滇东南猛洞岩群斜长角闪岩成因及其构造意义

滇东南老君山地区发育猛洞岩群前寒武系斜长角闪岩,呈港湾状分布在片岩、片麻岩中,并为后期变质-变形作用叠加改造。斜长角闪岩SiO_2质量分数为47.0%~50.4%,西蒙尼图解、DF及A-K图解均显示其原岩为正变质岩,因此推测为一套变质基性岩。斜长角闪岩地球化学特征显示,稀土总量w(ΣREE)=(214~267)×10~(-6),高于洋岛玄武岩,其配分模式与洋岛玄武岩相似,为轻稀土富集的右倾曲线;蛛网图显示Ba、Zr、Hf、Y元素亏损,富集Rb、Th、Ta、Nb、Ce、Sm元素,与板内碱性玄武岩特征类似;Zr/TiO_2-Nb/Y、TiO_2-10MnO-10P_2O_5、La/10-Nb/8-Y/15、Th/Zr-Nb/Zr、Th/Hf-Ta/Hf等判别图解显示,原岩为一套亚碱性—碱性玄武岩岩浆系列,其形成可能与大陆裂谷环境有关。角闪石及斜长石内部指示的(亚)显微结构较为发育,主要有自由位错、位错列,偶见位错环或位错偶极,与角闪石-斜长石矿物对获得的平均温度和压力(646℃、0.88GPa)一致,最高变质程度达低角闪岩相;同时,前人获得榍石U-Pb年代学及南温河片麻岩状花岗岩变质增生锆石均为230 Ma左右,代表印支期构造-热事件,此时老君山变质核杂岩雏形开始形成。

Genesis of Amphibolite in Mengdong Group-Complex in Southeastern Yunnan and Its Tectonic Significance

Existingin schist and gneiss in Mengdong Group-Complex,the amphibolite belongs to Precambrian Formation in Laojunshan area of southeastern Yunnan,on which metamorphism and deformation superposed.The content of SiO2 ranges from 47.0％ to 50.4％.In the discrimination diagrams of Simonen,DF and A-K,it falls in the series of orthometamorphite derived from meta-basite rock.Its geochemical characters show that the total REE contents of amphibolite (w (ΣREE)=(214-267) × 10 6) exceed those of oceanic island basalt,while its distribution pattern is similar to that of oceanic island basalt in steep right-dipping with light REE enrichment.The spider diagram shows that it is enriched in Rb,Th,Ta,Nb,Ce and Sm,while depleted of Ba,Zr,Hf and Y,similar to the characteristics of the alkaline basalts within plate.The discrimination diagrams of Zr/TiO2-Nb/Y,TiO2-10MnO-10P2O5,La/10-Nb/8-Y/15,Th/Zr-Nb/Zr,and Th/Hf-Ta/Hf suggest that their protolith should be of alkaline basalt magma series in a continental rift environment.The microstructures of amphibole and plagioclase indicate that the microstructure is relatively free,with free dislocations and dislocation columns,occasionally dislocation rings or dislocation dipoles,which is in accordance with the average pressure and temperature of hornblende-plagioclase minerals of 0.88 GPa and 646 ℃ respectively,confirming that the highest metamorphic grade is of low amphibolite facies.According to the U-Pb dating of sphenes in plagiogneiss and metamorphic overgrowth zircon in Nanwenhe gneissic granite (230 Ma),the deformation conditions were related to the Indosinian tectonothermal events,and during this period,the embryo of Laojunshan complex began to form.