阿尔泰可可托海3号伟晶岩脉中的铪质锆石

HAFNIAN ZIRCON FROM THE KOKTOKAY No.3 GRANITIC PEGMATITE VEIN, ALTAI, NW CHINA

利用电子探针技术研究了阿尔泰可可托海3号伟晶岩脉中铪质锆石的成分特征。结果表明:细粒花岗岩和正岩浆阶段结构带(Ⅳ带)的锆石相对贫Hf,w(HfO2)=3 06%～13 33%,主要属于普通锆石;岩浆 热液过渡阶段结构带(Ⅴ带和Ⅶ带)锆石相对富Hf,w(HfO2)=9 06%～35 21%,主要属于铪质锆石,少量具有普通锆石特征;同时本区发现的铪质锆石中HfO2最高含量是目前伟晶岩已报道铪质锆石中HfO2的最高含量。成分特征还显示,边部细粒花岗岩和正岩浆阶段锆石成分变化非常局限,岩浆 热液过渡阶段锆石成分变化巨大。认为锆石中HfO2含量从正岩浆阶段结构带到岩浆 热液过渡结构带的高度富集是挥发份从岩浆中分离出来独立成相的结果,而岩浆 热液过渡结构带锆石中HfO2含量在带内的巨大变化可能与热液流体的富集有关。

Zircons from the Koktokay No.3 granitic pegmatite, Altai, NW China, were further studied by electron microprobe. EPMA results revealed that zircons from the fine-grained albite granites and the textural zone (IV) of the magmatic stage are Hf-bearing (HfO_2 contents are 3.06wt%-13.33wt%), and they should be assigned to zircons; while zircons from the textural zones of the magmatic-hydrothermal transitional stage are Hf-rich (HfO_2 contents are 9.06wt%-35.21wt%), and they belong mainly to hafnian zircons, apart from a few zircons; moreover the highest HfO_2 content in this district is produced in hafnian zircon. The chemical composition of zircons from the fine-grained albite granites and the textural zone of the magmatic stage varies restrictly, while hafnian zircons from the textural zones of the magmatic-hydrothermal transitional stage show an extensive compositional variation. This study reveals that the extensive compositional variation of zircons formed in the magmatic-hydrothermal transitional stage may be due to the exsolution and enrichment of fluids in pegmatite melt during the crystallization of zircons.