【作者】 段亮；

【导师】 孟庆任；

【作者基本信息】 西北大学 ， 矿物学、岩石学、矿床学， 2010， 硕士

【摘要】 分隔华北板块和华南板块的秦岭造山带内部存在两条缝合带,指示其经历了多期裂解和碰撞。南秦岭位于中古生代商丹缝合带与三叠纪勉略缝合带之间,主要由古生代沉积岩、浅变质沉积岩和晚元古代变质火山岩组成。秦岭南侧的华南板块实际上是一个复合地体,由华夏板块和扬子地体在820 Ma左右沿江南造山带拼接而成。中古生代的裂解事件及随后的洋壳扩张使得南秦岭从华南板块中分离。与此同时,东亚的许多地块也开始从冈瓦纳大陆逐步裂解分离,并形成古特提斯洋。由此可见,中古生代是南秦岭,华南板块和冈瓦纳大陆地质演化过程中一个非常重要的时期。本论文通过对扬子地体西北缘泥盆系和南秦岭志留—泥盆系中碎屑锆石的U-Pb年代学和Hf同位素的分析,约束了其这一时期沉积物的物源,并在此基础上探讨了南秦岭、华南板块和冈瓦纳大陆之间的构造古地理关系。扬子地体下泥盆统石英砂岩中碎屑锆石的年龄谱主要显示三组年龄区间：0.85-1.0Ga(晚元古代早期),680-800 Ma和650-500 Ma(泛非造山时代),除此之外还有少量但广泛分布的大于1.0 Ga(前格林维尔时代)的年龄。通过对华南板块西北缘下泥盆统的碎屑锆石的U-Pb年龄与华南板块西南缘、印度北缘特提斯喜马拉雅和西澳大利亚珀斯盆地同时期地层碎屑锆石年代学的对比,发现它们的碎屑锆石年龄组成具有高度的相似性。结合对东冈瓦纳大陆格林威尔和泛非造山带的分析,认为上扬子地块下泥盆统石英砂岩物源主要是来自East African Orogen, Kuunga Orogen和华南板块内部(包括江南造山带、汉南-攀西弧和扬子基底)。以上结果显示华南板块在冈瓦纳大陆聚合时期应是东冈瓦纳大陆的一个重要组成部分。这一研究结果质疑了目前被大多数国内外学者所持有的一个观点,即在冈瓦纳大陆聚合时期,华南板块是一个远离冈瓦纳大陆的块体。虽然每个年龄组分的锆石Hf同位素均显示新生地壳生长和早期地壳物质的改造,但是物源流域区域范围内的幔源新生地壳生长主要集中于3.1 Ga,1.9 Ga和1.0 Ga,并且在新生地壳生长之后都经历了长时期对早期地壳物质的改造。另外值得注意的是,Hf同位素显示的物源流域区域范围内的幔源新生地壳生长时间和东冈瓦纳幔源新生地壳生长时间高度吻合[Kemp et al.,2006]。这种现象进一步支持我们所获结论的可靠性。南秦岭泥盆—志留系砂岩中大多数碎屑锆石的年龄聚集在以下三个年龄区间：ca.400-500 Ma,ca.750-900 Ma和ca.900-1000 Ma,峰值分别为450 Ma,820 Ma和970Ma。志留系中碎屑锆石的εHf(t)值位于球粒陨石均一线之下,意味着大多数碎屑锆石εHf(t)值指示其原岩产生于古老地壳物质的重熔。泥盆系碎屑锆石的分布特征与志留系碎屑锆石年龄的分布具有很强的相似性,证明在南秦岭泥盆系对志留系具有很强的继承性。泥盆系中除400-500 Ma大多数的碎屑锆石εHf(t)值位于球粒陨石均一线之上,其他年龄组分的锆石的εHf(t)绝大多为负值,与志留系中的一样。400-500 Ma和晚元古代中期(750-900 Ma)的碎屑锆石的形态主要为棱角状,显示出近物源的特点,而年龄为900-1000 Ma以及大于1.0 Ga的碎屑锆石大多数磨圆较好,暗示其物源区较远。已有的秦岭泥盆系沉积研究和北秦岭热年代学资料指示,北秦岭不可能作为南秦岭泥盆-志留系的沉积物源。我们认为南秦岭泥盆-志留系的沉积物提供年龄为加里东期(400-500 Ma)和晚元古代中期(750-900 Ma)碎屑锆石的物源应该为华南早古生代褶皱造山带和江南造山带。南秦岭志留系碎屑锆石Hf同位素特征显示,U-Pb年龄为400-500 Ma的碎屑锆石εHf(t)值指示其原岩为重熔的古老地壳物质,这与华南早古生代褶皱造山带中加里东期的花岗岩多为S型的事实相符。南秦岭下泥盆统西岔河组中的砾岩主要为棱角状黑色硅质岩或硅质泥岩,少数灰质砾石中含有志留系化石。因此,西岔河组的沉积物一部分应来自下伏志留系。其年龄为400-500 Ma且εHf(t)值为正值的碎屑锆石物源应为南秦岭与华南裂解分离过程中发育的幔源岩浆岩组合。我们的研究结果还指示南秦岭在志留纪时期可能与华南早古生代造山带距离较近,也就是说南秦岭很可能是在现今福建和两广地区与华南裂开分离出来的。此后,南秦岭通过商丹缝合带于华北地块相碰撞。与此同时,华南板块与冈瓦纳大陆相分离,并且在漂移过程中经历了顺时针旋转,最终于晚三叠世通过勉略缝合带于南秦岭地体相碰撞,形成秦岭造山带。更多还原

【Abstract】 The Qinling orogen, dividing the North China block and the South China block, is thought to have evolved by the multistage rifting and collision over a prolonged history, with two mountain chains and two sutures. Between the Middle Paleozoic Shangdan and Triassic Mianlue sutures is the South Qinling orogen, which is predominated by Paleozoic metasedimentary and metavolcanic rocks and Triassic granitoids. South to the Mianlue suture is the South China block, which is actually a composite terrane formed by the assembly of the Yangtze and Cathaysian blocks through the Jiangnan orogen around 820 Ma. Middle Paleozoic rifting and then seafloor spreading led to breakaway of the South Qinling belt from the South China block. Synchronously, separation of East Asian crustal fragments from Gondwanaland and openning the Palaeo-Tethys. Accordingly, it can be concluded that the Middle Paleozoic is a very important period in the evolution of South Qinling belt, the South China block, as well as Gondwana. This dissertation presents a study of detrital zircon U-Pb geochronology and Hf isotope geochemistry of quartz arenites of the Lower Devonian in northwestern margin of Yangtze terrane and Middle Paleozoic sandstones in South Qinling, putting some constraints on their provenance and exploring the tectonic and paleogeograhic relationship between the South Qinling, South China block and Gondwanaland.U-Pb detrital zircon geochronology from Lower Devonian quartz arenites of the northwestern margin of the Yangtze block yields dominant early Neoproterozoic (0.85-1.0 Ga), Pan-African (0.5-0.65 Ga) and middle Neoproterozoic (0.68-0.8 Ga) age populations and minor Mesoproterozoic to middle Mesoarchean (1.0-3.0 Ga) ages. Middle Mesoarchean to Mesoproterozoic rocks, however, are widespread in the South China block. Although Hf isotopic compositions show both juvenile crustal growth and crustal reworking for all the age groupings, the crust growth, essentially mantle-derived, occurred mainly around 3.1 Ga,1.9 Ga and 1.0 Ga, respectively. Zircon typology and youngest grain ages indicate that this suite of quartz arenites was the product of multiphase reworking. Abundant magmatic zircon detritus with concordant U-Pb Grenvillian and Pan-African ages, together with accompanying variousεHf(t) values, indicate an exotic provenance for the quartz arenite external to the South China block. Qualitative comparisons of age spectra for the late Neoproterozoic sediments of the Cathaysian Block, early Paleozoic sediments of pre-rift Tethyan Himalaya sequence in North India and lower Paleozoic sandstone from the Perth Basin in West Australia, show that they all have two the largest age clusters representing Grenvillian and Pan-African orogenic episodes. The resemblance of these age spectra and zircon typology suggest that the most likely source for the Lower Devonian quartz arenites of the South China block was the East African Orogen and Kuunga Orogen for their early Grenvillian and Pan-African populations, whereas the Hannan-Panxi arc, Jiangnan orogen, and the Yangtze block basements might have contributed to the detrital zircon grains of the Neoproterozoic and Pre-Grenvillian ages. Hf isotopic data indicate that the crustal evolution of the drainage area matches well with the episodic crust generation of Gondwana [Kemp et al.,2006]. These results imply that the previously suggested position of the SCB in Gondwana should be re-valuated, and the South China block should be linked with North India and West Australia as a part of East Gondwana during the assembly of Gondwana, rather than a discrete continent block in the paleo-Pacific.Collectively, the dominant detrital zircon age populations within Siluro-Devonian sandstones in South Qinling are Caledonian age (ca.400-500 Ma), ca.750-900 Ma and ca. 900-1000 Ma, with peak value of 450 Ma,820 Ma and 970 Ma, respectively. MostεHf(t)of detrital zircons from Silurian lie below the line of chondritic uniform reservoir (CHUR), indicating they are the products of reworking preexistent crust. The Hf isotopic character of Devonian resemble Silurian’s besides the presence of positiveεHf(t) value in the majority of Devonian detrial zircons with Caledonian age. Most zircons with Caledonian and 750-900 Ma age in Siluro-Devonian are angular or sub-angular, showing the provenance was near. After precluding the possibility of North Qinling behaved as the provenance,the Paleozoic orogenic belt and Jiangnan orogen are inferred as the provenance of providing the detrital zircons with Caledonian and 750-900 Ma age. The presence of Silurian fossils in Devonian strata and positiveεHf(t) value of Caledonian zircons reveal the Devonian’s source is the combination of underlying Silurian and igneous rocks formed in the process of rifting between South Qinling and SCB. Of importance, these facts solve the question Where and When South Qinling rifted from SCB, indicating the South Qinling are close to Cathaysian block before splitting.更多还原