内蒙古固阳地区角闪石等矿物分离结晶形成的新太古代TTG片麻岩——来自元素地球化学和热力学模拟的证据  

Neoarchean TTG Gneisses Generated Through Amphibole-Dominated Fractionation in Guyang,Inner Mongolia:Evidence from Elemental Geochemistry and Thermodynamic Modelling

作　　者：佟鑫 施建荣 王惠初 张家辉 任云伟 Tong Xin;Shi Jianrong;Wang Huichu;Zhang Jiahui;Ren Yunwei(Tianjin Center,China Geological Survey(North China Center for Geoscience Innovation),Tianjin 300170,China;Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]中国地质调查局天津地质调查中心(华北地质科技创新中心),天津300170 [2]中国地质科学院地质研究所,北京100037

出　　处：《吉林大学学报(地球科学版)》2023年第6期1734-1759,共26页Journal of Jilin University:Earth Science Edition

基　　金：中国地质调查局地质调查项目(DD20230205,DD20230003,DD20221631-3);国家自然科学基金委员会联合基金项目(U2244211)。

摘　　要：内蒙古固阳地区存在大量的新太古代末期(石英)闪长质-英云闪长质-奥长花岗质-花岗闪长质(DTTG)片麻岩组合。LA-ICP-MS锆石U-Pb同位素测年得到了石英闪长质片麻岩和英云闪长质片麻岩的结晶年龄分别为(2504±11)和(2514±7)Ma,在石英闪长质片麻岩中同时得到了约2.6 Ga的继承锆石年龄。这些石英闪长质(D)片麻岩具有相对较低的w(SiO_(2))(62.08%~62.62%)和Sr/Y值(23.3~39.8),w(MgO)(2.08%~3.06%)和w(TFe_(2)O_(3))(5.43%~6.01%)相对较高,不具有或具有弱的Eu负异常(Eu/Eu^(*)为0.91~0.98),轻、重稀土元素分异相对不明显((La/Yb)N为7.59~21.60),显示岛弧钙碱性岩石的地球化学特征。结合其锆石Hf同位素组成,本文认为其形成于相对低压较浅层次古老下地壳的部分熔融,同时有幔源熔体的加入。固阳新太古代末期英云闪长质-奥长花岗质(TT)片麻岩则显示较高的w(SiO_(2))(67.95%~77.01%)和Sr/Y值(120.00~941.00),相对低的w(MgO)(0.14%~1.29%)和w(TFe_(2)O_(3))(0.63%~2.66%),具有显著的Eu正异常(Eu/Eu^(*)为1.48~8.73),轻、重稀土元素分异明显((La/Yb)N为29.70~140.00)。TTG岩石一般被认为是来自水化变质玄武岩的部分熔融。而固阳新太古代末期(石英)闪长质-英云闪长质-奥长花岗质(DTT)片麻岩的地球化学特征则指示了角闪石分离结晶过程。固阳堆晶角闪石岩和奥长花岗质片麻岩的混合模拟可很好地复现出固阳地区石英闪长质片麻岩的地球化学组成。热力学模拟和微量元素的定量模拟则揭示了固阳新太古代末期英云闪长质-奥长花岗质岩石是在含水且相对低压条件下,闪长质岩浆经过多阶段分离结晶后形成,主要分离结晶矿物相为角闪石,同时伴有少量斜长石和榍石的分离结晶;其高的全岩Sr/Y、Eu/Eu^(*)值并不是指示其具有相对高压源区的充分条件。In Guyang,Inner Mongolia,a significant occurrence of Late Neoarchean(quartz)diorite-tonalite-trondhjemite-granodiorite(DTTG)rock assemblages has been identified.LA-ICP-MS zircon U-Pb dating has yielded crystallization ages of(2504±11)and(2514±7)Ma for the quartz dioritic and tonalitic gneisses,respectively.Additionally,the dioritic gneisses exhibit inherited zircon ages of ca.2.6 Ga.The quartz dioritic(D)gneisses in Guyang display a relatively low w(SiO_(2))(62.08%-62.62%),Sr/Y ratios(23.3-39.8),and elevated concentrations of w(MgO)(2.08%-3.06%)and w(TFe_(2)O_(3))(5.43%-6.01%).Notably,these rocks demonstrate either negligible or weak Eu negative anomalies(Eu/Eu^(*)=0.91-0.98)and show subtle differentiation between light and heavy rare earth elements((La/Yb)N=7.59-21.60).These features are indicative of island arc calc-alkaline geochemical signatures.Combining these characteristics with zircon Hf isotope compositions suggests that the late Neoarchean diorites likely formed through crustal partial melting under relatively low-pressure and shallow conditions.In contrast,the late Neoarchean tonalitic-trondhjemitic(TT)gneisses in the Guyang area exhibit higher w(SiO_(2))(67.95%-77.01%),elevated Sr/Y ratios(120.00-941.00),and lower w(MgO)(0.14%-1.29%)and w(TFe_(2)O_(3))(0.63%-2.66%).They also feature significant Eu positive anomalies(Eu/Eu^(*)=1.48-8.73)and prominent differentiation between light and heavy rare earth elements((La/Yb)N=29.70-140.00).While the consensus attributes the origins of TTG rocks to partial melting of hydrated altered basalts,the geochemical signatures of the Late Neoarchaean(quartz)diorite-tonalite-trondhjemite(DTT)rocks in the Guyang area allude to fractional crystallization of amphibole.A mixing simulation involving Guyang cumulative hornblendite and trondhjemite accurately reproduces the composition of quartz dioritic gneisses in the Guyang area.Thorough thermodynamic and trace element quantitative modelling reveals that the Late Neoarchean tonalic-trondhjemitic rocks likely formed through

关 键 词：片麻岩 TTG 元素地球化学 热力学模拟 分离结晶 固阳 

分 类 号：P588.3[天文地球—岩石学] P595[天文地球—地质学]