伊宁地块早石炭世球泡流纹岩的发现及地球化学特征

The discovery of bubble rhyolite in the Early Carboniferous and geochemical characteristics in Yining Block

伊宁地块西段中部阿腾套山一带发现的球泡流纹岩,多数产于早石炭世海相无球泡流纹岩夹层中,整体呈层状、互层状,球泡通气孔顺层平行于流纹构造定向分布,推断是海相极浅水环境下的产物,因而具有精细指相意义。该球泡流纹岩与同期A型花岗岩类共生,属碱性岩类,SiO_2含量为69.18%~77.69%,K_2O大于4%,Al_2O_3为11.32%~16.24%,Ga/Al×10~4=2.1~7.8,∑REE=85.22×10^(-6)~348.8×10^(-6),(La/Yb)_N=6.7~14.8,相对富集轻稀土,δEu具明显负异常,稀土配分及大离子元素蛛网图均与A型花岗岩类地球化学特征总体相同。ε_(Hf)(t)均大于0(+12.5^+15.5),两阶段锆石Hf模式年龄大于锆石U-Pb年龄,表明源区可能为早古生代新生地壳物质部分熔融演化而来。A型球泡流纹岩与同期A型花岗岩类均形成于拉张构造环境,是区内早石炭世大哈拉军山组中迄今唯一发现的显示拉张构造环境的珍贵实例。为揭示和探讨伊宁地块总体处于汇聚大背景下的局部拉张环境,以及东窄西宽之楔形构造形态提供了可能。

The bubble rhyolite found in Atengtao Mountain is interlayered in marine no-bubble rhyolite of the Early Carboniferous in the middle part of the western Yining Block. They appear as layered or alternating beds and the bedding-parallel bubble holes are directionally distributed along with the rhyolitic structure. Therefore, the bubble rhyolite has precise facies significance, and this study conclude that they were formed at shallow sea level. The bubble rhyolite, formed with A-type granites in the same period, belongs to alkaline series. The contents of SiO2 vary from 69.18% to 77.69%, with K2O of over 4%, Al2O3 of 11.32%～16.24%, Ga/Al×10^4=2.1～7.8, ∑REE=85.22×10^-6～348.8×10^-6 and （La/Yb）N=6.7～14.8, and the bubble rhyolites show LREE enrichment, HREE depletion with strong negative Eu anomalies. In addition, the REE patterns and the spider diagrams are consistent with the A-type granites. Because of εHf（t）〉0 （＋12.5～＋15.5） and the two-stage zircon Hf model age is older than the zircon U-Pb age, they might be formed the partial melting of juvenile crustal materials from the Early Paleozoic. So far, it is the only precious example, which is found in the Early Carboniferous Dahalajunshan Formation, shows extensional setting where A-type bubble rhyolite and granites were formed. Therefore, the Yining Block, which appears as scissor-shaped forms with east narrow and west widen, were formed in local extensional setting in the background of convergent tectonics.