博格达造山带东段萨尔乔克地区早石炭世双峰式  

Geochemical Characteristic and Its Geological Significance of the Early Carboniferous Bimodal Volcanic Rocks in Saerqiaoke Area,East of the Bogda Orogenic Belt

作　　者：汪晓伟[1,2] 徐学义[1,2] 马中平[1] 陈隽璐[1] 朱小辉[1] 孙吉明[1] 崔方磊 

机构地区：[1]中国地质调查局西安地质调查中心,西安710054 [2]长安大学地球科学与资源学院,西安710054

出　　处：《地质科技情报》2015年第3期58-70,共13页Geological Science and Technology Information

基　　金：中国地质调查局项目(1212011220649);国家自然科学基金项目(41202077;41272089);中央高校基本科研业务费专项资金项目(2014G1271059)

摘　　要：博格达造山带东段萨尔乔克地区出露大量早石炭世晚期双峰式火山岩组合,其岩石类型主要有亚碱性玄武岩、英安岩和流纹岩,主体属拉斑系列。玄武岩w（SiO2）为48.69%~52.09%,w（TiO2）（1.16%~1.34%）略高于N型大洋中脊玄武岩,w（Al2O3）高（17.44%~20.86%）、w（MgO）低（3.05%~4.98%）、富钠贫钾（Na2O/K2O=1.82~3.04）,表明其原始岩浆发生过明显的橄榄石和辉石的分离结晶作用;玄武岩具有近于平坦的稀土元素配分模式,Eu异常不明显（δEu=0.87~1.06）,相对富集Rb、Ba、P,亏损Nb、Ta、Th等不相容元素。火山岩地球化学特征表明研究区玄武质岩浆来自于亏损石榴石地幔橄榄岩在一定深度（约75km深度）情况下发生10%~20%的部分熔融,且受到一定程度的地壳物质混染,显示了板内玄武岩的地球化学特征,形成于陆内裂谷环境。中酸性火山岩具有较高的w（SiO2）（70.29%~75.46%）和全碱[w（Na2O＋K2O）=5.13%~9.90%],以及较低的w（TiO2）（0.25%~0.30%）、w（Al2O3）（12.85%~15.88%）和w（MgO）（0.36%~1.01%）;显示右倾负斜率稀土元素配分模式,铕负异常明显（δEu为0.36~0.86,平均为0.64）,富集Rb、Ba、Th等大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素,指示中酸性火山岩起源于地壳物质的部分熔融,源区有斜长石残留。萨尔乔克地区双峰式火山岩的地球化学特征表明该套火山岩应形成于大陆裂谷环境,同时获得中酸性火山岩中锆石的U-Pb年龄为（320.3±1.1）Ma,表明该套火山岩形成于早石炭世末期。结合进一步的地壳w（Rb）-w（Sr）和Zr/Y-w（Zr）判别图解,表明早石炭世早期开始的博格达初始裂谷在早石炭世晚期进一步快速拉张,发展到成熟阶段。这一发现进一步证实了博格达造山带在石炭纪处于大陆裂谷演化的观点,为进一步理解博格达地区晚古生代构造格局及板块构造体制提供了重要的地质依据。The end of early Carboniferous bimodal volcanic rocks were revealed recently in Saerqiaoke area of the eastern section of Bogda orogenic belt, which primarily comprise of Asian alkali basalt, rhyolite and dacite rocks, mainly belonging to the tholeiitic series. The basalts have high A1 （17. 44%--20. 86%）, Na2O/K2O （1.82--3.04）, TiO2（1.16%--1.34%）, slightly higher than N-MORB＇s, and low Mg （MgO =3.05%- 4. 98%） contents, indicating obvious fractional crystallization of olive and pyroxene during basaltic magmatism, which was the evolution product of original magma＇s fractional crystallization. The basalts slightly enriched in trace elements Rb, Ba and P, almost flat REE distribution patterns, no Eu anormalies （SEu=0.87--1.06）, but relatively depleted in Nb, Ta and Th. These characteristics of volcan- ic rocks indicated that the basalts have geochemical features indicative of intraplate hasalts formed within continent and originated from depleted garnet peridotite mantle, which occurred 10 to 20 percent partial melting at the certain depth about 75 kilometers, and suffered a certain degree of crustal contamination dur- ing the process. The dacites of bimodal volcanic rocks have high SiO2 （70. 29% -- 75. 46% ） and ALK （5.13%--9.90%）, but low TiO2（0.25%--0. 30%）, Al2O3（12.85%--15.88%） and MgO （0. 36%-- 1.01%）. Their trace elements generally have features of enrichment of Rb, Ba and Th, obvious depletion of P and Ti, and slight depletion of Ta and Nb, and REE distribution patterns were characterized by LREE enrichment, right-oblique shape and obvious depletion of Eu （SEu= 0. 36- 0. 86）, suggesting that the rocks originated from the melting of crustal material with plagioclase as an important relic mineral in the source. Geochemical characteristic of the bimodal volcanic rocks in Saerqiaoke area indicated that the vol- canic rocks were formed in a typical tectonic setting of continental rift. The acidic volcanic rocks with LA- ICP-MS zircon U-Pb age of （320.3＋1.1）Ma s

关 键 词：博格达造山带 早石炭世晚期 双峰式火山岩 岩石成因 大陆裂谷 成熟阶段 

分 类 号：P588.14[天文地球—岩石学]