| The North China craton, is an Archean crustal fragment with a complex history evolved from~1800 Ma cratonization to the formation of the Yanshan intraplate orogenic belt. During the late Paleozoic, the ancient Central Asian ocean subducted beneath the North China Plate, resulted in the northern margin of North China to become an active continental margin, forming a large number of intrusive rocks. The plutons on the both sides of the Fengning-Longhua ductile shear zone are magmatic rocks rather than gneissic rocks according to the microstructutal features. The plutons are Daguangding Pluton to the southwest of Longhua city, Longmensuo pluton to the southwest of Fengning city, Sunjiazhuang pluton to the western Chicheng ciry, and Majuanzi pluton to the northwest of Luanping city. Two samples collected from the diorite yield zircon SHRIMP weighted mean 206Pb/238U ages of 320-290 Ma. The core ages are of 1900±30 Ma, indicating that the magmatic rocks may be re-melted from the gneiss without addition of arc-related magma. One sample collected from the gneiss yield zircon SHRIMP weighted mean 207Pb/206U age of 1868±7 Ma. The similar ages of the cores and the gneiss mean that the diorite re-melt from the old rocks. The magmatic plutons have been piled up the ductile shear deformation, and the muscovite 40Ar/39Ar dating yields the deformation occurred at ~250 Ma. The ductile shear zone presents to different rocks indicates that it has piled up the gneiss of 1800-1900 Ma and the plutons of 320-290 Ma.Continental margin subduction along the northern margin of the North China craton transferred the tectonic stress from the margin toward the interiors of the craton via middle-lower crustal flow. Transpositional foliation, transpositional folds, and biotite/hornblende growth parallel to mineral lineation mean middle-lower crustal flow. Magma flew from north to south during their emplacement, forming a unique structural characteristics of the magmatic flow.
|
PDF Full Text Request