Geochemistry and boron isotopic compositions of tourmaline from the Paleoproterozoic amphibolites, NE China: Implications for the origin of borate deposit

The Paleoproterozoic metasediments and metavolcanics in the Jiao-Liao-Ji belt, northeastern China, host a large number of borate deposits. To trace the source of boron in these borate deposits, we carried out in situ chemical and boron isotopic studies on tourmalines of amphibolites from the Lieryu Formation of the Liaohe Group in the Jiao-Liao-Ji belt. The petrology and the chemical compositions of tourmalines of amphibolites show that these tourmalines formed by influx of B-rich fluids during different stages of metamorphism. The delta B-11 values of tourmaline continue to decline during the successive metamorphism. Tourmalines that formed during the prograde metamorphism show higher delta B-11 values (+ 9.52 similar to + 10.18 parts per thousand) than those formed during the peak metamorphism of amphibolite facies (+ 7.22 similar to + 9.43 parts per thousand). Subsequently, tourmalines with lower delta B-11 values formed during the retrograde metamorphism of greenschist facies, which have slightly different boron isotopic compositions in cores (+ 4.18 similar to + 5.50 parts per thousand) and rims (+ 2.66 similar to + 4.66 parts per thousand), mainly due to the temperature effect. The boron isotopic variations of tourmalines of amphibolites match well with our modeling study of boron isotope. The results suggest that tourmalines may be precipitated from fluids released from dehydration of marine sediments with relative high delta B-11 values (+ 9 parts per thousand) during prograde metamorphism. In the same way, the borate deposits formed from the liberated B-rich fluids that interacted with the Mg-rich carbonates and/or silicates.