桂东南大容山-十万大山S型花岗岩带的成因:地球化学及Sr-Nd-Hf同位素制约

Origin of the Darongshan-Shiwandashan S-type granitoid belt from southeastern Guangxi:geochemical and Sr-Nd-Hf isotopic constraints

本文报道桂东南大容山-十万大山花岗岩带浦北岩体(东北带)、旧州岩体(中部带)和台马岩体(西南带)全岩的主、微量元素、Sr-Nd同位素和锆石的LAM-MC-ICPMS原位Hf同位素分析结果。岩石学及元素地球化学结果显示:上述三个岩体为典型S型花岗岩;高I_(Sr)(>0.721)和低ε_(Nd)(t)(-13.0～-9.9)意味着它们可能来自古老地壳的重熔。岩浆结晶(～230Ma)锆石的ε_(Hf)(t)值主要集中在-11～-9,相应的T_(DM2)模式年龄为1.9～1.8Ga;少数结晶锆石的ε_(Hf)(t)值逐渐升高到-4.5,T_(DM2)降低为～1.5Ga。捕获锆石(1681～384Ma)的的ε_(Hf)(t)值分布在-17.1～+3.4,T_(DM2)主要集中在2.4Ga、1.9Ga和1.5Ga。大部分岩浆结晶锆石ε_(Hf)(t)值与根据"全岩ε_(Nd)(t)值和‘地壳Hf-Nd相关’预测值"基本一致,表明平均地壳存留年龄为1.9Ga的地壳是最重要的物源区。部分岩浆锆石与捕获锆石具有相同的T_(DM2)～1.5Ga,表明平均地壳存留年龄为1.5Ga的物源区参与了该花岗岩带的形成;由于缺少T_(DM2)>2.0Ga的岩浆锆石,少量平均地壳存留年龄为2.4Ga的再循环地壳物质参与了该花岗岩带的形成。因为缺少显著幔源特征的高ε_(Hf)(t)值锆石,本文认为地幔物质基本没有参与该S型花岗岩带的形成。

Abstract Comprehensive geochemical, Sr-Nd isotopic and in-situ zircon Hf isotopic data are reported for the Pubei pluton, the Jiuzhou pluton and the Taima pluton, which consist dominandy of the Darongshan-Shiwandashan granitoid belt in SE Guangxi. Petrographic and element geochemical features show that these plutons are typical S-type granites. High lsr（〉0.721） and low БNd （t）values （-13.0 ~ -9.9） indicate that they were originated from evolved crustal materials. Among the ~ 230Ma magmatic zircons, most have a limited range of БHf（t） values between -11 and -9 corresponding to TDM2 ages of ca. 1.9~1.8Ga, and the remainder have relatively high БHf（t） values up to -4.5 corresponding to younger TDM2 ages of ca.1.5Ga. Inherited zircons in age of 1681~ 384Ma have БHf（t）of -17.1 ~ ＋3.4, and TDm2 ages clustering at 2.4Ga, 1.9Ga and 1.5Ga. Considering that most ~ 230Ma magmatic zircons have БHf（t） values consistent with the estimated values in terms of Hf-Nd isotopic array, we interpret that the crust with average residential time of 1.9Ga is the dominant material source for the studied granites. Some magmatic and inherited zircons have nearly the same TDM2 ages of ~1. 5Ga, indicating that the crust with average residential time of 1.5Ga is also involved in formation of these granitoids. Given that no igneous zircons with TDM2 older than 2.0Ga have been detected, minor amount of recycled crustal materials with average residential time of 2.4Ga could also be involved. Because new mande inputs have not been identified in terms of zircon Hf isotopes, mantle-derived magmas might not have been involved in formation of this S-type granitoid belt.