Carbonatites are rare mantle-derived igneous rocks often economically enriched in rare earth and high field strength elements and are crucial archives of mantle and crustal processes. Zircon U-Th-Pb geochronology provides a powerful means of constraining their magmatic and hydrothermal evolution, but interpretation is often hindered by discordant data arising from open-system behavior. Discordant data are often dismissed as meaningless, yet hydrothermally recrystallized zircon can in fact record the timing of fluid-rock interaction, offering a means to date important orogenic-related or economic deposit-forming hydrothermal events. Here, we show that integrated U-Th-Pb geochronology, textural observations, trace element analysis, and Pb isotope data from the Mount Grace carbonatite and associated carbonatites and syenites in the southeastern Canadian Cordillera resolve both magmatic and hydrothermal histories. Four samples from meta-carbonatite and meta-alkaline rocks in the Monashee complex and Selkirk allochthon in the southern Omineca belt were studied to clarify the magmatic ages through new zircon U-Th-Pb geochronology. Concordant U-Pb zircon ages of ca. 360 Ma from the pyroclastic Mount Grace carbonatite confirm a Late Devonian depositional age for protoliths of the Monashee cover gneiss. Ca. 360 Ma magmatic ages for all four carbonatite-syenite samples expand the record of Late Devonian−Early Mississippian alkaline magmatism in the Monashee complex and Selkirk allochthon. Hydrothermally altered zircon domains that correspond with convolute zoning in cathodoluminescence and elevated light rare earth element contents yield a Th-Pb age of ca. 163 Ma from the Mount Grace carbonatite and U-Pb weighted mean and upper intercept ages of ca. 130 Ma from the Three Valley Gap carbonatite and syenite in the Monashee Complex and the Trident Mountain syenite in the Selkirk allochthon. Our data identifies a Jurassic hydrothermal event that affected carbonatites in both the Monashee complex and Selkirk allochthon, argued by some authors to have been tectonically separated until the Upper Cretaceous−Paleocene. Mineral Pb isotopic analyses from the same four carbonatites and syenites and one sediment-hosted magnetite-rich ore deposit reveal a highly radiogenic linear trend inconsistent with traditional closed-system models, likely indicating mixing between radiogenic Pb derived from clastic sedimentary sources and Pb in the alkaline bodies. This, along with the robust Late Devonian zircon age of the underlying Mt. Grace carbonatite, calls into question the validity of the shale curve model age for the Cottonbelt Pb-Zn deposit and helps resolve a longstanding age conundrum. Our findings refine the timing of Late Devonian carbonatite-alkaline magmatism along the western Laurentian margin, reveal a previously unrecognized Jurassic hydrothermal event that affected both the Monashee complex and Selkirk allochthon during Cordilleran orogenesis, and address a decades-old age conundrum in the Monashee complex leading to determination of a ca. 360 Ma maximum depositional age of the Monashee complex. More broadly, this study underscores the potential of hydrothermal zircon to date fluid-mediated events in carbonate-rich systems, offering a new framework for interpreting isotopically disturbed datasets in mineralized terranes.
Abdale et al. (Fri,) studied this question.