ANATEXIS OF JUVENILE MAFIC TO INTERMEDIATE CRUST –CONSTRAINTS FROM MAJOR AND TRACE ELEMENT AND SR, ND, PB ISOTOPES OF DIORITES TO GRANITES (DAMARA OROGEN, NAMBIA)
ANATEXIS OF JUVENILE MAFIC TO INTERMEDIATE CRUST –CONSTRAINTS FROM MAJOR AND TRACE ELEMENT AND SR, ND, PB ISOTOPES OF DIORITES TO GRANITES (DAMARA OROGEN, NAMBIA)
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幼年镁铁质对中地壳的深熔 – 闪长岩对花岗岩的主量元素和微量元素以及 SR、ND、PB 同位素的约束（纳米比亚达马拉造山带）
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发表时间：
2014
期刊：
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通讯作者：
N. Kastek
中科院分区：
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作者：
Stefan Jung;J. Berndt;A. Stracke;F. Hauff;N. Kastek
托姆索索布岩体(纳米比亚达马拉造山带中部)由石英闪长岩、花岗闪长岩和花岗岩组成。未变形和未变质的石英闪长岩和花岗岩的侵入年龄分别为541±3 Ma(石英闪长岩)和506±6 Ma(花岗闪长岩)。较老的年龄早于高T区域变质作用的主期，较年轻的年龄与区域变质主峰的推断年龄相吻合。元素和同位素变化是解耦的，表明联合同化-分离结晶过程在这些岩石的演化过程中并不重要。因此，锶-钕同位素组成(石英闪长岩：ɛNd(init.)：−1.5~−1.8；87Sr/86Sr(init.)：0.7049~0.7058；花岗闪长岩/花岗岩：ɛND(init.)：−2.7~−5.3；87Sr/86Sr(init.)：0.7044~0.7096)指示了石英闪长岩和花岗闪长岩的形成涉及不同的物源。石英闪长岩与花岗闪长岩/花岗岩的铅同位素比值(206Pb/204Pb18.36~18.38；207Pb20.6Pb15.66~15.67；208Pb208Pb38.09~38.12)与花岗闪长岩/花岗岩(206Pb20.4Pb18.39~18.78；207Pb15.62~15.67；208Pb204Pb37.90~38.17)有一定的重叠。但在相对恒定的206Pb204Pb值下，所有岩石类型的207Pb204Pb值都有较大的变化。根据与角闪岩无流体熔融实验结果的对比，下地壳变质玄武岩可能是石英闪长岩的源岩，可能富含K2O。对于花岗闪长岩，可能有高钾安山岩或英云闪长岩的来源。花岗岩被解释为花岗闪长岩的分馏产物。使用QTZ-AB-或系统学和Zr饱和温度的半定量压力-温度估计表明，压力超过5kbar，温度约为900°C，将熔化地点置于下地壳。弱演化的锶-钕同位素比值和中等的放射性成因铅同位素比值与石英闪长岩和花岗闪长岩的幼年镁铁质至中源相一致。这与以前的观点形成鲜明对比，即认为石英闪长岩和花岗闪长岩的起源要么是泛非时代的富集地幔来源在AFC过程之后熔融，要么是古老的、贫化的镁铁质地壳熔融。
The Tsomtsaub pluton (central Damara orogen, Namibia) consists of quartz diorites, granodiorites and granites. Intrusion ages of the undeformed and unmetamorphosed quartz diorites and granites are constrained by LA-ICP-MS U-Pb zircon analyses, which yielded ages of 541 ± 3 Ma (quartz diorites) and 506 ± 6 Ma (granodiorites), respectively. The older age predates the main phase of high-T regional metamorphism and the younger age fits to the inferred age of the main peak of regional metamorphism. Elemental and isotope variations are decoupled, indicating that combined assimilation-fractional crystallization processes were not important during evolution of these rocks. Therefore, Sr-Nd isotope compositions (quartz diorites: ɛNd(init.): −1.5 to −1.8; 87Sr/86Sr(init.): 0.7049 to 0.7058; granodiorites/granites: ɛNd(init.): −2.7 to −5.3; 87Sr/86Sr(init.): 0.7044 to 0.7096) indicate that generation of the quartz diorites and granodiorites involved distinct sources. Lead isotope ratios show some overlap between quartz diorites (206Pb/204Pb: 18.36 to 18.38; 207Pb/204Pb: 15.66 to 15.67; 208Pb/204Pb: 38.09 to 38.12) and granodiorites/granites (206Pb/204Pb: 18.39 to 18.78; 207Pb/204Pb: 15.62 to 15.67; 208Pb/204Pb: 37.90 to 38.17). However, a large variation in 207Pb/204Pb at relatively constant 206Pb/204Pb is apparent for all rock types. Based on a comparison with results from fluid-absent melting experiments using amphibolites, a lower crustal metabasalt, probably enriched in K2O, is a likely source rock for the quartz diorites. For the granodiorites, a high-K andesitic or tonalitic source is likely. The granites are interpreted as fractionation products of the granodiorites. Semi-quantitative pressure-temperature estimates using Qtz-Ab-Or systematics and Zr saturation temperatures indicate pressures in excess of 5 kbar and temperature of c. 900°C, placing the site of melting in the lower crust. The weakly evolved Sr-Nd isotope ratios and the moderate radiogenic Pb isotope ratios are consistent with juvenile mafic to intermediate sources for the quartz diorites and granodiorites. This contrasts with previous suggestions favouring an origin of quartz diorites and granodiorites by either melting of an enriched mantle source during Pan-African times followed by AFC processes, or by melting of ancient, depleted mafic crust.