The Late Paleozoic-Early Mesozoic Tectonomagmatic Activities in the Eastern Segment of the Inner Mongolian Uplift in the Yanshan Tectonic Belt and Their Geological Significance
|School||Chinese Academy of Geological Sciences|
|Keywords||Late Paleozoic-Early Mesozoic plutons ductile shear zones deformation and exhumation Yanshan tectonic belt Inner Mongolian Uplift (Inner Mongolian Axis) starting mechanism of the intraplate deformation|
The Yanshan tectonic belt, which is located at the northern margin of the North China block,has often been referred to as the cradle of Chinese geology. It is famous among Chinese andforeign geologists from its strong intraplate deformation in Mesozoic and Cenozoic. From thebeginning of the last century, many researches have been done on the geology and tectonics fromboth Chinese and foreign geologists in this area. Because these researches are mainly focused onthe geology and tectonics in Mesozoic-Cenozoic or early pre-Cambrian, the tectonic history of theYanshan tectonic belt in Late Paleozoic-Early Mesozoic and the uplift history of the InnerMongolian Uplift are still poorly understood.Some Late Paleozoic plutons were intruded into the Archean-Paleoproterozoic basementrocks in the Inner Mongolian Uplift in the northern part of the Yanshan tectonic belt. Though theyare thought to be Late Paleozoic, their ages are poorly constrained. In recent years, many ductileshear zones and a few records of Late Paleozoic deformation were found in the Inner MongolianUplift. These ductile shear zones, with the plutons nearby, formed a few Late Paleozoic-EarlyMesozoic tectonomagmatic belts in the Yanshan tectonic belt. Because some of the plutons and theductile shear zones are unconformably covered by the Jurassic-Cretaceous sedimentary andvolcanic rocks, they reflect the pre-Jurassic deformation and exhumation history of the Yanshantectonic belt, which is very important to understand the early history of the Yanshan tectonic belt.The main topic of this Ph.D thesis is the Late Paleozoic-Early Mesozoic tectonomagmaticbelts in the eastern segment of the Inner Monglolian Uplift. Many methods, such as SHRIMPU-Pb zircon dating, K-Ar thermochronometery, geobarometry, deformation analyses, tectonicchronology, rock geochemistry analyses, etc. are used in this study. The main advancementsachieved from the thesis are as followings:1. Many Late Paleozoic granitic plutons, which were though to be Archean-Paleoproterozoicmetamorphic rocks or plutons previously, were discovered from the so-called "crystallinebasement" （Ar-Pt1） in the northern margin of the North China. The SHRIMP zircon U-Pb ages ofthe Longhua quartz diorite, Daguangding quartz diorite and Boluonuo quartz diorite are 311±2 Ma,327±8 Ma and 303±4 Ma respectively. The SHRIMP zircon U-Pb ages of the Hushha granite bodyand granite vein are 304±3 Ma and 301±8 Ma respectively. Covered unconformably by theJurassic-Cretaceous sedimentary and volcanic rocks in some places, and the oldest stratum abovethe unconformity is Lower Jurassic Nadaling Formation, the Late Paleozoic plutons should havebeen exhumed to the ground before Early Jurassic. From Late Paleozoic to Early Jurassic, thetectonic exhumation of the Plutons and the Inner Mongolian Uplift is very clear. The discovery ofthese plutons also indicate that the widely distributed crystalline rocks in the Inner MongolianUplift were exhumed to the ground during the Late Paleozoic to early Mesozoic. It is also confirmed that Guanglingshan granite intruded near the end of Permian with a SHRIMP zirconU-Pb age of 253±4 Ma.2. On the basis of the SHRIMP U-Pb dating results and other reliable data published in recentyears, at least three stages of magmatism including Early Devonian （400±10 Ma）, late EarlyCarboniferous-Late Carboniferous （315±15 Ma）, late Late Permian-early Early Triassic （250±10Ma） have been found. The character of the Early Devonian magmatism is alkaline; theShuiquangou alkali complex is a good representative of them. Some basic and ultrabasic rockswere also intruded along the regional faults such as Shangyi-Chicheng fault and Damiao fault inthis stage. The character of the late Early Carboniferous-Late Carboniferous magmatism is mainlydiorite; the Longhua quartz diorite, the Daguangding quartz diorite, the Boluonuo quartz dioriteand the Gushan diorite are all good respresentives of them. The character of the late LatePermian-early Early Trissic magmatism is mainly granite; the Guanglingshan granit and theHanjiadian granite are good respresentives of them.3. Field geological surveys and microstructure analyses indicate that most of the LatePaleozoic-Early Mesozoic intrusions are strongly deformed in ductile or ductile-brittle shear zones.The deformation characteristics are a little different from the Plutons intruded at different stages.1) The deformation of the Shuiquangou alkali complex has much to do with theShangyi-Chicheng ductile shear zones just north of the complex. A few nearly E-W trendingductile shear zones are developed in compex. The temperature for the deformation is about 450℃—500℃. The deformation took place near the end of Late Carboniferous （320 Ma±）.2) The deformation of the late Early Carboniferous-Late Carboniferous is controlled both byductile shear zones and intrusion-related structures. Because the plutons intruded under a regionalN-S directed compression setting, the strong expending and side compression during intrusionresulted in the formation of the concentric foliations and radial lineations at the edge of the plutons.In the late period of the emplacement, the plutons were strongly deformed by NE-NEE ductileshear zones, which formed the NE-NEE trending mylonite foliations, stretch lineations and someleft-lateral ductile shear sense indicators. For example, the Longhua quartz diorite andDaguangding quartz diorite are both strong deformed by NE-NEE trending ductile shear zones,but the Boluonuo quartz diorite and Hushiha granite have not been strongly affected, and thedeformation fabrics in these rocks are intrusion-related structures. The temperature for thedeformation of these plutons is about 450℃—500℃. The deformation time of the plutons isprobably late Early Carboniferous-Late Carboniferous （330—300 Ma）.3) The temperature for the deformation of the late Late Permian-early Early Triassic granitesis about 300—450℃, and the rocks were deformed after 250 Ma, perhaps in Early Triassic.4. The deformation characteristics of the major ductile shear zones on the Inner MongolianUplift have been studied from both macrostructure analyses and microstructure analyses. Themain ductile shear zones are nearly E—W trending with a shear sense of thrust from north to south. The shear sense of the NE-NEE trending ductile shear zones which located between the nearly E—W trending ductile shear zones is thrust-sinistral. The formation of these ductile shear zones is aresult of the N-S directed maximum compressive stress from the subduction-collision processes ofthe Paleo-Asian Oceanic plate into the North China block. The results also show that thesuccession of the Mesozoic brittle faults and the Late Paleozoic ductile shear zones are very clear.5. Deformation analyses and tectonic chronology indicate that the deformation history fromLate Paleozoic to Early Mesozoic can preliminarily be divided into four stages, which are EarlyDevonian （400 Ma±）, late Early Carboniferous-Late Carboniferous （320—300 Ma）, EarlyPermian （280—260 Ma） and Late Permian-Early Triassic （after 250 Ma） respectively. The time ofthe tectonic deformation is clearly corresponded to the magmatisms in the area, but shows cleardifference in last two stages. The deformation and magmatisms in Early Devonian and late EarlyCarboniferous-Late Carboniferous probablly reflect two strong subduction episodes in thesubduction-collision processes of the Paleo-Asian Oceanic plate into the North China block. Thedeformation in Early Permian is probably a tectonic response to the final collision of theMongolian block and North China block. The deformations in Triassic probably reflect the startingof the intraplate deformation in the Yanshan tectonic belt. The multi-stage deformation, with thestrong magmatisms, formed a few Late Paleozoic-Early Mesozoic tectonomagmatic belts in theYanshan tectonic belt.6. Geobarometry was used to calucate the crystallized pressures and emplacement depths ofthe granitic intrusions. The results show that the emplacement depths of the Early DevonianShuiquangou akaline complex and the late Early Carboniferous-Late Carboniferous plutons areabout 20 km and 14 km respectively. So the exhumation amplitudes of the Inner Monglian Upliftwill be 20 km from early Late Paleozoic to early Early Jurassic and 14 km from LateCarboniferous to early Early Jurassic respectively. The uplift and exhumation history of the InnerMongolian Uplift is also discussed on the basis of thermochronometery of the LatePaleozoic-Early Mesozoic intrusions.7. Geochemical （major elements, trace elements, REE） and isotopic data （Sr, Nd, Pb, O, et al）are used to study the tectonic setting, sources and petrogenesis of the Late Paleozoic-EarlyMesozoic intrusions. The results are as followings:1)The sources of the Early Devonian Shuiquangou alkaline complex are mantle or mixture ofmantle and lower crust, perhaps with some melted Archean basement rocks involved. Thecomplex is probably formed by the crystallization differentiation of the basalt magmas. Theformation of the complex is probablly related to the subduction of the Paleo-Asian Oceanic plateunder the North China block and the movement of the Shangyi-Chicheng thults or ductile shearzones.2) The late Early Carboniferous-Late Carboniferous intrusions are calc-alkaline, I-type andvolcanic arc granites from geochemical composition. The source of the granites is a mixture of mantle and crust, perhaps with some melted Archean basement rocks involved. The rocks intrudedin a strong compression setting that is controlled by the subduction processes of the Paleo-AsianOceanic plate under the North China block.3) The late Late Permian-early Early Triassic granites are post-orogenic alkaline graniteswhich intruded under a extensional setting after the collision. The widely appears of this kind ofgranites indicate that the finial collisional between the Mongolian block and the North Chinablock has been finished before Late Permian.4) Through the sources of the Late Triassic granites are mantle sources or a mixture of crustaland mantle sources, they show clear characteristics of post-collisional granites and intraplategranite. So this kind of granites intruded after the collisional between the Mongolian block and theNorth China block.Because the ages of the youngest volcanic arc granite and the oldest post-collisional groundis around 300 Ma and 260 Ma respectively, the final collision between the Mongolian block andthe North China block is happened between 300 Ma and 260 Ma. Considering the time ofdeformation, the finial collision is supposed to happen during the period of 280—260 Ma in EarlyPermian. The results also indicate that the northern margin of the North China block is an activecontinental margin in Late Paleozoic, and the Paleo-Asian Oceanic plate is subducted southwardunder the North China block.8. The starting mechanism of the Mesozoic intraplate deformation has also been discussed.The Yanshan tectonic belt is located in an active continent margin in Late Paleozoic before thecollision between the Mongolian block and the North China block. Because of the subduction ofthe Paleo-Asian Oceanic plate under the North China block and collision between the Mongolianblock and the North China block, the deformation and magmatisms are very strong from earlyLate Paleozoic. This deformation belts, with strong magmatisms, formed many pre-Mesozoiccrustal heterogeneities and thermally weakened zones, which all played a very important role onthe starting of the intraplate deformation in Yanshan area in Mesozoic. The Mesozoic intraplatetectonic belt is developed from a late Paleozoic plate boundary tectonic belt in Mesozoic.