The effect of amphibolite facies metamorphism on the U–Th–Pb geochronology of accessory minerals from meta-carbonatites and associated meta-alkaline rocks

In this study we apply laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to the accessory minerals apatite, baddeleyite, monazite, pyrochlore, titanite, zircon and zirconolite from five meta-carbonatite and one meta-alkaline rock samples in order to obtain U–Th–Pb ages. We thereby assess the effect of amphibolite facies metamorphism on their U–Th–Pb isotopic systems. The results indicate that for the investigated samples the relative retentivity of Pb follows the order baddeleyite > zircon ≥ apatite ≥ titanite > zirconolite > monazite ≥ pyrochlore. Accordingly, the emplacement ages of the magmatic protoliths are best constrained by data from baddeleyite, zircon, apatite and titanite. In contrast, age data obtained from zirconolite, monazite and pyrochlore are interpreted to constrain the timing of metamorphism and/or cooling.

► Comparative U–Th–Pb geochronology of seven accessory phases ► Assessment of effect of amphibolite grade metamorphism ► Magmatic intrusion ages constrained by baddeleyite, zircon, apatite and titanite ► Resetting of the U–Th–Pb system in zirconolite, monazite and pyrochlore