Boron isotopic constraints on the Nb and Ta mineralization of the syenitic dikes in the ~ 260 Ma Emeishan large igneous province (SW China)
Boron isotopic constraints on the Nb and Ta mineralization of the syenitic dikes in the ~ 260 Ma Emeishan large igneous province (SW China)
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DOI：
10.1016/j.oregeorev.2014.09.009
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发表时间：
2015-03
影响因子：
3.3
通讯作者：
Fenlian Wang;C. Wang;T. Zhao
中科院分区：
文献类型：
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作者：
Fenlian Wang;C. Wang;T. Zhao
攀西地区的铌钽矿化脉岩和贫铌钽正长岩脉岩在时空上都与峨眉山大火成岩省的正长岩体有关。这些正长岩脉呈北西走向，宽度1 ~ 5 m，长度50 ~ 300 m。脉岩主要由钾长石、钠长石、霓石和钠铁闪石组成，但含铌钽矿脉岩和贫铌钽正长岩脉岩的矿物模式不同。岩脉中的主要铌钽矿物为烧绿石，与钠长石密切共生，产于钠长石化强烈的部位。铌钽矿化正长岩脉的岩石比贫铌钽岩脉含有更多的钠长石和更少的钾长石，并且比贫铌钽岩脉的成分更演化，表明铌钽矿化正长岩脉是由高度演化的岩浆形成的。采用单柱净化法、ICP-AES和MC-ICP-MS技术，分析了含铌钽矿岩脉和贫铌钽正长岩脉及其伴生岩体的B含量和B同位素组成。铌钽矿化正长岩脉样品的全岩B浓度范围为11.4 - 23.9 ppm，δ 11 B值范围为− 17.95-− 14.54‰，而贫铌钽岩脉和正长岩体样品的B浓度范围为3.32 - 16.5 ppm，δ 11 B值范围为− 13.45-− 10.02‰。铌钽矿化脉岩相对于铌钽贫脉岩的高B浓度与B不相容并倾向于在更演化的岩浆中富集一致。Nb-Ta矿化脉岩的δ 11 B值较低，表明B同位素在岩浆-热液过渡阶段可能在流体和岩石之间发生了分馏。我们建议，在一个过渡的，岩浆热液阶段的高度演化的岩浆可能有Nb-和Ta-氟络合物溶解在热液流体中的Na+的存在。在这一阶段，由于强烈的钠长石化，钠长石结晶导致流体中Na+的减少，分解Nb-和Ta-氟络合物。在钠长石化阶段，铌、钽从络合物中释放出来，溶解在流体中，最终进入烧绿石晶体的晶格中。
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 ~ 260 Ma Emeishan large igneous province. These syenitic dikes are NW-striking, and have width varying from 1 to 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 δ11B values from − 17.95 to − 14.54‰, whereas the samples of the Nb–Ta-poor dikes and syenitic plutons have B concentrations varying from 3.32 to 16.5 ppm and δ11B values from − 13.45 to − 10.02‰. The high B concentration of the Nb–Ta-mineralized 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 δ11B 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 Na+. 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.