摘要
锆石U-Pb年龄显示华北克拉通中部带西缘老山杂岩体和狐偃山正长岩形成时间接近(128.7～125.4Ma),代表华北克拉通中部带早白垩世岩浆活动。系统的地球化学研究显示两岩体具有如下特征:(1)SiO_2含量中等(57.3%～69.8%),高K_2O(3.0%～8.2%)和Na_2O(4.3%～6.4%)含量,低MgO(0.2%～1.4%)、CaO、TiO_2和P_2O_5含量;(2)富集大离子亲石元素(Ba、Sr、K)和LREEs,亏损高场强元素(Nb、Ta、Zr、Hf)和HREEs。全岩地球化学特征(如高Ba、Sr含量和K/Rb比值,低Rb、Y含量)与高Ba-Sr花岗岩相符;(3)根据主量元素,相容元素(Cr、Co、Ni、V)和微量元素(Ba、Sr)特征,研究样品可分为2组:组1具有高Ba特征,Sr(^(87)Sr/^(86)Sr(t)=0.7049～0.7053)和Nd(ε_(Nd)(t)=-11.5～-9.7)同位素组成均一,经历了以黑云母为主的分离结晶作用;组2具有低Ba特征,Sr(^(87)Sr/^(86)Sr(t)=0.7051～0.7070)和Nd(ε_(Nd)(t)=-18.7～-14.8)同位素组成更加富集且分散,经历了角闪石,斜长石及磷灰石、磁铁矿和榍石等矿物分离结晶。2组样品不同的源区组分和分离结晶组合会显著改变岩浆中特征微量元素(Ba、Sr、Rb)含量和比值(Sr/Y、Nb/Ta、Dy/Yb)。本文研究表明华北克拉通中部带岩石圈地幔富集机制主要与元古代东、西陆块向中部带碰撞拼合时俯冲板块派生流体交代作用有关,该区中生代破坏机制为古太平洋板块俯冲所引起的碰撞后伸展背景下富集岩石圈地幔减压熔融、底侵至下地壳并引起"壳-幔"岩浆相互作用所形成的大规模深部岩浆抽取。
Zircon U-Pb ages show that both Laoshan complex and Huyanshan syenite in the western part of the central North China Craton formed around 128.7 ～125.4Ma,representing the Early Cretaceous magmatism in the Central North China Craton （CNCC）.Systemtic geochemsty data of this tow rocks show that （1） moderate SiO2 （57.3% ～ 69.8%）,high K2O （3.0% ～ 8.2%） and Na2O （4.3% ～6.4%）,low MgO （0.2% ～ 1.4%）,CaO,TiO2 and P2O5 contents;（2） enrichment of LILEs （Ba,Sr,K） and REE,depletion of HFSEs （Nb,Ta,Zr,Hf） and HREEs.Their whole rock geochemical characteristics,such as high Ba,Sr contents and K/ Rb ratios,low Rb and Y contents,are consistent with high Ba-Sr granites;（3） Samples in this paper can be divided into two groups according to their major elements,compatible elements （Cr,Co,Ni,V）,and trace elements （e.g.,Ba,Sr） features：Group 1 shows high Ba contents,uniform Sr （87 S1/86 Sr （t） =0.7049 ～ 0.7053） and Nd （εNd （t） =-11.5 ～-9.7） isotopic compositions,suggesting biotite fractionation;Group 2 shows relatively low Ba contents,enriched and scattered Sr （S7Sr/S6Sr（t） =0.7051 ～ 0.7070） and Nd （εNnd （t） =-18.7 ～-1 4.8） isotopic compositions,suggesting fractionation of hornblende,plagioclase,apatite,and Fe-Ti oxides/sphene.The different source components and fractionation assemblages in above tow groups lead to distinctly different contents of some key trace elements （Ba,Sr,Rb） and their ratios （Sr/Y,Nb/Ta,Dy/Yb）.This paper suggests that the enrichment mechanism of the SCLM beneath CNCC was mainly associated with the fluid metasomatism derived from the subducted plates when the east and west part of NCC collided and sutured alone the CNCC during Proterozoic,and the destruction mechanism of CNCC in Mesozoic was possibly linked to the decompression melting and underplating of SCLM as well as ＂crust-mantle＂ interaction in the postcollisional lithospheric extension setting after the subduction of paleo-Pacific plate under the
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2017年第12期3873-3896,共24页
Acta Petrologica Sinica
基金
公益性行业科研专项(201511022)
国家自然科学基金项目(41503023
41425010
41073047)
中国地质调查项目(12120115027101
121201102000150005)联合资助
