科尔沁沙地风成细沙的物质组成特征及其对物源的指示

Material composition characteristics of fine particles of eolian sand in Horqin Sandy Land and its indication to provenance

科尔沁沙地是中国北方面积最大的半固定沙地,追踪其物质来源对认识沙地/沙漠的形成演化和重建碎屑物质的迁移路径至关重要。为此,本研究对科尔沁沙地风成沙的细颗粒组分(<63μm和<11μm)进行了岩石学分析、元素地球化学分析、Sr-Nd同位素分析、TIMA自动化矿物识别以及碎屑锆石U-Pb测年分析,全面表征科尔沁沙地物质组成并对其物源进行定量约束。结果表明:科尔沁沙地经历了较低的化学风化,分选和再循环的程度较低。重矿物组合以钛铁矿、绿帘石、锆石、石榴子石、磁铁矿+赤褐铁矿为主,与地球化学母岩判别图解共同指示科尔沁沙地沉积物主要来自中酸性岩浆母岩。Sr-Nd同位素进一步约束主要源区为中亚造山带南缘和华北克拉通北缘,少量粉尘来源于中国北方边界沙漠。沙地的碎屑锆石U-Pb年龄集中在中生代至晚古生代(200~600 Ma)和古元古代(1 518~2 000 Ma、2 200~2 600 Ma)。与潜在源区锆石年龄谱的对比显示,科尔沁沙地细颗粒组分既有来自中亚造山带南缘大兴安岭经风力搬运和河流搬运而来的近源物质供给,也有经河流搬运自华北克拉通北缘燕山山脉出露的古老基岩。逆向蒙特卡罗模型的定量物源结果显示中亚造山带的贡献占53.7%、华北克拉通贡献占46.3%。风与河流的共同作用解释了科尔沁沙地风成沙细颗粒组分的形成。

The Horqin Sandy Land is the largest semi-fixed sandy land in northern China.Tracing its material sources is of great importance to understand the formation and evolution of deserts and reconstruct the migration paths of fragments.Therefore,the composition of fine particles(<63μm and<11μm)of eolian sand in the Horqin Sandy Land was analyzed by petrology,elemental geochemistry,Sr-Nd isotope,TIMA automatic mineral identification and U-Pb dating of detrite zircon.The material composition of the Horqin Sandy Land was characterized and its provenance was quantitatively constrained.The geochemical indicators show that the Horqin Sandy land has experienced low chemical weathering and fractional recycling degree.The heavy mineral assemblages are mainly ilmenite,epidote,zircon,garnet,magnetite+hematite limonite,which together with the discriminant diagram of geochemical motherrock indicate that the sediments in the Horqin Sandy Land are mainly derived from medium acid magmatic mother rocks.Further constrained by Sr-Nd isotopes,the main source areas are the southern margin of the Central Asian orogenic belt and the northern margin of the North China Craton,and a small amount of dust originates from the northern border desert of China.The U-Pb ages of detrital zircons in sandy land are mainly from Mesozoic to Late Paleozoic(200-600 Ma)and Paleoproterozoic(1518-2000 Ma and 2200-2600 Ma).The comparison with the zircon age spectrum of the potential source area shows that the fine particles in the Horqin Sandy Land contain both near-source material from the Greater Khingan Mountains in the southern margin of the Central Asian orogenic belt by wind and river transport,and ancient bedrock from the Yanshan Mountains in the northern margin of the North China Craton by river transport.Quantitative provenance results of inverse Monte Carlo model show that the Central Asian orogenic belt contributes 53.7%and the North China Craton contributes 46.3%.The synergistic effect of wind and river explains the formation of fine particles of wind-sand in the Horqin Sandy Land.