呼伦贝尔沙地矿物岩石及碎屑锆石U-Pb年龄特征对物源及构造的指示

Mineral petrology and detrital zircon U-Pb age characteristics of the Hulunbuir Sandy Land:Implications for the provenance and the structure

沙地的物质组成(如矿物岩石学和锆石U-Pb年代学等)对于沙地的形成演化、物源、粉尘释放、古气候变化以及区域构造岩浆事件等具有极其重要的意义。本文对呼伦贝尔沙地风成沙进行了重矿物(63~125μm)和岩石学、碎屑锆石U-Pb年龄(<63μm)分析。结果表明,风成沙的岩石学类型主要为岩屑长石砂岩,其次为长石岩屑砂岩。Dickinson三角图解显示其碎屑组分主要来源于再旋回造山带和陆块物源区的混合物源区域;重矿物组合以角闪石、磁铁矿、绿帘石、钛铁矿和石榴子石为主,表明其物源母岩主要为中酸性火成岩和变质岩。沙地的碎屑锆石U-Pb年龄主要集中在100~600 Ma、750~2150 Ma、2300~2600 Ma。通过与潜在源区年龄谱对比,说明中生代锆石主要来自中亚造山带东段的大兴安岭经河流搬运而来的近源物质供给;较老的两组前寒武纪锆石可能经盛行的西北风搬运自西伯利亚克拉通东南部地块出露的古老基岩;值得注意的是,风成沙与海拉尔河河流沙的锆石年龄组成极其相似,说明风成沙是海拉尔盆地上覆松散河湖沙就地起沙堆积的结果。结合区域构造演化历史,呼伦贝尔沙地前寒武纪早期的锆石年龄峰值响应了西伯利亚古陆新太古代末期地壳增长以及西伯利亚克拉通的拼合和Columbia超大陆的形成;古生代以来的锆石年龄峰值记录了东北微地块与西伯利亚克拉通的拼合及裂解、蒙古-鄂霍兹克洋的后撤俯冲及闭合过程以及古太平洋板块向西俯冲等区域构造事件。

The material composition of sandy land(such as mineralogy,petrology and zircon UPb chronology)is of great significance for the formation and evolution of sandy land,provenance,dust release,paleoclimatological changes and regional tectonic magmatic events.Petrology,heavy minerals(63-125μm)and detrital zircon U-Pb ages(<63μm)of aeolian sand in the Hulunbuir Sandy land were analyzed in this paper.The results show that the petrological type of aeolian sand is mainly lithic feldspar sandstone,followed by feldspar lithic sandstone.Dickinson triangle diagram shows that the detrital components are mainly located in the mixture source area of the recyclic orogenic belt and the continental block.The heavy mineral associations are mainly composed of hornblende,magnetite,epidote,ilmenite and garnet,indicating that the parent rocks are mainly the intermediate-acid igneous rocks and the metamorphic rocks.The U-Pb ages of detrital zircons in the sandy land mainly range from 100 to 600 Ma,from 750 to 2150 Ma,and from 2300 to 2600 Ma.By comparing the age spectra of detrital zircons with those of potential source areas,it is concluded that the Mesozoic zircons were mainly supplied by near source materials transported by rivers from the Greater Khingan Mountains in the eastern section of the Central Asian orogenic belt.The two older Precambrian zircons may have been transported by prevailing northwest winds from ancient baserock outcropping in the southeastern Siberian craton block.It is worth noting that the zircon age composition of the aeolian sand is very similar to that of the fluvial sand in Hailar River,indicating that aeolian sand is the result of in-situ deposition of loose fluvial and lacustrine sands overlying the Hailaer Basin.Combined with the regional tectonic evolution history,the early Precambrian zircon age peak in the Hulunbuir Sandy land responds to the crustal growth of the Siberian ancient terrain at the end of the Neoarchean,the combination of the Siberian craton and the formation of the Columbia supercontinent.The peaks of zircon age since Paleozoic have recorded regional tectonic events such as the fragmentation and splitting of the northeast microblock and the Siberian craton,the retreating subduction and closure of the Mongolia Okhotsk Ocean,and the westward subduction of the ancient Pacific plate.