安徽铜陵焦冲辉石闪长岩体中的两类锆石及其地质意义

Two Types of Zircons from the Jiaochong Pyroxene Diorite Pluton in the Tongling Area of Anhui Province and Their Geological Significance

安徽铜陵焦冲辉石闪长岩体中的锆石可以明显地区分为两类:Ⅰ类锆石CL图像颜色为灰白色,发育岩浆震荡环带,U、Th、Pb含量和REE含量相对较低,加权平均年龄为128.5±2.1Ma(MSWD=0.29),代表了该岩体在锆石封闭温度下的岩浆结晶年龄,这一年龄数据以及近年来发表的岩浆岩年龄测定数据指示铜陵地区不仅发育有145~135Ma之间的岩浆侵入作用,而且发育有以往不曾提及的、与宁芜和庐枞等火山岩盆地火山-侵入岩同时代的、同位素年龄介于134~123Ma之间的岩浆作用。Ⅱ类锆石CL图像颜色为灰黑色,岩浆震荡环带不明显,U、Th、Pb含量和REE含量高,显示为受到热液作用影响的"高U锆石",加权平均年龄为141.4±2.0Ma(MSWD=1.06),但该年龄没有确定的地质意义。同一岩体中发育两类不同特征的锆石,并且其同位素年龄值相差悬殊,如果不加区分地进行加权平均年龄计算,可能得出不正确的年龄数值以致掩盖或混淆地质事实。焦冲辉石闪长岩体中的两类锆石同时还具有另一个不同于铜陵地区其他岩体中锆石的显著特征,即普遍呈不完整的、熔蚀港湾状的晶形,这指示该岩体可能经历了锆石从岩浆中晶出到熔蚀并最终保持熔蚀晶形的岩浆演化过程。焦冲辉石闪长岩体中两类锆石的特征及其U-Pb年龄既记录了成岩过程早期中酸性岩浆的冷凝结晶作用和流体交代作用,又反映了晚期基性岩浆的注入和混合作用,揭示其寄主岩石的形成经历了一个较为复杂的过程,即岩浆结晶→流体交代→岩浆结晶→岩浆混合→岩浆侵位。

The zircon grains from the Jiaochong pyroxene diorite pluton in the Tongling area of Anhui Province can be categorized into two types based on the CL images. Type I zircon grains are offwhite, show clear oscillatory zoning, and have relatively low concentrations of U, Th, Pb and REE. All these characteristics indicate their magmatic origin. A weighted average 206Pb/238U age of 128.5±2.1 Ma （MSWD=0.29） yielded in this study may represent the crystallization age of the pluton in a close environment. Combined with the previous published age data, our result indicates there are two periods of magmatic activity. The age data along with the published age of magmatic rock indicate that the Tongling area not only contains the 145±135 Ma magmatic emplacement activity, but also the 134±123 Ma magma activity, which has not been acknowledged but occurred at the same time as volcanic-intrusive rocks in volcanic basins such as the Ningwu basin and Luzong basin. Type II zircon grains are greyish black and show unclear oscillatory zoning, and have extremely high concentrations of U, Th, Pb and REE. The type II zircon grains are called ＂high U zircon＂, and might be influenced by hydrothermal process. The weighted average 206Pb/238U age of 141.4±2.0 Ma （MSWD=1. 06） obtained in this study has no definite geological significance. The two types of zircon grains with two groups of ages occuring in the same pluton verify that if we do not distinguish zircon grains when calculating the weighted average U-Pb age, the results obtained may not be correct or confuse the geological facts. Another significant characteristic of zircon grains from the Jiaochong pyroxene diorite pluton is that the crystal forms of zircon grains are poorly preserved and fragmented, with embayed texture. This indicates that the formation of Jiaochong pluton may have experienced zircon crystallization, corrosion, and the preservation of corrosion morphology. The characteristics of the two types of zircon grains and the U-Pb age from the Jiaochong pyroxene diorite not only record the earlier crystallization and hydrothermal metasomatism of the intermediate-acidic magma, but also reflect the later mafic magma injection and mixing during the diagenetic stage. This reveals the formation of the host rocks has experienced a complicated process： magmatic crystallization hydrothermal metasomatism→magmatic crystallization →magmatic mixing →magmatic emplacement.