TITANITE FROM THE HIGH-PRESSURE BELT IN WESTERN GRENVILLE PROVINCE, CANADA RECORDS HOT EXHUMATION

Exhumed orogenic lower crustal rocks in the western Grenville Province recorded a high-pressure (HP) metamorphism with peak conditions of ~2 GPa, ~900 °C at ~1060 Ma. However, the exact exhumation conditions and timing remain poorly constrained. In this study, two samples, 19-45 and 19-60, from the Algonquin terrane and the Lac Dumoine terrane of the allochthonous HP belt, were studied to constrain the exhumation of orogenic lower crust. The garnet-bearing gneiss 19-45 from the Algonquin terrane contains a mineral assemblage of plagioclase, K-feldspar, quartz, garnet, amphibole, ilmenite, biotite, titanite and zircon. The migmatite 19-60 from the Lac Dumoine terrane contains an assemblage of garnet, amphibole, plagioclase, quartz, biotite, and alkali-feldspar along with minor titanite and accessory zircon. Wavelength dispersive spectrometer (WDS) mapping of titanite grains in both specimens shows an increase in Ce and Nd moving towards the rim. Laser ablation split stream ICPMS analyses were conducted to reveal the trace element geochemistry and timing of titanite. Rare earth element (REE) plot of titanite in 19-45 shows a shallow downward slope from MREE to HREE (with the average Gd/Yb ratio as 4.1 ± 1.6) along with a negative Eu anomaly. Titanite in 19-60 shows a steep downward slope from MREE to HREE (the average Gd/Yb ratio as 17.5 ± 21.5) with a positive Eu anomaly for rim analyses. U-Pb geochronology of titanite yields ages of ~1045 Ma for 19-45 and ~1027 Ma for 19-60. A second generation of titanite in 19-60, which displays high positive Eu anomaly, yields an age of 970 Ma. Zr-in-titanite thermometer yields temperatures ranging from 795 °C to 820 °C for 19-45 when pressure is assumed 1.0–1.2 GPa based on upper titanite boundary in pseudosection. The same Zr-in-titanite thermometer yields a range from 800 °C to 880 °C for the first generation of titanite in 19-60, assuming a pressure range of 1.1–1.8 GPa. The second generation of titanite in the same specimen yields a systematically lower temperature (~30 °C less than the first generation). The study shows that titanite in the two specimens from the allochthonous HP belt recorded a hot exhumation, until 1045 and 1027 Ma, at the temperature of titanite crystallization and/or its U-Pb closure temperature.