Recycling of subducted upper continental crust: Constraints on the extensive molybdenum mineralization in the Qinling–Dabie orogen

The Qinling–Dabie orogenic belt is the most important molybdenum ore belt in the world, with a proven reserve of over 9 million tons metal Mo. More than 92% of the reserves come from porphyry-skarn type deposits that are closely related to Late Mesozoic granite intrusions (158–101 Ma) occurring mainly along the southern margin of the North China Craton, where the majority of the continental crust was formed before 2.5 Ga with crustal Hf model ages of 2.8 to 3.2 Ga. The granite intrusions, which consist mainly of granodiorite, monzogranite, and syenogranite, intrude an Archean to Paleoproterozoic basement. The granitic rocks are metaluminous to peraluminous with C/CNK values mostly in range of 0.9–1.2, commonly alkaline rich with high-K calc-alkaline and shoshonitic features. The linear trends of major and trace elements on Harker diagrams suggest a common magma source for the granitic rocks. Abundant inherited zircons with Neoproterozoic U–Pb ages and substantial younger crustal Hf model ages (averaging 2.4 Ga and 2.0 Ga for granitic rocks with U–Pb ages older and younger than 125 Ma, respectively) demonstrate that these granitic rocks were probably derived from partial melting of the subducted northern Yangtze continental crust with Hf model age younger than 2.2 Ga. Integrating the recent progresses in geological, geochronological, and geophysical investigations of the Qinling–Dabie orogenic belt, we propose that it is the conjunction of the decoupling of the subducted plate at the easternmost part and the slab rollback and subsequent breakoff at the westernmost part of the orogenic belt that resulted in the westward shallowing continental subduction along the Manlue suture at the southern margin of the Qinling–Dabie orogenic belt. This subduction took place during westward propagating continental collision in the Late Triassic and the prolonged relative rotation of the North China Craton and Yangtze Block that lasted until the Middle Jurassic. The molybdenum source for the extensive Mo mineralization in the Qinling–Dabie orogenic belt was most likely derived from Mo enriched components of subducted continental crust. The decoupling of the subducted plate that occurred in the easternmost part of the orogenic belt, where only less Mo-enriched components of the Yangtze Block have been subducted, may account for the lack of economically significant Mo mineralization there.