大兴安岭中南段查布其日上侏罗统玛尼吐组火山岩地质及年代学特征

通过1∶5万区域地质调查，运用“火山构造—岩性岩相—火山地层”三重填图方法，对大兴安岭中南段内蒙古查布其日地区上侏罗统玛尼吐组火山岩进行了系统的野外地质特征、岩石学、地球化学和锆石U-Pb年代学研究，分析玛尼吐组火山岩形成时代和构造背景。将内蒙古查布其日地区玛尼吐组划分为2个岩性段，一段为火山碎屑沉积岩和少量中性、中酸性火山碎屑岩，二段为中性火山熔岩、火山碎屑岩和少量火山碎屑沉积岩、酸性火山碎屑岩，构成一个完整的沉积喷发旋回；划分出爆发相（碎屑流堆积相、空落堆积相）、溢流相、火山通道相、喷发—沉积相等火山岩相，并圈出4个V级火山机构。获得查布其日玛尼吐组二段安山岩的LA-ICP-MS锆石U-Pb年龄为157. 8±1. 4 Ma，表明其形成时代为晚侏罗世。岩石地球化学特征为，玛尼吐组二段粗安岩的SiO2为64. 04%~65. 40%，高Al2O3(16. 02%~16. 96%)，岩石富碱(K2O+Na2O=8. 07%~8. 38%)，低TFe2O3(1. 38%~4. 43%)，贫TiO2(0. 6%~0. 81%)，A/CNK值介于1. 02~1. 12之间；稀土丰度总量??REE为177. 60×10-6~254. 61×10-6，轻、重稀土分异较明显，(La/Yb)N=8. 13~12. 63，具弱负Eu异常(δEu=0. 71～0. 78）； 微量元素以富集大离子亲石元素(Rb、Ba、Th、U、K)和相对亏损高场强元素(Nb、Ta、P、Ti)为特征。主量元素、微量元素地球化学特征显示查布其日玛尼吐组二段粗安岩为钾玄质岩。研究区玛尼吐组火山岩岩浆来源于伸展作用触发先前俯冲板片释放流体交代的大陆岩石圈地幔发生部分熔融，且受地壳混染影响较小，可能形成于蒙古—鄂霍次克洋造山后的伸展构造环境。

Geological and chronological characteristics of the Upper Jurassic Manitu Formation in Chabuqiri, central-south Great Hinggan Mountains

Based on 1∶50000 regional geological survey, the field geological characteristics, petrology, geochemistry and zircon U-Pb chronology of the volcanic rocks of the Upper Jurassic Manitu Formation in Chabuqiri area, Inner Mongolia, central-south Great Hinggan Mountains, were systematically studied by using the triple mapping method of “volcanic structural—litho—lithofacies and volcanic stratigraphy”. The Manitu Formation in Chabuqiri area of Inner Mongolia is divided into two lithologic members. The First Member consists of pyroclastic sedimentary rocks with a small amount of neutral and moderately acidic pyroclastic rocks, and the Second Member consists of neutral lava, pyroclastic rocks with a small amount of pyroclastic sedimentary rocks and acidic pyroclastic rocks, forming a complete sediment-eruption cycle. The volcanic facies are divided into eruption-sedimentary facies, excrete facies, clastic flow accumulation facies, void fall accumulation facies and volcanic channels, and four Ⅴ-class volcanic edifices are outlined. The LA-ICP-MS U-Pb age of the zircons from the andesite in the Second Member of the Manitu Formation in Chabuqiri-manitu Formation is 157. 8±1. 4 Ma, indicating that it was formed in the Late Jurassic. The geochemical characteristics of the trachyandensite in the Second Member of the Manitu Formation are: SiO2= 64. 04%~65. 40%, high Al2O3(16. 02%~16. 96%), rich alkali (K2O+Na2O=8. 07%~8. 38%), low Fe2O3(3. 12%~3. 96%), poor TiO2(0. 6%~0. 81%), A/CNK values between 1. 02~1. 12; the REE ranges from 177. 6×10-6 to 254. 6×10-6, with obvious differentiation between light and heavy rare earth, (La/Yb)N=8. 13~12. 63, and a weak negative Eu anomaly (δEu= 0. 71~0. 78); the trace elements are characterized by rich large ion lithophile elements (Rb, Ba, Th, U, K) and relatively deficient high field strength elements (Nb, Ta, P, Ti). The geochemical characteristics of main elements and trace elements show that the trachyandensite of the Second Member of the Manitu Formation in Chabuqi area is shoshonitic series rock. The volcanic magma of the Manitu Formation in Chabuqi area originates from the partial melting of the continental lithospheric mantle triggered by extensional process, and is less affected by crustal mixing, which may have formed in the extensional tectonic setting after the Mongolian-Okhotsk Ocean orogeny.