Geochemical characteristics, forming age and tectonic implications of the igneous rocks in Qikou depression
Ren Yunfei1, Luo Jinglan1, Zhang Chengli1, Liu Huaqing2, Li Shuangwen2
1. State Key Laboratory of Continent Dynamics/Department of Geology, Northwest University, Xi'an 710069;
2. Research Institute of Petroleum Exploration & Development-Northwest Branch, Petro China, Lanzhou 730022
Petrology and geochemistry study based upon core observation, slice observation and major elements analysis indicates that the main rock types of the Jurassic igneous rocks are intermediate-acid rhyolite, trachyte and trachyandesite, while the Cretaceous volcanic rocks mostly consist of alkali basalt and basaltic trachyandesite.The trachybasalt, alkali basalt and diabase are dominated among the Tertiary igneous rocks.The U-Pb and Ar-Ar isotope studies indicates that the Jurassic basalt formed at 133±20 Ma and the age of trachyte and andesite are 122.1±3.1 Ma and 111.8±0.9 Ma, respectively.The age of diabase is 16.57±0.23 Ma.Geochemical research shows that the Jurassic volcanic rocks are formed in a compression environment while the Tertiary intraplate basalt and diabase are formed in extension setting, and the magma comes from mantle plume-related enriched mantle and was contaminated by crustal material.Therefore, the tectonic setting transformation occurred between the Late Jurassic and the Early Cretaceous.This research is of great importance in the theory for understanding volcanic formation and origin, tectonic setting and its evolution under which the volcanic activity occurs.