政和-大埔断裂带内变基性岩成因及构造属性研究

摘要

The NE-SW trending Zhenghe-Dapu fault zone is an important fault belt in the Cathaysia block,within which metabasite rocks are extensively exposed.The study focused on zircon U-Pb geochronology and geochemical analysis of the metabasite rocks along the Zhenghe-Dapu fault zone from Zhenghe County to Jianou County,to clarify the petrogenesis and the tectonic attributes of the metabasite rocks,and to explore tectonic evolution of Cathaysia block during the late Neoproterozoic period.Zircon U-Pb dating yields 807±4 Ma and 812±7 Ma for protolith of hornblende plagioclase schists and rhyolite of the bimodal volcanic rock eruption age,respectively,suggesting that the protolith of plagioclase amphibolite schists Zhenghe County should have formed at 725±22 Ma.Geochemical analysis shows that the amphibolite schists of Dikou Town are significant enrichment in LILEs and LREEs and depletion in HFSEs,with a slight positive Eu anomaly (δEu =0.98 ～ 1.09),low initial 87 Sr/86 Sr ratios ranging from 0.685523 ～0.707149,and negative εNd(t) values of-1.2～-1.0.The tectonic discrimination diagram suggests that the amphibolite schists belong to intraplate basalt.The plagioclase amphibolite schists from Zhenghe County exhibit flat REE patterns,with LREE/HREE=1.77～2.08,(La)N/(Yb)N =0.99～1.30,(La)N/(Sm)N=0.88～1.17 and slight positive Eu anomalies (δEu=1.05～1.14),sharing the characteristic of NMORB.The plagioclase amphibolite schists have low initial 87Sr/86 Sr ratios ranging from 0.698279 ～0.701964 and εNd(t) values of +3.8 to +4.3,indicating the magma originated from relatively depleted mantle source.Both protolith forming age of the amphibolite schists from Dikou Town and the bimodal volcanic rock of Shuiji Town are roughly the same.The dated age of ca.810 Ma bimodal volcanic rock constrain the initial time of the rifting during the late Neoproterozoic of Cathaysia block.The protolith of the plagioclase amphibolite schists may be derived from the asthenospheric mantle upwelling,indicating that Neoproterozoic rifting may continue to ca.725 Ma.%NE-SW向政和-大埔断裂带是华夏板块内部一条重要的断裂带,该带内出露大量变基性岩,本研究主要针对该断裂带内政和-建瓯段变基性岩开展锆石U-Pb年代学及地球化学分析,意在厘清变基性岩成因及构造属性,并以此来探讨华夏板块新元古代晚期构造演化.锆石U-Pb定年显示,水吉镇双峰式火山岩角闪斜长片岩原岩和流纹岩喷发年龄分别为807±4 Ma和812±7 Ma,政和斜长角闪片岩原岩形成时代为725±22 Ma.地球化学特征显示迪口镇角闪片岩富集LILE和LREE,亏损HFSE,并且具有Eu略正异常特点(δEu=0.98～1.09)和低87Sr/86 Sr初始比值(ISr=0.685523～0.707149)以及负εNd (t)值(εNd(t)=-1.2～-1.0)特征,构造判别图解显示角闪片岩为板内玄武岩.政和斜长角闪片岩稀土元素蛛网图为“平滑型”,LREE/HREE为1.77～2.08,(La/Yb)N=0.99～1.30,(La/Sm)N =0.88～1.17,Eu略有异常(δEu=1.05～1.14),微量元素分配图及构造判别图解显示其具有N-MORB特征,低87Sr/86 Sr初始比值(ISr=0.698279～0.701964)和正εNd (t)值(εNd(t)=+3.8～+4.3)表明岩浆来自相对亏损地幔源区.角闪片岩原岩与双峰式火山岩形成时代大致相同,表明华夏板块新元古代裂谷作用开始于约810 Ma.而政和斜长角闪片岩原岩可能来源于软流圈地幔的上涌,表明新元古代裂谷作用可能一直持续到约725 Ma.