赣中南城县辉绿岩锆石LA-ICPMS年代学及岩石地球化学特征

燕山期是华南地区最主要的成矿阶段。近年,越来越多的学者认为该时期形成的众多金属矿床与基性岩脉关系密切。赣中南城地区Au-U等多金属矿化异常明显,区域范围内辉绿岩脉发育。本文针对地表辉绿岩样品进行岩石地球化学和年代学等方面研究。分析结果表明,研究区内岩石主量元素表现相对富铝(w(Al2O3)=12.07%~13.71%),富镁和富钙(w(MgO)=8.77%~10.86%;w(CaO)=10.77%~12.21%),低钛和低碱(w(TiO2)=1.18%~1.58%;w(Na2O+K2O)=3.09%~4.01%),Mg^#=8.91~55.78,w(FeO^T)=14.41%~16.87%,表明研究区内辉绿岩的基性程度较高,初始岩浆演化程度不高,早期没有发生明显的分离结晶作用,岩浆来源具有壳幔混合特征。样品整体富集Rb,Ba,Th,La,Sm等元素,亏损K,Hf,Ti,Yb,Lu等元素。REE配分曲线呈平缓的右倾轻稀土元素富集型,样品(La/Yb)N值在41.25~71.73之间且Ce,Eu无异常。其成岩年龄为158.0 Ma±3.4 Ma,形成于晚侏罗世,辉绿岩的成岩时间与该时期赣中南古陆大部分辉绿岩形成大地构造背景差异性较大,说明赣中南古陆在该时期大地构造背景复杂,推测当时研究区辉绿岩形成于华南和华北地块碰撞引起的多阶段岩石圈伸展拉张的构造环境。

ZIRCON LA-IC-PMS CHRONOLOGY AND PETROGEOCHEMICAL CHARACTERISTICS OF DIABASE IN NANCHENG AREA,CENTRAL JIANGXI PROVINCE

Polymetallic mineralization abnormalies of Au-U in the Nancheng area of central Jiangxi Province are obvious. Geochemical and chronological study for the well developed diabase dikes in the area is carried out so as to understand the important metallogenic stages of Yanshanian.It shows that the main elements of the diabase in the study area are relatively rich in aluminum(w(Al2O3)=12.07%~13.71%),magnesium(w(MgO)=8.77%~10.86% and calcium w(CaO)=10.77%~12.21%),low in titanium(w(TiO2)=1.18%~1.58% and alkali w(Na2O+K2O)=3.09%~4.01%),with the Mg^#=8.91~55.78 and w(FeO^T)=14.41%~16.87%,characteristic of high basic extent of the diabase,low evolution extent of magma,unclear fractional crystallization in the early stage and crust-mantle mixing magma.The diabase samples are enriched in Rb,Ba,Th,La and Sm,and depleted in K,Hf,Ti,Yb and Lu with gentle and right-incline REE distribution pattern.(La/Yb)N value of the diabase sample is in the range of 41.25 to 71.73 and there is no abnormality for elements Ce and Eu.Zircon LA-IC-PMS age dating for the diabase yields a emplacement age of 158.0 Ma±3.4 Ma,a late Jurassic period magmatic product,which is quite different from the geotectonic setting of diabase in most of the ancient continents of central-south Jiangxi paleo-continent during this period.It is considered that the tectonic setting of the central-south Jiangxi paleo-continent in this period was complicated and the diabase in the study area was formed in the tectonic setting of multi-stage lithosphere extension caused by the collision between the South China block and North China block.