冀北北岔沟门地区中生代侵入岩地质年代学和地球化学特征研究  

Studies on the geochronology and geochemical characteristics of Mesozoic intrusions in Beichagoumen area, northern Hebei province.

作　　者：毛德宝[1] 陈志宏[1] 钟长汀[1] 左义成[1] 石森[2] 胡小蝶[1] 

机构地区：[1]天津地质矿产研究所,天津300170 [2]中国地质调查局,北京100035

出　　处：《岩石学报》2003年第4期661-674,共14页Acta Petrologica Sinica

基　　金：国土资源部"九五"资源与环境科技攻关项目(950200302)资助

摘　　要：北岔沟门地区广泛分布中生代侵入岩体。本文选取了该地区有代表性的8个岩体进行地质年代学和地球化学研究,旨在建立其年代格架并探讨岩石的成因演化。主要岩石类型为闪长玢岩、花岗闪长岩、石英二长岩、二长花岗岩、钾长花岗岩等。颗粒级锆石的U-Pb同位素测年数据表明所研究的岩体主要有三期:早三叠世(245～250Ma)、晚侏罗世(140～147Ma)和早白垩世(125～137Ma)。地球化学数据表明岩石普遍具有较高的钾含量、准铝质,主要属高钾钙碱性系列,部分为橄榄玄粗岩系列。岩石不同程度地显示轻稀土元素富集、高场强元素(HFSE)和大离子亲石性元素(LILE)解偶等特点。进一步的岩石地球化学研究表明区内侵入岩可划分为高BaSr和低BaSr两种类型,其中高BaSr型具有高Al_2O_3(≥15％)、Sr(≥400 ×10^(-6)和低Y(≤18×10^(-6))、Yb(≤1.9×10^(-6)),高的Na_2O/K_2O(>1),Sr/Y(>20)、La/Yb(≥10)比值;较高的Mg~#(38.47～57.78)、具正的或弱的铕负异常等地球化学特征,这些特征与埃达克岩具有某些相似性;而低BaSr型岩石SiO_2、K_2O、Y、Yb含量相对较高、Al_2O_3、Sr、Ba含量相对较低、具明显的铕负异常等地球化学特征。尽管两类岩石在时空上密切共生,但可能具有不同的源区和成因机制。综合研究表明本区中生代侵入岩总体形成在?Abundant Mesozoic intrusions occur in the Beichagoumen region, northern Hebei Province, north China. Eight composite intrusions were chosen for geochemical and isotopic study in order to determine their emplacement ages and petrogenesis. Petrographically, they range from diorite porphyry, granodiorite, quartz monzonite, monzogranite to alkali-feldspar granite. U-Pb single zircon analyses indicate that the intrusions were emplaced in three periods: the Early Triassic (245similar to250Ma), the Late Jurassic (140similar to147 Ma) and the Early Cretaceous (125similar to137 Ma). Geochemical data suggest. that the rocks are potassium-rich, metaluminous, and are predominantly high-K calc-alkaline and subordinately shoshonitic. They invariably show enriched light rare earth elements (LREE), decoupled high field strength elements (HFSE) and large ion lithophile elements (LILE). Advanced study of geochemistry shows that the rocks can be divided into high Ba-Sr type and low Ba-Sr type. The high Ba-Sr type is characteristic of high Al2O3 (greater than or equal to15%), Sr(greater than or equal to400x10(-6)) and low Y(less than or equal to18x10(-6)), Yb(less than or equal to1.9x10(-6)) aboundances, high Na2O/K2O (>1), Sr/Y (greater than or equal to20) and La/Yb (greater than or equal to10) ratios, relatively high Mg-# (38.47similar to57.78), as well as positive or weakly negative Eu anomalies. All these characteristics are similar to adakite. Compared with the high Ba-Sr type, the low Ba-Sr type demonstrates relatively high SiO2, K2O, Y, Yb, and low Al2O3, Sr, Ba, as well as marked negative Eu anomalies. Although the two types of intrusions are associated with each other in space and time, they probably have different source rocks or diverse magma-generating mechanisms. Based on comprehensive research, we suggest that the Mesozoic intrusions in the region were formed under the condition of thickened lower crust and related to underplating or mantle-crust interactions.

关 键 词：中生代侵入岩 冀北 岩石学 地球化学 

分 类 号：P588.12[天文地球—岩石学]