新疆博格达白杨沟的枕状熔岩:岩石地球化学和Sr-Nd-Pb同位素特征

摘要

新疆博格达造山带北段东侧白杨沟地区分布有大面积的晚石炭世至早二叠世的火山岩,岩石主要由亚碱性玄武岩和安山岩组成,其中分布有大量保存极好的枕状熔岩.枕状熔岩主要为基性熔岩,SiO2含量50.17％～54.66％,Al2O3为13.10％～16.44％,MgO为4.62％～.7.89％,TiO2为1.23％～2.79％,CaO为7.06％～13.07％,Na2O为2.76％～4.99％,K2O为0.07％～0.82％.基性熔岩的大离子亲石元素(LILE)一组为富集如Rb和Ba,另一组相对亏损；高场强元素(HFSE)如Nb和Ta在N-MORB标准化图解上都具有明显的负异常.熔岩的稀土元素在球粒陨石标准化配分图解中表现为总体平坦,轻稀土元素(LREE)轻微富集的右倾曲线,6 Eu为0.89～1.12.从Sr-Nd-Pb同位素组成来看,εNd(t)为+2.8～+3.1,143Nd/144Nd①为0.512407～0.512422,8 Sr(t)为-14.0～-17.5、87r/86Sr①为0.70293～0.70317、206Pb/204Pb①为17.622～17.835、207pb/204pb①为15.374～15.463、28pb/204pb①为38.163～38.339,具有亏损地幔端元的特征.本区火山岩在构造环境图解上具有MORB与WPB的特点,综合区域地质资料和前人研究成果,推断研究区晚石炭世一早二叠世火山岩可能来源于亏损地幔源区,是在古亚洲洋关闭过程中主动大陆边缘的弧后盆地环境形成.%The late Carboniferous-Early Permian volcanic rocks in Baiyanggou of northeast Bogda orogenic belt mainly consist of sub -alkaline basalt and andesite, in which large quantities of pillow laves are excellently preserved. Pillow lavas are mainly basic lavas, whose SiO2 content is 46.65% to 52.50%, Al2O3=13.10%~16.44%, MgO=4.62%~7.89% ,TiO2=1.23%~2.79% ,CaO=7.06% ~13.07% ,Na2O=2.76%~4.99%,and K2O=0.07%~0.89%. A group of basic lavas is enriched with large ion lithophile elements (LILE) such as Rb and Ba, and the other group is relatively depleted in LILE. In the N-MORB diagram, high field strength elements (HFSE) have significant negative anomalies of such elements as Nb and Ta. In the chondrite-normalized REE patterns, lavas show right-oblique curve with overall performance of flatness and slight enrichment of light rare earth elements (LREE); 6 Eu=0.89~1.12. The composition of Sr-Nd-Pb isotopes shows that lavas have characteristics of the depleted mantle: εnd (t) =+2.8~+3.1, 143Nd/144Nd (I) =0.512407-0.512422, εsr (t)=-14.0-17.5, 87Sr/86sr= 0.70293-0.70317, 206Pb/204Pb1=17.622~17.835, 207Pb/204Pb(1)=15.374~15.463, and 208Pb/204Pb(I)=38.163~38.339. In the tectonic settin diagram, Baiyanggou volcanic rocks have the characteristics of MORB and WPB. The comprehensive regional geological data and the results of previous studies have led the authors to conclude that Late Carboniferous-Early Permian volcanic rocks of the study area might have been derived from the depleted mantle during the closure of the back-arc basin in the active continental margin of Paleo-Asian Ocean.