Zircon U-Pb ages, geochemical characteristics and tectonic implications of Early Carboniferous granites in Huolongmen area, Heilongjiang Province

Abstract：The Huolongmen area in Heilongjiang Province is tectonically located at the juncture between the Songnen Block and the Xing’an Block in the east of the Xingmeng oregenic belt. Large volumes of Neopaleozolc granitic rocks are distributed in this area. The granitic rocks are composed mainly of monzogranite, syenogranite and alkali-feldspar granite. The zircon U-Pb SHRIMP dating of monzonite granite yielded the concordant age of 351.5 ± 3.5Ma. Major element analyses show high SiO2（73.38%~78.68%）and alkali content（K2O+Na2O=6.58%~9.61%）, slightly high Al2O3（11.25%~14.50%）content and low MgO（0.1%~0.57%）and CaO（0.07%~0.89%） content. Trace element studies demonstrate the enrichment of Th, Zr, Nd, Rb, K and depletion of Ba, Sr, Nb, P, Ti. The samples are enriched in LREE (light rare earth elements) and depleted in HREE (heavy rare earth elements) with pronounced Eu anomalies（δEu=0.09~0.72）, suggesting a high degree of fractionation (（La/Sm）N=2.12~6.60，（Gd/Yb）N=0.71~2.48）. The granites generally belong to the high-K calc-alkaline series and might have been produced by the magma that experienced a relatively high degree of crystallization differentiation. Most Neopaleozoic granites fall in the post-orogenic zone and the volcanic arc granite zone, respectively, in the R1-R2 structural setting discrimination diagram and the (Yb+Nb)-Rb versus (Yb+Ta)-Rb discrimination diagram, suggesting that the Xing’an Block and Songnen Block might have experienced collision and suturing during Late Paleozoic.