pressure increase has been attributed to crustal thickening by tectonic loadi...