【作者】 王建刚；

【导师】 胡修棉；

【作者基本信息】 南京大学 ， 矿物学、岩石学、矿床学， 2011， 博士

【摘要】 印度和亚洲大陆碰撞形成的喜马拉雅造山带是目前地球上最典型的陆-陆碰撞造山带,对其研究所获得的构造模型直接影响着我们对其它造山带的认识。然而,目前我们对于喜马拉雅造山作用的早期过程却知之甚少。本论文以藏南日喀则地区的晚白垩世-早中新世沉积地层为研究对象,运用地层学、沉积学、岩石学、碎屑锆石U-Pb年代学和Hf同位素分析、碎屑铬尖晶石地球化学等多种方法对代表性的剖面进行了详细的研究,为印度-亚洲大陆初始碰撞和喜马拉雅造山带的早期隆升提供了新的证据。本论文重点研究地层包括：萨嘎桑单林剖面晚白垩世-始新世地层、柳区和夏鲁剖面始新世-渐新世柳区砾岩和日喀则地区晚渐新世-早中新世秋乌组和恰布林组。这三套地层是藏南喜马拉雅造山作用不同阶段的代表性沉积记录,时代从晚白垩世延续到早中新世,完整记录了喜马拉雅造山作用的早期构造-盆地演化。桑单林剖面位于萨嘎县城南侧约10km的泥沙质混杂岩带之中。剖面中的地层从下至上可分为三个部分：晚白垩世-古近纪蹬岗组主要由石英砂岩和粉砂质页岩组成,顶部出现少量的硅质岩和硅质页岩。早始新世桑单林组位于蹬岗组之上,由岩屑砂岩、硅质岩、硅质页岩和少量石英砂岩组成。者雅组整合于桑单林组之上,以黑色页岩与灰绿色岩屑砂岩互层为特征,下部含少量杂色硅质岩。者雅组中缺少具有年代意义的化石,根据碎屑锆石年龄和地层接触关系认为其时代为早-中始新世。蹬岗组石英砂岩中的碎屑锆石年龄主要分布在奥陶纪-前寒武纪(94%),并在早白垩世出现一个年龄峰(6%),其年龄分布显示典型的特提斯喜马拉雅特征,物源区为印度被动大陆边缘。而早-中始新世桑单林组和者雅组岩屑砂岩中的碎屑锆石年龄大多小于200Ma(85%),主要集中在～54-70、～80-125和～180-196Ma三个年龄区间,与冈底斯岩浆弧的锆石年龄一致。另外,这些砂岩中含有大量的碎屑铬尖晶石,其低Ti低Al的地球化学特征同样指示拉萨地体的物质输入。桑单林组和蹬岗组之间物源区的突然变化,为印度-亚洲大陆碰撞提供了最小年龄约束。桑单林组底部的最年轻碎屑锆石年龄和放射虫化石(RP9,49-50.3Ma)指示印度-亚洲初始碰撞至少早于Ypresian晚期(～50Ma)。柳区砾岩主要沿雅鲁藏布缝合带南侧分布,从拉孜至白朗东西延展超过150km,主要由沉积于冲积扇环境的粗粒碎屑岩组成。柳区砾岩由于不含冈底斯来源的中酸性火成岩砾石,被部分学者认为是印度和大洋岛弧碰撞的产物。为了确定柳区砾岩的物源区,本研究进行了详细的碎屑组分分析和碎屑锆石U-Pb年龄和Hf同位素分析。柳区砾岩的砾石组成包括石英砂岩、岩屑砂岩、板岩、硅质岩、玄武岩和少量的千枚岩、辉长岩和蛇纹岩。硅质岩、基性-超基性岩组合指示雅鲁藏布蛇绿岩套为重要物源区。柳区砾岩的碎屑锆石年龄主要分布在80-150、200-400和-450-1250Ma。年龄为80-150Ma的锆石可以分为两组：一组具有正的εHf(t)值,与冈底斯弧的岩浆锆石相似,为日喀则弧前盆地的再旋回锆石；另一组具有负的εHf(t)值,其物源区为特提斯喜马拉雅白垩纪地层或日喀则弧前盆地(最初来自北冈底斯)。年龄为200-400Ma的锆石的εHf(t)值为-4.3-+9.1,其唯一可能的物源区为朗杰学群。年龄大于450Ma的锆石可以来自特提斯喜马拉雅、朗杰学群以及日喀则弧前盆地。柳区砾岩中出现大量源自亚洲大陆(日喀则弧前盆地)的碎屑锆石,表明其沉积于印度-亚洲大陆碰撞之后。因此,新的数据不支持柳区砾岩为印度-大洋岛弧碰撞的沉积记录。日喀则地区的内磨拉石包括秋乌组和恰布林组,恰布林组可进一步划分为江庆则段、德日段和屯穷段。秋乌组由下部黑色页岩、厚层砂岩和上部杂色泥质岩、中薄层砂岩组成,沉积于湖泊和三角洲环境。江庆则段由互层状的红色泥质岩和灰绿色砂岩组成,沉积于河流环境。德日段主要由巨厚层粗粒砾岩组成,夹少量砂岩和紫红色泥质岩,沉积于以辫状河为主的冲积扇环境。屯穷段由红色泥岩、含砾泥岩和少量砾岩、砂岩组成,沉积于冲积扇扇中-扇端环境。最新的孢粉化石资料和年轻的碎屑锆石年龄指示秋乌组和恰布林组的沉积时间为晚渐新世-早中新世。物源区分析表明秋乌组和江庆则段的沉积物主要源自北侧的冈底斯弧,仅包含少量来自南侧雅鲁藏布缝合带的碎屑物质。而德日段和屯穷段中硅质岩、基性-超基性岩和沉积岩碎屑大量出现,指示南侧雅鲁藏布蛇绿岩套和日喀则弧前盆地碎屑物质的大量输入。晚渐新世时期沿冈底斯南麓大量出现湖相沉积地层,可能反应了这一时期上地壳的拉张作用,而之后南侧物质大量向盆地输入则是大反向断裂发育的结果。晚渐新世-早中新世沿雅鲁藏布缝合带短暂的拉张和之后迅速的挤压可能是印度板块俯冲角度反转(低角度向高角度变化)和最终断离的结果。这一研究结果表明内磨拉石并非印度-亚洲大陆碰撞的直接产物,而是后碰撞时期区域地质作用的结果。基于本论文的研究结果和最新的地质资料,笔者提出了一个新的沉积-构造模式来解释雅鲁藏布缝合带晚白垩世-第三纪时期的沉积演化：印度-亚洲大陆初始碰撞发生在白垩纪末期-早始新世,初始碰撞发生后,由于亚洲大陆地壳加载到印度大陆边缘之上,之前的被动大陆边缘盆地向喜马拉雅欠充填前陆盆地演化。萨嘎地区的桑单林组和者雅组沉积于前陆盆地的前渊位置。大约在始新世中期,喜马拉雅前陆盆地由欠充填阶段进入过充填阶段,特提斯海完全关闭。同一时期,特提斯俯冲板片的断离,导致陆-陆缝合部位快速均衡隆升,可能是盆地性质转化的深部控制因素。雅鲁藏布缝合带的隆起,还导致了柳区砾岩的沉积。研究认为柳区砾岩为前陆盆地楔顶沉积物,记录了喜马拉雅的早期隆升。渐新世-早中新世时期,俯冲印度板块的反转,引起冈底斯南缘短暂的拉张,形成以秋乌组为代表的湖相地层。之后由于俯冲印度板块的断离,导致迅速的地壳缩短和大规模逆冲断裂的发育,形成了以恰布林组为代表的内磨拉石沉积。更多还原

【Abstract】 Himalayan orogen created by the India-Asia collision and the subsequent crustal thickening along the Indus-Yarlung Zangbo suture zone is the most famous collisional mountain belt on the earth. The knowledge about the evolution of the Himalayas strongly influences our interpretation of the tectonics of other orogenic belts. However, our knowledge about the early history of the Himalayan orogenesis is rather limited. In this study, we investigated the Late Cretaceous to Tertiary sedimentary strata in Xigaze area, southern Tibet. A integrated study of stratigraphy, sedimentology, sandstone petrology, detrital zircon U-Pb ages and Hf isotopes, detrital Cr-spinel geochemistry was carried out on the the Late Cretaceous-Eocene strata in the Sangdanlin Section, the Middle Eocene-Oligocene Liuqu Conglomerate in Liuqu and Xialu localities and the Oligocene-Early Miocene Qiuwu and Qiabulin formations in Xigaze area to provide new constraints to the initial India-Asia collision and the early uplift history of the Himalayan orogen. The studied strata are nearly continuous from Late Cretaceous to Early Miocene, therefore comprise an integrated record for the early Himalayan orogenesis.The Sangdanlin section, located at about10km south of Saga city, consists of a subaerially exposed tectonic block of sedimentary strata embedded within a mud matrix tectonic melange zone. The sedimentary strata outcropping at the Sangdanlin section can be subdivided into three lithologic units. The Upper Cretaceous-Paleocene Denggang Formation is dominated by quartzarenites and silty shales, with red siliceous shales and cherts occurring near the top. The Early Eocene Sangdanlin Formation overlying the Denggang Formation is composed of litharenites, siliceous shales, chert, and minor quartz sandstones. The Zheya Formation, which conformably overlies the Sangdanlin Formation, is characterized by the presence of dark gray shales interbeded with light green litharenites, and minor variegated cherts occurring near the base of the formation. The age of the Zheya Formation is poorly constrained due to a lack of fossils, but might be of early-middle Eocene age based on the stratigraphical relationship and detrital zircon isotopic analyses. The Denggang Formation quartzarenites contain zircons with dominant Proterozoic-Ordovician U-Pb ages, with an additional age peak of Early Cretaceous, which we interpret to be derived from the northern Indian margin. By contrast, the lithic sandstones of the Sangdanlin and Zheya formations are dominated by zircons younger than200Ma, showing one major peak at～80-125Ma and two subdominant peaks at～54-70and～180-196Ma, comparable to those from the Gangdese magmatic arc. Cr-spinels in the Sangdanlin and Zheya formations are abundant and characterized by extremely low TiO2wt%, also indicating material input from the Lhasa terrane. The abrupt sedimentary provenance transition from the north Indian margin to the Lhasa terrane between the Denggang and Sangdanlin formations provides a minimum age constraint for the timing of India-Asia continental collision. The youngest zircon age cluster (～54Ma) combined with the occurrence of RP9(49-50.3Ma) radiolarians at the base of the Sangdanlin Formation suggest the collision happened at least prior to the late Ypresian (～50Ma).The Liuqu Conglomerate, which extents over a distance of150km (from Lhaze to Bainang) immediately south of the Yarlung-Zangbo ophiolite, is comprised by coarsen-grained clastic rocks mainly deposited in alluvial fan environments. As no volcanic clasts derived from the Gangdese magmatic arc were observed within the conglomerate, Davis et al.(2002) suggested that the Liuqu Conglomerate is a molasse record of the India and intra-oceanic arc collision. To extend and test their findings, we undertook detailed petrographic studies and analyses of U-Pb and Hf isotopes of detrital zircons from the Liuqu Conglomerate. Clasts in the conglomerate consist of quartz-arenite, litharenite, slate, radiolarian chert, and basalt, along with minor phyllite, gabbro, and serpentinite. Radiolarian chert, and mafic and ultramafic detritus are clearly derived from the Yarlung-Zangbo ophiolite. Detrital zircon ages from the Liuqu Conglomerate are concentrated in three clusters at80-150,200-400, and～450-1250Ma. Zircons of80-150Ma in age can be subdivided into two groups:a group with positive εHf(t) values shows a Gangdese affinity and is considered to have been recycled from sedimentary strata of the Xigaze forearc basin, while another group with negative εHf(t) values was derived either from Cretaceous strata of the Tethyan Himalaya or from the Xigaze foreac basin (originally derived from the north Lhasa terrane) of the Asian plate. Zircons with ages of200-400Ma and εHf(t) values of-4.3to+9.1were derived from Triassic clastic rocks of the Langjiexue Group, as this is the only possible source, to the best of our knowledge. Zircons older than450Ma may have multiple sources, including the Tethyan Himalayan sequences, the Langjiexue Group, and even the Xigaze forearc sediments. The occurrence of Asian-derived detritus in the Liuqu Conglomerate, deposited above the Indian plate and ophiolite, indicates that the conglomerate deposited after India-Asia collision and recorded the early erosion of the Himalayan-Tibetan orogen. Thus, the results are inconsistent with the proposal that the Liuqu Conglomerate records India and intra-oceanic arc collision.The interior molasse belt in the Xigaze area comprises the Qiuwu Formation and the overlying Qiabulin Formation. The Qiabulin Formation could be further subdivided into three units, which are from bottom to top, the Jiangqingze Member, the Deri Member and the Tunqiong Member. The Qiuwu Formation, comprises a lower part of dark shales and thick-bedded sandstones and an upper part of variegated mudstones and thin to medium-bedded sandstones, was deposited in a lake with intercalated delta sandstones. The Jiangqingze Member consists of intercalated greenish gray sandstones and red mudsotones, deposited in fluvial or braided fluvial environments. The Deri Member is composed mainly by coarse-grained conglomerate, with subordinate intercalated sandstone and mudstone, which was interpreted to have deposited on alluvial fans that were dominated by braided environments. The Tunqiong Member comprises red mudstones or pebbly mudstones with a few sandstone and conglomerate beds, deposited in distal alluvial fan environments. Palynofloras and detrital zircon ages suggest the Qiuwu and Qiabulin formations were deposited at late Oligocene-Early Miocene. Provenance analyses indicate that the Qiuwu Formation and the Jiangqinze Member were mostly derived from the Gangdese arc to the north, with only very limited detritus derived from the Yarlung-Zangbo suture zone to the south. By contract, gravels of radiolarian cherts, mafic and ultramafic rocks and sedimentary rocks are abundant in the Deri and Tunqiong members, which indicate significant influx of material from the Yarlung-Zangbo ophiolites and the Xigaze Forearc basin to the south. Occurrence of Late Oligocene-Early Miocene lake sediments parallel the Yarlung-Zangbo suture zone might record a short period of extension, which was suggested to be related to southward rollback of the hinge line in the subducting/underthrusting Indian continental lithosphere. The influx of detritus from the south in the upper part of the Qiabulin Formation was interpreted as a result of activation of the great counter thrust, which might be caused by the Indian continental slab break-off. Our data provide an alternative interpretation for deposition of the Interior Molasse belt and do not support the notion that these deposits record initial shortening owing to the India-Asia collision.Based on our studies and recent published geological data, a tectonic-depositional model was constructed to illustrate the Late Cretaceous-Tertiary depositional history along the Yarlung-Zangbo suture zone. In the new model, the India-Asia collision was considered to have occurred during the latest Cretaceous-Early Eocene, when the Indian passive margin basin changed to the underfilled Himalaya foreland basin in response to flexural subsidence driven by loading of Asian crust. The Sangdanglin and Zheya formations in Saga are interpreted to have deposited in the foredeep depozone of the foreland basin. A transition of the foreland basin from underfilled stage to overfilled stage occurred at about Middle Eocene, accompanied by the final closing of the Tethyan seaway. It is suggested that Eocene Tethyan slab breakoff, which may generate rapid isostatic uplift along the suture zone, was a deep-level dynamic control for these events. The Liuqu Conglomerate, deposited at this time, was interpreted as having deposited in a wedge-top basin and being a sedimentary record of early Himalayan uplift. During Oligocene-early Miocene, regional extension and subsequent activation of the great counter thrust which might be generated by the rollback and subsequent break off of the subducting Indian continental lithosphere occurred at the south margin of the Gangdese arc, accounting for deposition of the Interior molasse belt.更多还原