Magmatism And Regional Tectonic Evolution Of Orogenic Belt

Integrated with the lastest achievements in tectonic, basin evolution and deep structural detection, this paper mainly studies tectono-magmatic sequence, genetic types and tectonic setting of the Yanshanian magmatites within Dabie orogen and its neighbouring areas, and then discusses the relationship between magmatism and regional tectonic evolution and geodynamic process of Dabie orogenic belt. The main progresses in this paper are as follow.1. Established a regional tectonic evolution model. The Mesozoic tectonic evolution in study area is divided into four stages, namely, the Indosinian subduction-collision stage; the Jurassic intracontinental compression and uplift stage; the early Cretaceous extension stage of the thermal dome and the late early cretaceous- late Cretaceous rifting extension stage.2. Combined with isotopic data, spatial distribution, tectonic deformation and cutting across relationship of intrusive bodies, this paper discusses the magmatic activities and tectono-magmatic compositions and establishes tectono-magmatic event sequences. Five periods of magmatic activity in the Yanshanian orogenic belt have been recognised: middle Jurassic (166-154 Ma); late Jurassic (138-135 Ma); early period of the early Cretaceous (134-126 Ma); early-middle period of the late early Cretaceous (130-120 Ma) and middle-late period in the early Cretaceous .3. Discussed the genesis of different types of intrusive rocks. The author suggests that high Sr, low-Y (Yb)-type intrusive rocks produced by partial melting at the bottom of the thicken crust are similar to adakites with main characteristics. High Sr, Y (Yb)- type intrusive rocks are mixtures of magmas derived by partial melting in the thickened crust with alkali-basaltic magmas produced by low degree partial melting in the enriched lithospheric mantle. Mg-rich, high Sr and low Y (Yb)-type intermediate- acidic intrusive rocks are generated by partial melting in the delaminated lower continent crust, and metasomatized and contaminated with mantle peridotites during ascent of the magma. Low Sr-type granites are produced by partial melting in the middle-lower crust at normal depths whereas the peralkali syenites are results of fractional crystallization of the upper mantle-derived alkali-basaltic parental magmas under the condition of low pressures.4. Based on the Sr, Nd isotopic composition of the intrusive rocks, the author shows that the contributions of mantle material in intrusive rocks evidently increased in Dabie orogen from middle Jurassic to late Jurassic. Magma source may migrate from the lower crust with more mantle material to the middle-upper crust with dominated continental crust material from early to late Cretaceous.5. Break-off of the subducted slab resulted in thermal asthenosphere upwelling and induced partial melting at the bottom of the thickened crust in the orogenic belt in the middle Jurassic. The author firstly puts forward that underplating of the mantle-derived magmas initiated in the late Jurassic, and mixed with the ones produced by partial melting at the bottom of the lower crust. In the early period of the early Cretaceous, underplating reached its peak, and magmas, which derived by partial melting in the upwelling asthenospheric mantle and enriched lithospheric mantle, underplated in the crust-mantle transition zone and interacted with the lower crust. Large-scale delamination occurring in the early-middle period of the early Cretaceous gave rise to the extension, collapse and hot asthenosphere upwelling in the orogenic belt and the large low-Sr-granitic and mafic-ultramafic magmatism.6. Demonstrated the existence of the independent Yanshanian orogenic cycles in Dabie orogenic belt. The Yanshanian intracontinental orogen overlapped on the Indosinian subduction orogenic belt.7. The Indosinian intercontinental orogeny, a "cold" one with low geothermal gradients, was produced by subduction-collision of the cold plates whereas the Yanshanian orogeny may be related to asthenosphere upwelling, and evolve to become hot.8. The intracontinental orogenic belt in the Early Yanshanian was still controlled by paleotethys tectonic system, and initiated to conversion to marginal Pacific tectonic domain in the early period of the early Cretaceous. The transformation from compression to extension of the Yanshanian Dabie orogenic belt is essentially the result of conversion of two tectonic system domains.