冀北窟窿山流纹斑岩锆石SHRIMP U-Pb年龄、地球化学特征及地质意义  

Zircon SHRIMP U-Pb ages, geochemical characteristics of the Kulongshan rhyolitic porphyry in northern Hebei and their geological significances

作　　者：林天发 巫建华[2] 郭佳磊 牛子良 达朝元 周舰 LIN Tianfa;WU Jianhua;GUO Jialei;NIU Ziliang;DA Chaoyuan;ZHOU Jian(No. 243 Geological party, CNNC, Chifeng, Inner Mongolia, 024000;State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang, 330013)

机构地区：[1]核工业二四三大队,内蒙古赤峰024000 [2]东华理工大学核资源与环境国家重点实验室,南昌330013

出　　处：《地质论评》2019年第3期713-728,共16页Geological Review

基　　金：中国核工业地质局铀矿资源调查评价项目(编号:2018-01);国家自然科学基金项目(编号:41372071);中国核工业集团公司项目(编号:中核地计[2008]74)的成果~~

摘　　要：冀北窟窿山流纹斑岩位于赤峰—开原断裂以南、尚义—平泉断裂带以北的华北克拉通北缘隆起带和沽源—红山子铀成矿带的西南段,主要分布于窟窿山北西部,出露面积约25 km^2。本文通过SHRIMP锆石U-Pb定年和主量元素、微量元素、稀土元素、Sr—Nd—Pb—O同位素组成测试,查明流纹斑岩形成的地质时代,探讨流纹斑岩的物质来源和形成的构造背景,分析流纹斑岩与铀矿化的关系。分析结果显示:流纹斑岩SHRIMP锆石n(^(206)Pb)/n(^(238)U)年龄为138.4±1.3 Ma(n=12,MSWD=0.25),属早白垩世早期;流纹斑岩具有高硅、富钾、钠,高FeO^T/MgO值,贫Al、Mg、Ca、P的高分异特征,属准铝质—弱过铝质高钾钙碱性系列岩石,在SiO_2—[(Na_2O+K_2O)—CaO]和SiO_2—FeO^T/(FeO^T+MgO)图解中落于A型花岗岩区;∑REE含量较低,Eu负异常明显,具轻稀土富集型稀土配分曲线特征,富集Rb、Th、U、Nb、Ta、Zr、Hf、Y等元素,亏损Sr、Ba、P、Ti等元素,10000Ga/Al=3.74～4.29(>2.6),Zr+Nb+Ce+Y=374×10^(-6)～402×10^(-6)(>350×10^(-6)),显示A型花岗岩微量元素的特征。流纹斑岩的[n(^(87)Sr)/n(^(86)Sr)]_i值变化范围较大(=0.690385～0.724000),ε_(Nd)(t)值较低(=-16.2～-14.9)、T_(DM2)较大(=2163～2244 Ma),Pb同位素比值较低{[n(^(206)Pb)/n(^(204)Pb)]_i=16.893～16.922、[n(^(207)Pb)/n(^(204)Pb)]_i=15.393～15.424、[n(^(208)Pb)/n(^(204)Pb)]_i=37.716～37.771},在铅同位素图解中窟窿山流纹斑岩同时带有下地壳物质和富集地幔印记;δ^(18)O_(V-SMOW)=3.5‰～4.9‰,低于正常地幔的δ^(18)O_(V-SMOW)值(=5.3‰±0.3‰),具低氧流纹岩特征。Sr—Nd—Pb—O同位素特征显示,流纹斑岩可能是源于DMM型、EMI型地幔和少量古老下地壳混合形成的年轻下地壳部分熔融的产物,而且与高温热液蚀变有关。对比表明,窟窿山流纹斑岩与中国东部绝大多数与火山岩有关的热液型铀矿的赋矿围岩时代一致,特别是与沽源—红山子铀成矿带西南Kulongshan rhyolite porphyries lie in the northern margin uplift belt of North China Craton and Southwest section of the Guyuan-Honshanzi uranium belts, which between the Chifeng-Kaiyuan fault zone and Shangyi-Pinquan fault zone. Kulongshan rhyolite porphyries on located at the Kulongshan area, Fengning, northern Hebei, and it exposes 25 km2. SHRIMP zircon U-Pb dating indicate that the zircon of rhyolite porphyries n(206Pb)/n(238U) age was 138.4 Ma±1.3 Ma(N=12,MSWD=0.25), which belongs to the early stage of Early Cretaceous. Kulonghsan rhyolite porphyries have high SiO2, K2O, Na2O, high ratio of FeOT/MgO, poor Al2O3, MgO, CaO, P2O5. A/CNK=0.94~1.09, K2O/Na2O=1.03~1.30, which belong to the Peraluminous-weakly peraluminous high potassium calcium alkaline series rock. In the SiO2 vs(Na2O+K2O)-CaO and SiO2 vs FeOT/(FeOT+MgO) diagrams, they all fall into the A type granite area. The total amount of REE is 474×10^-6~531×10^-6,δEu=0.02~0.05, which show that strong negative Eu anomaly, they belong to light REE enrichment type in the standardized REE distribution curve of chondrite. Kulongshan rhyolite porphyries is Enrichment Rb, Th, U, Nb, Ta, Zr, Hf, Y and other elements, loss Sr, Ba, P, Ti and other elements, Z+Nb+Ce+Y=374×10^-6~ 402×10^-6(>350×10-6), 10000 Ga/Al=3.74~4.29(>2.6), which are characteristics of trace elements for A type granite. Kulongshan rhyolite porphyries have abnormal initial ratio of Sr,[n(87Sr)/n(86Sr)]i=0.690385~0.724000, relative low Nd(εNd(t)=-16.2^-14.9) and young Nd modle age(tDM2=2163~2244 Ma), they exhibit low radiogenic Pb istopic compositions,[n(206Pb)/n(204Pb)]i=16.893~16.922,[n(207Pb)/n(204Pb)]i =15.393~15.424,[n(208Pb)/n(204Pb)]i =37.716~37.771. In the Pb isotope model diagram, they fall into the lower crust region, which above the tropic of cancer. Meanwhile, they also fall in the EM I enriched mantle region.δ18OV-SMOW in rhyolite porphyries are 3.5‰~4.9‰, lower than the normal value of mantle(5.3‰±0.3‰), it may be related to water-rock interaction at high temperatur

关 键 词：流纹斑岩 Sr-Nd-Pb-O 同位素 锆石 SHRIMP U-PB 年龄 早白垩世早期 冀北窟窿山 

分 类 号：P588.141[天文地球—岩石学] P597.3[天文地球—地质学] P619.14