东天山卡拉塔格晚石炭世岛弧火山岩发现的地质意义

The discovery of Low-Carboniferous arc volcanic rocks and its tectonic significance at the Kalatag area in the eastern Tianshan

东天山大南湖岛弧带内古生代火山岩地层时代一直存在争议,其北部卡拉塔格地区核部火山岩尤为复杂。本研究通过详细的野外工作,在该区识别出一套晚石炭世(299.6±2.1 Ma)流纹岩地层。地球化学研究表明:流纹岩具有高SiO_(2)(75.83%~77.96%)、低Al_(2)O_(3)(10.41%~11.94%)和TiO_(2)(0.30%~0.34%)含量,富钠贫钾(Na_(2)O/K_(2)O=52~215),莱特碱度率(A.R.)为2.84~3.28,属钙碱性低钾系列岩石;所有岩石显示轻微富集大离子亲石元素和轻稀土元素(La/Yb)_(N)=2.12~2.43、亏损高场强元素Nb、Ta和Ti,并呈现Eu负异常(Eu/Eu^(*)=0.64~0.82),与俯冲相关的钙碱性岛弧岩浆岩特征相似。结合区域研究成果,本文认为该流纹岩是北天山洋向北俯冲过程中岩浆活动的产物,晚石炭世北天山洋尚未闭合。本研究成果为东天山大南湖岛弧带岩浆活动时限和构造演化提供了新资料。

Kalatage area is located in the Dananhu island arc belt on eastern Tianshan,Xinjiang,China,and volcanic age was extensively controversial in the cluster area.Based on fieldwork,we have discovered a new set of rhyolite strata of Late Carboniferous(299.6±2.1 Ma).Geochemical data shows that they are characterized by high SiO_(2)(75.83%~77.96%),low Al_(2)O_(3)(10.41%~11.94%)and TiO_(2)(0.30%~0.34%)contents.Additionally,Na_(2)O/K_(2)O and Wright alkalinity ratio(A.R.)range from 52~215 and 2.84~3.28,respectively,indicating that rhyolites belong to calc-alkaline and low potassium series.The rhyolites are negative Eu anomalies(Eu/Eu^(*)=0.64~0.82)and relatively enriched in large ion lithophile elements(LILE)and depleted in high field strength elements(Nb,Ta,Ti)and exhibit LREE-enriched(La/Yb)N=2.12~2.43,which is similar to the characteristics of subduction-related calc-alkaline island-arc magmatic rocks.Combined with previous findings,the rhyolites formed in an island arc environment related to the northward subduction of the North Tianshan Ocean,indicate that the North Tianshan Ocean was not closed during the Late Carboniferous.This discovery provides new data in understanding the time limit of magmatic activity and tectonic evolution of the Dananhu arc belt of the eastern Tianshan.