宁镇矿集区安基山花岗闪长斑岩和韦岗花岗闪长岩的锆石U-Pb年龄、Hf同位素特征及其地质意义

Zircon U-Pb Dating and Hf Isotopic Compositions of the Anjishan Granodiorite Porphyry and Weigang Granodiorite in the Ningzhen Ore Cluster Area and Their Geological Implications

宁镇矿集区位于长江中下游成矿带东部,发育了大量与Cu、Fe矿产有关的中酸性侵入岩,如区内安基山岩体和韦岗岩体分别与安基山铜矿及伏牛山铜矿、韦岗铁矿等Cu、Fe矿床成矿关系密切。尽管前人在宁镇矿集区开展了大量的岩石学工作,但是对这些花岗质岩石源区特征方面的研究却略显薄弱。本文对该矿集区内的安基山岩体和韦岗岩体开展了详细的LA-ICP-MS锆石U-Pb测年和锆石Hf同位素研究,进而确定成岩年龄、探讨岩浆源区性质及与成矿作用的关系。锆石U-Pb定年结果显示区内安基山花岗闪长斑岩和韦岗花岗闪长岩成岩年龄分别为108.9～110.9Ma和107.3～109.6Ma,宁镇矿集区晚中生代岩浆岩的成岩时代在110～100 Ma之间,明显晚于长江中下游其他矿集区成岩时代范围近20Ma。两个岩体的锆石Hf同位素研究显示,韦岗花岗闪长岩的εHf(t)值为-47.4～-16.9,平均为-24.9,二阶段Hf模式年龄(TDM2)为2.24～4.14Ga,平均为2.73Ga,可能是中太古代至古元古代下地壳部分熔融形成的;而安基山花岗闪长斑岩的εHf(t)值为-16.8～-8.9,平均为-12.4,二阶段Hf模式年龄(TDM2)为1.74～2.23Ga,平均为1.96Ga,可能是富集岩石圈地幔物质与下地壳物质混合的产物。本文研究结果结合前人研究显示,矽卡岩型Fe矿床有关的岩体相比于矽卡岩型Cu矿床成矿岩体含有更多的地壳物质;岩浆源区壳、幔物质含量比例可能是控制长江中下游成矿带Cu、Fe成矿作用的重要因素。

Ningzhen ore cluster area,located in the eastern section of the Middle-Lower Yangtze River Reaches metallogenic belt,hosts a large amount of intermediate-felsic intrusive rocks related to copper and iron deposits.For example,the Anjishan granodiorite porphyry and Weigang granodioritethe in the study area are closely related to copper and iron deposits such as Anjishan copper deposit,Funiushan iron deposit.Many petrological studies have been conducted in the Ningzhen ore cluster area,but study on source characteristics of these granitic rocks is relatively poor.In this study,we conducted LA-ICP-MS zircon U-Pb dating and zircon Hf isotope analyses for samples from the Anjishan complex and Weigang complex,to determine diagenetic ages and discuss characteristics of the magmatic source and their relation with mineralization.Our zircon U-Pb dating yields diagenetic ages of 108.9～110.9 Ma and 107.3～109.6 Ma respectively for Anjishan granodiorite-porphyry and Weigang granodiorite,indicating the diagenetic age of late Mesozoic magmatic rocks in the Ningzhen ore cluster area should be 110～100Ma,about 20Ma later than that in other area of the Middle-Lower Yangtze River Reaches metallogenic belt.Hf isotope analyses-47.4to-16.9with a mean,suggesting that the Weigang granodiorite resulted from partial melting of the lower crust during thefromHf age ranges from 1.74to 2.23Ga with a mean value of 1.96Ga,suggesting that the Anjishan granodiorite porphyry was probably mixing of the materials from mantle and lower crust.This study,combined with previous results,concludes that the complex related to skarn-type iron deposit contains more crustal material than the complex related to skarn-type copper deposit.The ratio of material contents between crust and mantle in the source area of magma is likely an important factor controlling the mineralization of copper and iron in the Middle-Lower Yangtze River Reaches metallogenic belt.