The Metamorphic Evolution History of High Pressure Granulites in Chengde Area, Northern Margin of North China: Zircon Chronology and Geochemical Evidence
华北北缘承德地区高压麻粒岩的变质演化历史——锆石年代学和地球化学证据

High pressure granulites in Chengde area of northern Hebei province occur as tectonic lenses with weak deformation in a wide composite shear zone composed dominantly of granitic gneiss. The deep crust thermal dynamics and exhumation mechanism demonstrated by these granulites play a critical role in the study of the collisional orogenic model of North China craton and the tectonic evolution model of North China craton as a whole. According to the result of U-Pb isotopic dating for these high pressure granulites and their surrounding rocks, the age of the surrounding protolith is ca. 2500 Ma. At around 2297 Ma, basic rock intruded granitic gneiss. At around 2381 Ma, subduction started, and then there occurred high-pressure-granulite-phase metamorphism. Following the subduction and a long-term crust thickening process, the exhumation at a certain point began after 2001 Ma. During the exhumation process, another basic dike invaded at around 1896 Ma. At 1885 Ma, the uplifting process and granulite-phase retrograde metamorphism took place. At 1850 Ma, a large part of North China experienced high-amphibolite phase metamorphic event including even partial melt. This event destroyed most part of the Paleoproterozoic metamorphic evolution. It can thus be seen that Paleoproterozoic geological evolution history in Chengde area or even the whole northern part of North China was not one or two phases of independent events that happened in different places. Instead, it was a complex and continuous process. Previous studies only revealed events of a few stages, whereas the high pressure granulites in Chengde area have recorded much more information.