Abstract: The Dongjikou pyroxene sysnite, appearing as irregular ovoid, intruding into the Palaeoproterozoic high-grade metamorphic rocks of Taihua group, and developing gneissic schistosity in Huayangchuan region, Xiaoqinling, is the only known Caledonian rock body. Detailed petrography and field work studies on Dongjikou pyroxene sysnite indicate that inner parts of the rock body, according to the different structural characteristics, can be divided into the earlier-formed fine-grained pyroxene syenite, developing optalic boeder and a little later-formed medium-grained porphyritic pyroxene syenite, developing chilled border. The mineral association in both types of rocks is composed of plagioclase, potassium feldspar, pyroxene, amphibole, quartz, etc. The phenocrysts are mainly composed of pyroxene and plagioclase. Pyroxene and amphibole have experienced the retrogressive metamorphism. LA-ICP-MS zircon U-Pb ages of the fine-grained pyroxene syenite and medium-grained porphyritic pyroxene syenite are (214.8±3.1) Ma（MSWD=0.48）and (212.9±3.1) Ma（MSWD=5.6） respectively，which represents that the formation age of the rock is Late Triassic instead of the Caledonian period. The isotopic dating results show that the Caledonian magmatism event was extremely weak or inexistent in Xiaoqinling area. According to isotopic dating in this paper and a systematical summary of the previous research on the geological characteristics, it can be concluded that the age of the Dongjikou pyroxene sysnite is very close to that of the Indosinian magmatic and metallogenic event in Xiaoqinling area. It’s concerned that the Indosinian magmatism is related to the transition regime from oceanic slab subduction to intercontinental collision. Moreover, intensive crust-mantle interactions caused upwelling of the deep material and triggered the gold enrichment.

Key words: pyroxene syenite, LA-ICP-MS zircon U-Pb dating, Late Triassic, Caledonian, Xiaoqinling, gold mines