Late Paleoproterozoic magmatism in North Hengshan: Final collapse of the Trans-North China Orogen

Abstract

Post-collisional magmatism carries important information for understanding when and how an ancient continental collisional orogen unrooted and collapsed. However, the post-collisional magmatic response to the final collapse of the Trans-North China Orogen, one of the 2.1–1.8 Ga collisional orogens that formed in coincidence with the assembly of the Paleo–Mesoproterozoic Columbia supercontinent, remains controversial. Here we present the results of a combined geochronological and geochemical investigation on a suite of Late Paleoproterozoic potassic granites in the North Hengshan terrane of the Trans-North China Orogen, the North China Craton. In situ zircon U-Pb dating shows that these potassic granites, as the last phase of Precambrian magmatism in this terrane, were emplaced between 1790 Ma and 1774 Ma. Their enriched bulk-rock Nd and zircon Hf isotopic compositions point to an ancient crustal source, and their zircon O isotopic variation indicates the involvement of weathered supracrustal rocks in their sources. Considering their bulk-rock major and trace elements, the Late Paleoproterozoic potassic granites in North Hengshan are best explained to be produced by melting of immature metagraywacke that belonged to the Neoarchean–Early Paleoproterozoic sedimentary sequences, at middle–lower crustal depths. The 1.79–1.77 Ga potassic granites in North Hengshan postdated the main phase of the Late Paleoproterozoic collisional orogeny, and were a magmatic response to the final collapse of the Trans-North China Orogen. These 1.79–1.77 Ga potassic granites, along with other 1.80–1.75 Ga magmatism in the Trans-North China Orogen, were most likely generated in a post-collision extensional setting, possibly superimposed by an Andean-type continental arc at its southern margin. The 1.80–1.75 Ga magmatism marked the end of collision between the Western and Eastern Blocks that gave rise to the final consolidation of the North China Craton and the assembly of the Columbia supercontinent, and recorded the outgrowth of this supercontinent through accretion at its continental margins.