辽西兴城地区首山南花岗伟晶岩地质特征及其地质意义

Geological characteristics of granitic pegmatite at south of Shoushan,Xingcheng area in western Liaoning and its geological significance

通过对辽西兴城地区首山南花岗伟晶岩进行岩相学、锆石LA-ICP-MS U-Pb年代学及岩石地球化学的研究,揭示其形成年代、与其围岩成因关系以及其形成环境。研究结果表明:花岗伟晶岩的形成时代为(2 486±8)Ma(NSWD=3.1)。锆石CL图像形态特征以及微量元素特征显示,伟晶岩锆石属于岩浆成因但受到热液改造。首山南伟晶岩同绥中花岗岩的黑云母二长花岗岩作对比,在主量元素、微量元素及稀土元素方面表现出与围岩极其相似的特征。伟晶岩样品表现出高硅(SiO_2=77.28%~77.9%)、富碱(K_2O+Na_2O=5.5%~5.69%)、过铝质(A/CNK=2.53~2.54)特征,在SiO_2-K_2O判别图解上属钾玄系列。稀土元素总量低(33.1×10^(-6)~41.01×10^(-6))。球粒陨石标准化配分模式显示出LREE相对富集,HREE相对亏损;(La/Yb)_N=16.33~32.41,平均值为24.37;δEu值为0.92~1.11,表现为无异常至轻微正异常。这些地化特征总和表明首山南伟晶岩是黑云母二长花岗岩高度演化的结果。结合前人Hf研究结果,认为伟晶岩及其母岩形成均与底板垫托作用有关。

Petrography,LA-ICP-MS zircon U-Pb geochronology and geochemistry were carried out for the granitic pegmatite in the south of Shoushan in Xingcheng to explore the formation age,its relationship with the Neoarchean granite in Suizhong area,and its tectonic setting. Dating results reveal that the pegmatite were formed at（ 2 486 ± 8） Ma（ NSWD = 3. 1）. CL images of zircon morphology and characteristics of trace elements indicate that zircon pegmatite belongs to magmatic origin but later suffered hydrothermal alteration. Compared with biotite granite of Suizhong granite,the pegmatite shows similarities in major elements,trace elements and rare elements.The samples are characterized by high silica（ SiO_2= 77. 28% - 77. 9%）,rich alkali（ K_2O ＋ Na_2O = 5. 5% -5. 69%）,and are peraluminous（ A/CNK = 2. 53 - 2. 54）. The SiO_2-K_2O discrimination diagrams indicate that the granite pegmatie belongs to shoshonite series. They have low rare elements contents（ ΣREE = 33. 1 × 10-（-6）-41. 01 × 10-（-6））. Chondrite normalized REE patterns reveal that the samples are relatively rich in light REE but de-pleted in HREE.（La/Yb）_N= 16. 33 - 32. 41,average is 24. 37%. δEu = 0. 92 - 1. 11,representing none anomalies or slightly positive anomalies. The geochemical characteristics suggest that the pegmatite is related to the evolution of the biotite monzogranite. Combining with previous research results of Hf isotopic,the pegmatite and its parent rock are related to the bottom pad effect.