Zircon U-Pb Geochronology and Geochemistry of the Early Devonian Syenogranite in the Xiarihamu Ore District from East Kunlun, with Implications for the Geodynamic Setting

In this paper, we discuss the zircon U-Pb age, major and trace element geochemistry of the syenogranite intrusion in the north of the Xiarihamu ore district. Zircon U-Pb dating demonstrates that the weighted mean206Pb/238U age for the zircons from the syenogranite is 391.1±1.4 Ma(MSWD=0.06), i.e., the Early Devonian. The syenogranite is slightly peraluminous and belongs to the high-K calc-alkaline series. The syenogranite is characterized by high SiO2(75.55%~76.10%), high alkali(Na2O+K2O=8.23%~8.46%), high FeOT/MgO(FeOT/MgO=17.40~42.59), low CaO(0.54%~0.69%) and MgO(0.03%~0.09%) contents. The syenogranite is characterized by a "sea-gull" REE pattern and significant negative Eu anomaly(δEu=0.09~0.12), as well as enrichment of Ga(24.3×10–6~26.9×10–6), Zr(132.0×10–6~363.0×10–6) and Y(86.1×10–6~97.0×10–6), and depletion of Sr(8.6×10–6~19.5×10–6) and Ba(14.0×10–6~37.9×10–6).And thus, the syenogranite belongs to aluminous A-type granite. The high Rb/Sr(ranging from 14.97 to 38.26, and the average value is 22.63) and Rb/Nb ratios(ranging from 13.84 to 16.13, and the average value is 14.54) of the rock indicate a crustal origin. Combined with the regional tectonic evolution and geochemical discrimination, we suggest that the syenogranite was formed in a post-orogenic setting. During the Late Silurian-Early Devonian, the tectonic regime in the East Kunlun area changed from compression to extension due to asthenosphere upwelling. Under the extensional setting, the upwelling asthenosphere provided enhanced heat and triggered the partial melting of the overlying felsic crust, and produced the A-type syenogranite.