【作者】 童英；

【导师】 王涛； 洪大卫；

【作者基本信息】 中国地质科学院 ， 矿物学、岩石学、矿床学， 2006， 博士

【摘要】 阿尔泰造山带位于世界上最典型的增生造山带—中亚造山带的南缘,是中亚造山带的重要组成部分。它北侧和东北侧是西萨彦岭—蒙古湖区早古生代古岛弧带,西侧为斋桑晚古生代碰撞造山带,南侧以额尔齐斯断裂带与准噶尔即哈萨克斯坦板块相接。同整个中亚造山带一样,它也发育有大量的花岗岩,花岗岩约占全区分布面积的40%。新的锆石U-Pb年龄测定表明,阿尔泰造山带很多原被来认为是海西期的花岗岩,实际上形成于早古生代,而晚古生代花岗岩的时空展布、岩石组合和成因目前尚不清楚,因而有必要加强对晚古生代花岗岩的研究。本文在前人研究基础上,通过对区内晚古生代花岗岩的野外特征、岩石学、锆石U-Pb年代学、地球化学以及Sr、Nd、Pb同位素示踪研究,得出如下几点认识:1、布尔根碱性花岗岩含有碱性岩暗色矿物钠铁闪石和霓石。高硅(71.46%~77.07%)、富钾(4.41%~5.73%)、富钠(3.91%~5.65%),贫铁、锰、镁(FeOT = 0.3%~1.6%,MnO = 0.03% ~0.13%,MgO = 0.05%~0.44%),具有高的铁镁比值(FeOT/MgO= 14.3~60.4),低磷、钛(P2O5=0.06%。轻稀土富集,重稀土亏损,具有明显的负铕异常,微量元素表现出明显的Ba、Sr、P、Ti的负异常,不具有Nb异常。具有低的Sr初始值(Isr<0.705)和高的εNd(t)值(+1.6~+6.3)。表明布尔根碱性花岗岩可能是由地幔岩浆底侵,导致早期俯冲的洋壳或弧物质重熔,同时加入一部分地幔物质,在经历了强烈的结晶分异作用之后形成的。2、本次研究对布尔根碱性花岗岩及其伴生的黑云母二长花岗岩采集了3个样品,进行SHRIMP和La-ICPMS锆石U-Pb测年,分别获得358±4Ma(碱性花岗岩)、353±3Ma(碱性花岗岩)和343±3Ma(黑云母二长花岗岩)。表明布尔根碱性花岗岩形成时代应为早石炭世。布尔根碱性花岗岩与区内早古生代同造山钙碱性花岗岩构成一个从钙碱性-碱性的岩浆旋迴。这意味着在晚古生代早期,阿尔泰造山带构造体制的发生了变化,即由同造山期的区域挤压体制转入到拉张体制,预示着主造山运动的结束。结合区内其它地质事实,一致表明,布尔根碱性花岗岩形成于后造山环境,此时造山作用已经结束,额尔齐斯洋应已闭合。　3、本文通过对12个岩体(脉)(11个花岗岩体和1个基性岩脉)的15个样品的锆石U-Pb(TIMS、SHRIMP、La-ICPMS)测定,表明阿尔泰晚古生代花岗岩除早石炭世的布尔根碱性花岗岩外,基本形成于二叠纪,可以分为两期(300Ma~280Ma和280Ma~250Ma)。主要分布在造山带南缘,少量分布在造山带中部。4、阿尔泰造山带的二叠纪花岗岩主要岩石类型基本可以分为三种:I型花岗岩、铝质A型花岗岩-正长岩以及I-A过渡型花岗岩。I型花岗岩、铝质A型花岗岩-正长岩主要形成于320Ma~280Ma。岩体内部发育基性岩包体,同时还伴生有大量基性/超基性岩,构成“双峰式”岩浆组合;I-A过渡型花岗岩主要形成280Ma~250Ma。这套钙碱性—碱性的岩浆组合,是在伸展背景下,由统一的地幔底侵,导致不同区块,不同部位的物质发生熔融,并发生不同程度的岩浆分异作用,形成不同类型的花岗岩;同时,一部分底侵的地幔岩浆通过分异作用和岩浆混合作用直接参与了花岗岩浆的形成。5、阿尔泰二叠纪花岗岩分布于造山带的南缘和中部,大部分岩体为近圆形或椭圆形,未发生变形,呈被动侵位于围岩之中,往往切割区域构造线,总体显示出典型的后构造特点。说明在这些花岗岩形成之后,阿尔泰造山带在区域尺度上,未发生强烈的区域变形。因此,阿尔泰二叠纪的构造环境是拉张环境而非挤造山环境。尽管额尔齐斯断裂带内的一部分岩体呈线性并发生变形,但变形为走滑性质,未见强烈的推覆挤压。因此,它们限定二叠阿尔泰未发生区域尺度上强烈挤压变形,而仅在额尔齐斯断裂带内发生走滑变形,且该期走滑事件主要发生290Ma~270Ma。6、22组Sr、Nd同位素和36组Pb同位素研究显示,阿尔泰造山带晚古生代花岗岩具有高(正)e Nd(t)值(+1.3~ +7.3)、年轻的Nd同位素模式年龄TDM2(546Ma ~995Ma)和低206Pb/204Pb、207Pb/204Pb和208Pb/204Pb特点,说明年轻幔源物质参与了花岗岩的形成,表明阿尔泰造山带在后造山阶段存在一定规模的地壳生长;同时,一部分花岗岩的eNd(t)值大于该时期洋壳的值,暗示阿尔泰造山带不仅存在地壳的水平生长,也存在一定规模的垂向生长。估算的最大生长量约为33,000Km3,约占阿尔泰地区上地壳体积的2.1%;最小生长量为25,000Km3,约占阿尔泰地区上地壳体积的1.5%。通过以上的研究,笔者提出阿尔泰造山带晚古生代构造演化模式如下:1)早石炭世,俯冲接近尾声,阿尔泰南缘的D-C1地层发生褶皱变形,主造山期已近结束。约350Ma布尔根碱性花岗岩侵入到已褶皱的D-C1地层,标志着该造山运动的结束。区域转入拉张背景,进入后造山阶段,此时,额尔齐斯洋可能已闭合(?),局部可能存较小的陆表海盆;2)中-晚石炭世,区域可能一直处于相对平静期,目前未见区域变质变形,该时期的花岗岩目前报道也很少;3)晚石炭世末—早二叠世初,发生强烈的岩浆作用,大型走滑等构造运动。区域进入全面的拉张。该时期,中亚大部分地区,包括东、西准噶尔都发生这期伸展事件。更多还原

【Abstract】 The Altai orogen in China is a typical region of the Central Asian Orogenic Belt （CAOB）, the world’s largest Phanerozoic accretion orogenic belt and the most important site of crustal growth in the Phanerozoic. It is situated between the Sayan and Gorny Altai early Paleozoic arc belt in southern Siberia to the north and the Junggar block to the south. The Altai orogen in China contains very large volumes of granitoids and related rocks as CAOB. The granitoids occupy more than 40% of the present area of the orogen. New zircon U-Pb ages show that these granitoids were mostly emplaced during the early Paleozoic, and few in the late Paleozoic and Mesozoic.Based on previous studies, we made detailed field investigating, and study on zircon U-Pb chronology, mineralogy, petrology, REE and trace elements geochemistry, Nd, Sr and Pb isotopic geochemistry of the late Paleozoic granitoids and basic rocks from the Altai orogen. The following achievements and conclusions have been made.1、The Bulgen alkaline granite contains arfvedsonite and aegirine. The rocks are enriched in K ,Na, Al, REE and LILE （Rb , Th , Ba） with negative anomaly in Ba, Sr, Ti ,P and poor in Mg, Fe, Mn with high FeOT/MgO values. They have low Isr and high （positive） eNd（t） value（+ 1.3 + 6.4）.All these characteristics suggest that the Bulgen alkaline granite could be formed by the mantle magma underplating, which induced the early subducted oceanic crust or arc remelting, and offered some mantle materials with strong crystal fractionation.2、Zircon U-Pb （SRHIMP） dating yielded a ²⁰⁶Pb/²³⁸U age of 358±4 Ma （MSWD=1.7） of alkaline granite （4001）. Zircon U-Pb （La-ICPMS） dating yielded a ²⁰⁶Pb/²³⁸U age of 354±3 Ma（MSWD = 0.96）of alkaline granite （40042） and a ²⁰⁶Pb/²³⁸U age of 343±3 Ma （MSWD = 1.6） of biotite monzonitic granite （4053）. They are same, so, the Bulgen alkaline granite formed in the early Carboniferous, which just follow the early Paleozoic calcalkali granitoids in time, and have some relationship in space with them. They constitute a calcalkali-alkali magmatic sequence, which show that magma characters changed from calcalkali to alkali during the late Devonian-early Carboniferous. It means that the tectonic setting changed in this period. Accounting for other geological events, all of them indicate that the Bulgen alkaline granite pluton formed in a post-orogenic setting, in which orogenic movement had been finished, and the Erqis Ocean had been closed at that time. After that, the Altai region changed to an extensional setting.3、Fifteen zircon U-Pb （TIMS,SHRIMP and La-ICPMS） ages show that, except for the early Carboniferous Bulgen alkaline granite, the late Paleozoic granitoids from the Altai orogen were mainly emplaced in the Permian. The Permian granitoids can be approximately divided into two stages （300 Ma 280 Ma and 280 Ma 250 Ma）, they are mostly distributed in the southern part of the Altai orogen and few intruded into the central part of the orogen4、The Permian granitoids are composed of three groups: I-type granite, aluminous A-type granite-syenite and I-A intermediate type. I-type granite and aluminous A-type granite-syenite mainly formed during 300 Ma 280 Ma and distribute in the Erqis fault belt, some plutons have been deformed. At the same time, large volume mafic plutons （dykes） were emplaced. These mafic intrusions were composed of a“Bimodal”magma group with the granitoids. I-A intermediate type granitoids were mainly formed during 280 Ma 250 Ma, they were not only emplaced in the southern part of the orogen, but also intruded in the central part of the orogen, and undeformed. This Permian calcalkali-alkali serial magma formed under an uniform mantle underplating background, the mantle magma offer the heat and the material in different positions of different terrane , partly melted of mantle magma directly took part in the form of granitoids by magmatic differentiation and magmatic mixing.5、Most plutons of Permian granitoids in the Altai orogen are approximately circular, without deformation, showing passive emplacement. They usually cut the tectonic lineament, showing a typical post-tectonic feature. It shows that there is no strong regional deformation in this area at that time. So, the Permian granitoids in the Altai orogen were emplaced in a post-orogenic setting. Although there are some linear deformed granitoids in the southern part of the Altai orogen, they are only localized in the Erqis fault belt, and the deformation character is strike-slip. So, the dating of the deformed and undeformed pluton in the fault belt defines the Erqis fault belt deformation time （290 Ma 270 Ma）.6、The late Paleozoic granitoids in the Altai orogen have high （positive） e Nd（t） values （e Nd（t） = + 1.3 + 7.3）,and low TDM（2546 995 Ma）. It indicates that mantle-derived material took part in the form of granitoid in the Phanerozoic. The Pb isotopic feature provides further support evidence for this. In addition, the e Nd（t） values of some granitoids are higher then the coeval ocean crust. It indicates that the Altai orogen occurrenced certain scale crustal growth and have both horizontal growth and vertical growth. The maximum amount of growth is 33,000 Km3,about 2.1% volume of the Altai area upper crust, and the minimum amount of growth is 25,000 Km3,about 1.5% volume of the Altai area upper crust.Based on the above study, some conclusions about the tectonic evolution modal of the Altai orogen in the Paleozoic can be drawed as follow:1) During the early Carboniferous, the Erqis Ocean wane to the close, the D-C1 stratum in the Altai orogen had been folded, the main Paleozoic orogenic epoch wane to the end. At 350 Ma, the Bulgen alkaline granite pluton intruded in the folded D-C1 stratum. It indicates the end of the orogenic event. The setting in this region changed to an extensional background, and turned into post-orogenic stage. The Erqis Ocean would have been closed,there would be some inland seas in some place.2) During the middle-late Carboniferous, this region might be turned into a relative quiet stage. There have not been found regional metamorphism and deformation and few granitoids emplaced at this stage have been discovered now.3) During the late Carboniferous-early early Permian, there are strong magmatic activities and large scale strike-slip tectonic movement, etc. The area entered in a full pull-apart setting. Actually, Most parts of the CAOB have strong magmatism during this period更多还原