RELEASE OF TRACE ELEMENTS THROUGH THE SUB-GREENSCHIST FACIES BREAKDOWN OF DETRITAL RUTILE TO METAMORPHIC TITANITE IN THE OTAGO SCHIST, NEW ZEALAND

Abstract

Detrital rutile is observed in trace amounts in the sub-greenschist facies, graywacke, and argillite lithologies of the Torlesse Terrane-derived Otago Schist, New Zealand. Trace element analyses reveal that mean compositions of detrital rutile grains are characterized by high concentrations (500–3600 ppm) of V, Cr, Fe, Zr, Nb, and W, moderate amounts (100–500 ppm) of Sn and Ta, and minor amounts (50–100 ppm) of Mn and Hf. Recrystallization of detrital rutile to metamorphic titanite is first observed in the prehnite-pumpellyite facies rocks, with recrystallization largely complete by upper chlorite greenschist facies. Dissolution, material transport, and precipitation reactions were crucial in the progression of this recrystallization reaction, with pore spaces generated allowing the transportation of Ca, Si, and O to the titanite-rutile interface. Redistribution of trace elements during the mineral transition of detrital rutile to metamorphic titanite was assessed using mass-balance techniques. These calculations indicate that V, Cr, Zr, Nb, Ta, and W were released during the prograde mineral reaction. The liberation of this suite of trace elements potentially provides a source for the W and Cr enrichment observed in the orogenic deposits of the Otago Schist.