Evolving magma sources during continental lithospheric extension: Insights from the Liaonan metamorphic core complex, eastern North China craton
Abstract The relation between magmatic activity and detachment faulting during continental lithospheric extension remains a persistent question. This paper takes the Liaonan metamorphic core complex (mcc) in the eastern North China craton as an example to explore how magma generation relates to detachment faulting in response to continental lithospheric extension due to the subduction of the Paleopacific plate beneath the Eurasian continent in the Early Cretaceous. Dating of lower plate granitic intrusions and trachy-dacitic volcanic rocks in the supradetachment basin constrains a progressive exhumation of the mcc between ca. 134 Ma and 113 Ma. Syn-tectonic magmatism is grouped into three types, i.e., early syn-kinematic, late syn-kinematic, and post-kinematic magmatism, in reference to detachment faulting. The magmatic rocks exhibit obvious differences in whole-rock Sr–Nd isotopes, implying varying sources, e.g., lithospheric mantle, juvenile and ancient lower crusts. Evolving magma sources during the progressive exhumation of the mcc are shown from one with a lithospheric mantle signature for the early syn-kinematic intrusions to one with mixed derivations from both juvenile and ancient lower crust for the late syn-kinematic plutons and volcanics and, finally, to a juvenile lower crust source for the post-kinematic intrusions. The source evolution of magmatism during the continental lithospheric extension implies that both crustal and mantle processes may have been involved in the magma generation. The Parallel Extension Tectonics model is applied to explain both the tectonic faulting and magmatic activities. In the model, the evolving magma sources accompanying the mcc exhumation were attributed to thinning of the cratonic lithosphere as a consequence of coupled crust–mantle detachment faulting during tectonic extension. The model can be applied to the variably extended provinces in the North China craton, but integrated structural, geochronological and geochemical studies are needed to present a comprehensive tectono-thermal (magmatic) history of these provinces.