大兴安岭北段小柯勒河花岗斑岩脉成因及地质意义:锆石U-Pb年龄、岩石地球化学及Hf同位素制约

Petrogenesis and Geological Significance of Granite Porphyry Dike from Xiaokelehe in North Da Hinggan Mountains: Constraints from Zircon U-Pb Age, Geochemistry and Hf Isotopic Composition

为了探讨大兴安岭北段小柯勒河花岗斑岩脉的岩石成因、构造背景及地质意义,本文对其进行了锆石U-Pb年代学、岩石地球化学与Hf同位素的研究。岩石地球化学结果显示:花岗斑岩富硅(w(SiO;)为69.85%~70.48%)、富碱(w(Na;O+K;O)为9.74%~9.89%),贫镁(w(MgO)为0.34%~0.40%)、贫钙(w(CaO)为1.04%~1.20%),A/CNK为0.98~1.02,属于准铝质-弱过铝质系列岩石;具有轻稀土元素富集和重稀土元素亏损的特征,中等Eu负异常,富集大离子亲石元素Rb、K和高场强元素Th、U、Hf、Zr,亏损大离子亲石元素Sr、Ba和高场强元素Nb、Ta、Ti、P。花岗斑岩中的10^(4)Ga/Al值为2.32~3.68,分异指数I;为87.63~89.01。综合分析认为小柯勒河花岗斑岩属于分异的Ⅰ型花岗岩。花岗斑岩锆石U-Pb加权平均年龄为(124.0±0.6)Ma,属于早白垩世晚期。锆石ε;(t)值为0.5~3.3,二阶段Hf同位素模式年龄T;为1 150~970 Ma。结合岩石地球化学特征分析认为花岗斑岩源岩为由亏损地幔衍生的中—新元古代新增生地壳部分熔融的产物,并有少量古老地壳物质的加入。该花岗斑岩形成于早白垩世晚期蒙古—鄂霍茨克洋闭合背景下的伸展环境,此时太平洋板块持续向欧亚大陆俯冲,但对本区影响有限。

In this paper, the granite porphyry geochemistry, zircon U-Pb geochronology and Hf isotopes are studied to discuss the petrogenesis, tectonic setting of the granitic porphyry dike, and the relationship between the rock vein and mineralization of Xiaokelehe. Our data show that the samples are characterized by high content of SiO;(69.85%-70.48%) and alkali(9.74%-9.89%), and low content of MgO(0.34%-0.40%) and CaO(1.04%-1.20%). The A/CNK values range from 0.98 to 1.02, the granitic porphyry is quasi-aluminum-weak per-aluminum. The rock is rich in LREE and poor in HREE, with obviously negative Eu anomalies. They are rich in LILEs(Rb and K) and HFSEs(Th, U, Hf, and Zr), and depleted of LILEs(Srand Ba) and HFSEs(Nb, Ta, Ti, and P). The samples have 10^(4)Ga/Al values ranging from 2.32 to 3.68 and I;values ranging from 87.63 to 89.01. All these characteristics indicate that the granite porphyry is fractionated Ⅰ-type granite. The zircon U-Pb dating indicates that the rock was formed in the Early Cretaceous((124.0±0.6) Ma). The Hf isotope results show that the ε;(t) values of the granite porphyry range from 0.5 to 3.3, and two-stage Hf model ages(T;) range from 1 150 to 970 Ma, which indicates that the magma originated from the partial melting of Meso-Neoproterozoic juvenile crust that was derived from the depleted mantle, and there was a small amount of ancient crust added. The granite porphyry was formed in the extensional environment under the background of the Mongolia-Okhotsk closure in the Late Early Cretaceous. At that time, the Paleo-Pacific plate continued to subduct toward the Eurasian continent, but its influence on this area was limited.