锆石U-Pb和Lu-Hf同位素研究内蒙乌努格吐山斑岩型铜钼矿岩浆岩特征

Zircon U-Pb and Lu-Hf Isotopic Dating of Magmatic Rocks in the Wunugetushan Porphyry Copper-Molybdenum Deposit,Inner Mongolia

位于额尔古纳—呼伦断裂北西侧的乌努格吐山铜钼矿为一大型斑岩型铜钼矿床,矿区岩浆岩发育,为确定赋矿围岩和成矿母岩的侵位时代,探讨岩石成因,本文选择该矿床代表性的岩石样品,采用激光剥蚀多接收器电感耦合等离子体质谱法(LA-MC-ICP-MS)和激光剥蚀电感耦合等离子体质谱法(LA-ICP-MS),对岩浆岩的锆石U-Pb同位素和锆石Lu-Hf同位素进行了分析测定。研究表明:①研究区锆石稀土元素含量较高,重稀土强烈富集,具δEu负异常和δCe强正异常,稀土配分曲线具左倾特征;②不等粒二长花岗岩为赋矿围岩,锆石^(206)Pb/^(238)U加权平均年龄为200.96±0.88Ma(MSWD=3.0),岩体形成于早侏罗世,锆石Lu-Hf同位素特征表明岩浆源区为幔源物质和少量古老壳源物质的混合;③流纹质碎斑熔岩为成矿母岩,锆石^(206)Pb/^(238)U加权平均年龄为179.58±0.91Ma,形成于早侏罗世,锆石Lu-Hf同位素特征显示岩浆源区以幔源物质为主。研究结果揭示了从赋矿围岩到成矿母岩岩浆源区从壳源到幔源过渡的演化过程。

BACKGROUND:The Wunugetushan copper-molybdenum deposit located in the northwest of the Erguna—Hulun fault is a large porphyry copper-molybdenum deposit.Magmatic rocks are widespread in the mining area.OBJECTIVES:In order to determine the emplacement age and tectonic setting of ore-hosting surrounding rock and ore-forming parent rock,and to discuss the petrogenesis and metallogenic dynamics.METHODS:Zircon U-Pb isotopes and zircon Lu-Hf isotopes of rocks were studied by laser ablation multi-collector inductively coupled plasma mass spectrometry(LA-MC-ICP-MS)and laser ablation inductively coupled plasma-mass spectrometry(LA-ICP-MS).RESULTS:(1)The zircons in the study area have a high content of REE with strong enrichment of HREE,δEu negative anomaly andδCe positive anomaly,and the REE distribution curves are left-inclined.(2)The anisomitic monzogranites are the ore-hosting rocks with a weighted average value of 200.96±0.88Ma,indicating that the rock mass was formed in early Jurassic.ZirconεHf(t)ranges from 0.1 to 5.8,single-stage Hf model ages(TDM)range from 643Ma to 882Ma,and two-stage Hf model ages(TDMC)range from 874Ma to 1235Ma.(3)The rhyolite porphyritic lavas are the ore-forming parent rocks with a weighted average value of 179.58±0.91Ma,indicating that the rocks formed in the Early Jurassic.Zircon εHf(t)ranges from 4.3 to 6.6,single-stage Hf model ages(TDM)range from 570Ma to 663Ma,and two-stage Hf model ages(TDMC)range from 802Ma to 952Ma.CONCLUSIONS:The zircon Lu-Hf isotopic characteristics of the anisomitic monzogranites indicate that the magma source is a mixture of mantle and a small amount of ancient crust.The magma source area of the rhyolite porphyritic lavas is dominated by mantle-derived materials.The transition evolution process from crust source to mantle source from host rock to metallogenic parent rock is revealed.