东秦岭宝丰盆地大营组火山岩SHRIMP定年及其意义  

SHRIMP zircon U-Pb dating for volcanic rocks of the Daying Formation from Baofeng basin in eastern Qinling,China and its implications.

作　　者：谢桂青[1] 毛景文[2] 李瑞玲[3] 叶会寿[2] 张毅星 万渝生[5] 李厚民[2] 高建京[1] 郑熔芬 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,北京100083 [2]中国地质科学院矿产资源研究所,北京100037 [3]中国地质调查局发展研究中心,北京100037 [4]河省省地质调查院,郑州450007 [5]中国地质科学院地质研究所,北京100037

出　　处：《岩石学报》2007年第10期2387-2396,共10页Acta Petrologica Sinica

基　　金：国家自然科学基金(40434011;40402011);中国地质大学地质过程与矿产资源国家重点实验室基金(GPM40504);中国科学院地球化学研究所开放室基金(200402);中国博博士后项目(20043620)联合资助。

摘　　要：宝丰盆地是东秦岭北部的重要火山岩盆地,主要发育早白垩世大营组火山岩,熔岩主体为玄武质粗面安山岩和粗面安山岩,为钾玄质火山岩,具有较高Na_2O+K_2O(8.04%～9.37%)、富集大离子亲石和轻稀土而亏损高场强元素特征,同位素(^(87)Sr/^(86)Sr)_i和(^(143)Nd/^(144)Nd)_i分别为0.7067～0.7079和0.5117～0.5118。本文在总结火山岩地球化学特征的基础上,应用SHRIMP锆石微区U-Pb测年技术对大营组玄武质粗面安山岩和粗面安山岩进行了精确测年,结果表明:玄武质粗面安山岩中锆石粒度较细且较少,阴极发光图像未显示明显环带结构,2个点SHRIMP谐和年龄为148.5～132.4Ma,可能代表残留锆石年龄。粗面安山岩中存在大量的锆石,大多数锆石的阴极发光图像显示明显的环带结构,暗示它们为岩浆成因的锆石,14个点SHRIMP谐和年龄加权平均值为117±2Ma(MSWD=1.3)。结合区域资料分析表明:东秦岭宝丰盆地火山岩形成于早白垩世(117±2Ma),与A型东沟花岗斑岩和太山庙花岗岩基、新发现的东沟和汤家坪斑岩钼矿的形成时代相当。秦岭造山带及邻区陆内造山深部过程存在俯冲组分,造山结束于早白垩世晚期,岩石圈伸展有助于下地壳拆沉作用。这些认识将为理解秦岭-大别造山带构造演化提供了重要信息。The Baofeng basin is one of the most important volcanic basins in the northern part of the East Qinling, where there mainly occur volcanic rocks of the Early Cretaceous Daying Formation. The lavas consist predominantly of trachyandesite and basaltic trachyandesite, which is geochemically equivalent to shonshointic rocks. They are characterized by higher Na2O + K2O (8.04% +/- 9.37%) and enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) and depletion in high field strength elements (HSFE). Initial Nd-Sr isotopic compositions are (Nd-143/Nd-144)(i) = 0.5117 similar to 0.5118 and (Sr-87/Sr-86)(i) = 0.7067 similar to 0.7079. On the basis of summarizing the geological and geochemical characteristics of volcanic rocks, precise dating was performed of the trachyandesite and basaltic trachyandesite of the Daying Formation by using the SHRIMP U-Pb zircon dating technique. The results indicate that: zircon grains in basaltic trachyandesite are fine in size and scarce; the cathodoluminescence images show no distinct zoning structure; the SHRIMP concordia ages at two analyzed spots of the zircon grains are 148.5 similar to 132.4 Ma, which probably represent the ages of relict zircons. However, there exist a large number of zircons in trachyandesite and the cathodoluminescence images of most zircons show distinct zoning structure, implying that they are magmatic zircons. The weighted average value of the SHRIMP concordia ages at 14 analyzed spots of the zircon grains is 117 +/- 2 Ma (MSWD = 1.3). These age data combined with the regional data indicate that volcanic rocks in the Baofeng basin in the East Qinling formed during the Early Cretaceous (117 +/- 2 Ma), synchronously with the formation of the A-type Donggou granite porphyry and Taishanmiao granite batholith, as well as the newly found Donggou and Tangjiaping porphyry Mo deposits. There occurred subducted components in the deep process of intracontinental orogeny in the East Qinling orogenic belt and its adjacent regions. The orogen

关 键 词：锆石SHRIMP U-PB 大营组 宝丰盆地 东秦岭 

分 类 号：P597.3[天文地球—地球化学] P588.144[天文地球—地质学]