Unveiling growth histories of multi-generational garnet in a single skarn deposit via newly-developed LA-ICP-MS U[sbnd]Pb dating of grandite

Abstract

Although garnet UPb dating method has been reported recently, yet the accurate concordia 206Pb/238U ages and growth histories of multi generation of garnets based on ages were still lacked. LA-ICP-MS UPb dating on multi-generational grandite (grossular-andradite) garnet from the large Tonglvshan Cu-Fe-Au skarn deposit was applied in this study. Based on petrographic observation, in chronological order, three generation garnets have been distinguished, namely homogeneous Grt1-exo (in the exoskarn zone) and Grt1-endo (in the endoskarn zone), oscillatory zoning Grt2 and vein-type Grt3 cutting magnetite ores. LA-ICP-MS UPb dating on four grandite samples from the Grt1-exo, Grt1-edno, Grt2 and Grt3 yields Tera-Wasserburg lower intercept 206Pb/238U ages of 139.1 ± 1.0 Ma (2σ, MSWD = 0.79), 134 ± 11 Ma (2σ, MSWD = 2.5), 143.4 ± 8.3 Ma (2σ, MSWD = 2.3) and 140.3 ± 1.4 Ma (2σ, MSWD = 0.95), respectively. More importantly, two concordia 206Pb/238U ages of 139.2 ± 0.6 Ma (2σ, MSWD = 1.4) and 139.8 ± 1.5 Ma (2σ, MSWD = 0.13) were firstly obtained from the sample of Grt1-exo with highest U concentrations ([U]avg > 80 ppm) contents. The precision UPb ages of 139–140 Ma from Grt1-exo and Grt3 can be considered as the timing of Cu-Fe-Au skarn mineralization, and consistent with the majority of published zircon UPb ages of the quartz dioritic stock and 40Ar39Ar plateau ages of phlogopite at Tonglvshan (142–140 Ma). The precision grandite UPb ages also indicate that the entire metasomatic hydrothermal mineralization activity in the Tonglvshan Cu-Fe-Au skarn deposit occurred within a relatively short time span of <1 (or 2.5 considering errors) Myr. In addition, we found that the grandite garnet is more easily to be enriched in U and can obtain the high-precision concordia UPb ages with higher andradite Mol%, euhedral and larger crystals, and relevant oxidized magmatic rocks or skarns.