Although the U-Pb isotopic system is widely used to date magmatism and medium- to high-grade metamorphism, dating low-grade metamorphic events has been hampered by a scarcity of suitable minerals. We show that monazite forms in shales during low-grade metamorphism and can be dated by in situ U-Pb analysis, providing precise ages for low- temperature thermal events. In Proterozoic shales from the Pine Creek inlier of northern Australia, monazite and xenotime crystals that formed in a contact aureole yield ages synchronous with granite intrusion, ca. 1830 Ma. In the Pilbara craton of northwestern Australia, monazite growth 2192 ± 5 m.y. ago coincided with basaltic volcanism and rifting, an event apparently responsible for hydrothermal iron-oxide mineralization in a nearby giant iron-ore deposit. The mineralizing environment was highly oxic and bathed by meteoric fluids; thus, our results imply the existence of an oxygen-rich atmosphere by 2200 Ma. Given the abundance of shales in the stratigraphic record, and the likely widespread distribution of monazite in such rocks, in situ U-Pb dating of monazite may become an important chronometer for low-grade metamorphic events.