ABSTRACT

The Dajing deposit is a large-scale vein-type polymetallic Sn deposit in the southern Great Xing'an Range of Northeast China. This study presents new geochronological and geochemical data for subvolcanic rocks associated with the Dajing deposit and discusses the petrogenesis and tectonic setting of these rocks. The subvolcanic rocks associated with the Dajing deposit are felsites, porphyritic andesites, and porphyritic dacites. Laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb Zircon dating undertaken during this study indicates that the porphyritic dacite was intruded during the late Permian (265.9 ± 5.4 Ma), whereas both the felsite and the porphyritic andesite were intruded during the Middle Jurassic (168.4 ± 5.0 and 175.5 ± 5.6 Ma, respectively). All of these subvolcanic rocks are of the high-K calc-alkaline series, with high A/CNK (=(Al₂O₃)/(CaO + Na₂O + K₂O)) molecular ratios (>1), and are LREE-enriched relative to the HREEs. The felsite is characterized by relatively high Rb, U, Nd, Zr, and Hf, and relatively low Eu, Ba, Sr, P, and Ti concentrations, and is geochemically similar to highly fractionated I-type granites. In comparison, the porphyritic andesite is characterized by relatively high Rb and K and relatively low Ba concentrations, and is geochemically similar to unfractionated I-type granites. The porphyritic dacite is enriched in Rb, Zr, and Hf, is depleted in Ba, Sr, P, and Ti, and is geochemically similar to A₂-type granites. The regional geological setting and geochemistry of these subvolcanic rocks indicate that both the felsite and porphyritic andesite formed during post-collisional extension within the Xingmeng Orogenic Belt. This extension was a far-field effect of the closure of the Mongol-Okhotsk Ocean and associated lithospheric delamination during the Middle Jurassic. In comparison, the porphyritic dacite was associated with post-collisional slab break-off.