甘肃北山柳园地区花岗岩类的年代学、地球化学特征及构造意义

Geochronology,geochemical characteristics and tectonic implications of the granitoids from Liuyuan area,Beishan,Gansu province,northwest China.

北山柳园地区分布大量的花岗岩类岩石,岩石类型有花岗闪长岩、二长花岗岩、钾长花岗岩和斑状花岗岩。锆石SHRIMP U—Pb 定年分析结果为:花岗闪长岩的侵位年代为423±8Ma 辉铜山以东(HT-)钾长花岗岩和二长花岗岩的侵位分别为436±9Ma 和397±7Ma。该区花岗质岩石都具有大离子亲石元素和轻稀土元素相对富集,K、Ni、Ta、P 和 Ti 负异常的特征,属于准铝质到过铝质的高 K 花岗岩。花岗闪长岩无 Sr 和 Eu负异常的特征,ε_(Nd)(t)=-2.5～-0.8,其岩浆源于岩石圈地幔或是软流圈与岩石圈地幔相混合的岩浆熔融,并受到了含有火山弧组分的年轻地壳的混染。钾长花岗岩和二长花岗岩具有 Sr 和 Eu 负异常的特征,ε_(Nd)(t)值分别为+1.4、-4.0～-2.0和-2.7～-0.3。HT-钾长花岗岩岩浆主要源于由于岩石圈地幔岩浆作用而导致上覆年轻地壳物质的部分熔融;花牛山附近(HN-)钾长花岗岩岩浆主要源于软流圈地幔部分熔融,可能受到了部分年轻地壳物质的混染;二长花岗岩岩浆主要源于年轻地壳的部分熔融。柳园地区4类花岗岩类岩石都是后碰撞构造背景下的岩浆产物,岩浆形成可能与俯冲板片断离有关。

Large amounts of granitoids, including granodiorite, monzogranite and potassium granite which divided into HT-potassium granite and HN-potassium granite on the base of where they locate, were emplaced in Liuyuan area, Beishan, Gansu province. The SHRIMP zircon U-Pb dating indicates that the emplaced age of granediorite, monzogranite and HT-potassium granite are 423±8Ma, 397 ± 7Ma and 436 ± 9Ma respectively. Four kinds of granitic rocks all have enriched large ion lithophile elements relative to high field strength elements, enriched light rare earth element relative to heavy rare earth element （HREE）, negative anomalies in K, Ni, Ta, P and Ti characteristics, and are metaluminous to peraluminous and high-K post-collisinal granite. In addition, granediorite has a lack of negative anomalies in Sr and Eu, БNd （t）=-2.5～-0.8, and its magmas resulted from partial melting of subcontinental mantle or admixture between partial melting of subeontinental mantle and partial melting of asthenospheric mantle, and contaminated by juvenile crust including orogenic components. HN-potassium granite, monzogranite and HT-potassium granite all have negative anomalies in Sr and Eu, and БNd （t）= ＋ 1.4,-4.0～-2.0 and -2.7～-0.3 respectely. All the geochemical characteristics indicate the magmas of HT-potassium granite were derived from partial melting of juvenile crust induced by subeontinental mantle. The magmas of HN- potassium granite were from partial melting of asthenospheric mantle, and likely contaminated by juvenile crust. And the magmas of monzogranite were from partial melting of juvenile crust. The four kinds of granitoid rocks are all outcomes of emplaced magmas under the post-collisional tectonic setting and genesis of magmas are likely related to slab break-off.