Discovery of the paleo-weathered mantle of the rapakivi granite covered by the Proterozoic Changzhougou Formation in the Miyun area，Beijing and their detrital zircon dating

Recently, the Proterozoic rapakivi granites (dike) with the paleo-weathered mantle-clastic rocks covered directly by the sandstones of the Changzhougou Formation of Changchengian System are discovered in the Miyun County, Beijing. The detrital components of the rocks embrace the in-situ weathered mantle saprolites and coarse-grained clastic rocks from rapakivi granites. Through SHRIMP and LA-ICP-MS dating methods, we have obtained the U-Pb dating ages of 1682±20 Ma and 1708±6 Ma from the detrital zircons sampled in the rocks; those age data are the same with the nearby Miyun rapakivi granite. The existence of this suite of weathered saprolites and clastic rocks, as well as the analysis of the dating results, have shown that the age of the base of the Changzhougou Formation should be younger than 1682 Ma.