Original paper

Paragenesis of titanite in metagreywackes of the Franz Josef-Fox Glacier area, Southern Alps, New Zealand

Grapes, Rodney; Watanabe, Teruo

European Journal of Mineralogy Volume 4 Number 3 (1992), p. 547 - 556

23 references

published: Jun 11, 1992
manuscript accepted: Sep 2, 1991
manuscript received: Dec 18, 1990

DOI: 10.1127/ejm/4/3/0547

BibTeX file

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Abstract Titanite occurs as an accessory mineral in schistose metagreywackes of the chlorite and biotite-albite zones in the Franz Josef-Fox Glacier area, Southern Alps of New Zealand. Two different types of titanite are recognized ; detrital and authigenic, (i.e. as a replacement phase of detrital titanite and ilmenite, and as single grains in the matrix). Detrital titanite persists into the higher grade part of the biotite-albite zone and is characterized by low Al2O3, high TiO2 and significant Fe2O3 in addition to containing Mn, Ce, La and Nb. With respect to authigenic matrix titanite, titanite that replaces detrital titanite and ilmenite has low AI2O3 and high TiO2 reflecting the host mineral compositions. Nb is present but Ce and La are below detection limit. Authigenic matrix titanite becomes depleted in Al, Fe and F and enriched in Ti with increasing metamorphic grade indicating the coupled substitution [Al, Fe]3+ + [F, OH]- ⇄T4+ + O2-. Decreasing Fe3+ can probably be related to decreasing bulk rock oxidation ratio ; decreasing Al to preferential uptake by coexisting epidote and decreasing F is associated with progressive F-enrichment of apatite with increasing metamorphic grade. At temperatures between 300-400°C at pressures of 3-4kbar the titanite + CO2 ⇄ calcite + rutile + quartz buffer reaction indicates that XCO2 in the fluid phase was below 0.02 in rocks of the chlorite and biotite-albite zones where calcite is only present as veins. In the biotite-albite-oligoclase zone, calcite + ankerite occur as matrix minerals and XCO2 may have been ~ 0.05 and coinciding with the disappearance of titanite and the incoming of oligoclase (An20) and authigenic ilmenite. Because the [Al, Fe]3+ + [F + OH]- ⇄ Ti4 + O2- substitution in titanite is expected to increase with increasing pressure and/or increasing temperature in rocks of essentially the same bulk composition, the early generation of Al-Fe-F-rich titanite in the metagreywackes may be the product of a higher P/T metamorphism than the later generation of Al-Fe-F-poor replacement titanite


titanitemetagreywackeSouthern AlpsNew Zealand