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Structural, metamorphic and geochronological works in the Paleoproterozoic Trans-North ChinaBelt: A new polyorogenic model.

The Paleoproterozoic Trans-North China Belt (TNCB) is a nearly north-south trending zone, of 1200 km long and 300 km wide, which resulted from collision between the two Archean eastern and western blocks, marking the amalgamation of the North China Craton. Previous models which account for the evolution of the belt are mainly based on geochronology, petrology and geochemistry but lack of structural constraints. Field trips in the best-exposed crustal segment of the TNCB – the Luliang, Hengshan, Wutai and Fuping Complexes - allow us to recognize several tectono-metamorphic units and to define the bulk geometry and kinematics of the belt. We decomposed the TNCB in two main lithological units, called the “Upper Wutai Unit” and the “Lower Wutai Unit”. The first one represents an ancient oceanic crust metamorphosed under greenschist facies conditions. The second unit consists of magmatic arc rocks which experienced an amphibolite facies metamorphism. Both units are interpreted as synmetamorphic nappes The Upper Wutai-Luliang nappe overthrusted to the SE upon the Lower Wutai Unit which one is thrust south-eastward upon the Fuping complex via the kilometric scale Longquanguang Thrust. Deformation related to the Longquanguang Thrust reworked and cross cut the typical dome like structure of the Fuping complex. We think it represents a huge intracontinental ductile flat lying shear zone. Therefore, Hengshan, Wutaishan and Fuping complexes all belong to the same terrane named the Fuping Block. The Lower Wutai Unit and Upper Wutai Unit Nappes are rooted in a suture zone recognized in the western Luliang massif, which resulted from closure of an oceanic domain (herein called Luliang ocean) between the Western Block and the Fuping Block. Precise U-Th-Pb chemical EPMA dating on monazite reported on a P-T-t path from a Ky-metapelite allow us to evaluate the timing of peak of metamorphism around 1880-1890 Ma. The Lower Wutai Unit is extensively migmatized and crustal melting occurred at 1850 ± 5 Ma as indicated by LA-ICP-MS U/Pb zircon dating of a migmatitic leucosome. Moreover, the Fuping massif shows evidence for a ca. 2100 Ma tectono-metamorphic and magmatic event, which we interpret as resulting from the closure of an oceanic domain (herein called the Taihang ocean), by westward subduction below the Fuping complex Our model involves three Archean blocks, namely from West to East, the Ordos, Fuping and Eastern blocks separated by the Luliang and Taihang oceans. Closure of the Taihang ocean is responsible for the 2100 Ma event whereas closure of the Luliang ocean accounts for the 1880 Ma one. Both events are controlled by westward directed subductions.