阴山—燕山北麓富碱侵入岩体的岩石地球化学特征、锆石SHRIMP U-Pb年龄及构造意义

Petrogeochemical characteristics and zircon SHRIMP U-Pb ages of the alkaline intrusions on northern side of the Yinshan Mountains—Yanshan Mountains and their tectonic significances

本文研究的达茂旗黄合少、察哈尔右后旗古城和河北尚义县八道沟岩体位于华北克拉通北缘阴山—燕山北麓,索伦缝合带以南,沿近东西方向展布。主要岩石类型为正长岩、二长岩和石英二长岩。岩石地球化学研究表明其SiO2含量59.08%~66.52%,Al2O3=15.34%~21.67%,A/CNK<1。属于准铝质中性岩范畴。全碱含量(Na2O+K2O)均在8%以上,在岩石的碱度率AR—SiO2图解上大多数样品落入碱性与过碱性区域,里特曼指数σ值从3.73~10.83均位于碱性或过碱性系列,属于碱性岩。岩体的微量元素以富集大离子亲石元素(LILE)如Rb、Ba和Sr等,亏损高场强元素(HFSE)如Ti、P、Nb和Ta等为特征。岩石稀土元素特征显示富集轻稀土元素(LREE),亏损重稀土元素(HREE)并且轻重稀土元素分馏比较明显的特征。除极个别样品外,岩石具有Eu负异常(δEu=0.35~0.91)。通过对察哈尔右后旗古城正长岩体和河北尚义县八道沟富碱侵入岩体的高精度测年研究,获得其SHRIMP锆石U-Pb年龄分别为413.2±3.3~415.0±5.9 Ma和366.8±5.2~372.3±4.7 Ma,表明其均为古生代泥盆纪富碱侵入岩浆活动的产物。结合前人关于阴山—燕山北麓其它富碱侵入岩体(内蒙古固阳东高家村碱性正长岩体、黄合少碱性正长岩和冀北水泉沟碱性杂岩体)的研究成果,推断在华北克拉通北缘阴山—燕山北麓一带,可能存在一条东西方向展布的晚古生代泥盆纪富碱侵入岩带。根据该岩带岩石地球化学特征,以及Ta/Yb—Th/Yb与Zr/Nb—Zr/Nb—Y/Nb判别图推测其岩浆物质来源可能与富集地幔有关,按照成岩温压环境指示,其形成深度位置约为下地壳,岩浆上升侵位过程中明显受到地壳物质的混染。依据前人关于碱性岩特征的研究,结合本文研究岩体的岩石化学特征和及Rb—Nb+Y构造判别图解指示,推测岩体形成于张性构造环境,可能与古生代志留纪末兴蒙造山带南部发育的白乃庙岛弧带和华北克拉通发生弧—陆碰撞后的伸展背景相关。

Objectives:For the important geological significance of alkali-rich intrusions,we studied the intrusions in north side of Yinshan Mountains and Yanshan Mountains on the petrogeochemistry and goechronology.Futhermore,we discussed petrogenesis and the magmatism evolution of these alkali-rich intrusions briefly.Methods:Our study is based on sample collection and field work,observe through a microscope and the data analysis of major elements,trace elements,and SHRIMP zircon U-Pb dating to get the high-precision ages of alkali-rich intrusions on the northern side of Yinshan Mountains and Yanshan Mountains.Results:The experimental data show that the SiO2 content of these whole intrusions account for 59.08%~66.52%,Al2O3= 15.34%~ 21.67%,A/CNK<1,signifying that these alkali-rich intrusions belong to intermediate rock with quasi-aluminous.Geochemical characteristics of trace elements shows that the rocks enriches lithophile elements(LILE),such as Rb,Ba and Sr;on the contrast,it’s relatively poor in high field strength elements(HFSE),such as Ti,P,Nb and Ta.The rare earth elements geochemistry shows distact fractionation of LREE and HREE,reveals a deficit of heavy rare earth elements and enriched the light one;with strong negative Eu anomaly(δEu = 0.35~0.91).SHRIMP zircon U-Pb ages of Gucheng pyroxene syenite(G1-2),hornblende syenite(G5-1),hornblende syenite(G6-1) are 414.6±5.0 Ma,415.0±5.9 Ma and 413.2±3.3 Ma respectively;SHRIMP zircon U-Pb ages of Badaogou masanophyre and ivernite are 372.3±4.7 Ma and 366.8±5.2 Ma respectively.Conclusions:These alkali-rich intrusions we studied are all distributed in the northern margin of the North China Craton,according to the same characteristics of geochronology and geochemistry,there is an alkali-rich intrusions belt distributed in the East-West direction in the northern foot of Yinshan Mountains and Yanshan Mountains.From west to east,it is composed of five intrusions.The geochemical characteristics analyses indicates the magmas came from the mantle depth,relating to enriched mantle.And the magmas may be contaminated by crustal materials in the process of its ascending emplacement.It is speculated that the corresponding diagenetic temperature and the pressure depths of Huangheshao,Gucheng and Badaogou intrusions are roughly from the bottom to the middle of the thickend continental crust,and the formation depths of the Gaojiacun and Shuiquangou intrusions are deeper.The petrogeochemical discrimination based on the tectonic environment indicates that the Paleozoic alkali-rich intrusions belt in north side of Yinshan Mountains and Yanshan Mountains was mainly formed in the extensional tectonic environment and was the product of collision-postorogenic magmatism.Combined with previous studies on the Late Paleozoic tectonic and magmatism in the study area,it is concluded that the tectonic environment formed by the Paleozoic alkali-rich intrusions in Yinshan Mountains and the northern foot of Yanshan Mountains may be closely related to the extensional tectonic background after the arc-continent collision between the northern margin of the North China Craton and the Bainaimiao island arc on its northern side in the Late Paleozoic from Late Silurian to Early Devonian,it proves conclusion of tectonic environment discrimination further.