阿尔泰青河伟晶岩中电气石成分和硼同位素对伟晶岩体系演化及其与围岩相互作用的示踪

Tracing the evolution of the pegmatite system and its interaction with the country rocks by chemical and boron isotope compositions of tourmaline in the Qinghe pegmatite from the Chinese Altay orogen

伟晶岩及其围岩蚀变带的电气石记录了伟晶岩体系的演化、流体出溶以及流体-围岩相互作用的过程。本文对阿尔泰造山带青河伟晶岩及蚀变围岩中电气石进行了系统的化学成分和硼同位素分析,以探讨伟晶岩体系的源区性质、岩浆演化、流体出溶规模等重要科学问题。根据矿物成分和结构构造不同,伟晶岩可分为边缘带、过渡带和核部带。蚀变围岩从接触带开始向外,依次出现带状蚀变围岩、细粒蚀变围岩和粗粒蚀变围岩。伟晶岩蚀变围岩电气石的形成与伟晶岩出溶流体(提供B、P等)和围岩(提供Fe、Mg)相互作用有关。电气石主、微量元素和硼同位素分析表明,所有电气石都属于碱性电气石中的铁电气石-镁电气石固溶体序列,电气石中(R1+Al)(Na+R2)_(-1)、(Al+O)(R+OH)_(-1)、(Na+Mg)(Al+X_(vac))_(-1)、(X vac Al)(NaR^(2+))_(-1)、FeMg_(-1)及MnMg_(-1)的替代关系记录了伟晶岩岩浆逐渐富集Fe、Mn、Al,亏损Na、Mg、Ca的演化路径。结合各分带V含量和Fe 3+-Al相关性,我们认为原始伟晶岩浆具低氧逸度的特征,而伟晶岩脉边缘带较高的氧逸度可能与伟晶岩浆与围岩相互作用有关。伟晶岩电气石具有比花岗岩电气石低得多的REE总量和(La/Yb)N比值,表明青河伟晶岩并不是花岗岩体系分离结晶的残余岩浆,而可能是云母片岩在中地壳角闪岩相条件下低程度部分熔融的结果。蚀变围岩电气石的硼同位素组成仅稍重于未蚀变围岩的电气石,显示青河无矿化伟晶岩演化晚期流体出溶比较有限。由于伟晶岩中的电气石硼同位素组成明显轻于围岩(云母片岩),推测云母片岩并非伟晶岩的唯一源区,而可能存在一定含量的黏土岩源区。

Tourmaline crystalized in pegmatites and its surrounding altered wallrocks record the evolution,fluid exsolution and fluid-rock interactions of the pegmatite system.We conducted systematic elemental and boron isotope studies on tourmalines from a pegmatite dyke and its surrounding country rocks in Qinghe(the Altay orogeny)to investigate the origin and evolution of the pegmatite-forming magma and the scale of its fluid exsolution.The pegmatite dyke can be divided into the border,the intermediate and the core zones according to its textures and mineralogy assembalge.Outwards from the contact,banded altered rocks,fine-grained altered rocks to coarse-grained altered rocks appear in sequence.The B and P in the pegmatite-derived fluids,together with Fe and Mg in the country rocks,provided necessary sources for the formation of the tourmaline.All tourmalines belong to the alkali group,which can be further classified into schorl to dravite solid-solution series according to the analytical results.Most tourmalines follow the exchange vectors of(R1+Al)(Na+R2)_(-1),(Al+O)(R+OH)_(-1),(Na+Mg)(Al+X_(vac))_(-1),(X vac Al)(NaR^(2+))_(-1),FeMg_(-1)and MnMg_(-1),which highlight the evolution trends of the increase of the Fe,Mn and Al contents and the decrease of the Na,Mg and Ca contents.Combined with the content of V and the correlation between Fe 3+and Al,we believe that the primary pegmatite magma was characterized by low oxygen fugacity,and that the high oxygen fugacity at the edge of the pegmatite dike could be related to the interaction between the pegmatite-forming magma and the surrounding rocks.In addition,tourmaline from pegmatite has much lower∑REE and(La/Yb)N than that from the granite,which suggests that the pegmatite was not a product after extreme fractionation of a parental granite.Instead,it originated from the direct melting of mica schist under amphibolite-faces conditions.The slight difference of boron isotope compositions between the altered and the unaltered wallrocks manifests a limited scale of fluid exsolution from the late-stage barren pegmatite.Tourmalines from the Qinghe pegmatite exhibit significantly lowerδ11 B values than tourmalines from the mica schist,implying that the mica schist was not the only source for the Qinghe pegmatite,and instead,there might be a certain amount of claystones.