Located in the eastern margin of Baoshan-Shan Thai block, the Ximeng Group was long regarded as the only Precambrian metamorphic basement rocks exposed in China of this block. Without scientific and reliable data of geochronology and paleontology, the age of the metamorphic rocks is still controversial. Based on detailed macro- and micro-structural observations, the metamorphic rocks of Ximeng Group which shows domelike uplift can be divided into granitic mylonites and low grade metamorphic rocks. The former are distributed in core of the uplift and have experienced intensively ductile deformation. The latter are distributed in the rim and have the similar features of Pake Formation and Wangya-Yungou Formation. According to EBSD fabric analysis of quartz in the deformed rocks of Ximeng Group, it can reveal that the rocks closed to the core have experienced an early stage of deformation at intermediate temperatures (550 similar to 650 degrees C, corresponding to amphibolite facies) and a late deformation at low temperature (400 similar to 550 degrees C, greenschist facies), while the rocks near the rim of uplift have been deformed at low temperature (400 550 degrees C). In this paper, three samples of granitic mylonites were chosen for LA-ICP-MS zircon U-Pb dating, giving ages of 455 +/- 3Ma, 456 +/- 3Ma and 454 +/- 3Ma respectively. Combined with previous research results of geochronology and paleontology, it indicates that the protolith of the Ximeng metamorphic rocks may be composed of two parts. The first part is the Ordovician granite ( similar to 460Ma) and the other mainly comprises Cambrian carbonate, elastic rocks and minor basic volcanic rock. The former is highly deformed into mylonites during the ductile shearing, and the latter is transformed into phylite, schist and marble due to low grade metamorphism at low temperature (400 550 degrees C, greenschist facies). It also can be concluded that Ximeng Group should not be regarded as the Precambrian metamorphic basement rocks of Baoshan block.