Petrogenesis and Tectonic Significance of Laohugou Mafic Dykes in Haicheng,Liaoning Province:Evidence from Zircon U-Pb-Hf Isotopes and Whole-rock Geochemistry

As one of important Paleo-proterozoic orogenic belts formed in North China craton, the Jiao-Liao-Ji belt was deposited massive meta-sedimentary rocks, underwent a complex tectonic evolution process with the modification by multiple stages of magmatic and metamorphic events, and it merged with Langrim Block in south and with Longgang Block in north to form the Eastern Block.A swarm of Paleoproterozoic metamorphic mafic dykes was exposed in the Haicheng-Caohekou area, Liaoning Province, including NE-SW trend Shensixian mafic dykes swarm and NW-SE trend Laohugou mafic dykes.Shensixian mafic dyke swarm was outcroped in the Jiao-Liao-Ji Belt, intercalated with the Langzishan and Lieryu Formations of the North Liaohe Group, while the Laohugou mafic dykes was intruded into the Neoarchean gneisses along the Southeastern margin of the Longgang block with high angle.LA-ICP MS zircon U-Pb dating and LA-MC-ICP MS Lu-Hf isotopic analyses on the sample from the Laohugou mafic dykes yielded a metamorphic age of (1 864±15)Ma, and T_DM1(Hf)ages (Hf model ages) of 2 040~2 293Ma, which are comparable with the record of the Shensixian mafic dyke swarm.Whole-rock major and trace element analyses display the comparability between the Shensixian and Laohugou dykes, both of them are characterized with the enrichment of the LREE and LILE and depletion of the HSFE, implying that the Shensixian and Laohugou mafic dykes were formed in the similar tectonic environment.In the Nb/Yb-Th/Yb diagram, the geochemical data of the Laohugou and Shensixian mafic rocks display Th enrichment, indicating a subduction-related metasomatism.But Laohugou mafic rocks have higher Nb/Zr ratios than the Shensixian mafic rocks, implying the metasomatismby silicate melt in the source region, while the low Nb/Zr ratios with variable Th/Zr ratios of the Shensixian mafic rocks indicate a fluid metasomatism in the source.In conclusion, Laohugou and Shensixian mafic dykes were formed in the same tectonic background, both of them were matasomatised by the silicate melts and fluids, respectively, which were metamorphosed during the subduction and collision event in 1 930~1 850Ma.