FLUIDS IN CALEDONIAN SHEAR ZONES, KWANGPING GROUP, QINLING MOUNTAINS,CHINA - A RECONNAISSANCE STUDY

In the Qinling mobile belt, Central China, Proterozoic metamorphic roc ks (age of metamorphism about 990 Ma) were deformed and partly reequil ibrated during the Caledonian orogeny (age about 440 Ma). The Caledoni an activity occurred mainly along prominent shear zones, which extend in an east-west direction. These relationships are well-illustrated in the Beikwangping-Banqiao area (Kwangping group), where three Proteroz oic metamorphic zones (lower and upper greenschist and lower amphiboli te grades) are bounded by two Caledonian shear zones (Maheko and Jaoan gou). Each structural unit shows a characteristic foliation: S1 (Prote rozoic) and S2a and S2b (Caledonian). Three types of fluid inclusions were observed during a detailed study of seven samples with comparable lithology (quartz-rich sediments) and well-defined microstructural ch aracteristics. The Proterozoic samples (S1) showed an evolution from p urely aqueous fluids in the lowest metamorphic grade to dominant gaseo us inclusions (pure CO2) in the highest grade (lower amphibolite). In the Caledonian shear zones, the earliest fluids (S2a) were low-salinit y, purely aqueous fluids introduced during large-scale thrusting. Late r fluids (S2b) were hotter, complex mixtures of gases (CO2 with variab le amounts of CH4 and N-2) and aqueous fluids of variable salinities e mplaced in discrete, less ductile zones of deformation, which were for med at the end of the thrusting.