SHRIMP U-Pb zircon ages of the Fuping Complex: Implications for Late Archean to Paleoproterozoic accretion and assembly of the North China Craton

Abstract

The Fuping Complex is situated in the central part of the North China Craton and consists of four major lithological assemblages: Fuping tonalitic-trondhjemitic-granodioritic gneisses, Longquanguan augen gneisses, Wanzi supracrustal assemblage and Nanying granitic gneisses. SHRIMP U-Pb geochronology combined with U-Th and cathodoluminescence (CL) imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late Archean to Paleoproterozoic history of the Fuping Complex. CL images reveal the coexistence of magmatic and metamorphic zircons in nearly all rock types of the Fuping Complex. The metamorphic zircons occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores, and are all characterized by nebulous zoning or being structureless, with extremely high luminescence and very low Th contents. These features make them distinct from magmatic zircons that are characterized by concentric oscillatory zoning, comparatively low luminescence and high Th and U contents. SHRIMP U-Pb analyses on magmatic zircons reveal that the tonalitic, trondhjemitic and granodioritic plutons of the Fuping gneisses were emplaced at 2523 ± 14 Ma, 2499 ± 10 Ma and 2486 ± 8 Ma, respectively; whereas the monzogranitic and granitic plutons of the Longquanguan augen gneisses were intruded, respectively, at 2510 ± 22 Ma and 2507 ± 11 Ma. Prismatic and oscillatory-zoned zircons dominate in the pelitic rocks of the Wanzi supracrustal assemblage and are interpreted as detritus from igneous source rocks. The concordant and discordant U-Pb ages of 2502 ± 5 Ma and 2507 ± 14 Ma obtained from two pelitic rock samples indicate these rocks must have been deposited no earlier than ∼2507 Ma ago. In addition, a zoned zircon grain in one pelitic rock sample has a near concordant age of 2109 ± 5 (1σ) Ma, which may provide a maximum depositional age for the Wanzi supracrustal rocks. SHRIMP results also reveal that granitic magmatism assigned to the Nanying granitic gneisses occured over a protected interval from ∼2077 ± 13 Ma to ∼2024 ± 21 Ma. The nebulously-zoned zircon grains and overgrowth/recrystallization zircon rims from different rocks yielded similar concordant 207Pb/ 206Pb ages in the range 1875 to 1802 Ma, interpreted as approximating the age of regional metamorphism of the Fuping Complex. Timing of primary zircon crystallization and regional metamorphism of the Fuping Complex is in general agreement with recent U-Pb zircon ion probe results for the Wutai and Hengshan Complexes that bound the Fuping Complex to the northwest. These areas are characterized by the emplacement of major granitoid bodies at around 2.50 Ga to 2.48 Ga ago, deposition of supracrustal rocks in the Paleoproterozoic, intrusion of Paleoproterozoic granitic bodies at 2.1 to 2.0 Ga, and regional metamorphism at 1.875 to 1.802 Ga. These data indicate that the Fuping and Hengshan Complexes do not represent an older crystalline basement to the Wutai Complex, as suggested in previous tectonic models but, together with the Wutai Complex, represent elements of a single late Archean to Paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western North China blocks at ∼1.85 Ga, which resulted in the final assembly of the North China Craton.