新疆博格达造山带晚古生代构造-岩浆演化过程:火山岩组合及地球化学证据  

Late Paleozoic tectono-magmatic evolution in Bogda Orogenic Belt,Xinjiang:Evidence from geochemistry of volcanic rocks

作　　者：王金荣[1] 李泰德 田黎萍[1] 于漫[1] 王怀涛[1] 赵志雄[1] 汤中立[1] 

机构地区：[1]兰州大学资源环境学院,兰州730000 [2]新疆有色地质矿产勘查局物探大队,乌鲁木齐830011

出　　处：《岩石学报》2010年第4期1103-1115,共13页Acta Petrologica Sinica

基　　金：国家自然科学基金重点项目(40534020);新疆国土资源厅项目(QD200509)联合资助

摘　　要：最近,笔者在博格达山东段北部的西地-伊齐-小红柳峡一带的地质调查中发现,该区发育有大量晚石炭世柳树沟组双峰式火山岩及早二叠世卡拉岗组酸性火山岩建造,但双峰式火山岩性质及成因有别于其南侧七角井早石炭世双峰式火山岩。玄武岩富钠贫钾(K_2O=0.18%～0.45%,Na_2O=2.24%～3.63%),属拉斑系列;TiO_2=1.6%～1.7%,略高于MORB,较高的Al(Al_2O_3=16.2%～16.7%)、高Mg(MgO=8.12%～9.54%,Mg~#=61～64),以及低K_2O/TiO_2和K_2O/P_2O_5比值(分别为0.1～0.27、0.63～1.68),反映了在岩浆演化过程中分离结晶作用不明显;Rb/Sr比值0.01～0.02,Zr/Nb=21.6～39.7,Zr/Y=5.38～7.47,以及不相容元素Ba、Zr、Hf相对略富集、Nb-Ta和Th相对亏损,显示岩石具有板内玄武岩的特点;稀土元素球粒陨石标准化配分图上整体接近于平坦型,(La/Yb)_N=1.8～1.9,Eu无异常至轻微正异常(δEu=1.07～1.12),正ε_(Nd)(t)值(+5.63～+5.89),(^(143)Nd/^(144)Nd)_I=0.512927～0.512944,Th/Yb<0.2,Ta/Yb=0.1,表明玄武岩浆源于亏损软流圈地幔,且在演化过程中不曾发生过斜长石的分离结晶作用,并暗示当时的大陆地壳可能由于拉张而变得较薄,玄武岩浆形成后快速上升至地表喷发。双峰式火山岩中的流纹岩Rb-Sr等时线年龄为296±2Ma(1σ),具高Si(SiO_2=76%～80%),富钾贫钠(K_2O=5.1%～5.7%,Na_2O=0.94%～2.03%);低Al(Al_2O_3=7.9%～10.4%);低Ti、Ca和P含量,属高钾钙碱性系列;微量元素Rb、Th、Zr、Hf、K相对富集,Ba、Sr、P、Ti、Nb、Ta为显著亏损;轻稀土元素适度富集且轻、重稀土分馏程度低,(La/Yb)_N=5.1～7.1,(La/Sm)_N=2、(Gd/Yb)_N=1.6～2.2,以及强烈的负Eu异常(δEu=0.17～0.2),(^(87)Sr/^(86)Sr)_I=0.7051～0.7052,δ^(18)O=11.6‰,指示岩石源于地壳物质的部分熔融,源区存在有斜长石残留,形成于大陆裂谷环境。早二叠世末流纹岩(Rb-sr等时线年龄为278±2Ma)具高Si(SiO_2=74%),富钾贫钠(K_2O/Na_2O>2),低Al(Al_2O_3=11.0%)以及较低的Ti和P含量的�The Late Carboniferous bimodal volcanic rocks and Early Permian acidic volcanic rocks were revealed recently in Yiqi-Xiaohongxia of the northern margin of the eastern section of Bogda Mountain,Xinjiang,but in petrogenesis the former different from those in the Qijiaojing Region south to the studied area.The bimodal volcanic suite in the studied area is composed of basalts and a less amount of rhyolites(rhyolites/basalts=～1/4).The basalts have high-Al(Al_2O_3=16.2%～16.68%),-Mg(MgO=8.12%～9.54%,Mg~#=61～64),and-Na(Na_2O=2.24%～3.63%),but low-K contents(K_2O=0.18%～0.45%),and low K_2O/TiO_2 =0.1～0.27 and K_2O/P_2O_5=0.63～1.68 ratios,belonging to tholeiitic series,and indicating no fractional crystallization of plagioclase during the basaltic magmatism;and have TiO_2=1.66%～1.71%,slightly higher than MORB's,and low Rb/Sr ratio (0.01～0.02),Th/Yb(<0.2) and Ta/Yb(～0.1),high Zr/Nb(21.6～39.7) and Zr/Y(5.38～7.47) ratios,slightly enriched in incompatible elements Ba,Zr,Hf,and LREE,(La/Yb)_N=1.8～1.9,no Eu anormalies(δEu=1.07～1.12),but relatively depleted in Nb-Ta and Th,and positiveε_(ND)(t) value(+5.63～+5.89),(^(143)Nd/^(144)Nd)_1=0.512927～0.512944, showing that the basalts have geochemical features indicative of basalts formed within continent and originated from depleted mantle, and that the continent crust became thinner and thinner due to extension for a long time.The rhyolites of bimodal volcanic rocks with Rb-Sr isochron age of 296±2Ma(1σ),have high-Si(SiO_2=76%～79.76%),-K_2O but low Na_2O(K_2O=5.14%～5.65%, Na_2O=0.94%～2.03%),Al(Al_2O_3=7.90%～10.39%),Ti,CaO and P_2O_5 contents,showing they are high-K calc-alkaline series;and exhibit relatively rich in trace elements Rb,Th,Zr,Hf,K and LREE,(La/Yb)_N=5.1～7.1,significantly depletd in Ba,Sr,P,Ti,Nb,Ta,and strongly negative Eu anomalies(δEu=0.17～0.2),(^(87)Sr/^(86)Sr),=0.7051～0.7052,δ^(18)O=11.6‰, suggesting that the rocks,similar to the geochemistry of the ryholites formed in continental rifts,originated from the melting

关 键 词：博格达造山带 晚古生代 构造岩浆作用 双峰式火山岩 大陆裂谷 后造山作用 

分 类 号：P588.14[天文地球—岩石学]