Geochemical characteristics and geological significance of the adakites from west Tibet

The occurrences of adakite or adakite-like rocks in the eastern part of the Qinghai-Tibetan region are well known (Qu et al. 2002, Hou et al. 2003). Here, we report the first occurrence of such rocks from the western part. The geochemical characteristics of adakite distributed in the west Tibet (between 88° longitudes and the western borders of China) and of less than 25 Ma in age are presented in this paper. The geological significance of the adakites is also discussed by comparing with the adakite-like rocks exposed in the east. Samples for geochemical study were taken from 3 sites from the southern, middle and northern Gangdese plutons where the trends are generally NNE. These samples, which belong to the series of continental alkaline magmatism, have the following characteristics: relatively high Si0 2 (56.00~71.51%) and A1 2 0 3 (13.04~16.85 %) and Na 2 O (Na 2 O/K 2 O=1.2 to 2.4); high Mg # ratios (57~64) based on petrochemistry; high Sr (420~1068 ppm); low Y (3.32~16.7 ppm). They show a REE pattern rich in light REE and abnormality in Eu (weak) and Sr (positive) compared to the standardized pattern. These geochemical characteristics are typical of adakite according to Defant and Drummond (1990) (Figure 1).