Early Cretaceous Tectono-magmatic Processes In The Liaodong Peninsula And Regional Tectonic Implications

Large scale lithosphere thinning is an important characteristic of the destruction of the North China Craton during the Late Mesozoic. A series of extensional structures were developed in the Liaodong peninsula, providing the possibility to discuss the dynamic model of craton destruction and lithosphere thinning in North China Craton. This work started with the analysis of the constitutes of the extentional structures, carried out the geochronologicial, geochemical analysis of synkinematic intrusions in the footwall and of volcanic rocks in extensional basins. Based on comparative study with the tectono-magmatic events in North Hebei, West Liaoning and East Shandong, the coupling and decoupling relationship between the shallow and deep processes during the lithosphere thinning and crust destruction in North China Craton are best explained by the multiple-layer crust-mantle detachment model.The Liaonan-Wanfu metamorphic core complex pair, Dayingzi-Huanghuadian detachment fault system, Benxi, Tongyuanpu and Dandong half-graben basins are distributed around Liaodong peninsula. The extensional basins were bounded by detachment faults, and the volcanic-sedimentary layers dip towards to the detachment faults. A mass of early Cretaceous plutons were outcropped in the foot wall of the detachment fault. The tectono-magmatic processes in the Liaodong peninsula occurred during 135~105.6Ma, i.e. volcanisms at 135~105.6, synkinematic emplacement at 130~113Ma, and extentional detachment faulting at 120~107Ma. Furthermore, the volcanic rocks in the extensional basins have identical geochemical characteristics with the synkinematic plutonic rocks, suggesting that these volcanic rocks are the result of eruption associated with the granitic magma in a NE oriented volcanic-intrusive magmatic zone. The sedimentary rocks are locally distributed overlying on the volcanic rocks. It is shown that the sedimentary structures were developed locally and bounded by the detachment faults.The synkinematic intrusions and volcanic rocks have characteristics reflecting various sources regions including lithosphere mantle, juvenile lower crust and ancient lower crust. From the west to the east in the area, ages of magmatic rocks with lithosphere mantle sources vary from 130±5Ma, 128±2Ma, 127±1Ma to 125±2Ma, and ages with juvenile lower crust from 113±2Ma to 106±3Ma, but ages of the mixed sources for magma from these end members developed through 131±3Ma~118±3Ma. In North Hebei, West Liaoning and East Shandong areas, igneous rocks were formed during 134~124Ma. It is also shown that from west to east, the source regions of magma change from simple lithosphere mantle to mixed source of lithosphere mantle, juvenile lower crust and ancient lower crust. It is therefore suggested a coherence with the transformation from graben basin (North Hebei) to the combination of metamorphic core complex, detachment fault system and graben basin (Liaodong peninsula).The coexistence of magma from multiple sources implies a complex material exchange during the development of magmas, and closed relationship with the crust-mantle decoupling and extensional detachment. These phenomena are best explained by the“crust-mantle detachment model”in which an important coupling between Late Mesozoic horizontal detachment faulting and vertical lithosphere thinning in North China Craton.