Thermochronologic constraints on evolution of the Linglong Metamorphic Core Complex and implications for gold mineralization: A case study from the Xiadian gold deposit, Jiaodong Peninsula, eastern China

The NNE-trending Linglong Metamorphic Core Complex hosts the majority of the gold deposits in the Jiaodong Peninsula of eastern China. Many of the deposits are hosted by the 163–155 Ma Linglong granite in the footwall of the Linglong detachment fault. Argon thermochronology suggests that the granite had cooled to 400 °C by 143 ± 1.5 Ma possibly as a result of normal movement on the detachment. Nine zircon fission track (ZFT) ages from samples collected along a NW–SE transect perpendicular to the central part of the Linglong detachment fault at the − 652 m level in the Xiadian deposit constrain the subsequent thermal evolution of Linglong Metamorphic Core Complex, which overlapped the period of major gold deposition. The ZFT ages vary from 136.9 ± 3.3 Ma (1σ) to 130.1 ± 2.2 Ma (1σ). The unaltered Linglong granite in the footwall and amphibolite in the hangingwall have similar ages at 136.9 ± 3.3 Ma (1σ) and 135.0 ± 3.0 Ma (1σ), whereas ages for the disseminated- and stockwork-style ores appear to be younger at ca. 131–130 Ma, although there is an overlap of ages when considering the 1σ precision. Interestingly, ZFT ages show no marked differences between the hangingwall and footwall of the Linglong detachment fault, although significant movement along the fault occurred. The results are best interpreted to indicate that the Linglong granite was emplaced at ~ 160 Ma, and cooled to 240 ± 50 °C at ~ 135 Ma, as recorded by unaltered rocks in the footwall. Hydrothermal alteration along the Linglong detachment fault led to annealing of zircon fission tracks and the consistent younger ages of ~ 131 Ma. Quartz aggregates associated with gold mineralization show evidence of recrystallization suggesting that the hydrothermal event was initiated at temperatures of at least 300–350 °C, near the brittle–ductile transition, but cooled rapidly to ZFT closure temperatures within a country-rock environment that was already relatively cool. Therefore these ZFT ages suggest that the timing of mineralization at the Xiadian deposit was post-135 Ma and likely very close to 131 Ma. The mineralization and cooling occurred in the footwall of a major detachment fault under an extensional regime, possibly related to the progressive slab rollback of the paleo-Pacific plate, and controlled by the Linglong Metamorphic Core Complex.