Calcic garnets as a geochronological and petrogenetic tool applicable to a wide variety of rocks

Abstract

This contribution presents U-Pb geochronological data for Ca-Fe-Ti garnets from compositionally diverse alkaline and carbonatitic intrusive rocks ranging from Neoarchean to Permian-Triassic (Cinder Lake and Eden Lake in Manitoba, Canada; Belaya Zima and Odikhincha in Siberia, Russia; Afrikanda in the Kola Peninsula, Russia) obtained using isotope-dilution thermal-ionization mass spectrometry (ID-TIMS) and their trace-element compositions measured by laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS). The studied garnets yield U-Pb concordant or subconcordant ages obtained with a precision of <0.5% owing to their relatively high content of U and negligible common Pb. The new ID-TIMS data are in excellent agreement with the previously reported zircon, baddeleyite and perovskite ages. The results of the present work demonstrate that postmagmatic alteration does not disturb or reset the U-Pb isotopic budget of these minerals, and that garnets representing the andradite-schorlomite-morimotoite system can serve as a robust reference material for micro-analytical geochronological studies of a wide spectrum of igneous and contact-metasomatic rocks. The new LA-ICPMS data demonstrate that the abundances of rare-earth, high-field-strength and other trace elements in calcic garnets from alkaline and carbonatitic rocks vary by at least two orders of magnitude and can therefore be used as reliable magma-evolution tracers. The rare-earth budget of these minerals is best described in terms of the chondrite-normalized ratios (Sm/La)cn and (Sm/Yb)cn, which are sensitive to lanthanide fractionation, and Y/Ho, which is interpreted to respond to garnet re-equilibration with a fluid.