阿尔泰诺尔特地区泥盆纪正格河火山岩的厘定及其地质意义

The determination of Zhenggehe Devonian volcanic rocks in Nurt area,southern Altay

阿勒泰南缘诺尔特地区晚古生代火山岩十分发育,其中正格河一带出露一套片理化蚀变流纹岩与含凝灰质英安流纹岩的岩石组合,呈北西-南东向带状展布,前人将其划为石炭纪红山嘴组a段。本次工作对其进行的LAICPMS锆石U-Pb测年结果显示为早泥盆世（206Pb/238U加权平均年龄值为（400.4±3.6）Ma）,时代不同于红山嘴组所属的石炭纪。其岩性明显区别于红山嘴组b、c段地层,且稳定成规模出露,故对此套流纹岩组合进行了重新厘定,暂命名为正格河火山岩。岩石Si O2含量在69.77%-75.16%,里特曼指数为1.26-1.70,（Na2O＋K2O）介于5.90%-7.28%,A/CNK值介于0.88-1.16,显示钙碱性、偏铝质岩石特征。微量元素富集Rb、Th等大离子亲石元素,亏损Nb、Ta、Hf等高场强元素,稀土元素总量变化较大,∑REE介于189.20×10^-6-308.47×10^-6,轻、重稀土元素分馏较强（（La/Yb）N=5.82-7.86）,铕负异常（δEu=0.45-0.75）,具有与俯冲作用有关的岩浆作用特征。结合区域地质资料,表明正格河火山岩形成于活动大陆边缘构造环境。

The Nurt area is located in the southem part of Altay, where Late Paleozoic volcanic rocks are well developed. There exists a set of rock assemblages of altered rhyolite and tuff dacite rhyolite in Zhenggehe area, which are in zoned distribution from northwest to southeast and were originally assigned to member a of Hongshanzui Formation. The LA-ICPMS zircon U-Pb dating has revealed that the volcanic rocks formed in the Early Devonian （the zircon 206pb/238U weighted average age is （400.4±3.6） Ma）, unlike Hongshanzui, whose age is of Carboniferous. The lithologic characteristics are obviously different from members b, c of Hongshanzui Formation, and are extensively stably distributed, so this set of rhyolites was re-determined, and tentatively assignedto Zhenggehe volcanic rocks. The SiO2 values range from 69.77% to 75.16%, and cr from 1.26 to 1.70, Na20＋K20 values range between 5.90% and 7.28%, A/CNK ratios range between 0.88 and 1.16, with sub-alkaline, calc-alkaline and meta-aluminum characteristics. The pluton is characterized by enrichment of trace elements Rb, Th and other large LILE, loss of high field strength elements Nb, Ta, Sr, Hr. The content of REE changes considerably, ∑REE range between 189.20×10^-6 and 308.47×10^-6, light and heavy REE show strong fractionation （（La / Yb）N being 5.82-7.86）, Eu exhibits negative anomalies （δEu = 0.45-0.75）, and the REE characteristics are similar to features of island- arc volcanic rocks. Combined with the data of southerner margin of Altay, the authors hold that the Zhenggehe volcanic rocks were formed in an active continental margin tectonic environment.