Prograde and near-peak zircon growth in a migmatitic pelitic schist of the southeastern Canadian Cordillera

Abstract

U–Pb dates and trace elements acquired simultaneously on zircon from a migmatitic pelitic schist from the southeastern Canadian Cordillera are linked with results of phase equilibria modeling. The first of three phases of zircon growth occurred between 77 and 72Ma along a prograde path in the kyanite+melt field between the wet-melting and muscovite-dehydration melting reactions (675–775°C). This protracted phase of prograde growth is characterized by a shallow positive slope in heavy rare earth elements (HREE). The second phase occurred at c. 69Ma, after significant garnet growth by biotite dehydration-melting at 775–875°C, which produced zircon depleted in HREE. The last phase occurred at c. 63Ma on the retrograde path as garnet and rutile were consumed at >780°C, as indicated by enrichments in HREE and Nb, but prior to feldspar crystallization in the melt, as suggested by the moderate Eu anomaly. This path is consistent with rutile and monazite petrogenesis, documented herein and in a previous study, respectively. In contrast with the high temperatures at which zircon grew (675–875°C), Ti-in-zircon thermometry yields unrealistically low estimates (470–739°C) with 80% below the wet solidus; there are no uncertainties in the buffering assemblage, as rutile and quartz were present throughout, and no effect of pressure because a pressure-dependent calibration was used. This study demonstrates that zircon growth along a prograde path is possible and warrants further investigation on factors controlling the Ti content of zircon thermometry in metamorphic rocks.