Disturbance versus preservation of U–Th–Pb ages in monazite during fluid–rock interaction: textural, chemical and isotopic in situ study in microgranites (Velay Dome, France)

Abstract

Monazite is extensively used to date crustal processes and is usually considered to be resistant to diffusive Pb loss. Nevertheless, fluid-assisted recrystallisation is known to be capable of resetting the monazite chronometer. This study focuses on chemical and isotopic disturbances in monazite grains from two microgranite intrusions in the French Central Massif (Charron and Montasset). Petrologic data and oxygen isotopes suggest that both intrusions have interacted with alkali-bearing hydrothermal-magmatic fluids. In the Charron intrusion, regardless of their textural location, monazite grains are sub-euhedral and cover a large domain of compositions. U–Pb chronometers yield a lower intercept age of 297 ± 4 Ma. An inherited component at 320 Ma is responsible for the scattering of the U–Th–Pb ages. The Montasset intrusion was later affected by an additional F-rich crustal fluid with crystallisation of Ca-REE-fluorocarbonates, fluorite, calcite and chloritisation. Pristine monazite is chemically homogeneous and displays 208Pb/232Th and 206Pb/238U concordant ages at 307 ± 2 Ma. By contrast, groundmass monazite shows dissolution-recrystallisation features associated with apatite and thorite precipitation (Th-silicate) and strong chemical reequilibration. 208Pb/232Th ages are disturbed and range between 270 and 690 Ma showing that the Th/Pb ratio is highly fractionated during the interaction with fluids. Apparent U–Pb ages are older due to common Pb incorporation yielding a lower intercept age at 312 ± 10 Ma, the age of the pristine monazite. These results show that F-rich fluids are responsible for Th mobility and incorporation of excess Pb, which thus strongly disturbed the U–Th–Pb chronometers in the monazite.