Abstract:

The Chongba leucogranite pluton is situated in the eastern part of the Greater Himalayan leucogranite belt. Geochemical data show that these rocks are characterized by high SiO2(73.87%74.95%), Al2O3(14.20%14.74%), K2O(4.44%4.89%), K2O/Na2O(1.191.42), A/CNK values (1.181.22), and enrichment in Rb, Th, U, depletion in Ba, Nb, Sr, Zr, and strong negative Eu anomalies (δEu=0.270.37). These features suggest that they are high potassium calcalkaline and peraluminous Stype granites. The high Rb/Sr (2.68.6) and low CaO/Na2O (0.180.20) ratios imply that the source rocks were probably pelites. The relatively high (87Sr/86Sr)i(0.7631990.778799) and low εNd(t) (-16.7-16.3) compared with those of the metasedimentary rocks in the Greater Himalaya crystalline complex (GHC), indicating that the leucogranites were generated from partial melting of the GHC metasedimentary rocks. The features of high (87Sr/86Sr)i and low Sr content, and the Rb/Sr values decreasing with increased Ba content indicate that the Chongba leucogranites were derived from muscovite dehydration melting of the GHC metapelite under waterabsent condition, which is probably closely associated with the structural decompression caused by the extensional decollement of the south Tibet detachment system (STDS).

Key words: southern Tibet, Chongba leucogranites, geochemistry, SrNd isotopes, formation mechanism