The Mashankou pluton in the eastern part of the Qinling Group in the North Qinling terrane, located in the Xixia area, Henan Province, is mainly composed of gneissic grandiorite. LA- ICP- MS zircon U- Pb dating shows that the pluton was formed at 929±7 Ma. The SiO2 and Al2O3 contents of the grandiorites are 65. 52%~66. 91% and 14. 63%~15. 41%, respectively. They have high contents of Na2O (2. 82%~3. 08%), K2O (2. 14%~3. 10%), TFe2O3 (5. 99%~7. 03%), CaO (2. 78%~ 3. 72%) and MgO (1. 35%~1. 87%), with Na2O/K2O ratios>1, A/CNK values of 1. 07~1. 14 and Mg# values ranging from 34. 4 to 38. 3. The grandiorites belong to peraluminous calc- alkaline and high- K calc- alkaline rocks, and show features of I- type granites. They are characterized by low content of Sr (102×10-6~175×10-6), high content of Yb (2. 13×10-6~3. 67×10-6) and Y (27. 0×10-6~38. 6×10-6), with low Nb/Ta, Rb/Y and Nb/Y ratios, indicating that the grandiorites may be products of partial melting of basaltic rocks in the lower crust. The rocks have moderate REE contents (∑REE=180×10-6~229×10-6), and are enriched in LREE and LILE such as Rb, Ba and Th, and depleted in HFSE such as Nb, Ta and Ti. They have the geochemical characteristics of volcanic arc granite and were formed in the Neoproterozoic continental marginal arc environment.