铜陵桂花冲花岗闪长斑岩地球化学特征、锆石U-Pb年龄及其地质意义

GEOCHEMICAL FEATURES AND U-Pb AGE OF THE GUIHUACHONG GRANODIORITE PORPHYRY IN TONGLING AND THEIR GEOLOGICAL IMPLICATIONS

针对对铜陵桂花冲花岗闪长斑岩进行岩石学、岩石地球化学及锆石U-Pb同位素年代学研究,结果表明该花岗闪长斑岩高硅富碱(w(SiO2)平均为63.60%,w(Na2O+K2O)平均为7.09%,K2O/Na2O为0.51~0.68),属高钾钙碱性系列岩石。岩石稀土总量低(ΣREE在187.43×10-6和209.19×10-6之间),但轻稀土富集而重稀土亏损(ΣLREE/ΣHREE为15.00~16.49,(La/Yb)N为22.37~28.45),且具弱负Eu异常(δEu在0.75~0.83,平均为0.80)。同时,岩石富集大离子亲石元素(Rb,Ba,Th和U),而亏损高场强元素(Nb和Ta)。锆石LA-ICP-MS U-Pb定年测得花岗闪长斑岩年龄为(138.3±1.4)Ma,表明该岩体的形成时代为早白垩世,与铜陵矿集区中生代主要成矿岩体的年龄(137.5 Ma±1.1Ma^151.8Ma±2.6Ma)一致。该花岗闪长斑岩形成于碰撞后构造环境,是由壳幔混合岩浆冷却结晶形成的。

Petrologic and geochemical features and zircon U-Pb isotope age of the Guihuachong granodiorite porphyry in Tongling are studied.It shows that the rock is rich in w（SiO2）（averagely 63.60%）and alkali（averagely 7.09%）,with smaller ratios of w（K2O/Na2O）（0.51-0.68）,indicating that it belongs to high-K calc-alkaline series.It is characterized by low contents ofΣREE（187.43×10-6-209.19×10-6）,high ratios ofΣLREE/ΣHREE（15.00-16.49）and（La/Yb）Nvalues（22.37-28.45）,and a little negative Eu anomalies（δEu 0.75-0.83,averagely0.80）.The rock is enriched in large iron lithophile elements（Rb,Ba,Th,and U）and depleted in high field strength elements（Nb and Ta）.LA-ICP-MS zircon U-Pb age of the granodiorite porphyry is defined to be（138.3±1.4）Ma,suggesting that the rock was formed at the beginning of the Early Cretaceous Period,corresponding to the age（137.5Ma±1.1Ma-151.8Ma±2.6Ma）of main Mesozoic metallogenic plutons in Tongling.It is considered that this granodiorite porphyry was formed by cooling and crystallization of magma with mixing source of mantle and crust in the post-collision tectonic environment.