内蒙古半沟地区紫苏碱长花岗岩的年代学、同位素地球化学及构造意义

The chronology,isotope geochemistry and tectonic significance of alkalic feldspar charnockite in Bangou area,Inner Mongolia

为进一步探讨华北克拉通古-中元古代构造演化过程,本文对位于华北克拉通北缘孔兹岩系与怀安片麻岩接触界限附近、侵入TTG片麻岩的紫苏碱长花岗岩进行了地质年代学及地球化学研究。锆石U-Pb测年结果表明,该紫苏碱长花岗岩的结晶年龄为1792±19Ma;此外,分析样品的主量元素显示高硅、贫镁铁、准过铝质-弱过铝质特征,属高钾钙碱性I型花岗岩;同时,微量元素具有右倾式稀土配分模式、Eu呈正异常、富集大离子亲石元素、亏损高场强元素的特征。锆石εHf(t)呈负值(-12.1~-6.2),εNd(t)=-4.5~-9.4,具有相对富集的同位素组成。锆石Hf和Nd同位素单阶段模式年龄分别为2.41~2.62Ga、2.40~2.61Ga,与年龄为~2.5Ga的TTG片麻岩对应值一致。岩石地球化学及Nd-Hf同位素证据指示紫苏碱长花岗岩源自碰撞后伸展抬升阶段TTG片麻岩的重熔,标志着华北克拉通古元古代造山运动结束。

The Bangou area is located in the contact position between the Trans-North China Orogen(TNCO)and the Khondalite Belt,and as a result,it is an important area for studying the tectonic evolution of the North China Craton(NCC).To enhance our understanding of the tectonic evolution of the NCC during the Paleo-Mesoproterozoic,this study investigates the geochronology and geochemistry of the alkalic feldspar charnockite that intrudes the tonalite-trondhjemite-granodiorite(TTG)near the contact boundary between the khondalite belt and TTG gneiss on the northern margin of NCC.Zircon U-Pb isotopic dating results indicate that the alkalic feldspar charnockite was formed around 1.79Ga.Major element analysis reveals the granite is high in silicon,low in magnesium and iron,and ranges from metaluminous to slightly peraluminous with characteristics of high-potassium calc-alkaline I-type granite.Trace element analysis shows that it has a right-leaning rare earth element(REE)distribution pattern with a positive Eu anomaly,enrichment in large ion lithophile elements,and depletion in high field strength elements.ZirconεHf(t)andεNd(t)values for the analyzed samples of the alkalic feldspar charnockite are-12.1~-6.2 and-4.5~-9.4 respectively,suggesting a relative enrichment of these isotopic compositions.The Hf and Nd isotope single-stage model ages of the analyzed zircons are 2.41~2.62Ga and 2.40~2.61Ga,which are consistent with those of the~2.5Ga TTG gneiss.Geochemical and Nd-Hf isotope evidence indicate that the alkalic feldspar charnockite was originated from remelting of the TTG gneiss.During this time,the Western Block and Eastern Block of the NCC had collided around 1.85Ga,leading to an overall uplifting at 1.82~1.80Ga which was followed by a post-collisional extension between 1.80~1.78Ga.The alkalic feldspar charnockite is an I-type granite formed in a post-collisional environment.Such a granite activity has not been reported so far in this region,and thus,the study of the alkalic feldspar charnockite in this paper provides new evidence for the evolution of the Paleoproterozoic craton,which marks the end of the Paleoproterozoic orogeny in the NCC.