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The greenschist facies volcano-sedimentary sequence of the Bainaimiao Group in central Inner Mongolia hosts the Bainaimiao Cu–Mo–Au deposit and the Bainaimiao lode gold deposit. But the ore classification and genetic process of the Bainaimiao Cu–Mo–Au deposit is still disputed. In this study we have selected the interbedded metamorphosed volcanic rocks of the Bainaimiao Group for LA-ICP-MS zircon U–Pb analysis. 17 analyses form a tight cluster on the concordia and yield a weighted mean 206 Pb/ 238 U age of 465 ± 1 Ma (95% confidence level, MSWD = 0.95, n = 17), which represents the best estimate of the crystallization age of the volcanic rocks of the Bainaimiao Group. Biotite from the greenschist of the Bainaimiao Group yielded a well-defined plateau with an age of 429.2 ± 4.1 Ma. The inverse isochron age is 429.1 ± 4.2 Ma with an elevated ( 40 Ar/ 36 Ar) i ratio of 299.9 ± 22.7, which represents the time of regional greenschist facies metamorphism. Previous studies showed that the Re–Os isochron age of the molybdenite from the Bainaimiao Cu–Mo–Au deposit is 445.0 ± 3.4 Ma and the mineralized granodiorite porphyry yielded a weighted mean 206 Pb/ 238 U age of 445 ± 6 Ma. These data indicate that the Bainaimiao volcanic rocks were formed at about 465 Ma and the porphyry Cu–Mo–Au mineralization took place at about 445 Ma. Volcanic rock emplacement is 20 Ma older than the Cu–Mo–Au mineralization indicates it is a porphyry deposit rather than VMS deposit. The dynamic setting for the porphyry mineralization is the Paleo-Asian Oceanic plate subduction along the northern margin of the North China Craton in early Paleozoic. Subsequently, the collision between the Bainaimiao arc and the North China Craton led to regional metamorphism and deformation at about 430 Ma and the old porphyry system may have been overprinted during this metamorphic process. [ABSTRACT FROM AUTHOR]

The Baolidao and Halatu plutons are located in the Northern Orogenic Belt (NOB) in Sonidzuoqi area of Inner Mongolia, which has an important significance for the tectonic evolution of Xing-Meng Orogenic Belt (XMOB). The two plutons have been intensively studied but the conclusions are still controversial. Combined with the previous study, this paper gives new geological data about the two correlative plutons for gaining a better understanding of their petrogenesis and the geodynamic setting. The Baolidao granitoids contain two different series, calc-alkaline series mainly formed in the Ordovician and high K calc-alkaline series mainly formed in the Carboniferous. The Halatu granites are formed in the Triassic and belong to high- K calc-alkaline series. This study got the zircon U–Pb ages of 316–322 Ma for the Baolidao granitoids and 233 ± 2 Ma for the Halatu syenogranites, respectively. In the tectonic discrimination diagrams, they mainly fall into the area of post-orogenic granites (POG). Hf isotopic analyses for the Baolidao granitoids (Sample BLD-1 and 3) shows ε Hf ( t ) = 3.0–14.0, with two-stage Hf model age ( T DM2 ) of 436–1138 Ma. The Halatu syenogranite (Sample HLT-1) also shows a depleted ε Hf ( t ) = 3.8–8.2, with T DM2 of 741–1024 Ma, suggesting the major involvement of juvenile crustal components. The various ε Hf values of the Carboniferous Baolidao and Triassic Halatu granitoids indicates a hybrid magma source of juvenile material with old crustal component, and the ε Hf ( t ) values decrease from the Carboniferous to Triassic, suggesting the increasing proportion of old continental material during this period. Combined with the regional geology, the Carboniferous Baolidao granitoids are possibly not arc rocks, but originated from the post-collisional setting. The Triassic Halatu granites were formed in the subsequently extensional environment. [ABSTRACT FROM AUTHOR]