【作者】 李明；

【导师】 高山；

【作者基本信息】 中国地质大学 ， 地球化学， 2010， 博士

【摘要】 我国东北地区在构造上位于华北克拉通和西伯利亚克拉通之间,属于中亚造山带的最东段,中亚造山带被认为是世界上最为重要的显生宙新生地壳单元,而中国东北地区则可能是显生宙以来全球地壳增生最强烈的地区之一。碎屑沉积物和沉积岩是大范围大陆地壳的最具代表性的样品,是研究大陆地壳形成、演化理想的对象。年轻沉积物或是现代河流沉积物的碎屑锆石记录了源自地壳物质的信息,而这些地壳物质却可能并没有被保存下来,或是已经没有再出露了。嫩江、松花江、黑龙江、乌苏里江,及其各支流海拉尔河、呼玛河、呼兰河等流域基本涵盖了中国东北地区的主要构造单元。本文对采自这些河流的13件河沙样品中的碎屑锆石进行LA-ICPMS U-Pb定年和LA-MC-ICPMS Hf同位素测定,所得1761颗谐和锆石的年龄和其中1381颗锆石的Hf同位素组成用来讨论中国东北地区地壳形成与演化的历史。文章取得的主要认识如下：1)在复杂构造带背景下,河流系统难免要流经多个构造单元,使碎屑锆石来源变得复杂,使研究结果出现偏差。在松花江南源第二松花江和松花江干流样品中均存在有相当数量的-2.5 Ga年龄的锆石,但着并不意味着在中国东北地区存在大量-2.5 Ga的结晶基底。由于第二松花江发源于华北克拉通东北缘,它将大量源于华北克拉通的锆石汇入东北地区内部,使得在其下游各样品中均出现-2.5 Ga,原属于华北克拉通特征年龄的锆石。对全松花江水系各主要河流均采取上、中、下游分段采样的方式,发现随着各采样点所能代表流域面积的增大,-2.5 Ga年龄的锆石在各样品中所占比例明显降低,并显示与流域面积对数值呈反比关系。这一趋势对于东北地区内部不同时间和空间分布的锆石源区也表现明显。该趋势是由外源汇入锆石的供给有限,在向下游迁移过程中锆石不断沉降却得不到进一步的供给,而且增大的流域面积也使汇入的其它年龄组成的锆石逐渐增多,这也促使外来的锆石在下游各样品中所占比例进一步降低。与之相反的是,真实源于流域内主要岩浆事件的锆石,会得到周边物源持续的供给而保持在样品中所占比例在一定范围内波动,这也意味着对于流域内的特征岩浆事件,在流域内分布的各采样点对其均是有代表性的。2)在现今中国东北地区中部(松辽盆地东北缘)、西部(大兴安岭中北部西侧)和东部(佳木斯地块东部)均可能存在一定规模的-1.8 Ga的结晶基底,它们可能源自改造的太古宙(2.5-3.2 Ga)地壳。中-新元古代岩浆锆石在现今中国东北地区内部各地均有零星出现,亏损地幔模式年龄显示该时期是地壳形成的主要阶段,这些地壳可能在该地区古生代和中生代的数次构造运动中被大量改造。但在东北地区复杂造山带背景下,锆石源区可能经历了数次改造与混染,此时的模式年龄可能记录的是新生地壳与改造或再循环的老地壳物质多次混合的结果,仍需结合锆石氧同位素组成对Hf模式年龄所代表的地质意义进一步加以限制。3)古生代期间西伯利亚克拉通和华北克拉通之间经历了复杂的造山运动,表现为不同地块间的碰撞拼合,最终导致两大板块拼合在一起,大量正εHf(t)值显示在各块体碰撞拼合过程中有大量新生地壳物质的加入,它们来源于地幔物质的部分熔融。各碰撞事件中还伴随对老地壳的改造,但熔融的地壳物质与幔源岩浆发生了不同程度的混合。在早古生代发生的兴安与额尔古纳地块碰撞事件峰期为-495 Ma,而在碰撞带附近发生的峰期为450-460 Ma和-440 Ma的岩浆事件则可能是地块碰撞拼贴作用结束后的后造山阶段活动的反映,碰撞事件还波及现今松嫩地块中部,导致该地区-485Ma的岩浆事件的发生。古生代中晚期松嫩与兴安地块间洋壳俯冲所引起的岩浆事件峰期为-355 Ma,两地块碰撞可能发生在350-300 Ma间。地块碰撞过程中有大量新生地壳产生,它们可能来源于受洋壳俯冲影响而部分熔融的地幔物质。碰撞后伸展环境下的岩浆事件集中于300-280 Ma,此次碰撞所造成的岩浆事件主要集中于兴安板块内部。古生代晚期西伯利亚边缘已拼贴的复合块体与华北克拉通碰撞,并可能导致复合块体下岩石圈地幔的部分熔融,产生大量新生地壳物质,碰撞后的岩石圈伸展作用导致复合块体内部不同位置发生的多起岩浆事件,主要峰值集中于-250 Ma、-240 Ma、-235 Ma和-225 Ma。4)中生代中期,中国东北地区已由古亚洲洋构造域转为古太平洋构造域控制,佳木斯地块与松嫩地块的碰撞拼合事件峰期为-190 Ma,受古太平洋板块低角度俯冲作用的影响,中国东北地区岩浆活动逐渐由东部大陆边缘向西部内陆迁移,岩浆事件峰值年龄由东部松嫩与佳木斯地块拼合带区域的-190 Ma到西北部额尔古纳和兴安地块的-160 Ma逐渐年轻。洋壳的低角度俯冲可能导致复合块体下部软流圈地幔上涌,产生大量新生地壳。中生代晚期,古太平洋板块俯冲方向改变,区域构造背景由挤压加厚转换为伸展减薄,挤压加厚的岩石圈地幔重力失稳发生拆沉,并导致软流圈上涌,熔融的软流圈地幔产生大量新生地壳。5)分别基于锆石的U-Pb年龄、Hf同位素二阶段模式年龄TDM2、由εHf(t)值划分的U-Pb年龄和模式年龄组合,以及新、老地壳物质的简单二单元混合模型可以得到不同的地壳生长曲线。由U-Pb年龄绘制的曲线主要反映了流域内岩浆活动期次随时间的累积分布；而模式年龄却由于锆石源区的岩浆混合作用而难以解释；由εHf(t)值区分锆石的新、老地壳来源可进一步反映新生地壳的产生时间,该模式显示中-古元古代(1.2-2.2 Ga)和显生宙(100-500Ma)是中国东北地区新生地壳产生的重要时期；混合模型区分了各点Hf同位素数据中新、老地壳物质所占的贡献,可能会更真实的反映新生地壳的生长,但由于本文中缺少氧同位素数据对新生地壳来源的制约,该曲线尚不完善,但相比由εHf(t)值区分锆石的新、老地壳来源绘制的地壳生长曲线,该模型更突显了中国东北地区太古宙和元古宙期间大陆地壳的幕式生长模式。更多还原

【Abstract】 Northeastern China (NE China) is an eastern part of the Central Asian Orogenic Belt (CAOB). It is generally believed that the growth of continental crust was largely achieved in the Precambrian time and that little new crust has been added during the Phanerozoic. However, significant productions of juvenile crust in the CAOB have been demonstrated. NE China has probably undergone two stages of tectonic evolution related to the closure of the Paleo-Asian Ocean in the Paleozoic, and the subduction of Paleo-Pacific Ocean in the Mesozoic-Cenozoic.Clastic sediments and sedimentary rocks, are representative samples of the continental crust derived from large areas, and are ideal for studies of formation, evolution, and chemical composition of the continental crust. Detrital zircons from younger sedimentary, or in modern river sediments, may record crustal material that has not been preserved or is no longer exposed.River basin of Nen River, Songhua River, Heilongjiang River, Ussuri River and their tributaries, including Hailar River, Huma River and Hulan River cover the most part of tectonic units (Erguna and Xing’an blocks in the northwest, Songnen block in the middle, Jiamusi blocks in the Southeast) of the NE China. In order to characterize the crustal growth and evolution process of the NE China,1761 concordant detrital zircons in thirteen sand samples from the above mentioned rivers were measured for U-Pb age by excimer laser-ablation ICPMS.1381 zircons from them were measured for Hf isotopic compositions by excimer laser-ablation multi collector ICP-MS.The main understanding of this study made the following:1) In complicated tectonic background, the river catchment flows over several tectonic units inevitably and result in the complicated source of detrital zircons and fallacy of research. There are significant-2.5 Ga age signatures in all samples from the main stream of the Songhua River and its south headstream, the Second Songhua River. However, there is no large scale-2.5 Ga crystalline basement in NE China. The Second Songhua River drains from the northern part of North China Craton (NCC), and bring a mass of zircons from there.-2.5 Ga is one of the characteristic age of NCC.In this study, a series of samples from upper, middle and lower reaches of the main tributaries and the main stream of the Songhua River system were measured for U-Pb age. The proportions of-2.5 Ga in detrital zircon age spectrum decrease with increasing drainage areas represented by sampling points along the Songhua River. The proportion of-2.5 Ga zircons shows significant negative correlation with logarithmic values of the drainage area. Such trends are also significant for detrital zircon source with different times and spaces in NE China. These trends are caused by limited supply of exotic zircons. The exotic zircons deposited during downstram migration but without further supply, and that the increasing drainage area also import other age composition of zircons, it reduce the proportion of exotic zircons further in the lower reaches.In contrast, the native zircons, real from the main magmatic events in basin, will be supplied by surrounding source ceaselessly, and keep their proportions drifting within a certain range, which means that various sampling points distributing in the basin are all representative for the characteristic magmatic events within the basin.2) The-1.8 Ga crystalline basement might exist in the middle (northeastern Songliao basin), the west (west of the middle-northern Great Xing’an Range) and the east (eastern Jiamusi massif) of present NE China. It might originate from reworked Archean crust (2.5-3.2 Ga). A few scattered Meso- and Neo-proterozoic magmatic zircons exist in the internal parts of NE China. A significant production of juvenile crust is demonstrated by two-stage depleted mantle model ages in this period. Voluminous of this crust was reworked in successive tectonic movements in Phanerozoic. In the complicated tectonic background of NE China, it should be pointed out that the source of zircons may undergo several rework and contamination. The model ages, at this time, may record the results mixed by juvenile and reworked or recycled crustal material. The geological significance represented by zircon Hf model ages must be combined with oxygen isotope composition for further restrictions.3) In Paleozoic, the region between Siberian Craton and NCC undergone complicated orogenic movement. Masses of juvenile crust suggested by positiveεHf(t) values were yielded during collision and assemblage between microcontinental blocks and the process of amalgamation between the two cratons. The collision might induce partial melting of asthenospheric or lithospheric mantle, and yielded juvenile crust. Some older crust was reworked during collision and assemblage events, and melted crust and mantle were mixed in various degree.The peak time of collision between the Erguna and Xing’an blocks is -495 Ma. Magmatic events occured at 450-460 Ma and -440 Ma by the collision belt reflected the extensional tectonic environment in post-collision process. The collision also induced the -480 Ma magmatic event happened in centre of present Songnen blocks.Subduction of the oceanic plate between the Songnen and Xing’an blocks induced magmatic events with peak time of-355 Ma. The assemblage of the two blocks might occur between 350 Ma to 300 Ma. The asthenospheric mantle upwelled subsequently and yielded juvenile crust. At post-orogenic extensional tectonic environment, magmatic events occured between 300 Ma to 280 Ma, all in Xing’an blocks.The collision between NCC and the assembled block at margin of Siberian Craton occurred in Late Paleozoic. The asthenospheric mantle under the assembled block upwelled and yielded juvenile crust. The post-orogenic tectonic environment induced several magmatic events in different time and space in NE China. The age peak values were-250 Ma,-240 Ma,-235 Ma and-225 Ma.4) In Middle Mesozoic, the NE China had been controlled by Paleo-Pacific Ocean tectonic domain. The peak time of collision between the Jiamusi and Songnen blocks is -190 Ma. Controlled by the subduction of the Paleo-Pacific plate, magmatism migrated westward from the continental margin to intracontinent, and the peak time of magmatic events became younger from-190 Ma at assemblage belt between the Jiamusi and Songnen blocks to-160 Ma at Erguna and Xing’an blocks. The subduction of oceanic plate induced the upwelling of the asthenospheric mantle, and subsequent productions of juvenile crust. In Late Mesozoic, the delamination of thickened crust also induced the upwelling of the asthenospheric mantle, and the mantle melted partially and yielded juvenile crust.5) Based on zircon U-Pb ages, Hf isotope two-stage model ages (TDM2), U-Pb age and model age groups distinguished by theεHf(t) value, as well as juvenile and older crustal material end-number mixing model, four crustal growth curves were drawn. Curve drawn by U-Pb ages reflected the cumulative distribution of magmatic activity with age. The model ages are difficult to explain for complex magma source. The distinguished juvenile and older crustal source by theεHf(t) value can be used for further reflecting the growth of juvenile crust. This model showed the Meso- and Paleo-proterozoic (1.2-2.2 Ga) and the Phanerozoic (100-500 Ma) are two of the most important periods of juvenile crustal additions in Northeastern China. The mixing model calculated the proportion of juvenile and older crustal material in each Hf isotope datum, and may be more realistic to reflect the growth of juvenile crust. For lack of oxygen isotopes to restrict the source of juvenile crust, the crustal growth curve based on mixing model was incomplete in this study. Compared to the curve drawn byεHf(t) value distinguishing, the curve by mixing model highlighted the episodic crustal growth of Northeastern China in Archean and Proterozoic.更多还原