Zircon U-Pb geochronology, geochemistry, and Hf isotopic compositions of the trachyandesite in the Dong’an Au deposit, Lesser Xing’an Range, northeastern China

摘要

Abstract The Dong’an Au deposit is a large-sized epithermal Au deposit discovered in the Lesser Xing’an Range, Northeastern China. Intermediate-acid volcanic rocks (e.g., trachyandesite, andesite, dacite, rhyolite, and rhyolitic tuff) of the Lower Cretaceous Fuminghe Formation are the important surrounding rocks for Au mineralization in the Dong’an Au deposit. However, the relationship between the intermediate volcanic rocks and the acidic volcanic rocks is unclear. The authors present new geochemical, zircon U-Pb and Hf isotope data for the trachyandesites in the Dong’an Au deposit. The whole-rock geochemical data indicate that the trachyandesite samples are high-K calc-alkaline. They are enriched in LILEs (e.g., K, Rb, and Ba), LREEs, and incompatible elements (e.g., Th and U), but are depleted in HFSEs (e.g., Nb, Ta, P, and Ti), showing characteristics of volcanic arc magmas. They have low Mg# values (32.77–48.12), Cr, Ni, and Co contents. Zircons U-Pb dating of the trachyandesites from sample DA-N1 and DA-N2 yield weighted average ages of 108.0 ± 1.1 Ma (MSWD = 0.91) and 104.7 ± 4.3 Ma (MSWD = 17). In situ zircon Hf isotope analyses of the trachyandesites from sample DA-N1 and DA-N2 yield εHf(t) values of −3.2 – 1.2 and −2.3 – 2.6, and two-stage model age (TDM2) of 1372-1090 Ma and 1321-1009 Ma, respectively. These suggest the trachyandesite were derived from partial melting of the juvenile lower crust with involvement of the ancient crustal materials. Combined with previous geological and petrogeochemical characteristics of the Lower Cretaceous Fuminghe Formation volcanic rocks in the Dong’an Au deposit, the Fuminghe Formation volcanic rocks are believed to be comagmatic, and fractional crystallization played an important role in the differentiation of these volcanic rocks. They were likely formed during the retreat of the subducted Paleo-Pacific Plate.