Pumpellyite-actinolite and contiguous facies metamorphism in part of Upper Wakatipu District, South Island, New Zealand

Abstract

Metamorphic minerals from pumpellyite-actinolite and contiguous facies in the Upper Wakatipu district, South Island, New Zealand, have been studied using optical and X-ray diffraction methods, and electron microprobe analyser. Analyses are given for chlorite, garnet, prehnite, stilpnomelane, alkali-amphibole, actinolite, pumpellyite, epidote, and muscovite. The area is divided into four progressive metamorphic zones. Mineralogical change is summarised. Plagioclase feldspars are all low albite. Muscovite is ubiquitous in all zones except for a few mafic rocks. Analysed muscovites are mildly phengitic. The composition is not related to grade, but is related to host rock composition. Refractive indices of muscovites are rather low and indicate a lower celadonite content than those of eastern Otago and the Sambagawa zone. Basal spacing of muscovites do not differ much between zones or rock types. Epidote is common throughout the area, especially in mafic rocks. The compositional range is fairly narrow having a maximum at around 29 molecular percent of Ca₂Fe₃Si₃O₁₂ (OH). The change in the higher grade part of Zone IIIb from ferric to aluminous variety may correspond to the disappearance of pumpellyite in Zone IV. Epidotes from the area tend to show higher refractive indices than those from hydrothermal and metasomatic environments. Lawsonite, which occurs in Zone 11, is low in ferric iron and has unusually low refractive indices. Pumpellyite is predominantly blue-green in Zones I, II, and IIIa, and predominantly colourless in Zone IIIb. The colour change between iron-rich and aluminium-rich is supported by chemical analyses. A prehnite almost free of ferric iron has been analysed in Zone I. Actinolite occurs in Zones IlIa, IlIb, and IV. There is no obvious correlation between the composition of actinolite and metamorphic grade, and composition seems to be largely controlled by the host rock composition. Some of the analysed minerals are barroisitic (subcalcic), which is characteristic of glaucophanitic metamorphism. Occurrence of alkali-amphiboles is restricted to Zone IIIa. Alkali-amphiboles are high in Fe and are classified as riebeckite-magnesioriebeckite while crossite and glaucophane are also identified optically. The latter minerals are mantled by actinolite or other alkali-amphiboles. Glaucophane and crossitc appear to be unstable in Zone IIIa. Ferristilpnomelane is common throughout Zones I to IV. The composition of stilpnomelane seems to be related to host rock composition. Chlorite is one of the commonest and most abundant minerals in rocks of all grades. Si-Al substitution is very limited while Fe-Mg substitution is fairly large. Chlorites in psammitic and pelitic rocks have higher Fe content than those in mafic rocks. The basal spacing of chlorite shows a good inverse correlation with Al content. There is no definite variation of chlorite composition. with metamorphic grade. A garnet unusually high in the andradite molecule is found in Zone IIIa. Almandine is present. Axinite and tourmaline are also commonly found. Except for the actinolite-chlorite pair, the co-existing mineral pairs do not show consistent distribution coefficients. The reason for this is briefly discussed. Probable reactions defining isograds are presented and Al-Ca-Fm-Fe⁺³ diagrams are used to explain the mineralogical change.