【作者】 李淼；

【导师】 刘晓春；

【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】 东南极普里兹带是近几年在南极大陆上识别出的一条重要的泛非期构造带,因其成因涉及到冈瓦纳超大陆在寒武纪的形成与演化问题,因此一直是国际地学界关注的焦点。目前对普里兹带的构造属性还存在很大争议,一些人认为是陆内造山,而大部分人认为是板块缝合带。本论文通过对东南极普里兹带(包括普里兹湾和格罗夫山)花岗岩类地球化学特征、年代学及岩石成因方面的研究,确定其岩浆源区、形成的构造环境,并进一步探讨东南极地盾普里兹带的构造属性及其在冈瓦纳古陆形成和演化中所起的作用。通过笔者对普里兹带花岗岩的研究,取得以下几点认识:(1)通过对花岗岩的锆石U-Pb定年,确定普里兹带花岗岩类的侵位年龄在550~490 Ma之间(泛非期)。格罗夫山紫苏花岗岩的侵位最早(547Ma),其次是紫苏花岗岩脉(533Ma),然后是花岗岩(526~503Ma)和花岗岩脉(501Ma),在时间上表现出连续演化的特点。普里兹湾拉斯曼丘陵的进步花岗岩和达尔克花岗岩晚于格罗夫山紫苏花岗岩(约530~500 Ma),穆如科尔山花岗岩和兰丁花岗岩侵位最晚(约500Ma)。普里兹带花岗岩类的侵位年龄近似等于或晚于泛非期区域变质作用的年龄60Ma,是泛非期强烈构造热事件的产物。东南极普里兹带花岗岩类岩石类型多样,包括紫苏花岗岩、二长闪长岩、二长岩、石英二长岩和花岗岩,构成一套准铝质-弱过铝质、橄榄玄粗岩岩石系列。这些花岗岩类侵位于不同的高级变质岩中,并发育后期面状的伟晶岩脉,局部地区发育晚期花岗岩岩墙。(2)利用电子探针分析确定出普里兹带花岗岩类暗色矿物的种属:角闪石属于钙质角闪石亚族中的铁绿钙闪石;黑云母以铁质黑云母为主,少数样品中黑云母为镁质黑云母和铁叶云母,表现出相对封闭、贫水环境下的壳幔混合型花岗岩的矿物特征。利用矿物的共生组合确定普里兹带花岗岩的岩浆侵位的温压条件:P=5~6.6kbar, T =774~832℃。岩体侵位时氧逸度较低,在?12.16~ ?13.65之间,略高于FMQ(铁橄榄石+磁铁矿+石英)这一平衡反应的缓冲曲线。(3)在地球化学特征上,普里兹带花岗岩类具有较高的全碱含量、Fe*值,K2O/Na2O和Ga/Al比值,以及较低的Mg、Cr和Ni丰度,表现出A型花岗岩的特点。拉斯曼丘陵的进步花岗岩并非是S型花岗岩,而是一种特殊的铝质A型花岗岩,由于经过强烈的分异演化和同化混染了泥质岩围岩而富含石榴石等富铝矿物。在微量元素标准化蛛网图中,富集大离子亲石元素和稀土元素,不同程度亏损Sr、Nb-Ta、P和Ti,显示出与俯冲作用有关的岩浆特点。在多个构造判别图解上,大部分普里兹带花岗岩类落在同-后碰撞的花岗岩区域,还有些落在板内花岗岩的区域。(4)普里兹带花岗岩类的87Sr/86Sr初始比值高,变化范围较大(0.7075~0.7246);εNd(t)值低,变化范围在?13.37~?9.17之间。Nd同位素模式年龄在2.0~2.3Ga之间,可能与早元古代古老地壳物质的再循环有关。同位素分析结果结合地球化学特征表明,普里兹带花岗岩类岩浆源区来自富集地幔,可能受到壳源物质不同程度的混染。拉斯曼丘陵地区花岗岩的87Sr/86Sr初始比值变化范围较大,可能反映原岩Sr同位素组成的不均一,而Nd同位素模式年龄与该区片麻岩的Nd同位素模式年龄趋于一致,表明花岗岩在成因上可能与该区片麻岩相联系。(5)普里兹带花岗岩类应形成于同-后碰撞构造环境下向板内环境过渡的转折期,这时原来被加厚的地壳发生伸展、减薄,先前含金云母和钾质角闪石的富集岩石圈地幔部分熔融底侵于下地壳,这些物质在下地壳又发生减压熔融形成一系列的A型同-后碰撞花岗岩。后碰撞花岗岩的形成预示着造山活动趋于尾声和新一轮的威尔逊旋回的开始。(6)普里兹带A型同-后碰撞花岗岩的确定支持普里兹带为碰撞造山带的构造属性,进一步说明东南极本身是由不同的前泛非期微陆块拼接形成的。在泛非期之前东南极乃至东冈瓦纳地盾并不是统一的陆块。更多还原

【Abstract】 The Prydz Belt is one of the most important Pan-African mobile belts within the East Antarctic Shield. The belt provoked great interest to many geologists in recent years because it was related to the formation and evolution of the Gondwana suppercontinent during the Cambrian. However, at present the tectonic attribute of the Prydz Belt remain highly debated. Some argued for an intraplate orogen, whereas others argued for a suture. In this paper, Geochronology, geochemistry and petrogenesis of granitoids from the Prydz Belt have been investigated and their tectonic significance and behavior during the assembly of Gondwana suppercontinent have been discussed. The main conclusions from the thesis are as follows.1. SHRIMP U-Pb zircon analyses reveal that granitoids from the Prydz Belt in East Antarctica were intruded from 550 Ma to 490 Ma, immediately to about 60 Ma after the metamorphic peak. In the Grove Mountains, charnockite, charnockite dykes, granite and granite dykes were dated at 547 Ma, 533 Ma, 526~503 Ma and 501 Ma, respectively. In the Prydz Bay area, the Progress granite and the Dalkoy granite were dated at 530~500Ma, whereas the emplacement ages of the Landing granite and the Munro Kerr Mountains granite are as young as 500Ma. These diverse granitoids from charnockite, monzonite, quartz-monzonite to granite constitute a metaluminous to weakly peraluminous granitic serious. They also show the same chemical characteristics as the shoshonitic suits.2. The results of representative minerals by electron microprobe analyses suggest that orthopyroxene is ferrohypersthene in charnockites, and eulite in charnockites dykes, whereas amphibole is ferropargasite, and biotite is annite (expect for a few Mg-rich biotite) in all granitoids. The mineralogical characteristics indicate an affinity of crust-mantle mixed granites in a relatively H2O-poor and reduced environment. P-T calculations suggest that granitoids from the Prydz Belt were emplaced at 5~6.6kbar and 774~832℃. Using the equation given by Wones (1989), oxygen fugacities are estimated to be -12.16 to -13.65, which are slightly above the QFM (quartz-fayalite-magnetite) buffer.3. Granitoids from the Prydz Belt are geochemically characterized by having high K2O + Na2O contents, K2O/Na2O and Ga/Al ratios and Fe* values, and low MgO, Cr, Ni contents, in agreement with A-type granites. The Progress granite in the Larsemann Hills should be defined as aluminous A-type granite, rather than S-type granite as considered by formers. This granite may have undergone extremely fractionation and contamination by the country pelites. In the primitive-mantle-normalized trace elements diagrams, all the granitoids show LILE and HREE enriched patterns with variably trough at Sr, Nb-Ta, Ti and P, a distinctive feature of subduction-related magmas. In a few tectonic diagrams, most granitoids plot the field of within-plate granites, and some belong to the syn- and post-collision granites.4. Isotopically, granitoids from the Prydz Belt display very low initial Nd isotope compositions and high initial Sr isotope compositions. The initial Nd values vary over a narrow range of -13.37~ -9.17, while the initial Sr values vary considerably from 0.7075 to 0.7246. Nd-depleted mantle model ages of 2.0~2.3Ga imply their derivation from old palaeoproterozoic crustal sources. Petrographical, geochemical and isotopic evidence indicate that granitoids from the Prydz Belt were generated from a phlogopite-bearing subcontinental enriched lithospheric mantle. Granitoids in the Larsemann Hills have variable initial Sr values, probably implying an inhomogeneity of Sr isotope composition in the source. On the other hand, these granitoids have Nd-depleted mantle model ages same as those of paragneisses, suggesting a close genetic link between them.5. Both magmatism and high-grade metamorphism in the Prydz Belt are probably related to the continental collision, including lithospheric thinning, magmatic underplating and crustal relaxation. The granitoids were derived by partial melting of the underplating materials of mantle magmas from enriched subduction-modified lithosphere, accompanying by the contamination of crustal components. The Landing granite and Murro Kerr granite may the final represent welding of different terranes and therefore sign the end of the post-collisional orogenic episode in the whole Pan-African belt and the beginning of a next Wilson cycle.6. Syn- and post-collisional A-type granitoids from the Prydz Belt support the idea that Prydz Belt represents a collisional orogen and the East Antarctic Shield itself was finally amalgamated by different terranes during the Pan-African period. Accordingly, East Antarctic Shield as well as East Gondwana were not a united continental blocks before the Pan-African time.更多还原