宁芜盆地钟姑矿田典型铁矿成矿岩体Hf-O同位素特征及地质意义

Hf-O isotope characteristics for ore-related intrusions of typical iron deposits in Zhonggu ore field,Ningwu basin

钟姑矿田位于宁芜盆地南部,受矿田内格状构造控制产出一系列早白垩世(129~132Ma)闪长岩及二长岩类侵入体,与铁矿床的形成密切相关,而区域内成矿岩体的研究尤其是岩浆岩的起源和演化有待进一步探讨。本次工作以矿田内的姑山辉石闪长玢岩、龙山辉石闪长岩、白象山闪长岩、钟九闪长岩和太平山二长岩等与铁成矿有关的岩体为研究对象,开展LA-ICP-MS原位锆石Hf同位素和SHRIMP原位O同位素研究。结果显示,钟姑矿田各成矿岩体具有较一致的锆石Hf-O同位素组成,^(176)Hf/^(177)Hf值为0.282425~0.282695,εHf(t)值为-12.3^-3.9,tDM2范围在1.26~1.86Ga;锆石δ18O集中于3.34‰~8.05‰,捕获锆石的176Hf/177Hf比值为0.282324~0.282487,εHf(t)=-15.8^-10.1,锆石tDM2年龄在1.72~20.8Ga之间,锆石δ^(18)O相对集中于5.90‰~6.51‰。钟姑矿田岩浆岩起源于富集的岩石圈地幔,同位素特征指示其携带板块俯冲和蚀变洋壳信息,钟姑矿田的成岩成矿作用应与伸展背景下古太平洋板块俯冲有关。与长江中下游其他矿集区岩浆岩对比研究表明,包括宁芜在内的长江中下游地区岩浆岩具有相同的地幔源区,以铁矿床为主的宁芜和庐枞盆地岩浆岩源区成分以交代岩石圈地幔为主,而以铜陵、繁昌为代表的铜多金属成矿区的岩浆岩受长江中下游新元古代基底的更多影响。

Zhonggu iron ore field is located in south part of Ningwu basin. The ore filed occurred diorite and monzonite intrusions （132~129Ma） which are related to iron deposits. The origin and evolution of these intrusions still need to be further studied. Here, we conduct LA-ICP-MS zircon Hf isotope and SHRIMP zircon O isotope test focus on the Gushan porphyritic gabbrodiorite, the Longshan gabbrodiorite, the Baixingshan diorite, the Zhongjiu diorite and the Taipingshan monzonite. 176Hf/177Hf ratios of these intrusions are 0.282425~0.282695 （tDM2=-1.26~1.86Ga）. The range of zircons have δ18O ranging from 3.34‰ to 8.05‰ and εHf（t） ranging from -12.3 to -3.9. 176Hf/177Hf ratios of inherited zircons in Taipingshan monzonite are 0.282324~0.282487 （tDM2=1.72~20.8 Ga）. The range of inherited zircons have δ18O ranging from 5.90‰ to 6.51‰ and εHf（t） ranging from -15.8 to -10.1. Zhonggu magmatic rocks are derived from enriched lithospheric mantle. Hf-O isotope imply the magma in this area are consistent with the Early Cretaceous period of lithospheric thinning and extension influenced by the subduction of the Paleo-Pacific Plate. Magmatic rocks in Middle and Lower Yangtze Metalloic Belt include Ningwu have the same origin, magmatic rocks in iron districts such as Ningwu and Luzong are mainly directely fromed by enriched lithospheric mantle, while magmatic rocks in copper-polymetallic districts represented by Tongling and Fancang contaminated with more Paleoproterozoic and neoproterozoic earth crust.