CHEMICAL AND BORON ISOTOPIC COMPOSITIONS OF TOURMALINES FROM THE HOUXIANYU BORATE DEPOSIT IN THE LIAOJI PALEOPROTEROZOIC FOLDBELT (NE CHINA): IMPLICATIONS FOR CYCLING OF BORON IN SUBDUCTION ZONE AND ORIGIN OF BORATE DEPOSIT
Tourmalines from the meta-volcanic rocks are chemically homogeneous, showing relatively high Fe and Na and low Mg, with δ11B values ranging from +1.2‰ to +2.6‰. Tourmalines from the tourmaline-rich rocks, however, commonly show compositional zoning, with an irregular detrital core and a euhedral overgrowth, and have significantly higher Mg, REEs, V and Sr than those from the meta-volcanic rocks. They show varied B isotope values ranging from +4.5‰ to +12.4‰, which plot intermediate between those of the terrigenous sediments and arc rocks with low boron isotope values and those of marine carbonates and evaporates with high boron isotope values. In addition, the rim of the zoned tourmaline shows notably higher Mg, Ti, V, Sn, and Pb, and REE, but lower Fe, Co, Cr, Ni, Zn, Mn, and lower δ11B values than the core. These data suggest that (1) the sources of boron of the borate ore-body are mainly from the Paleoproterozoic meta-volcanic and sedimentary rocks that were formed in a continental arc setting, and (2) the ore-forming fluids should be the high temperature metamorphic fluids related to the amphibolite-facies metamorphism of the foldbelt during a process of arc-continental collision at ca. 1.9Ga, which leach boron from the boron-rich meta-volcanic and sedimentary rocks, and the boron-rich metamorphic fluids subsequently interacted with the marine Mg-rich carbonates and evaporates, forming the borate deposits.