Petrogenesis of Variscan and Yanshanian magmatic rocks in the Longjiang region of Heilongjiang province and implications for ore exploration

Abstract

The Longjiang region is located on the eastern slope of the southern section of the Great Xing'an Range, in the transition zone between the Songliao Basin and Great Xing'an Range uplift. As it is an extension of the Inner Mongolia Linxi-Tuquan Cu-Mo-Pb-Zn metallogenic belt in the Heilongjiang region, it has conditions conducive to the formation of porphyry deposits. We report zircon U-Pb dating and whole-rock geochemistry data of magmatic rocks in representative mineralized points in the Longjiang region. Zircon U-Pb age shows that the age of magmatic rocks is concentrated in the Variscan period (312 ± 5 ∼ 294 ± 2Ma) and Yanshanian period (134 ± 2 ∼ 123 ± 1Ma). According to whole-rock geochemical analysis, the granites have SiO₂ contents of 61.2–69.16 % (66.9 % on average), Al₂O₃ contents of 13.87–15.98 %, MgO contents of 0.6–2.22 %, TiO₂ contents of 0.25–0.88 %, K₂O contents of 2.70–3.63 %, Na₂O/K₂O ratios of 1.05–2.05, and high K₂O+Na₂O contents (5.8–7.17 %), which are consistent with adakitic rocks. Yanshanian granites have A/CNK values of 1.02–2 and an average differentiation index of 84.7, which indicates that they are highly differentiated I-type granites. Variscan I-type granites represent the post-collisional setting of the Paleo-Asian Ocean and are relatively scarce in the region. Yanshanian highly differentiated I-type granites are products of the Paleo-Pacific Ocean's post-collisional retreat and are widely distributed in the region. As the materials and fluids that formed the Liujiushan, Haiyang, and Fendou deposits were derived from magmatic activities, and Variscan and Yanshanian magmas have excellent ore-forming potential. The highly differentiated I-type granites formed by intense Yanshanian magmatism are the most promising candidates for ore exploration in this region.