ABSTRACT

D/H ratios and H₂O contents have been measured for 9 samples of alkaline granites from Kuiqi, Fuzhou and 11 samples of alkaline and monzonitic granites from Qingdao, both on the eastern coast of China. The δD values range between -99 ∼ -115‰ for the alkaline granite from Kuiqi with H₂O contents of 0.08 ∼ 0.13 wt%. For Qingdao granitic complex, the alkaline granite exhibits a δD range of -116 ∼ -145 ‰ with H₂O contents of 0.07 ∼ 0.15wt%, in contrast to -98 ∼ -104‰ and 0.16 ∼ 0.59wt%, respectively, for the δD and H₂O contents of the monzonitic granite. The genetic correlations of the two alkaline granites with extensional fault belts, the miarolitic cavities disseminated in the granites and particularly the steep correlation of δD vs. H₂O contents of the alkaline granites suggest that degassing may be a characteristic process during the magmatic stages of alkaline granites, leading to depletions of deuterium and water contents in the granites. It is proposed that the enrichment of deuterium in the vapor during magmatic degassing is due to the D/H isotopic fractionations among the hydrogen-bearing species in melts, i.e., H₂O, OH^-, H⁺ etc., in which H₂O is the most D-enriched one.