PALEOPROTEROZOIC METAMORPHISM AND CRUSTAL EVOLUTION OF THE HENGSHAN-WUTAI-FUPING REGION, NORTH CHINA CRATON

An arguable point in regarding the Neoarchean and Paleoproterozoic crustal evolution of the North China Craton (NCC) is that whether the tectonic setting in the central belt during the mid-Paleoproterozoic (2.35–2.0 Ga) is dominated by extensional or oceanic subduction–arc regime. A review of the mid-Paleoproterozoic magmatism and sedimentation for the Hengshan-Wutai-Fuping region suggests that a back-arc extension regime was dominated in this region. This conclusion is consistent with the observations that the 2.35–2.0 Ga magmatism shows a typical bimodal distribution where the mafic rocks mostly have arc affinities and the acidic rocks are mainly comprised of highly-fractioned calc-alkaline to alkaline (or A-type) granites, and this magmatism was coeval with development of extensional basins characteristic of transgressive sequences with volcanic interlayers such as in the Hutuo Group. Although the final amalgamation of the NCC was believed to occur at ~1.85 Ga, recent zircon U–Pb age dating for mica schist in the Wutai Group suggests a collision event may have occurred at ~1.95 Ga. As the metamorphic ages of ~ 1.85 Ga mostly obtained from the high-grade rocks using zircon U–Pb approach, it is most probable that they indicate uplifting and cooling of these high-grade terranes. This is because (i) phase modelling suggests that newly-grown zircon grains in high-grade rocks with melt can not date the age of peak pressure and temperature stages, but the age of melt crystallization in cooling stages; (ii) the metamorphic P–T paths with isobaric cooling under 6–7 kbar for the Hengshan and Fuping granulites suggest their long-time stay in the middle-lower crust; and (iii) the obtained metamorphic age data show a continuous distribution from 1.95 Ga to 1.80 Ga. Thus, an alternative tectonic scenario for the Hengshan-Wutai-Fuping region involves: (i) formation of a proto-NCC at ~ 2.5 Ga; (ii) back-arc extension during 2.35–2.0 Ga resulted in bimodal magmatism and sedimentation in rifting basins on an Archean basement; (iii) a crust thickening event in the extended region resulted in a kyanite-type metamorphism at ~1.95 Ga, and (iv) uplifting and cooling of the thickened crust from 1.93 to 1.80 Ga.