内蒙古苏尼特右旗白乃庙地区徐尼乌苏组的形成时代及其地质意义

Age and geological significance of Xuniwusu Formation from Bainaimiao area of Sonid Youqi,Inner Mongolia

本文对白乃庙地区徐尼乌苏组沉积特征、原岩建造、变质火山岩及变质碎屑岩的年代学和地球化学进行了研究,探讨了白乃庙地区早古生代构造演化。本次研究采集了徐尼乌苏组中具有代表性的变质含砾粗粒杂砂岩、变质英安质晶屑凝灰岩和变质长石石英细砂岩样品,进行了锆石LA-ICP-MS U-Pb年代学和岩石地球化学分析。结果显示,2个变质英安质晶屑凝灰岩锆石的加权平均年龄分别为440.9±1.8Ma(MSWD=0.10)和440.9±1.7Ma(MSWD=0.15),锆石Th/U比值为0.46~1.59,自形程度较好,发育有典型的岩浆锆石振荡环带结构,显示为岩浆成因锆石的特点,表明徐尼乌苏组的形成时代为早志留世。变质含砾粗粒杂砂岩的碎屑锆石年龄在452.0±1.3Ma^535.0±1.0Ma之间,最高峰值年龄为490Ma左右;变质长石石英细砂岩碎屑锆石年龄则在440.1±5.7Ma^3268.9±57.7Ma之间,最小谐和年龄为440.1±5.7Ma,峰值年龄为490Ma左右,另有1.0Ga、1.6Ga、1.8Ga和2.5Ga四个明显的峰值年龄。根据研究区徐尼乌苏组岩性组合与结构构造,可将该组划分为3个不同的沉积旋回。结合白乃庙地区徐尼乌苏组测年结果、岩石地球化学特征、原岩建造及区域岩浆岩资料,白乃庙弧后盆地沉积可划分为三个演化阶段:早期快速堆积阶段(452.3~440.9Ma),形成了一套成熟度较低的粗碎屑岩建造,物源主要来源于白乃庙岩浆弧中的岩浆岩;中期火山喷发阶段(440.9~440.1Ma),以火山沉积作用为主,为火山碎屑岩建造夹有碳酸盐建造和少量碎屑岩建造,碎屑物质主要来源于该时期的火山活动;晚期稳定沉积阶段(<440.1Ma),形成一套细碎屑岩建造和碳酸盐建造,为浅海相稳定沉积,此时物源广泛,分别来源于华北克拉通基底、兴蒙造山带和白乃庙早古生代火山弧。根据徐尼乌苏组的沉积建造和火山建造特征,结合白乃庙火山弧岩浆活动特征,认为徐尼乌苏组形成于早古生代弧后盆地中,此时华北板块北缘属于安第斯型活动大陆边缘。

This paper is based on the sedimentary characteristics,protolith formation,geochronology and geochemistry of the metamorphic volcanic and clastic rocks of Xuniwusu Formation,to discuss the Early Paleozoic tectonic evolution in Bainaimiao area of Sonid Youqi,Inner Mongolia.Four representative samples of Xuniwusu Formation（metamorphic gravel coarse greywacke,metamorphic dacite crystal tuff and metamorphic feldspar quartz sandstone） are analyzed by LA-ICP-MS zircon U-Pb geochronology and geochemistry.The U-Pb dating rusults show that the age of the two metamorphic dacitic crystal tuffs are 440.9±1.8 Ma（MSWD=0.10） and 440.9±1.7 Ma（MSWD=0.15）.The zircons form two metamorphic dacitic crystal tuffs are euhedral-subhedral in shape,display fine-scale oscillatory growth zoning and have high Th/U ratios（0.46~1.59）,indicating a magmatic origin.It is indicated that the formation epoch of Xuniwusu Formation is Early Silurian.The detrital zircon ages of the metamorphic gravel coarse-grained greywacke are between 452±1.3 Ma and 535.0±1.0 Ma,and the highest peak age is about 490 Ma.The detrital zircon ages of the metamorphic feldspar quartz sandstone are between 440.1±5.7 Ma and 3268.9±57.7 Ma.The minimum concordia age is 440.1±5.7 Ma,and the highest peak age is about 490 Ma,but there are also four obvious peak ages（1.0 Ga,1.6 Ga,1.8 Ga and 2.5 Ga）.According to the sedimentary rhythm,Xuniwusu Formation can be divided into three sedimentary cycles.Combining with the dating results,petrological and geochemical characteristics,protolith formation and regional magmatic data,three sedimentary stages were used to represent the evolution of Bainaimiao back-arc basin.The initial stage of rapid accumulation（452.3~440.9 Ma）,formed a suite of coarse clastic rocks with low maturity,belongs to the coastal environment.The provenance is mainly derived from the magmatic rocks of Bainaimiao volcanic arc.The intermediate stage（about 440.9~440.1 Ma）,should be the Early Paleozoic subduction orogenic stage,which was dominated by volcanic sedimentation,mainly composed of volcanic clastic rock formation with carbonate rocks and a small amount of clastic rocks interlavers.These clastic materials were mainly derived from volcanic activity.Late stable sedimentary stage（ 〈440.1 Ma） formed a fine clastic rock formation and carbonate formation,belongs to the unchanging shallow marine sedimentary facies.At this time,the provenance is derived from the North China Craton,Xing-Meng Orogenic Belt and Early Paleozoic volcanic arc.According to the characteristics of sedimentary formation and volcanic formation,combined with the characteristics of the magmatic activity in the volcanic arc of Bainaimiao,we considered that the formation of Xuniwusu Formation is in the Early Paleozoic back-arc basin.At this time,the northern margin of the North China plate was the Andean-type active continental margin.