【作者】 向必伟；

【导师】 朱光；

【作者基本信息】 合肥工业大学 ， 矿物学、岩石学、矿床学， 2009， 博士

【摘要】 大别造山带位于中国东部,是印支期扬子板块俯冲于华北板块之下形成的陆—陆碰撞造山带。该造山带北部区域又经过造山后穹隆作用,最终形成了大别造山带现今的构造格局。北大别穹隆构造主要由混合岩化片麻岩和大量早白垩世岩体组成,南、北边界分别为五河-水吼剪切带与晓天-磨子潭剪切带。野外观察显示,北大别穹隆北部边界晓天-磨子潭剪切带总体走向NWW-SEE,倾向NNE或NE,糜棱面理倾角多为20～40°,矿物拉伸线理呈现一致的NWW-SEE向。野外S-C组构、旋转残斑,显微构造(S-C组构、残斑拖尾、“云母鱼”构造)以及石英C轴组构均指示了该剪切带一致的上盘向NWW的剪切指向。穹隆南部边界五河-水吼剪切带东段走向NWW-SEE,倾向SEE-SE,糜棱面理倾角多为60°左右,矿物拉伸线理陡倾,侧伏角约为60°S甚至倾向。野外和显微构造观察以及糜棱岩中石英C轴组构均指示五河—水吼剪切带与晓天-磨子潭剪切带具有一致的上盘向NW-NWW的剪切指向。北大别穹隆南、北边界剪切带内糜棱面理和矿物拉伸线理与其相邻的灰色片麻岩面理和拉伸线理产状协调,糜棱岩与片麻岩的变形连续。南—北方向上横切北大别穹隆的构造剖面上,穹隆北部边界临近区域片麻岩面理倾向NE-NNE,倾角20～40°左右;腹部片麻理倾角逐渐变缓多在10°左右,且倾向不稳定;继续向南片麻理逐渐转变为高角度南倾,至南部边界区域倾角为50～60°。片麻岩内S-C组构、旋转残斑以及石英C轴组构测量均指示了上盘向NWW的运动特征。北大别穹隆内片麻岩与其边界剪切带糜棱岩连续、协调的变形组构产状及岩石变形特征,均指示片麻岩与糜棱岩一起经历了同一流动变形过程。依据片麻理产状在穹隆内的分布特征,推测北大别穹隆内片麻岩与边界剪切带糜棱岩形成于穹隆作用早期的近水平状态,遭受造山后隆升作用改造后呈现为中部上拱的“背形”形态。北大别穹隆内南—北方向上混合岩化片麻岩多分布于穹隆构造腹部,东—西方向上西部较东部分布广泛,指示北大别穹隆的隆升幅度中部较南、北两侧大,西部隆升幅度较东部大。对晓天-磨子潭剪切带和五河-水吼剪切带内糜棱岩矿物变形的显微观察表明,糜棱岩中石英普遍动态重结晶,重结晶型式以颗粒边界迁移(GBM)为主,甚至出现高温条件下颗粒边界缩减动态重结晶型式。糜棱岩中长石也发生了广泛的动态重结晶,主要以鼓胀式重结晶型式(BLG)与颗粒边界旋转重结晶型式(SR)共存。利用矿物动态重结晶型式与变形温度直接的关系,这两条剪切带糜棱岩变形温度均在600～650°之间。按早白垩世大别造山带地温梯度(>30°)计算,两剪切带均形成于中地壳深度。尝试用角闪石Al压力计算表明,晓天—磨子潭剪切带变形平均压力为5.21 Kba,也指示了18.3 km的中地壳变形深度。野外对晓天-磨子潭剪切带与其相邻岩体及火山岩接触关系的观察表明,剪切带的形成至少不晚于早白垩世岩浆活动时间。位于磨子潭镇南部的早白垩世火山岩直接覆盖于剪切带糜棱岩之上,说明原本处于地壳深部的韧性剪切带被抬升至地表之后,火山岩喷发至地表,表明剪切带应形成于该期岩浆活动之前。本次工作通过对受控于晓天-磨子潭剪切带发育的未变形岩体的U-Pb锆石年代学的测试也证实,剪切带至少在130 Ma时已停止活动。从晓天—磨子潭剪切带糜棱岩中选取了角闪石和黑云母矿物进行了40Ar/39Ar年代学研究,获得的年龄结果介于120-142 Ma之间,皆被解释为冷却年龄。剪切带糜棱岩所记录的年龄在空间上表现出明显的东早西晚的规律,揭示了晓天-磨子潭剪切带西段的平均抬升速度小于东段。剪切带中段的XM31-2糜棱岩中角闪石给出了142 Ma的坪年龄,记录剪切带活动的开始时间应早于142 Ma,同时也揭示了北大别穹隆活动早期中部首先冷却抬升。北大别穹隆内早白垩世变形(片麻状)岩体都给出了早于132 Ma的年代学数据,沿晓天—磨子潭剪切带东段分布的未变形岩体给出了130 Ma结晶年龄,指示下地壳流动的截止时间为132 Ma。本次测试样品XM24-3和52-1都分别给出了界于132~127 Ma之间的十分相近的角闪石和黑云母冷却年龄,表明在此期间北大别穹隆经历了十分快速的抬升和冷却。前人从五河-水吼剪切带中获得的40Ar/39Ar年龄数据(~130 Ma)与晓天-磨子潭剪切带冷却时间相近。这些年代学研究指示了晓天-磨子潭与五河-水吼剪切带形成于同一时代,共同经历了早白垩世北大别穹隆作用的改造。北大别的下地壳流动与水平拆离剪切带的活动发生在145-132Ma期间,132 - 127 Ma期间发生了快速抬升与穹隆形成,这时早期的剪切带被动抬升而不再发生活动。本次研究利用原始水平剪切带模型,通过变形模拟进一步定量计算了晓天-磨子潭与五河-水吼剪切带受北大别穹隆改造详细过程,并揭示了穹隆构造的几何学、运动特征。模拟计算获得了原始水平剪切带剪切方向为上盘向NWW(280°)运动,运动学涡度为0.95。通过将理论水平剪切带绕各自不同的空间轴旋转适当角度,理论计算所获得的剪切带面理、拉伸线理产状在赤平投影上很好拟合了晓天-磨子潭剪切带东段的晓天剖面、中断的磨子潭剖面、西段的丁铺剖面和五河-水吼剪切带的水吼剖面上实测剪切带糜棱面理与线理产状分布特征,指示了两边界剪切带早期为同一韧性剪切带,也表明晓天-磨子潭剪切带各段内产状与运动学的现今不协调特征是遭受后期不均一改造的结果。理论计算的面、线组构产状与晓天-磨子潭剪切带实测面、线组构产状的拟合,需要绕东—西向水平轴将理论水平模型南部向上旋转,而拟合五河-水吼剪切带时需要绕东—西向水平轴将理论模型北部向上旋转,指示了北大别穹隆是中部上拱的运动过程,证实了穹状隆升的“背形”构造形态。模拟晓天-磨子潭剪切带西段剖面的产状时,理论模型绕东—西向轴南部向上旋转的角度大于东段,并且模拟剪切带中段剖面的产状分布时,需绕南—北向水平轴将理论模型西部向上旋转,表明北大别穹隆过程中西部抬升幅度大于东部。北大别穹隆的运动学性质体现了典型的下地壳韧性流动的特征。大别造山带印支期陆—陆碰撞以及高压-超高压折返过程使得造山带地壳显著加厚。相对于造山带东部的扬子克拉通,北大别穹窿区域“储存”巨量侧向重力势能。在中国东部岩石圈减薄背景下,随着北大别区域下地壳的逐渐升温、软化,造山带内下地壳物质韧性流动性增强,北大别下地壳岩石在重力的驱动下发生了重力跨塌,产生了下地壳物质向南东东流动,并在韧性流动的上部边界产生了近水平的、上盘向北西西的韧性剪切带。在重力均衡作用下,下地壳物质的流出是北大别穹隆区域发生了“背形”隆升的动力来源之一。更多还原

【Abstract】 The Dabie orogenic belt (DOB), located in eastern China, was developed due to continent-continent collision between the South China block and North China block. After post-orogenic doming, the tectonic framework of the northern DOB formed.The North Dabie Dome (NDD) mainly consisted of migmatitic gneiss and the Early Cretaceous plutons. The NDD is bounded by the Xiaotian-Mozitan shear zone (XMSZ) to the north and the Wuhe-Shuihou shear zone (WSSZ) to the south. Based on field observation, the XMSZ, the northern boundary of NDD, strikes NWW-SEE and dips at 20-40°toward NNE or NE. The lineation in mylonite plunges NWW. The macrostructures (the S-C structure andσ-type feldspar porphyroclast) and microstructures (the S-C structure,σ-type feldspar porphyroclast and“mica fish”structure) and quartz C-axis fabrics all show top-to-NWW shear sense. The east segment of the WSSZ strikes NWW-SEE and dips at about 60°toward SE or ESE. The lineation in mylonite plunges about 60°toward S. The field structures, microstructures and quartz C-axis fabrics in the WSSZ all show top-to-NWW shear sense. The attitude of the foliation and lineation gradually varies from mylonite in the two shear zones to gneiss in the NDD and the interior of the NDD between the two shear zones also exhibit widespread, top-to-WNW ductile deformation dated as the Early Cretaceous. Across the NDD from north to south, Gneiss foliation in the northern part of the dome dips at 20-40°toward NE or NNE whereas that in the southern part dips at 50-60°towards SE or SSE. Foliation in the middle part of the dome shows variable strikes and dip angles while the lineation consistently plunges WNW or ESE. The S-C structure,σ-type feldspar porphyroclast and quartz C-axis fabrics all show top-to-NWW shear sense. All of these demonstrate that the two shear zones experienced the same flow deformation as the interior of the NDD. The gradually varying gneissositic foliation inside of the NDD indicates that the XMSZ and WSSZ were originally one flat-lying detachment shear zone with uniform top-to-WNW shear sense before doming of the NDD. The migmatitic gneiss exposes more often in the central part of the NDD than in the south and north sides and the NDD is more widely in the west than in the east, which indicated the maximum uplift happened in the central NDD and the west part of the NDD uplifted more intensively than the east part.By microscopic observation of oriented thin-sections the quartzes in mylonite all dynamic recrystallized, mostly by grain boundary migration recrystallization and some with grain boundary reduction recrystallization under high temperature condition. Feldspar in the mylonites shows widespread dynamic recrystallisation mainly by both bulging recrystallisation and subgrain rotation recrystallisation. All of these indicate the deformation temperature of 600~650°C. Using a geothermal gradient of >30°C/km normally for post-orogenic extension setting, the XMSZ and WSSZ formed at a middle curst level. Calculating by Al-in-hornblende geobarometer, the average pressure of 5.21 kbar was acquired, indicating depth of 18.3 km.The mylonite in the XMSZ was covered by the Early Cretaceous volcanic rocks north to the NDD, which suggests that the magmas erupted after mylonite in the XMSZ uplifted to surface. The zircons from the undeformed plutons along the XMSZ gave U-Pb age of 130 Ma, which shows that the movement of XMSZ ended at least before 130 Ma. A number of biotite and hornblende from mylonite in the XMSZ were collected for 40Ar/39Ar dating and a series of plateau ages from 120 Ma to 142 Ma were acquired, which were all explained as cooling age. The obtained 40Ar-39Ar ages from the western segment are significantly younger than those of the same minerals from the eastern one, denoting the west segment of the XMSZ experienced slower uplift. The middle segment of the XMSZ gives the oldest 40Ar/39Ar cooling age of 142 Ma (hornblende). On the basis of the cooling age distribution along the XMSZ, it is inferred that along an EW transverse the middle part of the NDD uplifted first during the doming. The deformed plutons in the NDD gave ages of older than 132 Ma while the undeformed ones along XMSZ gave U-Pb age of 130 Ma (this work), which shows that the middle-lower crustal ductile flow ended at about 132 Ma. Hornblende and biotite ages from the same samples of XM24-3 and XM52-1 in the XMSZ only show slight difference, indicating the NDD experienced fast cooling from 132 Ma to 127 Ma. The 40Ar/39Ar ages of the XMSZ are consistent with the 40Ar/39Ar ages of 126 Ma (biotite) and 130 Ma (muscovite) from the WSSZ, demonstrating the XMSZ and WSSZ were originally one shear zone. The lower crustal ductile flow and sub-horizontally detachment in NDD had lasted from about 145 Ma to 132 Ma and the fast uplift had been occurred between 132 Ma and 127 Ma in the NDD.To explore the geometric and kinematic pattern, a flat-lying shear zone theoretic model was used to quantitatively model the process that the XMSZ and WSSZ had been rebuilt by the dome. By modeling, the top-to-NWW (280°) shear sense and the kinematic vorticity number (0.95) for the original flat-lying shear zone were acquired. After rotation around a certain axis, the attitude of foliations and lineations by theoretic calculation can be respectively consistent with the natural attitude varying in the Xiaotian cross-section in east segment, the Mozitan cross-section in middle segment, the Dingpu cross-section in west segment of XMSZ and the Changpu section of WSSZ.The results from modeling not only prove that the XMSZ and WSSZ were originally the same flat-lying shear zone, but also indicates that the geometrical differences of the whole XMSZ resulted from asymmetrical rebuilt during doming. The south of the theoretic model must be rotated up around an east-west axis to correspond with the sections of XMSZ, and the north of theoretic model must be rotated up around the same axis to correspond with the section of WSSZ. These progresses indicated that the interior of the NDD had been uplifted higher than the north and south sides and the NDD exhibits an arch-shape as a whole. The rotation angle in the theoretic model of the Dingpu cross-section is larger than the Xiaotian cross-section and an additional rotation around a south-north horizontal axis was done in the Mozitan cross-section. These information verified that the west part of the NDD had more uplift than the east part.The kinematics of the NDD shows lower crustal flow. During the content-content collision and exhumation of ultra-high pressure metamorphic rocks, the crust of the NDD had been thickened and therefore a huge gravity potential energy was stored in the NDD crust relative to the adjacent South China block. During the continental-scale, extensional regime in East China, the lower crust was heated and softened and hence gravitational collapse would take place in lower crust with increasing plasticity of lower crustal rocks. When the lower crust flowed towards SEE, the flat-lying ductile shear zone with top-to-NWW shear sense came to being. Just for the lower crustal flow, the lithosphere isostatic rebound became one of the most important factors resulting in development of arch-shape of the NDD.更多还原