Age, geochemistry, and Sr–Nd–Hf–Pb isotopes of the Caosiyao porphyry Mo deposit in Inner Mongolia, China

Abstract The giant Caosiyao porphyry Mo deposit (Yinshan area, central Inner Mongolia) is recently discovered in the northern margin of the North China Craton. The mineralization is newly molybdenite Re–Os dated to be Late Jurassic (ca. 148 Ma). LA-ICP-MS zircon U–Pb dating of the two syenogranite porphyry intrusions in the Caosiyao deposit yielded ca. 148.5 Ma and ca. 140.9–140.1 Ma, respectively, and the Caosiyao monzogranite porphyry yielded ca. 145.0 Ma. These Late Jurassic to Early Cretaceous granitic porphyries are high-K calc-alkaline and featured by high Rb, Th, U, and Zr contents and low TiO 2 , P 2 O 5 , Nb, and La contents, with high K 2 O/Na 2 O and medium to strong negative Eu anomalies, all resembling post-orogenic granitoids. The Caosiyao granitic porphyries have high whole-rock initial 87 Sr/ 86 Sr (0.710417–0.718568), low zircon e Hf ( t ) (− 18.9 to − 12.5) and whole-rock e Nd ( t ) (− 17.8 to − 13.7), old single-stage and two-stage model ages for zircon Hf (1282–1552 Ma and 1989–2393 Ma, respectively) and for whole-rock Nd (1766–2431 Ma and 2046–2375 Ma, respectively), as well as low K-feldspar 206 Pb/ 204 Pb (16.603–17.177), 207 Pb/ 204 Pb (15.319–15.374), and 208 Pb/ 204 Pb (36.841–37.225) values. We suggest that the Late Jurassic and Early Cretaceous syenogranite porphyry may have been sourced from a highly and moderately mature lower crust, respectively, whereas the Early Cretaceous monzogranite porphyry may have been sourced from an immature lower crust. The Caosiyao porphyries may have formed during the post-orogenic extension after the Mongol–Okhotsk Ocean closure and coeval with the Paleo-Pacific plate subduction.