福州复式岩体的成因:锆石U-Pb年代学、地球化学及Hf同位素约束

Petrogenesis of Fuzhou composite pluton:Constraint from zircon U-Pb geochronology,geochemistry,and Hf isotopes

福州复式岩体I-A型复合花岗岩的岩石成因与源区性质目前尚未得到很好认识。本文对福州复式岩体花岗岩进行了岩石学、锆石U-Pb年代学、主量和微量元素地球化学及锆石Hf同位素研究。该复式岩体花岗岩可分为钙碱性和碱性系列。LA-ICP-MS锆石U-Pb定年结果表明,钙碱性系列花岗岩形成年龄为111~101Ma,是早白垩世多期次岩浆活动作用的产物;碱性系列花岗岩形成的年龄为95~93Ma,是晚白垩世岩浆活动的产物。两类岩石均具有Eu负异常、LREE富集和HREE亏损的特征,并且Rb、Th、U、K、Pb等大离子亲石元素相对富集,Ba、Sr以及高场强元素Nb、Ta、P、Ti相对亏损。其中,钙碱性系列花岗岩的轻重稀土分馏程度较之碱性系列花岗岩明显,而碱性系列花岗岩的负铕异常比钙碱性系列花岗岩明显。锆石Hf同位素组成表明,钙碱性系列花岗岩的ε_(Hf)(t)值为-3.9~0.2,地壳模式年龄表明花岗质岩浆源岩来自于新元古代古老地壳的部分熔融,并有少量的地幔组分卷入。碱性系列花岗岩的ε_(Hf)(t)值为0.04~4.8,地壳模式年龄指示花岗质岩浆源岩来自于新元古代古老地壳的部分熔融,并有大量幔源组分的混入。综合分析表明,福州复式岩体I-A型复合花岗岩具有相同的源区,其形成的差异主要是构造环境的变迁、幔源岩浆的加入以及岩浆分异演化等多种因素综合作用的结果。

The petrogenesis and origin of I-A type double granite in Fuzhou composite pluton have not been well known.A systematic study of zircon U-Pb chronology and Hf isotope,and whole rock geochemical were carried out on the composite pluton in Fuzhou.The pluton is divided into two categories:calc-alkaline and alkaline.LA-ICP-MS zircon U-Pb dating results show that the ages of calc-alkaline granites are 111~101 Ma,these plutons are the products of magma activity in the Early Cretaceous;the age of alkaline granites are 95~93 Ma,which are the products of the Late Cretaceous magmatic activity.All samples in the study have Eu negative anomaly,characterized by LREE enrichment and HREE loss,and they are all relatively enriched in Rb,Th,U,K,Pb and other LILE,depleted in Ba,Sr and high field strength elements Nb,Ta,P,Ti.Light-heavy REE fractionation of calc-alkaline granites is more obvious than alkaline granites.However,compared with calc-alkaline granites,the negative Eu anomaly of alkaline granites is more obvious.The results of zircon Hf isotope show that the ε_(Hf)(t)of calc-alkaline granites values from negative to positive,-3.9~0.2,and the crustal model ages indicate that the source rocks of granitic magma derived from the remelting of Neoproterozoic ancient crust,with a small amount of mantle components.The ε_(Hf)(t)of calc-alkaline granites value is 0.04~4.8,the crust model ages suggest granitic magma derived from the remelting of Neoproterozoic ancient crust and mixed with a large number of mantle components.Comprehensive analysis shows that the I-A double granite of Fuzhou composite pluton has the same source,but the difference of the formation is the results of comprehensive shaping of many factors such as the changes of tectonic environment,the addition of mantlederived magma and magmatic evolution.