摘要
猛洞岩群出露于中越边境滇东南老君山地区,是一套前寒武系变质沉积-岩浆杂岩,其所处大地构造位置特殊,对滇东南地质演化,乃至华南大陆西端以及特提斯构造域形成与演化具有重要的科学意义。在系统野外调研基础上,本文利用光学显微镜、电子显微镜、电子探针等微观研究手段,对猛洞岩群变质-变形进行了详细的研究。研究发现,猛洞岩群地层宏观构造发育同构造分泌结晶脉、顺层褶皱、透镜体等,微观构造发育云母鱼构造、膝折构造、石英矩形条带等现象。显微组构统计表明猛洞岩群经历过北东-南西、南东-北西向、近东西向、北东-南西(近南北向)挤压过程,与上述宏观、微观现象基本一致。同时,猛洞岩群片岩中黑云母-石榴石温压计分别为429℃、0.5GPa,片麻岩中电气石-黑云母温度计平均温度为546℃,为绿片岩相-低角闪岩相。石英斜长角闪片麻岩中榍石和金红石TIMS U-Pb同位素年龄分别为236Ma和170Ma。以上工作表明,滇东南猛洞岩群记录了多期变质-变形作用,其中主期变质-变形年龄为236Ma,控制了变质杂岩系的宏观构造格架,其应力(σ1)为南东-北西向挤压,近南北向、近东西向挤压可能为与上述主应变相匹配的次级应变方向;但北东-南西向挤压可能为主变质-变形之后的另一次变质-变形产物,时间限定为170Ma,代表老君山穹窿燕山早期的伸展-剪切活动。因此,本文认为老君山-Song Chay穹窿先后经历了印支期南北向推覆挤压作用,晚印支期-早燕山期伸展-剪切作用以及中晚燕山期接触变质作用;且老君山穹窿开始隆升时限为236Ma左右,与周边的特提斯闭合、碰撞时间基本一致;至170Ma穹窿发生垮塌后的伸展剪切活动。文山-麻栗坡NE方向广泛发育印支期飞来峰及异地岩,可能指示包括猛洞岩群在内的前印支期地层是一套从越南北部推覆而至的外来岩体。上述认识为华南西端的构造演化探讨提供了证据。
The Mengdong Group-Complex is a series of Precambrian metamorphic sedimentary and magmatic complex cropped out in the Laojunshan area,southeastern Yunnan Province. For its peculiar location, the Mengdong Group-Complex has the important scientific significance to understand the tectonics of SE Yunnan,and even of South China and Tethys. In order to probe the metamorphism and deformation characteristics,by means of microscale observation such as optical microscope, electron microscope and electron probe, etc., the characteristics of metamorphic-deformation have been studied in details. The results show that the macro styles of the Mengdong Group-Complex contain fold bedding, syntectonic secrete crystal veins, asymmetric shear lens, decollement sub-ductile structures, and the micro structures consist of mica fish, kink, quartz rectangular bank. Microscopic structure statistics data indicate that the Mengdong Group-Complex experienced the squeezing and stressing process in the directions of NE-SW, SE- NW, E-W and S-N, of which the result is similar to macro and micro structure. Meanwhile, the temperature of garnet-biotite mineral pairs in schist is 429~C in average and the pressure of the schist is 0.5GPa. The temperature of tourmaline-biotite temperature in gneiss is 546~C in average, corresponding to typical assemblages of the high green schist-low amphibolites facies. The U-Pb dating of titanite and rutile mineral suggests that the major metamorphism and deformation events occurred around ca. 236Ma and 170Ma. The above evidence shows that the Mengdong Group-Complex has experienced many metamorphic-deformation, and the principal stress of metamorphic-deformation （SE-NW） controls macrostructure framework of metamorphic Group-Complex（236Ca. ）, but S-N and E-W may be the directions of the sub-strain matched with the principal stress and NE-SW may be another direction after main metamorphic-deformation and age is confined to 170Ma. On behalf of the Laojunshan dome of early Yanshanian extensional-shear activity. Combined with other geochronological data,the Mengdong Group-Complex had undergone successively the large-scale thrust events in early Indosinian, the extension-shearing events in later Indosinian and early Yanshannian,and the contact metamorphism in later Yanshannian. Based on the study above, it can be outlined that the dynamic background of these metamorphic-deformation events is the Indosinian event, which is controlled the main features of the Mengdong Group-Complex. Laojunshan uplift of the dome began to activate in 236Ma, in consistent with the time of Tethys closure and collision, and stretched to 170Ma,after collapse of dome. NE directions widely developed klippe and offsite rock in Indosinian Wenshan-Malipo imply that pre-Indosinian formation belongs to the Mengdong Group-Complex and should be a set of foreigner rocky Nappe from Northern for the study of tectonic evolution in the west of Vietnam. Therefore, the above results provide evidence South China.
出处
《地质学报》
EI
CAS
CSCD
北大核心
2017年第1期15-42,共28页
Acta Geologica Sinica
基金
国家973项目(编号:G2007CB411408)
国家自然科学基金项目(编号:40972129)
矿床地球化学国家重点实验室自主选题项目(编号:KCZX20090106)联合资助的成果
关键词
变质-变形
组构
温压计
年代学
猛洞岩群
老君山
滇东南
Metamorphic-Deformation
Fabric
Temperature gauge
Chronology
Mengdong Group-Complex
Laoj unshan
Southeastern Yunnan
