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

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 72 Ma along a prograde path in the kyanite + melt field between the wet-melting and muscovite-dehydration melting reactions (675-775 degrees 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. 69 Ma, after significant garnet growth by biotite dehydration-melting at 775-875 degrees C, which produced zircon depleted in HREE. The last phase occurred at c. 63 Ma on the retrograde path as garnet and rutile were consumed at > 780 degrees 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 degrees C), Ti-in-zircon thermometry yields unrealistically low estimates (470-739 degrees 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. (C) 2017 Elsevier B.V. All rights reserved.