nt face="Calibri" size="3">The Siziwangqi area of Inner Mongolia lies in the middle segment of the northern margin of the North China plate, where the Mesozoic tec- tonic evolution was in an overall background of the transformation from compressional orogenesis to intraeontinental extension. From the Late Jurassic to the Early Cretaceous, compressional structures and extensional structures were overlapped in this region. The compressional structures were limited in the up- per crust, whereas the extensional structures took place in the middle crust and deeper lithosphere. They have different dynamic factors, of which the ex- tensional structures were related to the delamination and thinning of lithosphere, and the underplating of mafic magma. The Early Cretaceous shoshonite of Siziwangqi was the product of lithospherlc thinning, which reflected the extensional structures in deep lithosphere. Inverse isochron age nt>nt-family: 宋体; mso-ascii-font-family: calibri; mso-ascii-theme-font: minor-latin; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-hansi-font-family: calibri; mso-hansi-theme-font: minor-latin">nt size="3">（nt>nt face="Calibri" size="3">127 nt>nt-family: 宋体; mso-ascii-font-family: calibri; mso-ascii-theme-font: minor-latin; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-hansi-font-family: calibri; mso-hansi-theme-font: minor-latin">nt size="3">±nt>nt face="Calibri" size="3"> 2Mant>nt-family: 宋体; mso-ascii-font-family: calibri; mso-ascii-theme-font: minor-latin; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-hansi-font-family: calibri; mso-hansi-theme-font: minor-latin">nt size="3">）nt>nt face="Calibri" size="3"> and weighted average age nt>nt-family: 宋体; mso-ascii-font-family: calibri; mso-ascii-theme-font: minor-latin; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-hansi-font-family: calibri; mso-hansi-theme-font: minor-latin">nt size="3">（nt>nt face="Calibri" size="3">127. 1 nt>nt-family: 宋体; mso-ascii-font-family: calibri; mso-ascii-theme-font: minor-latin; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-hansi-font-family: calibri; mso-hansi-theme-font: minor-latin">nt size="3">±nt>nt face="Calibri" size="3"> 1. 1 Mant>nt-family: 宋体; mso-ascii-font-family: calibri; mso-ascii-theme-font: minor-latin; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-hansi-font-family: calibri; mso-hansi-theme-font: minor-latin">nt size="3">）nt>nt face="Calibri" size="3"> of the shoshonite were obtained by 40At/39 Ar laser probe dating, which indicate that the lithosphere thinning peak of the middle segment of the northern margin of the North China plate occurred before 127Ma, and that the lithosphere was thickened gradually after 127Ma.nt>
