Zircon U-Pb Geochronology, Geochemistry and Hf Isotopic Composition and Its Geological Implication of the Fine-Grained Syenogranite in Dong'an Goldfield from the Lesser Xing'an Mountains

The fine-grained syenogranite in the Dong'an goldfield is the early Yanshanian granite body in the Lesser Xing'an Mountains, it is one of the granite bodys of the granite belt which is related to porphyry and skarn-type molybdenum-polymetallic deposits in the east Jilin-Heilongjiang Province. In order to understand regional magmatic evolution in the early Yanshanian and cosmical molybdenum-polymetallic hydrothermal mineralization, thus further promote the research degree of the metallogenic background in the Dong'an goldfield. This paper presents geochemisty, zircon U-Pb geochronology, and Hf-isotopic data of these rocks, their petrogenesis, sources and tectonic implications have been investigated. The granitic rocks yield an average zircon LA-ICP-MS U-Pb age of 184±2 Ma (MSWD=1.2), i.e., the Early Jurassic. The granites have high SiO₂ (75.39%-78.84%) and K₂O (K₂O/Na₂O: 1.46~1.81), low CaO, MgO and Mg^# (12.79-23.52), with A/CNK values of 1.05-1.14, indicating high potassium calc-alkaline, weakly peraluminous granites. The granites are enriched in large ion lithosphile elements (Rb, K) and incompatibale elements (Th, U), and depleted in high field-strength element (Nb, Ti). They have strong fractionation between LREE and HREE, as well as slightly negative Eu anomalies (Eu/Eu^*=0.76-0.92). It was shown that these granitic rocks belong to highly fractionated I-type granites according to lithogeochemical constituents, Harker plots, the discrimination diagrams of Ce vs. SiO₂ and (K₂O+Na₂O)/CaO vs. Zr+Nb+Ce+Y. All zircons in the granites have ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.282 588 to 0.282 775, ε_Hf(t) values ranging from -2.35 to +3.94, and two-stage Hf model ages (T_DM2) ranging from 973 to 1 386 Ma. It was indicated that the magma was originated from the partial melting of Meso-Neoproterozoic juvenile accreted lower crust that was derived from the depleted mantle, with addition of ancient sialic crustal materials. The results show that the granites were generated from the extensional and lithospheric thinning environment due to the subduction of the Paleo-Pacific plate, and underplating of the mantle-derived magma provided the thermodynamic power for the partial melting of the crust. The small scale hypabyssal-middle high potassium calc-alkaline granitic intrusive bodies that were generated in the continental arc extension setting in the early Yanshanian are main targets for the prospecting of porphyry and skarn-type molybdenum-polymetallic deposits in the east Jilin-Heilongjiang Province.