Abstract
The Yuanjiang marble-hosted ruby deposit in the middle Ailao Shan-Red River (ASRR) shear zone, is the only economic ruby deposit in China. This deposit is hosted in Paleoproterozoic sillimanite gneisses and the ores are found in metamorphosed platform carbonates. Rubies are unevenly distributed within calcite-rich marble layers and mainly associated with phlogopite, scapolite, graphite, pyrite. Rubies contain zircon, rutile, apatite, calcite inclusions. The genesis of the Yuanjiang ruby deposit and its genetic link to the ASRR shear zone remain poorly understood. In this contribution, its formation age has been indirectly constrained by the in-situ LA-ICP-MS U-Pb dating of zircon inclusions in rubies and 40Ar/39Ar dating of syngenetic phlogopite from the ruby-phlogopite bearing marble, which yielded the ages of 36.2 ± 1.1 Ma (MSWD = 1.4, 2σ, n = 4) and 22.2–22.5 Ma, respectively. The 36.2 Ma U-Pb age of metamorphic zircon inclusions falls within the range of local crustal thickening related subduction-collision and coupled with the spatial distribution in which ruby deposits were not restricted in the shear zone, we propose that ruby mineralization was triggered by the local continental subduction-collision rather than the large-scale left-lateral shearing. Due to the estimated closure temperature of the 40Ar-39Ar system is lower than the ore-forming temperature, the obtained phlogopite 40Ar/39Ar ages of 22.2–22.5 Ma represent the minimum ore-forming age of the Yuanjiang ruby deposit. These phlogopite ages reveal a slower cooling rate of ruby deposits within the shear zone (including the Yuanjiang and the Yen Bai ruby deposits) than outside of the shear zone (the Luc Yen deposit), which can be attributed to the additional heat from the left-lateral shearing movement. Therefore, we propose that the left-lateral shearing plays an important role in transporting marble-hosted ruby deposits in the Ailao Shan-Red River shear zone to the subsurface.
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