浙江省大桥坞铀矿床赋矿围岩为一套火山-侵入杂岩,锆石U-Pb定年显示该套杂岩的成岩时代为138~125 Ma.Hf同位素分析结果显示随着成岩时代变新,这些岩石中锆石εHf(t)值从约-13.0升高到约-3.0.锆石饱和温度同样表现出升高的趋势,从~749℃升高到~846℃.以上特征表明大桥坞地区火山-侵入杂岩为壳幔岩浆混合成因,且幔源物质加入的比例随成岩时代变新而增多.综合前人对赣杭构造带相山、芙蓉山和沐尘地区花岗质岩石的研究结果,发现这些岩石的全岩εNd(t)值和锆石εHf(t)值在135~112 Ma期间分别从-9.0升高到-2.0和-10.0升高到2.0,表明其成因上可能同壳幔岩浆混合有关,且随着成岩时代变新幔源岩浆的加入逐渐增多.Sr-Nd同位素模拟显示幔源岩浆加入的比例在135~112 Ma期间从0升高到~60%.由于幔源岩浆较壳源岩浆贫U和Th,前者的加入会稀释壳源岩浆中U和Th的含量,降低其铀成矿潜力.幔源物质加入越多,铀成矿潜力越低.赣杭构造带壳幔岩浆混合作用呈西弱东强的地质事实,可能是该带上铀矿床的分布呈西大东小格局的重要原因之一.因此,赣杭构造带东段找矿需关注富铀基底(花岗岩或长英质变质岩基底)与早白垩世火山岩盖层相结合的地区.%The Daqiaowu uranium deposit is a volcanic-intrusive rock-hosted uranium deposit, where zircon εHf(t) values of the volcanic-intrusive complex (dated at 138~125 Ma) exhibit a remarkable rise from approximately-13.0 to-3.0 through time. Zircon saturation temperatures of these rocks also show an increase from ~749℃ to ~846℃. These characteristics suggest that volcanic-intrusive rocks in the Daqiaowu were generated by mixing of magmas derived from mantle and crust, and that more inputs of mantle-derived materials were added to younger rocks. Together with a compilation of granitic intrusive rocks associated with magma mixing in Xiangshan, Furongshan and Muchen, we found that whole-rockεNd(t) and zirconεHf(t) values of these granitic rocks increased from-9.0 to-2.0 and-10.0 to 2.0 during 135~112 Ma, respectively, suggesting more contributions of mantle-derived magma through time in the eastern part of Gan-Hang Tectonic Belt (GHTB) than in the western part. Sr-Nd isotopic modelling results indicate that the percentage of mantle-derived magma has increased from 0%to~60%during 135~112 Ma. Generally, mantle-derived magma has lower uranium contents than does the derived magma. The magma mixing/mingling between them would be expected to dilute the uranium contents of the crust-derived magma and lower its uranium metallogenic potentials. More mantle-derived materials in the commingled magmas would lead to much lower uranium metallogenic potential. The fact that magma mixing is more intense in the eastern part of the GHTB than in the western part probably is one of the reasons for the emplacement of larger uranium deposits in the western GHTB. Thus, in order to find larger uranium deposits in the eastern part of the GHTB, more attention should be paid to such areas with both uranium-rich basement (granites or felsic metamorphic rocks) and the overlying Early Cretaceous volcanic rocks.
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