Late Paleoproterozoic (1900-1800 Ma) nappe stacking and polyphase deformation in the Hengshan-Wutaishan area:: Implications for the understanding of the Trans-North-China belt, North China Craton

The Hengshan-Wutaishan area forms part of the Trans-North-China Belt developed in response to the final amalgamation of the North China Craton. The Hengshan-Wutaishan domain and the adjacent Fuping massif constitute the largest and the most representative rock exposure of the belt. A structural study allows us to redefine the lithological and tectonic units that compose the architecture of the Hengshan-Wutaishan domain and to propose that the Trans-North-China Belt was built up through a polyphase tectonic evolution within the period 1900-1800 Ma. The first event (D1) corresponds to the emplacement of lower and upper nappes herein called the Orthogneiss and Volcanites Unit (OVU) and the Low Grade Mafic Unit (LGMU), respectively. The OVU mainly consists of rocks formed in an arc system and metamorphosed under amphibolite facies conditions whereas the LGMU represents oceanic crust rocks with an arc component that underwent greenschist facies metamorphism. The syn-metamorphic D1 deformation is characterized by a NW-SE stretching and mineral lineation with a top-to-the SE sense of shear that corresponds to the sense of nappe motion. U-Th-Pb EPM chemical dating on monazite from three metapelites of the OVU gives isochron ages of 1884 +/- 11 Ma, 1886 +/- 5 Ma and 1887 +/- 4 Ma interpreted as the age of the prograde amphibolite facies metamorphism coeval with nappe stacking during the D1 event. The D2 event corresponds to a widespread crustal melting of the OVU and underlying basement rocks in Hengshan related to the exhumation of deep structures about 20-30 Ma after nappe emplacement. The migmatite contains blocks of retrograded eclogite or HP-granulitic restites, and is characterized by the development of a pervasive foliation and lineation. A late D2 event represents the folding stage of the migmatitic foliation during the completion of a dome. The Orthogneiss and Volcanics Unit and the Low Grade Mafic Unit are unconformably covered by weakly metamorphosed to unmetamorphosed sediments belonging to the Hutuo Super-group. These rocks were deformed by a ductile D3 event, characterized by a NW-SE trending stretching lineation, south verging folds associated with an axial planar slaty cleavage and an E-W crenulation lineation. Lastly, a left-lateral wrenching along the Zhujiafang Shear Zone separates the northern and southern parts of Hengshan. Our structural study shows that there is no significant difference between the southern part of Hengshan and the lower part of Wutaishan, both areas belong to OVU and show a structural continuity. We suggest that this polyphase deformation developed in response to a north-westward subduction of an old ocean, named the Luliang Ocean, beneath the Western Block, followed by collision with a micro-continent named the Fuping Block at similar to 1890-1880 Ma. (c) 2007 Elsevier B.V. All rights reserved.