内蒙古狼山地区中三叠世早期C型埃达克岩的发现及其构造意义

The Discovery of Earliest Middle Triassic C-Type Adakite from Langshan Area,Inner Mongolia and Its Tectonic Implications

狼山构造带位于索伦缝合带南西,华北北缘及中亚造山带南缘的结合位置,是研究中亚造山带晚古生代-早中生代构造-岩浆演化及地球动力学背景的重要场所.在该地区新识别出中三叠世早期具有埃达克岩特征的扎拉山岩体,该岩体岩石类型主要为花岗闪长岩及二长花岗岩,LA-ICP-MS锆石U-Pb定年结果显示该岩体的形成时代介于244.9±1.2Ma～244.1±2.3Ma.地球化学特征表明,该岩体具有较高的SiO_2(68.77%～72.58%)、Al_2O_3(14.48%～16.28%)、Sr(287×10^(-6)～455×10^(-6),平均值413×10^(-6))含量及Sr/Y比值(46.07～95.50),较低的Y(4.07×10^(-6)～8.01×10^(-6))、Yb(0.43×10^(-6)～0.78×10^(-6))、Cr(5.18×10^(-6)～8.92×10^(-6))、Ni(1.34×10^(-6)～7.71×10^(-6))含量及Mg#值(35.54～41.64),Na_2O/K_2O比值为0.86～1.19,重稀土元素强烈亏损,轻重稀土元素分馏明显(26.45<(La/Yb)N <56.13),铕异常较弱(0.82<δEu<1.02),具有C型埃达克岩特征.锆石Hf同位素分析结果显示εHf(t)值介于2.5～8.9,对应的二阶模式年龄TDM2介于707～1 115Ma,表明其源区主要为年轻地壳.结合区域地质背景,结果表明内蒙古狼山地区中三叠世早期具有C型埃达克岩特征的扎拉山岩体应为古亚洲洋闭合之后,西伯利亚板块与华北板块碰撞造山阶段的产物,为加厚的下地壳部分熔融形成.

The Langshan structural belt is located in southwest of the Solonker suture zone and as well as the junction between the northern margin of the North China Craton and southern of Central Asian orogenic belt,which is an ideal workplace to study on tectonic-magmatic evolution and geodynamics background from Late Paleozoic to Early Mesozoic.An earliest Middle Triassic pluton with Adakite characteristics was discovered in this study and was called as Zhalashan pluton.The main rock types of Zhalashan Pluton are granodiorite and monzogranite.LA-ICP-MS zircon U-Pb ages yield the formation time of Zhalashan pluton between 244.9±1.2Ma and 244.1±2.3Ma.Geochemical characteristics show that the Zhalashan granites are typical C-type adakite with high SiO2(68.77%-72.58%),Al2O3(14.48%-16.28%),Sr(287×10-6-455×10-6,413×10-6 on average)content and high Sr/Y ratio(46.07-95.50),low Y(4.07×10-6-8.01×10-6),Yb(0.43×10-6-0.78×10-6),Cr(5.18×10-6-8.92×10-6),Ni(1.34×10-6-7.71×10-6)content and low Mg#(35.54-41.64),Na2O/K2O ratio between 0.86 and 1.19.In addition,it exhibits significant fractionation between LREE and HREE[26.45<(La/Yb)N<56.13]and is depleted in HREE with slight Eu anomaly(0.82<δEu<1.02).Zircon Hf isotopic data displays thatεHf(t)values varyfrom 2.5~8.9,Hf model ages TDM2 vary from 707-1 115 Ma,indicating ajuvenile crustal material.Together with regional geological background,the authors hold that the Zhalashan pluton resulted from collision between Siberia Craton and North China Craton after the closure of the Paleo-Asian Ocean which is the product of partial melting of the thickened lower crust.