通化地区中生代大川花岗岩岩体成因与REE赋存状态——来自岩石地球化学、锆石U-Pb定年和Hf同位素制约

Petrogenesis and REE Occurrence State of Mesozoic Dachuan Alkaline Granite in Tonghua Area:Constraints from Geochemistry,Zircon U-Pb Dating and Hf Isotope

吉林南部地区位于华北克拉通北缘东段,是一个经历了太古宙、元古宙、古生代、中生代以及新生代漫长地质历史演化的地质构造区,广泛发育晚中生代富集稀土元素的碱长花岗岩。本文选取该区富集稀土元素的大川花岗岩体,开展岩相学、锆石U-Pb年代学与Hf同位素、岩石地球化学研究,结果表明:该岩体主要由细粒、中粒和粗粒黑云母碱长花岗岩所组成,其次是花岗伟晶岩;主量元素具有高硅(w(SiO_(2))=72.56%~84.09%)、低铝(w(Al_(2)O_(3))=8.15%~14.76%)、准铝—强过铝质(A/CNK=0.98~1.60)的特征,属于高钾钙碱性系列;样品稀土质量分数较高(w(ΣREE)=88.53×10^(-6)~839.95×10^(-6)),(La/Yb)_(N)=2.30~41.24,富集轻稀土元素,亏损重稀土元素,稀土配分模式为右倾型,Eu负异常比较明显,δEu值为0.05~0.60,分异指数(ID=86.72~97.69)较高,固结指数(IS=0.16~1.13)较低,显示在岩浆演化过程中经历了斜长石、黑云母、磷灰石等矿物不同程度的分离结晶作用,结合岩相学特征,初步认为大川碱长花岗岩为高分异花岗岩;粗粒黑云母碱长花岗岩单颗粒锆石U-Pb同位素年龄为(120.8±1.6)Ma;锆石ε_(Hf)(t)值全部为负值(-10.7~-5.2),对应的Hf同位素二阶段模式年龄为2 587~2 090 Ma;基于同位素与岩石地球化学数据,初始岩浆起源于新太古代—古元古代陆壳物质的部分熔融,岩体为同源岩浆房经岩浆结晶分异作用后岩浆上侵就位的产物。岩矿鉴定及全岩微量显示,随着岩浆的演化,石英含量逐渐升高,黑云母含量逐渐降低,稀土总量也逐渐降低,稀土主要富集在粗粒黑云母碱长花岗岩相中。

The southern part of Jilin is located in the eastern section of the northern margin of the North China craton,which has experienced a long geological evolution in the Archean,Proterozoic,Paleozoic,Mesozoic and Cenozoic.The alkali-feldspar granite enriched in rare earth elements in the Late Mesozoic is widely developed in this area.In this paper,the Dachuan granite enriched in rare earth elements was selected to carry out petrography,zircon U-Pb dating and Hf isotope,petrogeochemistry analysis.The results show that the granite is mainly composed of fine-grained,medium-grained and coarse-grained biotite alkali-feldspar granite,followed by granite-pegmatite.The main elements have the characteristics of high silicon(w(SiO_(2))=72.56%-84.09%),low aluminum(w(Al_(2)O_(3))=8.15%-14.76%),quasi aluminum-strong peraluminum(A/CNK=0.98-1.60),belonging to the high potassium calc-alkaline series.The samples have a high rare earth mass fraction(w(ΣREE)=88.53×10^(-6)-839.95×10^(-6)),(La/Yb)_(N)=2.30-41.24,enriched in light rare earth elements,depleted in heavy rare earth elements.The partition pattern of rare earth elements is right-leaning type,with obvious negative Eu anomaly andδEu value ranging from 0.05 to 0.60.The differentiation index(ID=86.72-97.69)is high,and the consolidation index(IS=0.16-1.13)is low,indicating that the magma evolution has experienced different degrees of separation and crystallization of plagioclase,biotite,apatite and other minerals.Combined with the petrographic characteristics,the Dachchuan alkali-feldspar granite is preliminarily considered as highly differentiated granite.The single zircon U-Pb isotopic age of coarse-grained biotite alkali feldspar granite is(120.8±1.6)Ma.Theε_(Hf)(t)values of zircons are all negative(-10.7--5.2),and the corresponding Hf isotope second-order model ages of the zircons are 2587-2090 Ma.Based on isotope and petrogeochemical data,the initial magma originated from partial melting of the Neoarchean and Paleoproterozoic continental crust,and the rock mass was the product of magma emplacement after magma crystallization differentiation in the homologous magma chamber.Mineral identification and whole rock trace elements show that with the evolution of magma,the quartz content gradually increased,the biotite content gradually decreased,and the total rare earth contents also gradually decreased.The rare earth elements are mainly enriched in coarse-grained biotite alkali feldspar granite phase.