Four-period calc-alkaline volcanic rocks are widespread in the Yanshan orogenic belt. The components of magma sources were controlled predominantly by the interaction between the continental crust and upper mantle near MOHO surface. The Tiaojishan Formation and Yixian Formation are the representative types of volcanic activity. The petrological features of Tiaojishan Formation volcanic rocks are relatively uniform, geochemical parameters varies at a narrow range and the AFC processes of magma are not strong enough, which indicates that the components of the magma source were not very complex. The estimated thickness of the crust was 40-45 km during the Early to Middle Jurassic. We infer that Tiaojishan trachyandesite magma was melted from amphibole bearing garnet granulite + underplating basalts at thickening crust condition. A possible model, delaminating lower continental crust into the underlying weakening lithospheric mantle and interaction between them, has been proposed as one means to explain the genesis of basic-middle basic magma in the Yixian Formation. The inferred source of these magmas was lower crust + weakening lithosphere. Rheological behavior of the lithospheric mantle has been heterogeneously translated from rigid to ductile. This translation process was induced by the effects of fluid, melt, water released from mantle minerals, shear stress and increasing temperature in the lithospheric mantle beneath the small extension basin and above the uplifted asthenospheric diapir after approximately 135 Ma. These local areas were the preferable sets to hold the delaminated lower crust. The interaction between the foundering continental lower crust and the underlying weakening lithospheric mantle may be compared with the interaction between the subducting ocean crust and mantle wedge at a subduction zone.
|Number of pages||7|
|Journal||Diqiu Kexue - Zhongguo Dizhi Daxue Xuebao/Earth Science - Journal of China University of Geosciences|
|Publication status||Published - Jan 2006|
- Lower crust
- Weakening lithospheric mantle
- Yanshan orogenic belt