冀北大滩盆地粗面岩的年代学、地球化学特征及成因研究

丰宁大滩盆地位于西拉木伦河断裂带以南的华北古板块北缘，燕辽Mo-U-Ag-Pb-Zn多金属成矿带西段和沽源-红山 子铀成矿带西南段，空间上与赋存张麻井大型铀钼矿床的沽源盆地毗邻。大滩盆地张家口组粗面岩-流纹岩组合构成盆地 的主体，是热液型铀矿的主要围岩。粗面岩SHRIMP锆石U-Pb年龄为140.3 ± 1.4 Ma（2σ, MSWD=1.09），地质时代属于早 白垩世早期。具有较高的SiO2、K2O+Na2O含量，SiO2=61.6%~63.1%，K2O+ Na2O=9.28%~10.2%，在K2O-SiO2图解中投影点落在橄榄玄粗岩系列范围，TAS图解上都落在碱性系列粗面岩或粗面英安岩范围，结合标准矿物Q含量为12.9%~18.4% （小于20%），属典型的粗面岩。稀土元素含量高，富集轻稀土，铕负异常不明显，ΣREE=（279~318） ×10-6，δEu=0.77~ 0.88，稀土配分曲线图表现为右倾的轻稀土富集型，具高压型粗面岩的稀土元素特征；富集大离子亲石元素K、Rb、Ba、 Pb和高场强元素Zr、Y，亏损Nb、Ta、Sr、Ti元素。较低的Ti/Zr（8.40~14.72）、Nb/Ta（14.07~15.26） 比值，较高的Rb/Sr （0.95~1.60），具有高Sr-Ba 英安岩-流纹岩的微量元素的特征；具有较高的（87Sr/86Sr）i （0.7077~0.7096）、较低的εNd(t)（-12.09~ -10.67），较大的TDM2 （1911~1796 Ma），较低的（206Pb/204Pb）t （16.92~17.04）、（207Pb/204Pb）t （15.39~15.42） 和（208Pb/204Pb）t （37.58~37.67），在（87Sr/86Sr）i -t/Ga和（208Pb/204Pb） t-（206Pb/204Pb）t 图解上投影点位于下地壳和地幔之间，并靠近EMⅠ富集地幔，在εNd(t)-（87Sr/86Sr）i 图解上投影点位于EMⅠ富集地幔演化线上，与汉诺坝二辉麻粒岩包体的区域一致。可见，大滩盆地粗面岩的成因可用两阶段模式进行解释：古元古代源于EMⅠ富集地幔的岩浆与少量古老下地壳物质部分熔融产物混合形成年轻下地壳，早白垩世早期在加厚地壳背景下年轻下地壳部分熔融形成的岩浆喷出地表形成粗面岩。

Geochronology,Geochemical Characteristics,and Genesis of Trachyte in Datan Basin,Northern Hebei

Datan volcanic basin in Fengning is located in the zone of the northern North China Block, south of Xar Moron river fault zone, west of the Yan-Liao polymetallic (Mo-U-Ag-Pb-Zn), and southwest of the Gu(yuan)-Hong(shanzi) uranium belts. It is adjacent to the Guyuan basin where there is a large Zhangmajing U-Mo deposit. The rhyolite-trachyte rock assemblage of Zhangjiakou was formed in the Datan volcanic basin and is the main wall rock of hydrothermal-type uranium deposit. The zircon SHRIMP U-Pb dating of trachyte yields an age of 140.3 ± 1.4 Ma (2σ, MSWD=1.09), which represents the emplacement age of the trachyte, and suggests that it was formed in the early Early Cretaceous. Trachyte contains high content of SiO2, K2O+Na2O ratio , SiO2=61.6%~63.1% , K2O+ Na2O=9.28%~10.2%, and falls in shoshonite series range of the K2O-SiO2 diagram and in alkaline series trachyte or dacite range of the TAS diagram. With the result of the CIPW and the content of Q =12.9%~18.4% (less than 20%), it belongs to the typical trachyte. Trachyte has high content of rare earth elements, enriched in light rare earth and the non-obvious negative Eu anomaly, ΣREE=(279~ 318)×10-6, δEu=0.77~0.88. The results show the right-leaning enrichment in light rare earth type with a high pressure type trachyte characteristics of rare earth elements, enrichment in large ion lithophile element (K, Rb, Ba, Pb) and high field strength elements (Zr、 Y), and loss of these elements(Nb, Ta, Zr, Ti ). The Ti/Zr (8.40~14.72),Nb/Ta(14.07~15.26) ratio is low and Rb/Sr(0.95~1.60) ratio is high with high Sr-Ba dacite-rhyolite the characteristics of trace elements; High (87Sr/86Sr)i(0.7077~0.7096), lower εNd(t)(-12.09~-10.67), the larger TDM2(1796~1911Ma), lower (206Pb/204Pb)t(16.92~17.04), (207Pb/204Pb)t(15.39~15.42),（208Pb/204Pb)t(37.58~37.67). In the (87Sr/ 86Sr)i -t/Ga and (208Pb/204Pb)t-( 206Pb/204Pb)t diagram, it is located between the lower crust and the mantle, and is close to the EMⅠ enriched mantle. In the εNd(t)-(87Sr/86Sr)i diagram, it is located within EMⅠenriched mantle evolution line and Hannuoba two-pyroxenne grain xenoliths area. So the trachyte in the Datan basin can be explained by a two-stage model: In the paleoproterozoic, EMⅠenriched mantle magma and a small part of the partial melting of ancient crust material were mixed and formed the young crust. In the early Early Cretaceous, the magma from partial melting of the young and thickened lower crust erupted and formed trachyte.