Abstract:

A lot of meta-basalts and mafic dykes occurred in the Northern Liaohe Group in Haicheng-Caohekou area, among which, the meta-basalts of Dashiqiao Formation in south Helan, Liaoyang, preserved a good pillow structure. The meta-pillow lava is chemically characterized by low alkaline, low K₂O/Na₂O ratios and iron-rich, which suggest that it belongs to island-arc tholeiite series. The REE and trace element characteristics indicate that the meta-pillow lava formed in the tectonic setting associated with the island-arc, likely back-arc basin rather than the continental rift. The study of isotopic chronology show that zircon SHRIMP U-Pb age of the meta-pillow lava is (1 869±28) Ma, of meta-diabase dyke is (1 828±13) Ma with a group of ~2 080 Ma captured zircon ages, and the zircon LA-ICP-MS U-Pb age of granite pegmatite is (1 869.7±7.6) Ma. Based on their occurrence, zircon dating and previous research data, it is suggested that the meta-pillow lava is formed at ~1880 Ma which could represent the forming age of Dashiqiao Formation, and the granite pegmatite is at ~1860 Ma which can be considered as a marker of the end of orogeny, and the meta-diabase dyke at ~1830 Ma which may indicates the beginning of post-orogeny extension.

Key words: Liao-Ji mobile belt, Liaohe Group, pillow lava, Zircon U-Pb dating, tectonic setting, tectonics, Liaoyang