澜沧江南带火山岩系侏罗纪火山岩的存在及其地质意义

The Existence of the Jurassic Volcanic Rocks in the Southern Part of the Lancangjiang Belt and Its Geological Significance

澜沧江南带火山岩系是古特提斯构造演化的产物,是反演古特提斯演化的利器。本文选择带内南段普洱地区那澜村剖面火山岩进行了系统的年代学和地球化学研究,为古特提斯洋演化过程提供了新的认识。那澜村火山岩为一套发育斑状结构、流纹构造的流纹岩,其斑晶由半自形板柱状钾长石和石英组成,基质为隐晶质长石、石英。主量元素中SiO2含量介于75.59%~77.40%之间,Al2O3含量介于11.69%~12.20%之间,K2O含量(4.13%)大于Na2O(3.72%),属过铝质高钾钙碱性-钙碱性系列火山岩。主量元素CaO/Na2O值为0.01~0.04,Al2O3/(MgO+TFeO)为0.03~0.06,CaO/(MgO+TFeO)为5.51~6.66,微量元素Nb/Ta比值集中在12.27~13.08,放射性同位素组87Sr/86Sr比值为0.714~0.719,207Pb/204Pb为15.733~15.760,206Pb/204Pb为18.903~19.400,表明火山岩源区为重熔的含(变质)泥岩地壳。微量元素标准化配分曲线呈不规则锯齿状且具高度一致性,Rb、Nb、Ta、Y、Yb等数据具富Rb低Nb、Ta、Y、Yb特征,表明澜沧江南带火山岩具有陆缘弧火山岩属性。LA-ICP-MS测定锆石U-Pb年代学表明那澜村火山岩谐和年龄为(200.6±2.8)Ma,和澜沧江南带北段火山岩活动(210–280 Ma)具有一致性,说明澜沧江南带火山活动至少延伸至早侏罗世,也即古特提斯洋的俯冲活动至少持续到早侏罗世。

The volcanic rock in the south of the Lancang River is a product of the evolution of the Paleo-Tethys and can serve as a tool for inverting the evolution of the Paleo-Tethys.In this paper,the authors selected the systematic chronology and geochemistry of the volcanic rocks in the Nanancun section of the Pu'er area as the study object,which can provide a new understanding of the evolution of the Paleo-Tethys Ocean.The volcanic rock in the area is a rhyolite of plaque-like structure and rhyolite structure.Its phenocrysts are composed of semi-automorphic slab-like K-feldspar and quartz.The matrix is cryptocrystalline feldspar and quartz.The main element analysis shows that the content of SiO2 is 75.59%~77.40%,that of Al2O3 is 11.69%~12.20%,and that of K2O(4.13%)is higher than that of Na2O(3.72%),indicating that the rhyolite is peraluminous high potassium calc-alkaline,calc-alkaline volcanic rock.The CaO/Na2O ratio is 0.01~0.04,Al2O3/(MgO+TFeO)is 0.03~0.06,and CaO/(MgO+TFeO)is 5.51~6.66,the ratio of Nb/Ta is concentrated at 12.27~13.08,with an average of 12.79,and the data of radioactive isotope 87Sr/86Sr are 0.714~0.719,207Pb/204Pb are 15.733~15.760,206Pb/204Pb are 18.903~19.400,indicating that the volcanic source was derived from the remolten(metamorphic)mudstone crust.The standardized trace element distribution curve is irregularly jagged and highly consistent,the data of Rb,Nb,Ta,Y and Yb are rich Rb and low Nb,Ta,Y and Yb,indicating that the volcanic rocks in the southern part of the Lancang River have the property of volcanic rocks in the continent marginal arc.The LA-ICP-MS zircon U-Pb chronology shows that the concordant age of volcanic rocks is(200.6±2.8)Ma.It is consistent with the volcanic rock activity(210–280 Ma)in the northern section of the southern Lancang River belt,indicating that the volcanic activity in the southern Lancang River belt lasted at least to the early Jurassic,that is,the subduction activity of the Paleo-Tethys Ocean lasted at least to the early Jurassic.