Preliminary study on the components of the lower crust in east China Plateau during Yanshanian Period: Constraints on Sr and Nd isotopic compositions of adakite-like rocks.

The composition of continental crust has always been an interesting subject for many geologists. However, the big difficulty is how to determine the composition of the deep crust, especially the composition of the lower crust. The identification of adakite-like rocks within the continental crust may provide a possible way to make sense of the problem. The geochemical characteristics of C-type adakite-like rocks in the eastern China show that they are the products of partial melting of intermediate to mafic granulites in the deep lower crust in high P-T condition, the composition of the lower crust therefore can be traced by these adakite-like rocks. Our preliminary studies show that the geochemical characteristics of adakite-like rocks are different from place to place in the eastern China, suggesting that the lower crust sources from which the adakite-like rocks are derived varying in the compositions from place to place too. Based on the Sr-Nd isotopic compositions of these adakite-like rocks, the lower crust of east China Plateau is argued to be formed by three different parts of Jiaodong-Dabei district, Beijing-Liaoxi district and the lower Yangtze district (the middle-lower reaches of Yangtze River). Following opinions are discussed in the paper: (1) The lower crust of Jiaodong area is characteristic of high SiO2 content, lowNa2O/K2O ratios (0. 8-1. 3), high ISr ratios (0. 709-0. 712), low eNd(t) values (-15-21) and high TDM ages (1. 9-2.1Ga), whereas the lower crust of Dabie area has relatively low ISr ratios, which is the upper Isr ratio limit of the whole area and may probably represent the composition of Mesozoic basement. (2) The lower Yangtze district lies in the Yangtze Block, in which Mesozoic adakite-like rocks have variable ISr ratios, eNd(t) values and TDM ages, suggesting the underplating of abundant young basaltic magma beneath the lower crust basement of Proterozoic. The adakite-like rocks from Tongling area have relatively high Isr ratios (0. 707- 0. 709), low eNd(t) values (- 11- 16) and high TDM age (1. 7 - 2. 2Ga), similar to that of the lower crust of Yangtze basement. (3) The Sr-Nd isotopic compositions of the adakite-like rocks in Liaoxi-Beijing area are characteristic of low ISr ratios (0. 705 - 0. 707), varying eNd(t) values ( - 4- 20) and TDM ages (0. 7 - 2. 1Ga), reflecting that the composition of the lower crust in Liaoxi-Beijing area is rather complicated. The Sr-Nd isotopic composition of the adakite-like rocks in Beijing-Zhangwu area is similar to that of granulite inclusions from Hannuoba basalts and far beyond the eNd(t) values ( - 32- 44) of the lower crust of the North China Block. These adakite-like rocks therefore are assumed not to be derived from the old Archean lower crust of North China Block, but the products of partial melting of young underplating basic granulite of Mesozoic age beneath the lower crust of North China. In addition, the shortcoming of using adakite-like rocks tracing the composition of the lower crust is also discussed in the paper.