Petrologic-geochemical-chronological Constraints To The Mesozoic Rotation In Middle-east Yanshan Orogenic Belt

During the Middle-Late Triassic time (Indonician Movement), the Yanshanorogenic belt in eastern China was in a NS-directed compression and extension. Sincethe middle-early Mesozoic time, the proto-Pacific plate subduction and collisionbeneath the Eurasian plate strongly influenced the eastern China continental marginand resulted in the tectonic framework transformation. The eastern Asian continentalcrust occurred rotated deformation, and tectonic lines transformed from EW toNE-NNE trending at present position. Many scientists have done lots of researchesregarding this key subject. They have various understandings concerning thebeginning time and triggering factors mechanism of the massive tectonictransformation. The author chooses the east segment of the Yanshan orogenic belt asto focus this thesis on studying tectonic deformation, granitic intrusion ,Archean-lower Proterozoic gneiss, as well un-deformation mesoproterozoic formationduring the period of tectonic framework transformation, and the responding rockgeochemistry and chronological limitation are also discussed in the paper.During 220-240 Ma (Triassic time), there were the Dushan granitic intrusion.The magmatic source was from the mid-lower crust. This granitic batholith had beendeveloped the mylonitization which was controlled by the sinistral strike-sliprotation. On the southeastern side of the Dushan granite pluton, the deformation isstrong, but on the northern side of the Dushan granite, there is no ductile sheardeformation pluton or some other thrust deformation can be found. But, within thesurrounding rocks, dextral strike-slip deformation had been found which iscorrespondent to the sinistral strike rotation out side of the pluton. On the southernside of the Qinlong fault system, ~190-180 Ma granite intruded and also experiencedthe rotation deformation, the top-to-the-NW-compression and NE-shearing andsinistral strike-slip motion, as well counterclockwise rotation occurred . At ~175-174 Ma, there developed the Yuerya granite intrusion with muscovitegrains. The magmatic source was from the re-melted mid-upper crust. Although thecountry rocks of this granitic intrusion has been deformed by NE-trending structures,the granite has no any ductile shearing. Granitic intrusion at ~165 Ma was throughthe Dushan granite as well cut through the NE-trending ductile shear zone on thesouthern side of the Dushan granitic batholith. This bio-hornblende granite has noany brittle or ductile shearing deformation.The study combined with the regional geological evolution and structuralframework, stratigraphic sequences and magmatic events, indicates that thecounterclockwise rotation in the eastern Yanshan orogenic belt from EW to ENE andNE-trending, which occurred prior to ~165 Ma. Principally the main sinistralstrike-slip rotation and correlated NE-trending ductile shearing took place at~174-175 Ma. Later, along the NE-trending tectonic belts and also within the belts,the compressive and extensional deformation, sedimentation and volcanic eruptionssuch as Tiaojishan Formation, all of them occurred post ~160 Ma.