新疆富蕴县苏普特一带双峰式火山岩的发现及其地质意义

The Discovery of Bimodal Volcanics in Supute Area of Fuyun County,Xinjiang,and Its Geological Significance

新疆阿尔泰苏普特一带中泥盆统阿勒泰组为一套以玄武质岩石为主的双峰式火山岩组合,下部以变质玄武岩为主,夹少量流纹岩,中部以变质玄武岩、玄武质凝灰岩、沉凝灰岩为主,夹少量酸性火山岩及其碎屑岩,上部流纹质岩石有所增加。玄武质岩石往往变质为角闪片岩、绿帘阳起石片岩等,属拉斑玄武岩系列,低SiO2、K2O,高Ti,富Na、Rb、Ba、Th、U等,轻稀土略富集,无明显铕异常,Nb负异常明显,相似于MORB和岛弧型玄武岩的某些特征,与弧后盆地玄武岩的特征基本一致。流纹质岩石属钙碱性系列,高SiO2,低K2O,为钠质型;大离子亲石元素富集,具明显的Nb、P、Ti负异常和弱的Th正异常,Sr含量较低;稀土元素含量较高,∑REE为135.67×10-6～407.71×10-6,轻稀土略富集,(La/Yb)N为2.55～3.73,铕负异常较明显,δEu值为0.34～0.54。流纹质岩石地球化学特征也与典型弧后盆地双峰式火山岩中的流纹岩相似。结合区域资料分析,苏普特双峰式火山岩是阿尔泰南缘俯冲撕裂型裂谷盆地演化晚期局部形成的类似于弧后环境的产物。

In Supute area, Middle Devonian Aletai Formation is a suite of basaltic bimodal volcanics assemblage. At the bottom, there are mainly the metamorphic basalts interstratified few rhyolites; in the middle, there are mainly the metamorphic basalts, basaltic tufts and tuffites with few acidic volcanics and its clastic rocks; at the top, there are more rhyolitic rocks. Basaltic rocks usually were metamorphic as amphiboleschist and epidote-antinolite schist, which belong to tholeiite series, with low SiO2 and K2 O, high Ti and rich Na, Rb, Ba, Th and U. The light rare earth elements of basaltic rocks are slightly enriched without an obvious Eu anomaly and with an obvious Nb negative anomaly, which are similar to the characteristic of MORB and arc-type basalts, and almost same as that of back-arc basin basalts. The rhyolitic rocks belong to the calc-alkaline series with high SiO2 and low K2O, which are the agpaitic type; its large ion lithophile elements are enriched with obvious Nb, P and Ti negative anomaly and slight Th positive anomaly and low content of Sr. The contents of rare earth elements are high with ∑REE of 135.67×10^-6-407. 71×10^-6 , light REE enriched slightly with （La/Yb）h of 2.55-3.73, Eu has an obvious anomaly with δEu of 0.34- 0. 54. The geochemical characteristic of rhyolitic rocks is also similar to that of rhyolite in the bimodal volcanics of the typical back-arc basin. Combined with the regional data analysis, Supute bimodal volcanics are the products during the evolution from the rift basin of subduction rip-up type in the southern margin of Altay to partially formed similar back arc at the later period.