大兴安岭北段小柯勒河花岗斑岩地球化学、Hf同位素组成及锆石U-Pb定年

Characterization of the Xiaokelehe granite porphyry in the Northern Great Xing’an Range

小柯勒河钼铜矿床是近年来大兴安岭北段发现的斑岩型矿床,成矿与花岗闪长斑岩有关。本文研究的花岗斑岩脉侵位到花岗闪长斑岩中,围绕花岗斑岩叠加了新的蚀变和矿化。花岗斑岩内部和两侧发育有带状分布的钾化、硅化和绢英岩化蚀变,蚀变带内发育浸染状和细脉状黄铁矿、辉钼矿和黄铜矿,辉钼矿矿化尤为强烈,总体表现为中高温热液蚀变和矿化特征。本文通过LA-ICP-MS锆石U-Pb年代学、全岩地球化学与Hf同位素研究,揭示了花岗斑岩岩石成因、岩浆源区和构造背景。获得岩石锆石U-Pb年龄为(131.42±0.39)Ma,为早白垩世。小柯勒河花岗斑岩的组成矿物主要为正长石、钠长石、微斜长石和石英,无碱性铁镁矿物及富铝矿物。岩石属于高钾钙碱性系列,具有高SiO 2(76.24%~77.55%)、富碱((K 2 O+Na 2 O)含量8.12%~8.44%)、贫MgO(0.09%~0.16%)、低CaO(0.30%~0.65%)的特点,A/CNK值为0.94~1.04,并富集Rb、Th、U,亏损Nb、P、Ti。岩石的(Zr+Nb+Ce+Y)含量和10000 Ga/Al比值均低于A型花岗岩的下限,综合矿物学及地球化学特征表明花岗斑岩属于高分异I型花岗岩。锆石的εHf(t)的值为-0.2~+4.0,Hf同位素二阶段模式年龄T DM2(Hf)=1199~931 Ma,表明岩浆主要起源于亏损地幔的新增生地壳,形成过程可能有古老地壳物质的混染。结合区域构造演化资料及构造判别图,认为岩石形成于古太平洋板块俯冲弧后扩张和蒙古—鄂霍茨克造山后的伸展环境。晚侏罗世末期—早白垩世是大兴安岭中北段重要的钼(铜)矿化爆发期,这一时期侵位的浅成高钾钙碱性花岗质侵入体,是寻找斑岩型钼(铜)的重要目标地质体。

The Xiaokelehe Mo(Cu)deposit is a newly discovered porphyry deposit in the Northern Great Xing’an Range and related to the granodiorite porphyry.The granite porphyry veins,intruding into the granodiorite porphyry,are superimposed with new alteration and mineralization.Potash feldspathization,silicification and sericitization are developed in the wall rock contacting the granite porphyry.In this alteration zone,it developed veinlet-disseminated pyrite,chalcopyrite and molybdenite—specially abundant,indicating an medium to high temperature of alteration and mineralization.In this paper,LA-ICP-MS zircon U-Pb geochronology,whole-rock geochemistry and Hf isotope were used to constraint the magmatic source and tectonic setting.The zircon U-Pb age obtained for granite porphyry was 131.42±0.39 Ma,corresponding to the Early Cretaceous.The Xiaokelehe granite porphyry is mainly composed of orthoclase,albite,microcline and quartz.No mafic alkaline and peraluminous minerals were observed.The granite porphyry belongs to high-K calc-alkaline series,characterized by high contents of SiO 2(76.24-77.55%)and(K 2 O+Na 2 O)(8.12-8.44%),and low contents of MgO(0.09-0.16%)and CaO(0.30-0.65%).The A/CNK ratio ranged between 0.94-1.04.The rocks also showed pronounced depletions of Nb,P and Ti,and enrichments of Rb,Th and U.The(Zr+Nb+Ce+Y)content and 10000 Ga/Al ratio were below the lower limits of the A-type granite.Integrated mineralogical,geochemical data suggest that the granite porphyry is a highly differentiated I-type granite.Hafnium isotopic analysis of zircons from the granite porphyry yieldedεHf(t)value of-0.2 to 4.0 and two-stage model age(T DM2)of 931 to 1199 Ma,indicating the magmas were derived from partial melting of juvenile crustal material,with addition of ancient crustal materials.It is believed that the porphyry deposit was formed under the extensional environment related to the paleo-Pacific and Mongol-Okhotsk Plates.The Late Jurassic Early Cretaceous was an important period of Mo(Cu)mineralization in the middle and northern Great Xing’an Range.The shallow high-K calc-alkaline granitic intrusion is important for exploring porphyry Mo(Cu)deposit.