In this paper, we studies the Datan granite of back Longmen Mountains area, in the northwest margin of the Yangtze block, using zircon U Pb geochronology and geochemical methods. The results show that the zirons of the samples from the Datan granite have internal oscillatory zonings and higher Th/U ratios (0.22～1.57), which indicates that the zirons are igneous in origin. The results of zirons LA ICP MS U Pb dating for the Datan granite were 806±19Ma (MSWD=0.56), which indicated that the Datan granite was formed in the Late Neoproterozoic. The Datan granite is high in SiO2 (74.12％～77.10％) and Al2O3 (11.86％～12.56％), A/CNK=0.95～1.05 (1.01 on average). It is a type of peraluminous granite, and is identified as I type granite. The abundance of ∑REE varies in the range of 97.69×10 6～189.60×10 6 (140.85×10-6 on average). The rocks show a slightly weak to intermediate negative Eu anomalies. The trace element geochemistry is characterized by evidently negative anomaly of Th, Nb, Ta, P, etc. and critically negative anomaly of Rb, Sr, Ti, etc. The granite emplaced by underplating of granitic magma, which was formed through partial melting of pyrogenic rock, and is typical crust source petrogenesis. The Datan granite shows the characteristics of normal continental arc granite, and the magma is derived from lower crust. The granite intrusions is formed in the continental margin setting which has no relation with the subduction. Thus the Datan granite is the product of partial melting of the lower crust due to the crustal thickening caused by active continental margin subduction and arc continent collision orogeny during the Neoproterozoic in the northwest margin of the Yangtze block. The granite is formed in a transitional environment from syn collision (compressional environment) to post collision (extensional environment), namely post orogenic period, and is the product of the initial break up of Rodinia supercontinent