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

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 loading in a plate boundary collisional regime.A much less common type of zoned garnet occurs in sillimanite + K‐feldspar‐bearing metapelites and is characterised by a core to rim decrease in CaO and MgO contents and a sympathetic increase in FeO and MnO near the rim. This zoning pattern is interpreted to result from post‐growth volume diffusion, and records metamorphism at significantly lower pressures than that indicated by rims of the growth‐zoned garnets described above. Overprinting relationships suggest that the diffusion‐zoned garnets formed prior to crystallisation of the growth‐zoned garnets. At least some of the diffusion‐zoned garnets are products of Early Cretaceous metamorphism, and, together with the growth‐zoned garnet, indicate two distinct episodes of garnet growth during this period. Isotopic evidence presented recently for mid‐Paleozoic metamorphism in western Fiordland suggests that separate metamorphic terranes of Early Cretaceous and mid‐Paleozoic age are probably present in this area. © 1991 Taylor & Francis Group, LLC.