关键词: 东秦岭—大别造山带, 岩石圈拆沉, 花岗岩类, 超高压变质岩, p-T-t轨迹, 挤压伸展转

Abstract: Lithosphere delamination is an important way for compositional and structural evolution of collisional orogens. Geotectonic research, geophysical survey and geochemical analysis have revealed that the lithosphere of East Qinling-Dabie Mountains had delaminated intensively in Mesozoic. However, the extensive Mesozoic metamorphism and magmatism have been weakly related to lithosphere delamination. Based on a comprehensive review on research results of metamorphic rocks, the authors argue that the formation and exhumation of UHPM (ultra-high pressure metamorphite) experienced two stages, i.e., cold slab subduction and exhumation during 240～200Ma, and lithosphere delamination and thermal exhumation during 196～163Ma. The rapid thermal exhumation, co-occurred with extensive regional metamorphism and intermediate-acidic magmatism, implied that the subducted slabs had broken off and delaminated. The p-T-t path of UHPM consists of two curves, i.e., a barrette-like curve reflecting the process of cold slab subduction and exhumation (cold overthrusting), and a crescent curve representing the thermal exhumation after slab breakoff. By statistically analyzing of isotopic ages for granitoids and commenting on previous studies, the authors preliminarily determine that a large volume of granitoids developed during 200～100Ma, predominately formed in the period of 150～100Ma, with the peak age at about 130Ma. The inferred lithosphere delamination from the development of granitoids occurred slightly later than that deduced from researches on metamorphic rocks. Distant effect of Jurassic collisions of Qiangtang terrane, Lasa terrane and Paleo-Pacific land onto the Eurasian land made East Qinling-Dabie Mountains in a compressional circumstance for a long time, and suppressed its orogenic extension; whereas, Cretaceous late-collisional extension and appearance of West Pacific trench-arc-basin system relieved East Qinling-Dabie Mountains of external compression, instantly result in violent orogenic lithosphere delamination-extension and depressurized-calefactive melting, and consequently, led to the formation of large scale Early Cretaceous granitoids and intermediate-acidic volcanic rocks. In conclusion the magmatism caused by lithosphere delamination in East Qinling-Dabie Mountains occurred in the transition from compression to extension in the period of J₃-K₂.

Key words: east Qinling-Dabie Mountains, lithosphere delamination, granitoid, UHPM(ultra-high pressure metamorphite), p-T-t path, transition from compression to extension