2.7 Ga granitic gneiss in the northern foot of Daqingshan mountain, central Inner Mongolia, and its geological implications

This study presents the SHRIMP U-Pb dating results of Archean biotite granitic gneiss found in the Early Precambrian metamorphic basement in Xi Ulanbulang area in northern foot of Daqingshan Mountain in central Inner Mongolia. Zircon characteristic of the rock is consistent and it obviously shows core-mantle-boundary structure from cathodoluminescence images. The core of zircons not only has intensive magmatic zoning but also has a smaller range of age distribution. The weighted average age (2 697±11 Ma) represents the diagenetic age of rock which is the oldest one of all rocks reported in the region. It shows that a magamatism occurred to 2.7 Ga years ago in the region. The mantle and of zirons is considered to be featured with metamorphic recrystallization causes. Both the weighted average age (mantle 2 561±18 Ma; rige 2 539±34 a) and Th, U content features of them are very close except for different cathodoluminescence images. Therefore, we consider that the ages of the mantle and boundary together reflect the tectono-thermal event which occurred to 2.5 Ga ago that has been proved to be an important geological event by substantial evidences during the formation and evolution of the North China craton and a large number of pre-rocks have been strongly transformed.