Tectonic Division of the Sulu Ultrahigh-Pressure Region and the Nature of Its Boundary with the North China Block

Abstract

Detailed geological, geochemical, and geochronological data suggest that the boundary between the North China Block and the Sulu UHP region in eastern Shandong Peninsula is not a single fault as previously considered. Instead, it is a complicated magmatic-metamorphic zone about 40-60 km wide bounded by the Muping (MP) and Mishan (MS) faults, and herein termed the Kuyushan boundary zone. The complicated zone consists of a granitoid complex containing abundant lenses and pods of metamorphic rock of different protoliths and sizes that are tectonically juxtaposed. The Sulu UHP region is subdivided into two tectonic units: the Haiyangsuo eclogitized granulite unit, and the Rongcheng-Weihai UHP unit bounded by a large-scale ductile shear zone. The Haiyangsuo unit is considered to be a tectonic slab of the lower crust of the Yangtze Block, in which granulites underwent an eclogite-facies overprint. The Rongcheng-Weihai unit represents an ancient subduction relict of the Yangtze crust, and consists of two subunits. The Rongcheng subunit is a typical UHP unit, with a P-T path recording rapid uplifting from mantle (coesite-eclogite facies) to upper lower-middle crustal (amphibolite-granulite facies) depths. The Weihai subunit is a tectonic slab that was detached from the UHP zone during exhumation, and was tectonically emplaced into the lower crust of the North China Block above the subduction zone. UHP rocks of this unit have been strongly modified by subsequent granulite-facies metamorphism. The North China Block, the Kunyushan boundary zone, and the Haiyangsuo and Rongcheng-Weihai units are imbricated slabs. The Rongcheng subunit may have been subducted northwestward under the North China Block, but a detached UHP slab of the Weihai subunit was overthrust onto the North China Block. This inference is supported by a recent geophysical study.