The processes and mechanism of lithospheric thinning in eastern North China Craton during Early Cretaceous: Evidence from Xishimen Complex, Hebei Province

The lithosphere of the Cretaceous North China Craton experienced tremendous thinning events, but the processes and mechanism of the thinning are still controversial. The Early Cretaceous Xishimen Complex is an important complex related to lithospheric thinning in the eastern North China Craton. According to its formation ages and rock structure characteristics, the Xishimen Complex can be divided into two sets of rock suites. The early suite is composed of monzodiorite-monzonite-syenite series with equigranular structures, whose formation age is about 135. 6Ma, with contents of SiO2 = 51. 72% similar to 68. 56% and MgO = 0. 17% similar to 4. 88%. Furthermore, a few samples from this suite show the characteristics of a C-type adakite. The late suite is composed of porphyry monzonite-porphyry syenite series with porphyry-like structures, and its forming age is about 125. 3Ma, with contents of SiO2 =57. 00% similar to 67. 51% and MgO =0. 61% similar to 3. 37% , and a large number of inherited zircons with an age of about 2500Ma occurred in this suite. Both two suites have intrusive rocks of high potassium calc-alkaline series, but most of the early suite rocks have the characteristics of adakitic rocks. The zircons in the late suite rocks have a negative epsilon(Hf)(t), which indicates that they have more crustal material compositions. The calculated data by the amphibole thermometer and barometer show that the early suite is formed under a P-T condition of T = 883. 66 degrees C and P =194. 95MPa, which is equivalent to an environment of 7. 36km depth underground. While the late suite is formed under a condition of T =642. 34 degrees C and P = 26. 81MPa, which is equivalent to a depth of 1. 01km underground. From 135. 6Ma to 125. 3Ma, the crust rapidly uplifts about 6. 35km. Thus, we believe that the intracontinental magmatism begins to affect the study area at about 135. 6Ma. The melting and mixing of crustal materials caused by the upwelling of asthenosphere mantle form the early suite, and the large-scale magmatic activity and crustal fold deformation result in the thickening of continental crust. Then, the lithospheric mantle detaches, and the crust uplifts rapidly. After about 125. 3Ma, the asthenosphere mantle continues to upwell, and the rebounding continental crust begins to melt in large scale to form the late suite. In a word, delamination is the main mechanism of lithospheric thinning in the Mesozoic North China Craton.