大兴安岭南端芝瑞盆地流纹岩年代学、地球化学及岩石成因  

Petrogenesis of early Late Jurassic rhyolites from the Zhirui Basin in southern Daxing'an Range:Their chronologic and geochemical constrains

作　　者：解开瑞 巫建华[1,2] 祝洪涛 吴仁贵[1] 刘帅[2] 

机构地区：[1]东华理工大学地球科学学院,江西南昌330013 [2]东华理工大学核资源与环境国家重点实验室培育基地,江西南昌330013 [3]核工业243地质大队,内蒙古赤峰024006

出　　处：《地球化学》2016年第3期249-267,共19页Geochimica

基　　金：国家自然科学基金(41372071);中国核工业集团公司项目(中核地计[2008]74号)

摘　　要：芝瑞火山盆地位于大兴安岭南端内蒙古克什克腾旗境内,处于西拉木伦河-长春缝合带以南、赤峰-开源断裂带以北的辽源地块,盆地中的长英质火山岩系不整合于下二叠统大石寨组之上、新近系汉诺坝组玄武岩之下,并见有多处花岗斑岩侵入其中。SHRIMP锆石U-Pb年龄表明,盆地火山岩系中流纹岩锆石的^(206)Pb/^(238)U年龄为(156.9±1.7)Ma,属于晚侏罗世早期岩浆活动的产物。岩石地球化学资料表明,流纹岩具富SiO_2、K_2O和高FeO_T/MgO值,低Al_2O_3、CaO和MgO的特征,属于高钾钙碱性系列岩石,∑REE含量较高,轻重稀土分馏明显,具右倾的稀土分布模式,负Eu异常明显,具有Ga、Zr、Nb、Y含量高和Ba、Sr含量低的微量元素特征,相对富集Rb、Th、U、Pb、Zr、Hf,相对亏损Ba、Sr、P、Ti及Nb、Ta等元素,具有A型流纹岩和低Sr-Ba流纹岩的微量元素特征;在Nb-Y-3Ga和Rb/Nb-Y/Nb图解上显示拉张构造环境的A2型花岗岩的特征;具有较高的Sr初始比值((^(87)Sr/^(86)Sr)_i=0.706510~0.709821),较低的Nd初始比值(ε_(Nd)(t)=-6.32^-4.44)和相对较年轻的Nd模式年龄(1304~1457 Ma)及较低的Pb同位素组成((^(206)pb/^(204)pb)_t=17.15~17.80、(^(207)pb/^(204)pb)_t=15.41~15.48、(^(208)Pb/^(204)Pb)_t=37.38~37.63),在Sr-Nd同位素示踪图解和Pb同位素构造演化图解上流纹岩同时具有下地壳和富集地幔印记。元素、同位素地球化学示踪指示芝瑞流纹岩可能由起源于富集地幔的中元古代年轻下地壳的部分熔融形成,且在岩浆上升过程中经历了结晶分异作用。在(Yb+Ta)-Rb和(Y+Nb)-Rb图解上,芝瑞流纹岩均显示板内拉张构造环境,结合区域上分布的同时代火山岩及A型花岗岩类可以限定流纹岩形成于伸展构造背景,并可能与北部的蒙古-鄂霍茨克缝合带的演化有关。The Zhirui volcanic basin is located in Hexigten Banner, Inner Mongolia, at the southern tip of the Daxing＇an Range. Its tectonic position lies in the Liaoyuan Block between the Xar Moron - Changchun suture zone and the Chifeng-Kaiyuan suture zone. The felsic volcanic series in the Zhirui Basin occurs unconformablely above the underlying Dashizhai Formation, and below the overlying basalt of the Hannuoba Formation, and has been intruded by granitic porphyry. The SHRIMP zircon 206pb/238U age estimates for the rhyolite yield weighted an average age of （156.9±1.7） Ma, indicating an early Late Jurassic age. The whole-rock geochemistry of the rhyolites from the felsic volcanic series shows an A-type magmatic affinity, with a typical enrichment in SiO2, K2O, Ga, Zr, Nb, Y and a high ratio of FeOT/MgO but low contents of Al2O3, CaO and MgO, belonging to K-high calc-alkaline series rocks. Furthermore, they also have significantly higher contents of trace elements such as Ga, Zr, Nb, Y but low levels of Ba and Sr. Relative enrichment in Rb, Th, U, Pb, Zr, Hf, but depletion in Ba, Sr, P, Ti and Nb, Ta, and fractionated REE patterns show strong negative Eu anomalies. In the Nb-Y-3Ga and Rb/Nb-Y/Nb diagrams, all of the six samples show the characteristics of A2-type granites that demonstrate an extensionally tectonic setting. The rhyolites share such features as relatively high （87Sr/86Sr）i valeus （0.706510-0.709821）, low eNd（t） values （eNd（t）= -6.32- -4.44） and young Nd-model ages （1304-1457 Ma）. They exhibit low radiogenic Pb isotopic compositions, with （206pb/208Pb）t= 17.15 - 17.8 0, （207pb/204pb）t= 15.41-15.4 8 and （208pb/204pb）t=3 7.3 8-3 7.6 3. The projection points are locateed between the lower crust and the depleted mantle as shown in the Sr-Ndisotope-tracer diagram and located in the lower crust area close to the enriched mantle as shown in the Pb isotope-tracer diagram.These elemental and isotopic characters argue for parental magmas from partial melting of mid-Mesoproter

关 键 词：流纹岩 岩石成因论 地球化学 Sr—Nd—Pb同位素 晚侏罗世早期 芝瑞 

分 类 号：P597[天文地球—地球化学] P581[天文地球—地质学]