【作者】 刘丙祥；

【导师】 陈福坤；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2014， 博士

【摘要】 北秦岭地体东段是秦岭造山带的重要组成部分,以复杂多样的岩石组成、多期次的构造岩浆作用、变质地体的相互叠置为特点,地体内不仅大量产出秦岭造山带中最古老的前寒武纪基底—秦岭群,而且广泛出露晋宁期和加里东-海西期的花岗闪长质侵入岩类,因而它既是探讨中国南北两大陆块(华南和华北)碰撞、拼合关系的关键位置,又是研究秦岭造山带地壳的形成和演化、花岗岩类的成因物源及其构造属性的最佳理想场所。为了较好地约束秦岭群的源区特征和构造背景,揭示北秦岭地体的岩浆作用和地壳演化规律,本文运用多种测试方法和分析手段,主要包括主、微量元素分析、全岩Rb-Sr和Sm-Nd同位素分析以及锆石原位U-Pb定年和O-Hf同位素测试等,重点研究了秦岭群基底片麻岩和角闪岩两种主要岩石类型、晋宁期和加里东-海西期花岗侵入岩类,尤其详细剖析了后者的岩石组合特征、时空分布规律,并对不同成因类型侵入岩的源区及岩石成因进行了系统讨论,最后建立了晋宁期和加里东-海西期岩浆作用的年代学格架,通过与其邻区华北南缘、南秦岭及扬子北缘地体岩浆作用和地壳演化的对比,系统总结了北秦岭地体地壳的生长和再造历史。研究结果不仅为岩石的成因提供可靠的数据支持和地球化学制约,而且有助于认识秦岭造山带基底的演化和构造的归属,对揭示岩浆形成的深部地球动力学背景和探索大陆碰撞造山过程中地壳的生长、分异和演化具有重要的科学意义。秦岭群主要由片麻岩、角闪岩和少量大理岩组成,记录了北秦岭地体形成和早期的陆壳演化过程,经历了多期变形和变质作用。其中,长英质片麻岩富集轻稀土元素和大离子亲石元素(Rb、Ba、Th、K和Pb),Eu负异常和亏损高场强元素(Nb, Ta, P, Ti),锆石δ18O值变化略大且位于6.5-9.3‰之间,全岩初始εNd值-4.6,两阶段Nd模式年龄TDM2=1.99Ga;斜长角闪岩也富集大离子亲石元素和轻稀土元素,且亏损Nb和Ta,锆石δ18O值均一为5.0-6.0‰,但具有较高的全岩初始ENd值(2.8～3.3)和较年轻的TDM2(1.29～1.24Ga)。锆石U-Pb年龄表明长英质片麻岩和斜长角闪岩均形成于～960Ma与火山弧相关的构造背景之中,而非前人所认为的裂谷环境,同时发现一期早古生代的变质热事件。秦岭群碎屑锆石年龄谱显示其可能为一个独立的地质单元,并在新元古代早期具有不同于南北两大陆块的演化历史,很可能为格林威尔造山带的一个残留体。晋宁早期(-940)的花岗岩侵入体在南秦岭地体中极为少见,在北秦岭地体中却出露相当广泛且几乎都经历了明显的变质变形作用,这些岩体记录了秦岭造山带前寒武纪基底地壳的形成和演化历史。本文选取方庄和德河两个岩体为代表。结果表明,方庄花岗质糜棱岩的锆石结晶年龄为933.4±9.2Ma,δ18O值8.3～11.9‰,初始87Sr/86Sr比值0.72455,初始εNd值-6.0,Nd模式年龄2.09Ga (TDM2)；德河黑云斜长片麻岩的锆石结晶年龄为939.7+7.6Ma和948.1+8.9Ma,δ180值8.6～10.0‰,初始87Sr/86Sr比值变化较大为0.70986～0.72455,初始εNd值-5.3--4.8,Nd模式年龄2.05～2.00Ga(TDM2)。结合已报道的15个新元古代花岗岩类岩体的年龄和地球化学数据,北秦岭地体新元古代岩浆作用可以划分为980～870Ma挤压碰撞作用和～844Ma伸展裂解作用两大阶段,包括～940Ma强烈变形S型同碰撞花岗岩、-880Ma弱-无变形后碰撞Ⅰ型花岗岩和～844Ma板内A型花岗岩三类花岗岩体。地球化学组成显示,这些花岗岩类岩石可能源自不同时期形成的秦岭群基底杂岩的部分熔融,但在后碰撞阶段幔源物质或年轻地壳物质的加入明显增加。北秦岭地体中新元古代岩浆活动与Rodinia超大陆演化基本同时代,可能记录超大陆形成过程中的地壳响应。在新元古代之前,北秦岭地体或许具有不同于华北陆块和华南陆块的演化历史。加里东一海西期岩浆活动在北秦岭地体东段广泛发育,在其周缘地体中几乎未见出露。本文对研究区内共10个中酸性侵入体19件样品和中基性富水杂岩体中的4件样品进行锆石U-Pb定年分析。结合近期公开报道的53件锆石U-Pb年龄数据,建立和查明北秦岭地体东段加里东-海西期岩浆活动的年代学格架和时空分布特征,将该期岩浆活动划分为3个阶段：第一阶段(-487Ma),主要分布于研究区东部；第二阶段(-450Ma),遍布整个研究区；第三阶段(-417Ma),分布于研究区中部。第一阶段(-487Ma)：Ⅰ型花岗岩以板山坪、西庄河及麻池河乡等岩体为代表,具有类似于俯冲洋壳成因埃达克质岩石的地球化学特征,具有较高的初始εNd值(0.19～3.17)、较低的初始87Sr/86Sr比值(0.70257～0.70524)、较高的初始εHf值(2.51～13.97)、TDM2(Nd)=0.92～1.21Ga和TDM2(Hf)=0.56～1.28Ga及相对低的全岩δ18O值(7.03-7.60‰),这些特征指示该阶段Ⅰ型花岗岩可能源自俯冲洋壳板片的部分熔融。结合空间上与之共生的岛弧成因的富水杂岩的产出,两种岩浆作用类型共同证实了研究区内早古生代板块俯冲-消减事件的存在。S型花岗岩以漂池和蛮子营岩体为代表,具有高硅、钾,低钛、镁的特征,属高钾钙碱性过铝质系列,其初始εNd值=-7.98～-10.05、初始87Sr/86Sr比值=0.70352～0.72737、初始εHf值=-13.75～-0.78、TDM2(Nd)=1.89～2.06Ga、TDM2(Hf)=1.46～2.33Ga、锆石δ180值较高位于7.88～8.65‰之间,略低于全岩618O值10.9-11.7‰,这些特征共同指示该S型花岗岩具有板块俯冲过程中诱发活动陆缘陆壳物质熔融成因特征。第二阶段(～450Ma):～460Ma I型花岗岩以灰池子岩体最为典型,具有加厚下地壳成因埃达克岩的地球化学特征,具有中等的初始εNd值(-0.51～1.54,多数大于0),较低的初始07Sr/86Sr比值(0.70463～0.70612),变化较大的初始EHf值(-10.89～12.80),TDM2(Nd)=1.08～1.25Ga和TDM2(Hf)=0.61～1.70Ga,全岩δ180值相对较高为7.60～9.08‰,这些特征结合继承锆石的大量出现和在岩体内部未见中基性暗色包体的地质事实共同指示该阶段Ⅰ型花岗岩可能是新生的加厚下地壳部分熔融的产物,其源岩物质与第一阶段Ⅰ型花岗岩基本类似,其形成背景与古生代期间商丹洋壳的俯冲-汇聚-碰撞,进而导致新生下地壳的缩短-叠置-加厚有关；-450Ma I-S型花岗岩,以五垛山、宽坪、枣园、黄柏岔及桃坪岩体为代表,岩体岩石兼具碰撞型和俯冲型岩石的成因特点,总体显示其成因及物源与-460MaⅠ型花岗岩相似,不同之处较前者富集Nd同位素和具有较高的全岩δ18O值(9.96-10.34‰),可能暗示了幔源物质的贡献相对早期较少；-440Ma Ⅰ型花岗岩以沿商丹缝合带分布的武关及小王涧岩体为代表,它们可能是造山过程中滞后的消减源区发生部分熔融的产物。S型花岗岩以安吉坪和土桥岗岩体为代表,具有类似于第一阶段中S型花岗岩的高钾钙碱性过铝质系列岩石的特征,其初始ENd值=-7.64～-8.20、高初始87Sr/86Sr比值=0.71025～0.71263、TDM2(Nd)=1.8l～1.88Ga、锆石δ18O值较高位于9.60～13.30%o之间,总体呈现挤压碰撞成因的岩石地球化学特点。第三阶段(-417Ma)：Ⅰ型花岗岩以侵入于二郎坪群的张家庄岩体为代表。该岩体岩石具有低钾(Na2O/K2O=3.30～28.83)拉斑玄武岩系列过铝质的特点,其高初始εNd值=3.19～5.07、低初始87Sr/86Sr比值=0.70331～0.70471以及TDM2(Nd)=0.74～0.89Ga,这些地球化学和同位素特征可能是新元古代期间形成的新生底侵基性岩浆或早期就位于造山带根部的早期基性岩浆混染了较少量的古老壳源物质熔融所致；物源方面与形成于新元古代的北秦岭第一类基性火山岩有较好的亲缘关系,暗示新元古代晚期存在又一期新生地壳的生长事件。S型花岗岩以丹凤岩体为代表,总体显示高硅和钾、富碱、低镁等特点,稀土和微量元素显示陆壳重熔S型花岗岩的特征。总体上,三个阶段中的Ⅰ型花岗岩,其物源基本与南秦岭火山岩或北秦岭第一类基性火山岩有很好的亲缘性,与华北物质基本无关；而S型花岗岩的物源则主要来自秦岭群片麻岩、宽坪群变沉积岩以及北秦岭第二类基性火山岩,华北基底物质对其也无贡献。北秦岭地体东段地壳生长以幕式增生为主,主要形成于古-中元古代及新元古代。在～2300Ma和～2000Ma主要表现为古老地壳物质的再造,在～1600Ma、1000～630Ma及～500Ma以地壳的增生为主,同时兼具古老地壳物质的再循环。通过与邻区地壳演化过程对比发现,华北南缘发生显著地壳增生时代为中太古代(～2700Ma);北秦岭地体在中元古代以后才开始发生大规模的地壳生长；南秦岭地壳增生事件可能主要发生在新元古代；扬子北缘主要在太古代和中元古代发生大规模的地壳生长或再造。在这四个构造地体中,仅有北秦岭地体缺少太古宙时期地壳的生长或改造事件。因此,元古代之前(>2000Ma),北秦岭地体可能为一个独立的地质体,具有不同于华北南缘、南秦岭以及扬子北缘的独特地壳演化历史。更多还原

【Abstract】 The North Qinling terrain, being an important part of Qinling orogenic belt, is characterized by diverse rocks, multiphase tectonic-magmatic-metamorphic events. This terrain is composed of not only basement rocks of the Qinling Group, but also numerous magmatic rocks in different ages of the Jinningian, Caledonian, and Hercynian periods. The Qinling Group is previously interpreted as the oldest Precambrian basement unit of the North Qinling Terrain. Therefore, North Qinling is a key location to reveal the relationship of collision and assemblage between the North and South China blocks, and also an ideal place to research formation and evolution of crustal terrains, genesis of granitoids and provenance of tectonic units.In order to better understand provenance and tectonic setting of the Qinling Group and to reveal magmatism and crustal evolution in the North Qinling terrain, a detailed study of major and trace elements and Rb-Sr and Sm-Nd isotopic composition of whole-rock samples, as well as U-Pb ages and Hf-O isotopic data of zircons from the same analytical spot by the SIMS and LA-MC-ICP MS techniques, was measured for different rocks collected from two major rock units of the Qinling Group (felsic gneiss and amphibolite) and granitoids (Jingningian, Caledonian and Hercynian periods) in eastern North Qinling. Rock associations and spatial-temporal distribution of the Jingningian and Caledonian-Hercynian magmatic rocks throughout North Qinling are investigated for petrogenesis, magma source, geochronological framework of different magmatic events. Further, by comparison with magmatism and crustal evolution of the adjacent areas, crustal growth and reworking in the North Qinling terrain are systematically summarized. Research results in this study not only place geochemical constraints on genesis of these rocks, but also provide insight into evolution of the basement and tectonic provenance of the Qinling orogenic belt, and further help to better understand geodynamic background during the formation and evolution of continental crust in orogenic processes.The Qinling Group consists predominantly of gneisses and amphibolites and minor marbles, and underwent multi-phase deformation and metamorphism. This geological unit records hence the formation and early crustal evolution of the North Qinling terrain. Felsic gneisses exhibit enrichment of LREEs and LILEs (Rb, Ba, Th, K, Pb), negative Eu-anomalies and depletion of HFSEs (Nb, Ta, P, Ti). The rocks have slightly elevated δ18O values (6.5～9.3‰) and initial εNd values of-4.6corresponding to two-stage Nd model age of1.99Ga. Amphibolites are also enriched in LILE and LREE and depleted in Nb and Ta, and have homogeneous δ18O values (5.0-6.0%o), but higher initiald values (2.8～3.3) and younger two-stage Nd model ages (1.29～1.24Ga) compared to the gneisses. These rocks were emplaced in a～960Ma volcanic arc environment rather than in a rift setting as previously suggested. A major metamorphic event took place in early Paleozoic. Based on the age spectrum of detrital zircons, the Qinling Group is interpreted as an independent geological unit, which was mainly derived mostly from1000～900Ma old granitoid rocks. The North Qinling terrain can be regarded as a remnant of the Grenville orogenic belt with an early Neoproterozoic evolution different from the North and South China blocks.Granitoids of Jingningian ages exposed both in the South and North Qinling terrains and record Precambrian crustal formation and evolution of the basements in the Qinling orogenic belt. However, Early Jingningian granitic intrusive are rare in South Qinling but numerous in North Qinling. In this study, the Fangzhuang and Dehe granitoid plutons emplaced into the Qinling Group in the North Qinling terrain are chosen as research object. Analytical results show that zircons of the Fangzhuang granite pluton formed in933.4±9.2Ma, zircons have δ18O values of8.3～11.9‰and whole-rock yields initial εNdd value of-6.0, Nd model age of2.09Ga (TDM2) and initial87Sr/86Sr ratio of0.7246, while zircons of the Dehe granite pluton crystallized in939.7±7.6Ma and948.1±8.9Ma and whole-rock has initial Nd values of-5.3～-4.8, corresponding to Nd model ages of2.05-2.00Ga (TDM2) and variable initial87Sr/86Sr ratios of0.7099～0.7246. Integrating with isotopic ages of fifteen Neoproterozoic granitoid plutons exposed within the North Qinling terrain previously reported, the Neoproterozoic magmatism in this terrain can be subdivided into two major stages resulted from980～870Ma compressional and～844Ma extensional tectonic settings, including granitoids of～940Ma syn-collisional,～880Ma post-collisional and～844Ma extensional origin. Geochemical and isotopic features of these granitoids indicate that they probably originated from partial melting mainly of crustal material similar to the Qinling Group, but contribution of mantle or juvenile material to the magmas became significant after～900Ma in the post-collisional processes. Neoproterozoic magmatic activities in the North Qinling terrain are broadly simultaneous with the formation of supercontinent Rodinia and hence, they record response of the basement in this terrain. The crustal evolution in North Qinling terrain prior to the Neoproterozoic was likely different from those in the North and South China blocks. Caledonian-Hercynian magmatic activities were extensively developed in the North Qinling terrain, but it is scarcely present in the adjacent terrains. In this study, nineteen samples from ten intermediate-acidic intrusions and four samples from the intermediate-basic Fushui complex in the North Qinling terrain were measured for zircon U-Pb ages using SIMS and LA-ICP MS techniques. Integrating with fifty-three zircon isotopic ages previously reported in North Qinling, a geochronologic framework can be established for spatial and temporal distribution of the Caledonian-Hercynian magmatism in this terrain. It can be subdivided into three major phases of granitoids:-487Ma mainly distributed in eastern part of the study area,～450Ma occurring throughout the study area, and～417Ma scattered only in middle part of the study area.Stage one (-487Ma):I-type granitoids in geochemical composition, represented by the Banshanping, Xizhuanghe and Machihexiang plutons, are similar to adakites related to subducted oceanic-crust. They have initial87Sr/86Sr ratios of0.7026～0.7052, high initial εNd values of0.19-3.17and young two-stage Nd model ages of0.92-1.21Ga, and δ18O values between7.03and7.60%o. Coexisting with the island-arc related Fushui complex in the terrain, two types of magmatism together confirm an Early Paleozoic subduction event, which points to this I-type granitoids may be derived from partial melting of subducted oceanic crust. S-type granitoids, particularly the Piaochi and Manziying plutons, have high SiO2and K2O contents and low TiO2and MgO contents and define a high-K calc-alkaline magma trend. They have initial87Sr/86Sr ratios of0.7035～0.7274, lower initial εNd values of-7.98～-10.05and two-stage Nd model ages of1.89～2.06Ga, and zircon δ18O values of7.88～8.65‰, lower than whole-rock δ18O values of10.9-11.7%o, likely indicating S-type granites resulted from melting of crustal material induced by the subduction.Stage two (-450Ma):I-type granitoids (-460Ma) represented by the Huichizi pluton is analogous to thickened lower crust derived adakitic rocks, which have initial87Sr/86Sr ratios of0.70463～0.70612, initial εNd values of-0.51～1.54and two-stage Nd model ages of1.08～1.25Ga and two-stage Hf model ages of0.61～1.70Ga, and whole-rock δ18O values between7.06and9.08%o. This type of plutons does not contain intermediate-basic magmatic enclaves but many inherited zircons, implying that granites might be products of partial melting of thickened juvenile lower crust, whose source is the same as I-type granites from Stage one and related to Paleozoic Sangdan oceanic crust subduction-converge-collision, resulting in juvenile lower crust shortened, superimposed and thickened. I-S-type granites (-450Ma), represented by the Wuduoshan, Kuanping, Zaoyuan, Huangbaicha and Taoping plutons, have some characteristics of both collision-related and subduction-related granitoids. They overall display similar feature and more enriched in Nd and O isotopic composition compared to I-type granite (-460Ma), indicating contribution of mantle-derived material is relatively less than the older granites. I-type granitoids (-440Ma), represented by the Wuguan and Xiaowangjian plutons distributed along the Shangdan suture zone, might be the product of partial melting of pre-existing crustal component in subduction zone. S-type granites represented by the Anjiping and Tuqiaogang plutons display a high-K calc-alkaline magma trend as the stage one, which have an initial87Sr/86Sr ratios of0.71025～0.71263, lower initiald values of-7.64～-8.20and two-stage Nd model age of1.81～1.88Ga, and zircon δ18O values of9.60～13.30‰, indicating feature of compression and collision origin.Stage three (-417Ma):I-type granitoids, represented by the Zhangjiazhuang pluton, display a low-K (Na2O/K2O=3.30～28.83), peraluminous tholeiite magma trend, and have lower initial87Sr/86Sr ratios of0.70331～0.70471, high initial εNd values of3.19-5.07, corresponding to two-stage model Nd age of0.74-0.89Ga, implying that another juvenile crustal growth may be occurred in late Neoproterozoic. Geochemical and isotopic features indicate that they probably originated from the partial melting of the underplating juvenile basic magma or magma that had already been emplaced into orogenic root during the early stage, mixing with small amounts of ancient crustal material. S-type granites, represented by the Danfeng pluton, have high SiO2and K2O contents and low MgO content. Their geochemical characteristics point to products of re-melting of continental crust.Overall, I-type granitoids of three stages originated from the sources closely related to basic volcanic rocks in the South and North Qinling terrains but irrelevant to the basement of North China. However, S-type granites were mainly derived from crustal material similar with metamorphic rocks of the Qinling and Kuanping Groups, without contribution of the basement of North China block.Crustal formation in the eastern North Qinling is characterized by episodic growth that mostly took place in Proterozoic through reworking of ancient crustal material at～2300Ma and2000Ma, crustal growth and recycling of ancient crust at-1600Ma,1000～630Ma and～500Ma, respectively. By comparison with the adjacent terrains, it can be observed that the North Qinling terrain can be interpreted as a separate geological entity with a special evolution different from the South and North China blocks before Proterozoic. Crustal growth in the southern margin of the North China block took place in Archean (～2700Ma), the South Qinling terrain mainly in Neoproterozoic and the northern margin of the Yangtze block in Archean and Mesoproterozoic, while the North Qinling terrain began massive growth after Mesoproterozoic.更多还原