Abstract:

The Changchengian System, with its basal boundary being a key to the delimitation between the Paleo and the MesoProterozoic in the North China Platform, serves actually as an absorbing Proterozoic research field in Chinese geological circle. For a long time it has been difficult to precisely delimit the intraProterozoic ages of North China and their global stratigraphic correlations has been controversial because of lack of available rocks or bodies for age dating. In resent years, the Proterozoic rapakivi granites (dike) with the paleoweathered mantleclastic rocks covered directly by the sandstones of the Changzhougou Formation of Changchengian System are discovered in the Miyun County, Beijing. The detrital components embrace the insitu weathered rocks of rapakivi granites in the lower and the rapakivilike coarsegrained clastic rocks in the upper. Using SHRIMP and LAICPMS dating methods, we have obtained the UPb dating ages of 1682±20 Ma and 1708±5.6 Ma from the detrital zircons sampled in the rocks; those age data are the same with the nearby Miyun Rapakivi Granite. Preliminary analysis has revealed that the Miyun Rapakivi Granite and its synchronic dikes had once been uplifted and outcropped in the air after they were emplaced. The processes of weathering and denudation resulted in both the saprolites and the almost insitu accumulated rapakivi debris. The formation of the weathered rocks was to be preChangzhougou time in age. The existence of this suite of weathered rocks and clastic rocks, and the analysis of the dating results, have shown that the age of the base of the Changzhougou Formation (which belongs to the Changchengian System) should be younger than 1682 Ma instead of the formerly reported 1800 Ma. According to the analysis of the dating results of the insitu rapakivisourced sedimentary clastics and the correlative age data, we here propose a new age of the lower limit of the Changchengian System to be about 16651670 Ma in North China.

Key words: paleoweathering mantleclastic rocks, rapakivi granite, Changzhougou Formation, Changchengian System of Proterozoic, Beijing