Inhomogeneous lithospheric thinning in the central North China Craton: Zircon U–Pb and S–He–Ar isotopic record from magmatism and metallogeny in the Taihang Mountains

Abstract

The large scale Mesozoic magmatism and related metallogeny in the Taihang Mountains (TM) provide important clues for the lithospheric thinning of the North China Craton (NCC). Among the ore deposits, the vein gold mineralization of Shihu in the Fuping region and the skarn ore deposit of Xishimen in the Wu'an region represent typical Mesozoic metallogeny in the TM. In the Shihu gold mine, the Mapeng batholith is dominantly composed of monzogranite and granodiorite, whereas, the Wu'an pluton in the Xishimen iron mine mainly comprises monzonite and diorite. Here we present zircon LA–ICP-MS U–Pb data from 8 samples which reveal the timing of magmatism in the TM as ca. 130Ma, which is contemporaneous with the large-scale metallogeny in the margins of the NCC. The δ34S values recorded in the sulfide minerals from the Shihu gold deposit and the Xishimen skarn iron deposit show a range of 2.2‰–5.0‰, and 11.6‰–18.7‰, respectively. Helium isotopic compositions of fluid inclusions in pyrite from the Shihu gold deposit vary from 0.12 to 1.98Ra (where Ra is the 3He/4He ratio of air=1.39×10−6), with calculated mantle helium values of 1.4%–25%, whereas, those of the Xishimen skarn iron deposit range from 0.06 to 0.19Ra, with calculated mantle helium of 0.7%–2.2%. The S–He–Ar isotopic data suggest a lower crustal origin for the ore-forming components, with variable inputs of mantle source. The large population of inherited zircons in our samples, with 207Pb/206Pb ages ranging between 2500Ma and 1800Ma, also supports crustal participation. Our data reveal that the Shihu gold deposit witnessed greater mantle input than the Xishimen skarn iron deposit, suggesting that the continental lithosphere is markedly thinner under the Fuping region than that under the Wu'an region. Our interpretation is also supported by published data from two ultra-broadband high-precision magnetotelluric sounding profiles across the TM region showing a variation in the lithosphere thickness from 155km to 70km while moving from the south (Wu'an region) to the north (Fuping region). Our study suggests that inhomogeneous lithospheric thinning in the central NCC occurred at least as early as ca. 130Ma ago.