| 摘要 | 中亚造山带是世界上最大的显生宙增生造山带，而阿尔泰造山带是中亚造山带的重要组成部分。据统计，阿尔泰造山带内的伟晶岩脉多达十万余条，且主要呈北西-南东向展布。但截止目前，有关阿尔泰造山伟晶岩成因、形成时代、岩浆物源以及形成大地构造背景等的研究还相当缺乏。此外，高度演化且完善分异的复杂稀有金属花岗伟晶岩通常经历了岩浆、岩浆-热液过渡和热液三个阶段的演化，目前对这三个阶段的地球化学演化特征及其持续时间仍然不完全清楚。本论文选择仅次于世界著名的可可托海3号伟晶岩脉的新疆阿尔泰柯鲁木特112号伟晶岩脉为研究对象，探讨伟晶岩与吉得克二云母花岗岩之间的成因联系，伟晶岩岩浆体系中岩浆、岩浆-热液过渡及热液阶段的地球化学特征及其演化时限，伟晶岩形成的物源及其大地构造背景等重要科学问题。 本文系统开展不同结构带中锆石U-Pb定年、Hf同位素组成、等价不相容元素对分异 (Zr-Hf, Nb-Ta, Y-Ho)、REE地球化学特征，石榴子石矿物学以及流体-熔体及流体包裹体显微测温学研究，主要获得以下重要认识： 锆石U-Pb年代学表明柯鲁木特112号伟晶岩脉主体形成于238.3±2.0 (I带)至210.7±1.6 Ma (VI带)，其围岩吉德克二云母花岗岩和黑云母花岗岩形成年龄分别为445.6±4.3和455.6±5.4 Ma，表明伟晶岩与花岗岩之间不可能存在成因上的联系。锆石Hf同位素研究则表明112号伟晶岩 (εHf(t)值为 + 0.03 ~ + 2.35)及其围岩二云母花岗岩 (εHf(t)值为- 1.41 ~ + 3.67)和黑云母花岗岩 (εHf(t)值为+ 1.18 ~ + 4.13)具有相似源区且以壳幔混合为特征。伟晶岩中锆石低的εHf(t)和相对较老的TDM模式年龄暗示初始岩浆与含有幔源物质的古老地壳熔融有关，表明伟晶岩初始岩浆起源于后碰撞构造背景下，增厚地壳伸展减薄导致的减压部分熔融，进一步指示中亚造山带南缘在三叠纪经历陆陆碰撞造山作用。锆石U-Pb定年结果显示形成柯鲁木特112号伟晶岩的岩浆经历了长期的结晶分异演化过程，其中的岩浆阶段演化了~ 5Ma，岩浆-热液过渡阶段持续了~ 23 Ma，而热液阶段则持续了~ 22Ma。 岩浆阶段(I和II带)锆石显示较高的HfO2含量 (平均12.67 ~ 16.04%)及较小的Zr/Hf比值 (平均8.49-5.98)，最高的∑REE+Y含量 (平均分别为4644 ppm和2086 ppm) 且不发育稀土“四分组效应”，最小Y/Ho比值且最接近CHARAC场 (CHArge and RAdius Control, 24 ~ 34)，平均分别为47.0和39.5，表明112号伟晶岩岩浆经历了高度的分异演化并富集一定量的挥发组分；岩浆-热液过渡阶段(III和V带)锆石具最高的HfO2含量(平均18.63 ~ 22.29%)，最低的Zr/Hf比值(平均5.24-3.93) 以及∑REE+Y含量 (分别为101 ppm和13 ppm)，最强的M型“四分组效应”(TE3,4平均分别高达1.70和1.49)，最大的Y/Ho比值且最大程度偏离CHARAC场，平均分别为128和122，表明F的富集加剧了Zr-Hf分异，并且富F流体相优先富集REE，尤其是Nd、Gd、Ho、Er及Lu(较相邻元素)，该流体相出溶加剧了锆石中不相容元素对分异以及显著的稀土“四分组效应”；热液阶段(VI带)锆石则显示最低的HfO2含量 (平均6.81%)及最高的Zr/Hf比值 (平均15.71)，较高的∑REE+Y含量 (平均为2779 ppm)，较强的M型“四分组效应”(TE3,4平均为1.52)，较低的Y/Ho比值52且轻度偏离CHARAC场，表明流体相出溶事件结束，锆石中不相容元素对的分异程度和四分组效应减弱。IV带锆石显示继承核和新生边结构，二者的HfO2含量 (平均10.12%)和Zr/Hf比值 (平均10.18)一致且介于岩浆-热液过渡和热液阶段结构带锆石之间，∑REE+Y含量和Y/Ho比值分别为290 ppm和1042 ppm以及73和53，前者显示中等M型四分组效应，后者则具有W和M混合型四分组效应，表明岩浆-热液过渡阶段早期成核及晚期流体作用交代的特征。锆石极低的Nb/Ta比值反映了各结构带矿化特征，表现为Nb、Ta显著矿化的IV带中锆石具最高的Nb(平均174 ppm)和较高Ta含量(平均364 ppm)，且Nb/Ta值较低 (平均0.45)；Ta矿化最强烈的V带锆石具最高Ta含量 (平均1257 ppm)、最低(Nb含量42 ppm)及最低Nb/Ta比值 (平均0.03)；Nb、Ta无矿化的VI带中锆石具有最低的Nb(平均75 ppm)和Ta(平均52 ppm)含量，且Nb/Ta值最高 (1.36)。I、II和III带的Nb、Ta矿化较弱且低于最低工业品位，锆石中Nb、Ta含量和Nb/Ta比值介于IV、V和VI带之间。 对石英、锂辉石、微斜长石、钠长石和绿柱石中包裹体研究发现，岩浆阶段形成的结构带 (I和II带)中以含气相结晶质熔体包裹体、次生流体包裹体为特征，岩浆-热液过渡阶段形成的结构带 (III、V带)中以大量的流体-熔体包裹体，原生和次生流体包裹体为特征，而热液阶段形成的石英核带 (VI)中以大量富晶体流体包裹体、原生和次生流体包裹体为特征，热液蚀变形成的IV带中则只存在流体包裹体。出溶流体相为H2O+CO2+NaCl体系，包括富CO2 (气相比10-50%, 0.35-7.59%NaCleqv)和富H2O+NaCl相 (气相比小于10%，5-12.85% NaCleqv)两类。结合熔融包裹体和流体-熔融包裹体的爆裂温度、流体包裹体均一温度及对应压力和Li硅酸盐矿物相变P-T，得出112号伟晶岩主体形成于600-300oC以及3.6-2.3 Kbar的P-T条件下。 |
| 其他摘要 | The Central Asia Orogenic Belt (CAOB) is known as the largest Phanerozoic accretionary orogenic belt in the world, and the Altay Orogenic Belt is one of its important parts. According to previous statistics, there are nearly one hundred thousand pegmatite veins exposed in the Altay area, and most pegmatites show northwest-southeast trending distribution. However, it is quite deficient of the studies on pegmatitic petrogenesis, formation age, provenance and geotectonic setting until now. Besides that, Granitic pegmatites with high grade evolution and perfect fractionation generally undergo three stages evolution, including magmatic, magmatic-hydrothermal transition and hydrothermal stages, it is not entirely clear the geochemistry evolution features and evolution time limitation. The Kelumute No.112 pegmatite, which ranks only second to the well-known Koktokay No. 3 pegmatite in Xinjiang, has been chosen for investigating the crucial science questions, including petrogenetic relationship between the pegmatite and Jideke two mica granite, geochemical features and evolution time limitation of different stages (mgmatic, magmatic-hydrothermal and hydrothermal stages) in pegmatite magmatc system, pegmatitic provenance and geotectonic setting. Zircon U-Pb chronology, Hf isotope composition, fractionation of Isovalent element pairs(Zr-Hf, Nb-Ta, Y-Ho), geochemical features, garnet mineralogy, microthermometry of fluid and fluid-melt inclusions have been carried out in this work, the following original understanding have been obtained: The Kelumute No. 112 pegmatite in the Chinese Altay was mainly formed from 238.3 ± 2.0 to 210.7 ± 1.6 Ma, and the wall rocks of the Jideke two-mica granite and biotite granite are dated at 445.6 ± 4.3 and 455.6 ± 5.4 Ma, respectively, indicating no genetic relationship between the pegmatite and its wall rocks. Parental magmas of the Kelumute No. 112 pegmatite (εHf(t) = + 0.03 ~ + 2.35) and its wall rocks of Jideke two-mica granite (εHf(t) = - 1.41 ~ + 3.67) and biotite granite (εHf(t) = + 1.18 ~ + 4.13) originated from a common source and characterized by mixing of crust and mantle-derived components. Low εHf(t) ratios and older TDM model ages suggest the pegmatite parental magma relates to melting of ancient crust with some mantle components, indicating parental magma stem from partial melting of thickened orogenic crust resulting from decompression under post-collisional geodynamic setting, further suggest that the orogensis of southern Central Asian Orogenic Belt proceeded in continental - continental collision during the Triassic. |
修改评论