Geochronology,Geochemical Characteristics And Geological Significance Of The Wulanba Pluton And Xiaohanshan Pluton, Inner Mongolia

The central and southern portion of the Greater Khingan Range is an important tin-polymetallic metallogenic belt in northern China.It has experienced multi-stage tectonic evolution and magmatism since the Paleozoic period.This paper presents detailed petrography,zircon U�Pb chronology,elemental geochemistry,and zircon Hf isotope compositions of the Wulanba and Xiaohanshan Plutons.The paper is aim to constrain the timing of emplacement,the nature and evolution of the magmas and of the corresponding regional tectonic background.The Wulanba pluton is composed of porphyritic monzonitic granite and fine-grained monzonitic granite.The LA�ICP�MS zircon U�Pb dating results show that the porphyritic monzonitic granite has age of 138 Ma and fine-grained monzonitic granite was emplaced at 149Ma.The Xiaohanshan pluton comprises porphyritic syenite,and its age is 138Ma.The Wulanba and Xiaohanshan plutons have the following geochemical characteristics:silicon-rich with SiO₂ content of66.63wt.%⁷8.2 wt.%.Alkali-rich with total alkali?K₂O+Na₂O?of 8.32 wt.%¹1.35 wt.%,and relatively potassium-enriched with the K₂O content of 4.20 wt.%⁸.83 wt.%.The 10000Ga/Al values are greater than 2.6,between 2.8⁴.2.The total rare earth elements of both plutons range from 83.69 to 344.85ppm,with LREE/HREE of 2.13¹2.48 and?La/Yb?_N of 1.10¹1.82.In the primitive mantle-normalized trace elemental spider diagram,they are enriched in Rb and Th,and depleted in Ba,Sr,P,and Ti.The zircon?Hf?t?values of the Wulanba pluton are all positive,varying from+5.7 and+11.5,corresponding to a relatively young two-stage Hf model age.The zircon Hf isotopic characteristics indicate that the source is composed of relatively young crustal materials.Combined with the regional geological background and previous research results,it is believed that the Wulanba and Xiaohanshan plutons have characteristics similar to A-type granites.Both plutons formed during the extension of lithosphere.The underplating of basaltic magmas induced partial melting of the lower crust.