Boron isotopic constraints on the Nb and Ta mineralization of the syenitic dikes in the ∼260 Ma Emeishan large igneous province (SW China)

Both Nb-Ta-mineralized and Nb-Ta-poor syenitic dikes in the Panxi region (SW China) are spatially and temporally associated with syenitic plutons, which are part of the similar to 260 Ma Emeishan large igneous province. These syenitic dikes are NW-striking, and have width varying from Ito 5 m and length from 50 to 300 m. The dikes are mainly composed of K-feldspar, albite, aegirine and arfvedsonite, however, mineral modes are different in the Nb-Ta-mineralized and Nb-Ta-poor syenitic dikes. The major Nb-Ta-bearing mineral in the dikes is pyrochlore, which is closely associated with albite and occurs in places with intensive albitization. Rocks of the Nb-Ta-mineralized syenitic dikes contain more albite and less K-feldspar than the Nb-Ta-poor dikes, and have compositions more evolved than the Nb-Ta-poor dikes, indicating that the Nb-Ta-mineralized syenitic dikes formed from a highly evolved magma. We analyzed the B concentrations and B isotopic compositions of the samples of both Nb-Ta-mineralized and Nb-Ta-poor syenitic dikes and associated syenitic pluton using a single column purification method and ICP-AES and MC-ICP-MS techniques. The samples of the Nb-Ta-mineralized syenitic dikes have whole-rock B concentrations ranging from 11.4 to 23.9 ppm and delta B-11 values from -17.95 to -14.54 parts per thousand, whereas the samples of the Nb-Ta-poor dikes and syenitic plutons have B concentrations varying from 3.32 to 16.5 ppm and delta B-11 values from -13.45 to -10.02 parts per thousand. The high B concentration of the Nb-Tamineralized dikes relative to the Nb-Ta-poor dikes is consistent with that B is incompatible and tends to be rich in more evolved magma. The relatively low delta B-11 values of the Nb-Ta-mineralized dikes indicate that the B isotopes may have fractionated between fluids and rocks in a transitional, magmatic-hydrothermal stage. We propose that the highly evolved magmas in a transitional, magmatic-hydrothermal stage may have Nb- and Ta-fluorine complexes dissolved in the hydrothermal fluids in the presence of Nat. Albite crystallization due to intensive albitization in this stage resulted in the decrease of Na in the fluids, decomposing the Nb- and Ta-fluorine complexes. The released Nb and Ta from the complexes were then dissolved in the fluids and finally entered the lattice of pyrochlore crystals in the stage of albitization. (C) 2014 Elsevier B.V. Al! rights reserved.