Abstract:

The data on lower crustal granulite xenoliths of the Cenozoic, the Mesozoic and the Paleozoic and their zircon ages were collected in this study. The results show that these xenoliths represent the compositions and formation ages of the bottom of lower crust in different domains and different times in NCC. The most useful information is from the Cenozoic basalts and their xenoliths such as from Hannuoba, Nushan, Junan and Qingdao. They may reflect almost the whole bottom of lower crust situations in NCC. Although the data from kimberlites provided the information only before 500Ma, they also can examine and confirm the accuracy of the results from the Cenozoic data. Granulites entrained by Mesozoic diorite in Inner Mongolia are cognate xenoliths, so they merely represent the bottom of lower crust of local area where these kinds of magmas distributed. The multistage geological events have occurred in NCC which were dominated by adding the juvenile mantle materials. The major events were happened in 2.83.0 Ga, 2.5 Ga, 1.8 Ga, 14090 Ma and 4547 Ma and the very important two lower crust growths appeared in ca. 2.5 Ga and 14090 Ma. Basic magma underplating at ca. 2.5 Ga was recorded in granulites of the Cenozoic and the Paleozoic from different places and it may relate to the “mantle plum” activity. Magma underplating in 14090 Ma is widespread in NCC including northern, eastern and southeastern parts. It is possible that the zircon ages of 2.3 Ga reported in granulite xenoliths from Nushan and Junan are the products of Mesozoic underplating magma mixed or/and interacted with the Archean lower crust. Data on the zircon ages of 1.92.0 Ga and 1.8 Ga attest it correlated to the tectonic movement at crust and the disturbance of instantaneous thermal event led to recrystallization of zircon. Tectonic background, a very important problem, must be pay attention to when you study the deepseat xenoliths, especially at the boundary of the firstorder tectonic units because of the complicated interaction between adjacent blocks, so it is necessary to distinguish which block be each xenolith derived from. This is helpful to the understanding of the complicated evolution history in different domain of NCC.

Key words: granulite xenolith, the bottom of lower crust, zircon age, magma underplating, North China Craton