Based on a detailed structural analysis and a preliminary study of major and rare earth elements in a suite of undeformed granites, mylonitized granites and leucogranitic dikes within the Yunmeng Mountains of northeastern Beijing, the authors have reached some conclusions: with increasing shear strain, SiO-2 decreases whereas Al_2O_3,TiO-2,H-2O,and LREE increase in mylonitized granites; compared with the undeformed granite, the leucogranitic dikes have lower LREE and P-2O-5 contents and (La/Gd)-N ratios, but elevated Sm/Nd ratios; and the REE distribution patterns of the mylonitized granites are parallel to the pattern of the undeformed granite, whereas the HREE distribution pattern of leucogranitic dike BH-2-5 and BH-2-6 is similar to that of the undeformed granite; the REE distribution pattern of BH-2-3 differs however from the patterns of all the other samples. The major and rare earth element geochemistry of BH-2-3 suggests that it was derived from partial melting of ancient metamorphic rocks. In such partial melting reactions, accessory phases that concentrated LREE like monazite occurred as residue phases, which resulted in extremely low LREE and P-2O-5 contents and elevated Sm/Nd ratios in the melts. The other two leucogranitic dikes may represent the latest crystallization differentiation product of the Mesozoic Yunmeng Mountain granites. The increasingly intense crystallization differentiation of a granitic magma resulted in the elevated SiO-2 and lowered LREE contents as well as the variation of Sm/Ndratios in the final silica saturated products .