大兴安岭早白垩世伸展作用: 内蒙古乌拉盖球状岩地球化学及锆石U-Pb定年的限制

大兴安岭火山岩带中南部乌拉盖地区新发现的球状岩石除因其漂亮的构造被人们关注外，其地球化学特征及锆石U-Pb定年对于探讨东北地区大兴安岭演化历史也具有重要意义.在详细的野外地质调查基础上，通过对内蒙古乌拉盖球状岩及其主岩地球化学及Sr-Nd同位素特征等进行综合研究，探讨其源区性质及形成机制.研究结果表明:乌拉盖球状岩及其主岩具有富硅（SiO₂=75.35%~76.21%）、富碱（K₂O+Na₂O=7.30%~7.45%）和低铝（Al₂O₃=9.82%~11.30%）、镁值（Mg#=4.52~14.47）、铬（Cr=14.5×10-6~22.5×10-6）、镍（Ni=1.84×10-6~3.97×10-6），指示其物质来源于地壳.微量元素和稀土元素具后造山火山岩特征:Rb、Th、K等大离子亲石元素和Nb、Ta、Zr、HREE等高场强元素明显富集，N-MORB标准化图解中Nb负异常不明显，轻重稀土元素分异明显，LREE/HREE=5.21~6.70.初始87Sr/86Sr（0.705 9~0.713 7）高于现代大洋MORB（0.702 29~0.703 34），也显示壳源特征；143Nd/144Nd（0.512 456~0.512 528）则低于现代大洋MORB（0.512 99~0.513 30），ε_Nd值为+4.8~+6.2，又表现亏损地幔的信息.研究认为，乌拉盖球状岩及其主岩岩浆来源于下地壳，且有亏损地幔物质的参与.Sr、Ba、Eu亏损强烈，反映了明显分离结晶作用的存在，岩石形成于板内稳定的构造环境；主量元素及微量元素图解又表现出后造山A型花岗岩特征.年代学研究显示，球状岩没有得到很好的年代学数据，其可能具低温富流体的特征、新生结晶锆石偏少，大量的捕获锆石或其他难容矿物可能为球状岩球粒的生长提供物质基础；球状岩主岩文象花岗岩锆石LA-ICP-MS U-Pb成岩年龄为142.2±2.7 Ma，侵入球状岩的辉绿岩脉年龄平均为140.5 Ma，主岩的围岩正长斑岩获得锆石LA-ICP-MS U-Pb成岩年龄为349.5±3.4 Ma，综合判断球状岩的成岩时代为早白垩世早期（~141 Ma）.综合研究表明，乌拉盖地区球状岩石及其主岩形成机制与大兴安岭火山岩带晚期岩浆活动有关，主要受控于白垩世早期蒙古-鄂霍茨克洋闭合后造山伸展体制下的构造背景，是大兴安岭大规模区域伸展作用、岩浆底侵的产物. 

Early Cretaceous Extension of Great Xing 'an Range: Constraints from Geochemistry and Zircon U-Pb Ages of Orbicular Rocks in Uragai Area,Inner Mongolia,China

In addition to its beautiful structure, the newly discovered orbicular rocks in the central and southern parts of the Great Xing'an volcanic belt in the Uragai region are of great importance to the exploration of the subduction history of the Great Xing'an Range in the NE China. On the basis of detailed field geological survey, in this paper it makes a comprehensive study on the geochemistry and Sr-Nd isotope characteristics of the orbicular rocks and its host rocks in Uragai area, Inner Mongolia, and discusses its magmatic source and formation mechanism. The analysis results show that the Uragai orbicular rock and its host rocks have high silica (SiO₂=75.35%-76.21%) and alkali (K₂O+Na₂O=7.30%-7.45%), but low aluminium (Al₂O₃=9.82%-11.30%), Mg# value (Mg#=4.52-14.47), chromium (Cr=14.5×10-6-22.5×10-6), and nickel (Ni=1.84×10-6-3.97×10-6), indicating a crustal magmatic source. Trace and rare earth elements (REEs) show post-orogenic volcanic rock features: obviously enriched in large ion lithophile elements (LILEs, e.g., Rb, Th, K, LREE) and high field strength elements (HFSEs, e.g., Nb, Ta, Zr, HREE). The Nb negative anomaly is not obvious in N-MORB standardized diagram, with significant differentiation between light rare earth elements (LREE) and heavy rare earth elements (HREE) (LREE/HREE=5.21-6.70). The initial 87Sr/86Sr (0.705 9-0.713 7) is higher than the MORB of modern ocean (0.702 29-0.703 34), which also suggests a crustal source. However, the 143Nd/144Nd (0.512 456-0.512 528) was lower than the value of modern oceans (0.512 99-0.513 30), and with ε_Nd value from +4.8 to +6.2, which shows depleted mantle signature. Study shows that the magmatic source of Uragai orbicular rock and its host rocks originated from the lower crust, and with participation of depleted mantle material. The obviously depleted Sr, Ba and Eu suggest the fractional crystallization course, and the rocks formed in a stable in-plate tectonic environment. Major and trace element diagrams also show the post-orogenic A-type granite features. The study shows that the orbicular rocks have few reliable geological ages, probably resulted from sufficient low temperature fluids which blocked the crystallization course. The abundant trapped zircons or other refractory minerals may provide a material basis for the formation of orbicular rocks. The zircon LA-ICP-MS U-Pb age of graphic granite in the main face of the orbicular rocks is 142.2±2.7 Ma, and the average age of diabase dikes in the orbicular rocks is 140.5 Ma, and the zircon LA-ICP-MS U-Pb age obtained from orthophyre, which is surrounding rocks of the main face of the orbicular rocks, is 349.5±3.4 Ma, These age information restricted the formation age of orbicular rocks at Early Cretaceous(141 Ma±), its host rocks is Early Carboniferous, which is totally different with host rocks of orbicular rocks in and abroad. Comprehensive research shows that the formation mechanism of orbicular rock and its host rocks in Uragai area is related to late stage magmatic activity in the Great Xing'an volcanic belt, largely controlled by the post-orogenic extension tectonic setting of the Mongol-Okhotsk Ocean in the Early Cretaceous, and it is a product of the large-scale regional extension and strong magmatic underplating in the Great Xing'an Range.