老挝沙耶武里地区晚石炭世火山岩地球化学及同位素年代学研究

THE RESEARCH OF GEOCHEMISTRY AND ISOTOPE CHRONOLOGY ON THE LATE CARBONIFEROUS VOLCANIC ROCK IN SAINYABULI OF LAOS

沙耶武里地区(老挝北部)大地构造上位于古特提斯造山带的东段,其内部地质构造十分复杂,在次级构造单元划分上尚存在较大的争议。研究区内岩浆岩主要以火山岩为主,局部出露超镁铁质和中酸性侵入岩。通过对晚石炭世火山岩地质特征、岩石学、岩石地球化学、同位素年代学进行综合研究,其中,岩石地球化学中主量元素显示w(SiO_2)在46.36%~63.30%之间,显示为中基性岩,K2O/Na2O介于0.10%~1.71%之间,总体上Na2O>K2O,并且在TAS图解中大部分样品落入玄武岩区、粗面玄武岩区;微量元素显示大离子亲石元素富集,高场强元素亏损,反映有壳源物质混染,由Nb强烈亏损显示出岩石形成是俯冲作用的结果;稀土元素LaN/YbN在4.67~13.54之间,LREE/HREE在3.14~7.48之间,说明岩浆分异程度高低不一,而δEu在0.93~1.17之间,反映出岩浆形成过程中有少量斜长石晶出;同位素U-Pb年龄值为297.9 Ma±5.0 Ma,显示其形成于晚石炭世,最后,结合构造环境判别图分析,得出火山岩在板块俯冲作用下喷发的形成过程。

The Sainyabuli area in the north of Laos is located on the eastern portion of the Guttthys orogenic belt with a very complex geological structure and there is a heated argument on the division of the secondary structural unit. The large proportion of the magmatic rocks is vol- canic rock in this research area with small scale intrusions and intermediate acid intrusive rock. The study of geological characteristics, petrology, petrogeochemistry, and isotope chronology of the late carboniferous volcanic rocks shows that the volcanic rocks belong to intermediate-basic category with w （SiO2） ranging from 46. 36% to 63. 30%, and K2O/Na2O from 0.10% to 1.71%. Most of samples fall into the range of the basalt and trachybasalt in the TAS diagram. The value of trace elements shows that it is rich contamination effect in volcanic rock. The strong volcanic rock formation. The value of the REE HREE is 3.14 to 7.48 respectively,characteristic in LILE and poor in HFS, as a result of crust- loss of Nb is duo to the plate subduction during shows that LaN/YbN is 4.67 to 13.54, LREE/ of difference of differential degree. The value of 3Eu from 0.93 to 1.17 demonstrates that there is a little plagioclase appears in the process of the magma formation. The U-Pb isotope dating yields an age of 297. 9 Ma±5.0 Ma,indicating that the volcanic rock formed in late Carboniferous epoch. The formation process of volcanic rocks in the area is established based on the tectonic discrimination analysis.