关键词: 通化地区, 元素地球化学, U-Pb同位素定年, Hf同位素, 岩石成因, REE赋存状态

Abstract:  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 (SiO2) =72.56%-84.09%), low aluminum (w (Al2O3) =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 and δEu 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. The εHf(t) values of zircons are all negative (-10.7--5.2), and the corresponding Hf isotope second-order model ages of the zircons are 2 587-2 090 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.

Key words: Tonghua area, element geochemistry, U-Pb isotopic dating, Hf isotope, petrogenesis, REE occurrence state