摘要
华北陆块南缘的崤山地区,呈北东向分布着龙卧沟、后河、白石崖等燕山期花岗岩体,岩石类型以二长花岗斑岩为主。主量元素、微量元素和Sr-Nd-Pb同位素地球化学研究表明,崤山地区燕山期花岗岩类总体成分呈酸性,岩石主要属于高钾-钾玄质系列;它们具有相似的微量元素(含稀土元素)组成模式,LREE、HREE分馏明显,(La/Yb)N=9.52～41.21,平均26.16,Eu异常不明显(δEu=0.82～1.35),大部分岩体呈现出微弱的负Eu异常,少部分样品显示出微弱的正异常;富集Rb、Ba、Th、K、Pb、Hf和Y等元素,亏损Ta、Nb、Zr、P和Ti等元素,岩体高Sr(392.8×10-6～916.9×10-6,平均678.8×10-6),低Y(8.12×10-6～21.34×10-6,平均14.86×10-6)和Yb(0.503×10-6～1.756×10-6,平均1.26×10-6),暗示源区有石榴石残留。岩体的锶同位素初始比值ISr=0.70645～0.71022,平均0.70828,εNd(t)=-19.7～-3.4,平均-14.6,二阶段Nd模式年龄t2DM集中在1827～2372Ma,具有高放射成因铅同位素组成特征,初始铅同位素比值为:206Pb/204Pb=17.228～18.720,平均17.905;207Pb/204Pb=15.444～15.656,平均15.544,208Pb/204Pb=37.519～38.707,平均38.187。元素和同位素地球化学特征表明崤山地区燕山期花岗岩类是华北陆块南缘加厚地壳底部熔融的产物,源岩以南秦岭结晶基底为主,同时有华北陆块的太华群、熊耳群及幔源物质的混入。岩体是扬子板块携秦岭微板块俯冲到华北板块之下,随着区域构造体制由挤压向伸展转换,岩石圈巨量减薄、软流圈上涌底侵上覆岩石圈,加热加厚地壳熔融形成,表明南秦岭板块的结晶基底在拆离式俯冲的作用下向北已楔入崤山地区结晶基底之下,而崤山北麓可能是其在东秦岭的北界。
Systematic and comprehensive petrological and geochemical study investigates the formation of eight northeastern Yanshanian granites in the southern margin of North China Block in Xiaoshan Mountain area,China. Petrologically,adamellite is domain. Geochemically,according to major elements,eight granites are silicate and belong to high K-shoshonite series; all granites have similar trace element patterns （including REE） ,obvious partition of LREE from HREE （La/Yb）N = 9. 52 ～ 41. 21 （avg. 26. 16）,and weakly abnormal Eu （δEu = 0. 82 ～ 1. 35）,and enrich in Rb,Ba,Th,K,Pb,Hf and Y,and deplete in Ta,Nb,Zr, P and Ti. High Sr （392. 8 × 10-6 ～ 916. 9 × 10-6,avg. 678. 8 ×10-6）,low Y （8. 12 ×10-6 ～ 21. 34 × 10-6,avg. 14. 86 ×10-6） and Yb （0. 503 × 10-6 ～ 1. 756 × 10-6,avg. 1. 26 ×10-6） characterize a garnet-bearing magma source in a thick lower crust under North China Block. Initial isotopic Sr ratio （ISr = 0. 70645 ～ 0. 71022,avg. 0. 70828） and initial epsilon Nd （εNd（t） = -19. 7 ～ -3. 4,avg. -14. 6） disclose a crustal magma source. The proterozoic Nd depleted mantle model ages （t2DM） of the granites are concentrated in 1827 ～ 2372Ma,and present high radiogenic Pb and initial Pb ratios （206Pb/204Pb = 17. 728 ～ 18. 720 （avg. 17. 905） ; 207Pb/204Pb = 15. 444 ～ 15. 656 （avg. 15. 544）; 208Pb/204Pb = 37. 519 ～ 38. 707 （avg. 38. 187）. All isotopic data suggest that the batholith was problebly formed by partial melting of the South Qinling crystalline basement with the participatioin of Taihua Group, Xiong’er Group and mantle materials. When extension environment substitute compression conditions caused by subduction of Yangtze Block with Qinling micro-block under North China Block,delamination of thick garnet-bearing crust and asthenosphere upwelling provide heat to melt the thick bottom crust,and forward,to form magma. This suggests that the crystalline basement under South Qinlin Orogen through detachment in north direction into the crystalline basement of Xiaoshan Mountain area,and the north of Xiaoshan Mountain area probably is the northern margin of East Qinling Orogen.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2013年第8期2635-2652,共18页
Acta Petrologica Sinica
基金
河南省国土资源厅2009年度地质矿产科技攻关项目(2010-61-6)资助
关键词
花岗岩类
化学组成
基底性质
构造属性
华北陆块南缘
Granites
Chemical composition
Basement features
Tectonic setting
Southern margin of the North China Block
