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[1] Late Mesozoic extension in NE Asia resulted in the development of a large extensional province. Metamorphic core complexes (MCCs) are the major features in this province and have 40 Ar/ 39 Ar ages of 130-110 Ma for the mylonites and U-Pb zircon ages of 150-110 Ma for the integral granitic intrusions. Based on this and previous studies, this paper summarizes major characteristics of these MCCs and recognizes a regional kinematic shear sense. Most MCCs in the Transbaikalia region, Sino-Mongolia border tract, and the northwest-central portion of the North China craton (NCC) show a top-to-the-southeast (SE) shear, whereas those in the eastern and southern NCC locally underwent top-to-the-northwest (NW) shear. The three largest basins (Songliao, Huabei and Ordos) in North China are located in the transitional zone between domains of opposing shear sense. We interpret the extension in the Transbaikalia, Sino-Mongolia tract and northwestern part of the NCC to reflect late-orogenic collapse of thickened crust following Middle-Late Jurassic collision along the Okhotsk suture. The southeastward extension is probably controlled by crustal-scale top-to-the-SE tangential shear. The transition from contraction to extension is marked by detachment faults that nucleated as extensional crenulation cleavage (ecc, i.e., C′) in sub-horizontal ductile shear zones late in orogenic crustal thickening. The combined effect of gravitational loading and thermal-uplifting is considered to be the origin of the late-or post-orogenic collapse. The top-to-the-NW extension in the NE of the NCC might reflect antithetic sub-extensional zones or Mesozoic back-arc extension as a far-field effect of Cretaceous Pacific plate subduction.

Integrated zircon U-Pb dating and whole rock geochemical analyses have been carried out for two typical S-and I-type granitoids in the northQinling. Zircon dating by SIMS of the Piaochi S-type granitoids yields an emplacement age of 495±6 Ma. The granitoids show whole-rock ε Nd (t)=−8.2-−8.8, zircon ε Hf (t)=−6-−39. The Huichizi I-type granitoids have emplacement ages of 421±27 Ma and 434±7 Ma established by LA-ICP-MS and SIMS methods, respectively. Their whole-rock ε Nd (t)=−0.9-0.9 and zircon ε Hf (t)＝−11-8.4. Combined with statistical analyses of 28 zircon ages of granitoid plutons collected from the literature, Paleozoic magmatism in the north Qinling can be divided into three stages. The first-stage magmatism (~505-470 Ma) mainly occurred in the east part of the north Qinling and has features of an I-type arc, associated with which are S-type granitoids such as Piaochi pluton. The early granitoids (~505-490 Ma) have close spatio-temporal relations to ultra-high-pressure (UHP) rocks, and thus are interpreted as an oceanic subduction system along a continental margin. The second-stage magmatism (~450-422 Ma) occured through the whole north Qinling, and is characterized by I-type granitoids represented by the Huichizi pluton. The magma is interpreted as partial melting of lower crust mixed by mantle-derived magma in a collisional setting with the uplift of terranes. The third-stage magmatism (~415-400 Ma) is dominated by I-type granitoids and only took place in the middle part of the north Qinling, and is regarded as a late-stage collision. The spatial and temporal variations of the Qinling Paleozoic magmatism reveal protracted subduction/collision. The subduction was initiated from the east part of the north Qinling, earlier than that in the Qilian-northern Qaidam, Kunlun, and northern Dabie regions. This demonstrates variations in time of subduction, accretion and collision of separate blocks or terranes in the orogenic systems in central China.

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