SUMMARY

Geochemical characteristics of granitoids of Ca Vịnh complex, Northwest of Vietnam: Evidence for the adakitic type of granite

Nguyễn Thị Xuân, Bùi Thế Anh, Nguyễn Thị Bích Thủy

The main petrographic composition of the Ca Vịnh magmatic rocks are trondhjemit, tonalite, granodiorite and granite. The rocks belong to calc-alkaline magmatic series and display geochemical characteristics of adakitic type with high content of Sr (206-791 ppm) and light rare earth elements (La=17.65-30.09 ppm), but very low concentration of heavy rare earth elements (Y=11.59-18.08 ppm; Yb=0.87-1.61 ppm). They have high ratios of Sr/Y (15.51-51.01) and La/Yb (10.96-34.58). The initial Sr isotope and ε_Nd values of Ca Vịnh granitoid are low, ranging from 0.700 to 0.702 and - 0.3 to -3.9, respectively. These characteristics are evidence for generation of magmatic melt through re-melting of the lower continental crust having mafic composition. This is supported by relative high magnesium values (Mg #=46.1-53.5). The emplacement age of the Ca Vịnh granitoid and its Sr-Nd isotopic composition are completely similar or closely to those of trondhjemit from the Kongling and the Kanding complexes forming during the Meso-Neoproterozoic time in the northern margin of the Yangtze plate.

Keywords: Ca Vịnh complex, granitoid, adakitic type, Northwest of Vietnam