华北克拉通胶—辽—吉带古元古代条痕状花岗岩成因及其构造意义  

Petrogenesis of Paleoproterozoic Gneissic Granites from Jiao-Liao-Ji Belt of North China Craton and Their Tectonic Implications

作　　者：杨明春[1,2,3] 陈斌[4] 闫聪[5] 

机构地区：[1]北京大学地球与空间科学学院,北京100871 [2]北京大学造山带与地壳演化教育部重点实验室,北京100871 [3]海洋出版社,北京100081 [4]合肥工业大学资源与环境工程学院,安徽合肥230009 [5]中化石油勘探开发有限公司,北京100031

出　　处：《地球科学与环境学报》2015年第5期31-51,共21页Journal of Earth Sciences and Environment

基　　金：国家重点基础研究发展计划(“九七三”计划)项目(2012CB416603)

摘　　要：华北克拉通胶一辽一吉带内发育大量古元古代条痕状花岗岩，岩性主要为含角闪石或磁铁矿的二长花岗岩，以暗色矿物定向排列为显著识别特征。基于系统的野外调查、岩石学、地质年代学和地球化学研究，对条痕状花岗岩成因和其形成的构造环境提出了不同的模式。LA-ICp-MS锆石U-Pb年龄显示这些条痕状花岗岩形成于2173～2205Ma，基性岩脉侵位于2159Ma，后期的角闪岩相变质发生于约1907Ma。条痕状花岗岩中发育基性暗色微粒包体，也发育钙质角闪石和镁铁质角闪石，副矿物为锆石、榍石和磁铁矿。主量和微量元素分析结果显示：条痕状花岗岩属于钙碱性岩浆系列，A／CNK值为0．9～1．2（多数小于1．1），A／NK值为0．9～1．4，SiO2质量分数为68．3％～76．9％，岩石具有低TiO2质量分数（小于0．3％）；岩石富集大离子亲石元素（如K、Rb、Sr、Cs），亏损部分高场强元素（如Nb和Ti）。这些特征都指示条痕状花岗岩可能是形成于俯冲带环境下的I型花岗岩。部分岩体（如大房身岩体）具有类似A型花岗岩的特征，这是岩浆体系富硼而引起的高分异所致的，岩体高SiO2质量分数（76．7％～77．1％）也是高分异的表现。硼的加入会导致岩浆体系固相线温度降低，使得岩浆具有更为充分的时间进行演化。全岩εNd（t）值（一8．6～1．5）和锆石εHf（t）值（一1．3～5．6）都具有非常宽的变化范围，结合基性暗色微粒包体的岩相学特征以及斜长石的环带，说明条痕状花岗岩形成于下地壳熔融的酸性岩浆和富集地幔熔融的基性岩浆的混合。花岗岩的弧岩浆属性及其老于2500Ma的太古代继承锆石指示胶一辽一吉带在古元古代可能发生了北向俯冲。Voluminous Paleoproterozoic gneissic granites, which are mainly composed of hornblende or magnetite monzonitic granites, occur in Jiao-Liao-Ji Belt of North China Craton. The typical recognition feature is the orientational melanoeratic mineral. The plutons are intruded by diorite dykes, and undergo a late-stage amphibolite facies metamorphism. These gneissic granites are considered as A-type granites and formed in a continental rifting setting. Based on the integrated field, petrographic, geochronological, and geochemical studies on several gneissie granite plutons, the different models for the origin and tectonic setting of these gneissic granites were proposed. LA-ICP-MS zircon U-Pb ages reveal that these gneissic granites emplace at 2 173-2 205 Ma, and the marie dykes emplace at 2 159 Ma followed by an amphibolite facies metamorphism at 1 907 Ma. The gneissic granites contain basic marie microgranular enclaves, calcium and mafic hornblendes, and the accessory minerals are zircon, titanite and magnetite. The analysis results of major and trace elements show that the gneissic granites are calc-alkaline magma series, A/CNK values are 0.9-1.2 （mostly less than 1.1）, A/NK values are 0.9-1.4, mass fractions of SiOz are 68.3%-76.9%, and mass fractions of TiO2 are low （〈0.3%） ; large ion lithophile elements such as K, Rb, Sr and Cs are rich, and high field strength elements such as Nb and Ti are depleted. These features suggest that these gneissic granites are I-type granites formed probably in a subduction zone. Because of the high differentiation caused by boron-rich magmatic system, part plutons （such as Dafangshen） have characteristics of A-type-like granites, and the high mass fraction of SiO2 （76.7%- 77. 1%） is also shown by the high differentiation. The addition of boron causes the decease of solidus temperature, and tends to prolong magma evolution. The large variation of whole-rock εNd （t） values （--8. 6-1. 5） and zircon εHf （t） values （--1.3-5.6）, together with the existe

关 键 词：条痕状花岗岩 古元古代 岩石成因 电子探针分析 弧岩浆 俯冲带 华北克拉通 胶—辽—吉带 

分 类 号：P588.121[天文地球—岩石学]