Trace element changes in rutile from quartzite through increasing P–T from lower amphibolite to eclogite facies conditions

Abstract

Low concentrations of Na, Ca, K, Fe, Mg and Al in quartzite commonly prevent the crystallization of index metamorphic minerals, inhibiting the obtainability of thermobarometric calculations. Quartzite typi-cally contains quartz, zircon and rutile; therefore, single-element thermometers, such as Zr-in-rutile, may be applied. We investigate changes in trace-element composition of rutile from quartzite through increasing metamorphic conditions. Studied samples derive from a quartzite package (Luminárias Nappe, Minas Gerais, Brazil) where previous thermobarometric constraints on metapelites showed an increasing metamorphic grade south-wards, from high-pressure lower amphibolite facies (580°C; 0.9 GPa) to eclogite facies (630°C; 1.4 GPa). Rutile from the lower-grade facies samples show a large spread in Zr concentrations, with the highest values corresponding to temperature estimates higher than metamorphic conditions affecting those units, and thus interpreted as inherited detrital signatures. A narrower spread in Zr concentration is observed in rutile grains from the higher-grade facies, and estimated Zr-in-rutile temperatures agree with previous thermobarometric constraints. Therefore, we show that at 630°C, Zr contents in detrital rutile from quartzites re-equilibrate. The comparison between the quartzite-and metapelite-hosting rutile grains from the same area shows that the resetting of the geothermometer in the latter seems to occur at slightly lower temperatures (∼50°C lower).