PORPHYROBLASTIC MANGANAXINITE METAPELAGITES WITH INCIPIENT GARNET IN PREHNITE-PUMPELLYITE FACIES, NEAR MEYERS PASS, TOULESSE TERRANE, NEW-ZEALAND

Moderately manganiferous siliceous pelagites near Meyers Pass, Torless e Terrane, South Canterbury, New Zealand, have been metamorphosed in t he prehnite-pumpellyite facies. A conodont colour index measurement su ggests T-max in the range 190-300 degrees C. Porphyroblastic manganaxi nite, manganoan pumpellyite, manganoan chlorite and trace spessartine- rich garnet and sphalerite have formed in an extremely fine-grained qu artz-albite-berthierine-phengite-titanite groundmass. Porphyroblastic manganaxinite semischists and schists are distinctive rocks in prehnit e-pumpellyite to lower-grade greenschist and blueschist facies of New Zealand and Japan. Mn in the manganoan pumpellyites substitutes for Ca in W sites. Total Fe/(Fe + Mg) ratios in chlorite are dependent on ox idation state, being less than or equal to 0.22 in red hematitic hemip elagites, and greater than or equal to 0.61 in low-fO(2) grey metapela gites. In the low-fO(2) metapelagites, manganoan berthierine with litt le or no chlorite is inferred in the groundmass and iron-rich chlorite occurs as porphyroblasts and veinlets, whereas in the red rocks, Mg-r ich chlorite occurs both in groundmasses and veinlets. Variably high S i in the manganoan chlorites correlates with evidence for contaminant phases. The Mn content of chlorite contributing to garnet growth is de pendent on metamorphic grade; incipient spessartine indicates a satura tion value of 6-8% MnO in chlorite in low-fO(2) rocks at Meyers Pass. Lower MnO contents are recorded for otherwise analogous rocks with inc reasing metamorphic grade, but at a given grade coexisting chlorite an d garnet are richer in Mn where fO(2) is high. Manganaxinite and manga noan pumpellyite also contributed to reactions forming grossular-spess artine solid solutions. Formation of garnet in siliceous pelagites is dependent on both Mn and Ca content. The spessartine component increas es with grade into the greenschist facies. Partial recrystallization o f berthierine to chlorite and the growth of porphyroblastic patches of other minerals was facilitated by brittle fracture and access of flui ds to an otherwise impermeable matrix; to this extent the very low-gra de metamorphism was episodic.