Dating magmatic and hydrothermal processes using andradite-rich garnet U-Pb geochronometry

Andradite-rich garnet is a common U-bearing mineral in a variety of alkalic igneous rocks and skarn deposits, but has been largely neglected as a U-Pb chronometer. In situ laser ablation-inductively coupled plasma mass spectrometry U-Pb dates of andradite-rich garnet from a syenite pluton and two iron skarn deposits in the North China craton demonstrate the suitability and reliability of the mineral in accurately dating magmatic and hydrothermal processes. Two hydrothermal garnets from the iron skarn deposits have homogenous cores and zoned rims (Ad(86)Gr(11) to - Ad(98)Gr(1)) with 22-118 ppm U, whereas one magmatic garnet from the syenite is texturally and compositionally homogenous - (Ad(70)Gr(22) to - Ad(77)Gr(14)) and has 0.1-20 ppm U. All three garnets have flat time-resolved signals obtained from depth profile analyses for U, indicating structurally bound U. Uranium is correlated with REE in both magmatic and hydrothermal garnets, indicating that the incorporation of U into the garnet is largely controlled by substitution mechanisms. Two hydrothermal garnets yielded U-Pb dates of 129 +/- 2 (2 sigma; MSWD = 0.7) and 130 +/- 1 Ma (2 sigma; MSWD = 0.5), indistinguishable from zircon U-Pb dates of 131 +/- 1 and 129 +/- 1 Ma for their respective ore-related intrusions. The magmatic garnet has a U-Pb age of 389 +/- 3 Ma (2 sigma; MSWD = 0.6), consistent with a U-Pb zircon date of 388 +/- 2 Ma for the syenite. The consistency between the garnet and zircon U-Pb dates confirms the reliability and accuracy of garnet U-Pb dating. Given the occurrence of andradite-rich garnet in alkaline and ultramafic magmatic rocks and hydrothermal ore deposits, our results highlight the potential utilization of garnet as a powerful U-Pb geochronometer for dating magmatism and skarn-related mineralization.