Lower crust granulite xenoliths were found from the Early Cretaceous alkaline basalt in Siziwangqi area of Inner Mongolia, which is located at the middle segment of the north margin of North China Craton. Most of the xenoliths are intermediate-acid hypersthene-feldspar granulites, while only a few ones are mafic two-pyroxene granulites. SiO2, MgO, and Al2O3 contents and Mg-# of these xenoliths vary in 49.9% similar to 69.24%, 0.92% similar to 3.56%, 13.91% similar to 22.79% and 43.0 similar to 62.9, respectively. The total REE contents vary in a large range, from 33.76 x 10(-6) to 184.13 x 10(-6). The REE patterns of the xenoliths show some variations, indicating that the xenoliths either have different source regions, or they have different evolution processes. However, the xenoliths have similar chondrite-normalized REE patterns, showing slightly enriched LREE and depleted HREE with obvious Eu positive anomaly in most of those xenoliths. Great differences in trace element compositions of the granulite xenoliths may due to their different source regions or different evolution processes. The geochemical features of these xenoliths are significantly different from those of granulite xenoliths and granulite terranes in North China Craton published before. Estimated temperature and pressure for a two-pyroxenes granulite are 940 similar to 947 degrees C and 1.2 +/- 0.1 GPa, respectively. And the estimated balance temperatures for hypersthene-feldspar granulites are between 927 degrees C and 715 degrees C. The composition and the evolution history of the lower crust of Siziwangqi area in Early Cretaceous can be inferred from the petrography and balanced P-T conditions of granulite xenoliths, and configuration features of zircons and U-Pb LA-ICP-MS dating on zircons: the boundary between upper part and lower part of the lower crust was at about 40km in depth; the upper part of lower crust was mainly composed of hypersthene-feldspar granulites, which was >= 9km in thickness; the lower part of the lower crust probably consisted of two-pyroxene granulites; the hypersthene-feldspar granulites that located at the upper part of lower crust were formed during a granulite facies metamorphism and partial melting, which was caused by the underplating of basaltic magma in Early Cretaceous(similar to 122Ma); The original rock of the hypersthene-feldspar granulites was magmatic rock that formed in Late Archaen (2509 +/- 35Ma); the starting time of lithosphere thinning of Siziwangqi area was 122Ma.
