Abstract:

Quartz porphyry veins and granite porphyry veins are developed in the Dubi area of Zhengxiangbai Banner, Inner Mongolia, and are structurally located in the Bainaimiao island arc belt on the northern margin of the North China craton. Through LA-ICP-MS zircon U-Pb dating, the diagenetic ages of the quartz porphyry and granite porphyry are (127.8±2.1) and (128.8±0.7) Ma, respectively, and both are products of the Early Cretaceous magmatism. Petro-geochemically, the quartz porphyry is rich in silica (w(SiO₂) 76.06%-77.20%) and potassium (w(K₂O) 4.66%-6.48%), high TFeO/MgO ratio (10.42-12.55), higher 10000Ga/Al ratio (2.71-3.27), low w(CaO)(0.58%-0.70%). w(Al₂O₃) is 11.63%-12.67%, and the aluminum saturation index(A/CNK) is 1.08-1.15. The granite porphyry is rich in silica (w(SiO₂) 70.67%-71.88%) and potassium (w(K₂O) 5.83%-6.45%), high TFeO/MgO ratio (10.91-12.10), higher 10000Ga/Al ratio (2.64-3.03), and low w(CaO) (0.33%-0.44%). w(Al₂O₃) is 14.47%-15.14%, and the aluminum saturation index is 1.14-1.16. Their REE distribution curves show “seagull” right-leaning  pattern with obvious negative Eu anomalies. They are relatively rich in Rb, Th, K, Zr, Hf etc., and depleted in Sr, Ba, Ti, P, etc., indicating the characteristics of aluminous A-type granite. The zircon εHf(t) values of quartz porphyry and granite porphyry are －1.56-1.85 and －0.87-2.14, respectively, and the two-stage model ages(TDM2) are 1 281-1 067 Ma and 1 240-1 046 Ma, respectively, indicating that the two rocks are the products of  mixed materials from the crust and mantle. Combined with the regional tectonic evolution, it is considered that the quartz porphyry and granite porphyry were formed in the intraplate extensional environment during the thinning of the Early Cretaceous lithosphere on the northern margin of the North China craton.

Key words: geochronology, geochemistry, quartz porphyry, granite porphyry, Dubi area