LA-ICP MS zircon U-Pb dating for the hidden intrusions in the Lu-Zong Basin and its tectonic significance

The Luzong Mesozoic Volcanic Basin is located in the Middle-Lower Yangtze River fault-depression zone, which is located on the northern margin of the Yangtze Block. The volcanic and intrusive plutons occur widely in Luzong basin, including four groups of volcanic rocks, such as Longmenyuan Group, Zhuanqiao Group, Shuangmiao Group, and Fushan Group and 34 intrusive plutons. Based on former geochronology research of outcropped plutons, LA-ICP MS zircon U-Pb dating for the hidden intrusions such as Qingshuitang, Liudun, Qiaochong, Niutoushan and Lijiazhuang intrusions in the north of the Lu-Zong basin was carried out in this paper. These intrusions consist of monzonite and quartz syenite which contain a lot of K-feldspar. LA-ICP MS zircon U-Pb dating yield the ages of 133.21.4Ma, 131.3±1.3Ma, 131.0±1.4Ma, 131.2±1.2Ma and 130.9±1.2Ma for five intrusions, respectively. The results indicate that all the hidden intrusions in the north of Lu-Zong basin formed in the Early Cretaceous. These intrusions are characterized by relative high silica and alkalis, ascribed to metaluminous or peraluminous, moderate REE and strong Eu depletion with a slightly right-inclined sea-gull REE patterns. The data, together with other accuracy dating data of volcanic rocks and intermediate-acid high-K calc-alkalic plutons in the Middle and Lower Reaches of the Yangtze River area, support the hidden intrusions formed in a within-plate tensional tectonic environment. The results of the study are important for discussing the geodynamic settings of the hidden intrusions in the Middle and Lower Reaches of the Yangtze River area.