Tectonics of SE China: New insights from the Lushan massif (Jiangxi Province)

In south China the Lushan massif forms a Jurassic through Early Triassic [e.g., Mattauer et al., 1985, topographic high of the South China Block south of the 1991; Okay et al., 1993; Hacker et al., 1996; Gilder et al., Qinling-Dabie belt. The Lushan massif consists of two main 1999]. The noah directed continental subduction of south lithotectonic units separated by a major tectonic ontact: a China was responsible for ultra-high pressure (UHP) Neoproterozoic (upper Sinian)-Paleozoic unit comprising metamorphism and related deformation in the Dabieshan. primarily unmetamorphosed sandstones overlies a Several interpretations have been put forward to account for Paleoproterozoic unit mainly composed of low-pressure, the geodynamic evolution of the Qinling-Dabie belt, but high-temperature gn isses and micaschists. Both units are cut whatever the proposed model, the southern foreland of the by Cretaceous granitic intrusions. Three primary tectonoDabieshan is seldom considered. Since it is likely that such metamorphic and magmatic events are recognized. The eastern intense deformation as that related to UHP metamorphism part of the Lushan massif is cut by a NNE-SSW trending should also exert a strong influence on the structural ductile normal fault (D3 deformation) coeval to the evolution of the South China Block south of the collision emplacement of a 100-110 Ma leucogranite dated by zone, we have conducted the first detailed structural and 4øAr/39Ar laserprobe on biotite and muscovite. D2 geochronologic investigations f the metamorphic rocks of deformation is responsible for the formation of a the Lushan massif, which is a key element for the decakilometer-scale NE-SW trending upright anticline understanding of the Mesozoic evolution ofsouth China. characterized by NE-SW stretching a d NW-SE shortening. While tectonic evolution f South China is generally The age of this folding event is defined by a 127+1 (2•) Ma poorly understood, in Jiangxi Province, south of the U/Pb titanite date obtained fora syntectonic grano•iorite and Dabieshan, three basement areas are recognized: the 4øAr/39Ar ages of 133 Ma for amphibole. This Cretaceous age Wugongshan, Jiulingshan, and Lushan massifs from south o also corresponds t the 4øAr/39Ar ages of 126 Ma found on noah respectively (Figure 1). The Jiulingshan anticline syntectonic muscovites at the base of the Sinian unit. An consists of Proterozoic slates, phyllites, and granitoids, older deformation event, D1, characterized by a top-to-theoverlain by sedimentary rocks ranging from latest Proterozoic NW extensional decollement of the Sinian-Paleozoic series (Sinian) to Early Triassic. Locally, along the south margin of above Proterozoic metamorphic ocks i related to the Triassic the anticline, the tectonic superposition of phyllites upon tectonics of the Dabieshan. Lastly, in the lower part of Sinian Triassic orPermian limestones was previously invoked as rocks, the occurrence of kyanite cataclased during D1 evidence forlarge-scale ov rthrusting [Hs•i et al., 1990]. This documents an older, poorly preserved, late Paleozoic-early interpretation is superseded by a mechanism of extensional Mesozoic tectonometamorphic event (Dx) related to a blind shearing, indicated by displacement of the allochthonous thrust inthe continental crust of the South China Block in the rocks top-to-the-south and top-to-the-noah along the south southern foreland ofthe Dabieshan. and noah margins, respectively; this allochthonous association s best explained interms of extensional shearing