Hannuoba intermediate-mafic granulite xenoliths revisited: Assessment of a Mesozoic underplating model
Hannuoba intermediate-mafic granulite xenoliths revisited: Assessment of a Mesozoic underplating model
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DOI：
10.1016/j.epsl.2010.02.042
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发表时间：
2010-05
影响因子：
5.3
通讯作者：
Neng Jiang;Jing-hui Guo
中科院分区：
文献类型：
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作者：
Neng Jiang;Jing-hui Guo
华北克拉通北缘第三系汉诺坝碱性玄武岩中的中镁铁质麻粒岩捕虏体通常被认为是由中生代玄武岩底侵形成的，因为它们含有丰富的中生代锆石和成分上比邻近的太古宙麻粒岩地形的岩石更高的Mg#（分子Mg/（Mg+Fe））。然而，这样的中生代底侵模型无法解释捕虏体中前寒武纪锆石的存在及其演化的Sr-Nd同位素组成，这些组成与预期的来自岩石圈和软流圈地幔的玄武岩熔体不同。相反，它们的 Sr-Nd 同位素组成与麻粒岩地形和中生代低 Mg# 花岗岩的同位素组成相似。这里将新获取的数据和已发表的数据结合起来，评估中生代玄武质底侵模型，并讨论汉诺坝中基性麻粒岩捕虏体的起源及其与中生代岩浆作用和麻粒岩地形的关系。来自镁铁质麻粒岩捕虏体 (JN0724) 的锆石，其地球化学和 Sr-Nd 同位素组成与其他镁铁质麻粒岩捕虏体无法区分，产生约 2.5Ga U-Pb 年龄，提供了明确的证据，表明镁铁质麻粒岩捕虏体代表了古代下地壳的一部分。综合数据表明，富含辉石的麻粒岩捕虏体可以最好地解释为古代下地壳部分熔融产生中生代低 Mg# 花岗岩后留下的休憩岩。对汉诺巴富含长石麻粒岩捕虏体中生代锆石的 Hf 同位素分析表明，这些锆石是从中生代花岗质岩浆结晶的，这些岩浆源自古代下地壳，而不是中生代底板岩浆。所提出的方案成功地解释了麻粒岩捕虏体和麻粒岩地形之间的年龄和成分差异，并为花岗质岩浆的起源和演化提供了新的见解。结论是汉诺坝中基性麻粒岩捕虏体和中生代花岗岩大部分反映了古代下地壳的改造而不是新生地壳的加成。它有力地证明了地壳深熔可能是控制大陆地壳化学分异的主要过程之一。我们的数据表明，来自许多其他太古代克拉通的麻粒岩捕虏体中较年轻的锆石年龄是否记录了太古代玄武岩底侵作用，可以通过 U-Pb 年龄和 Hf 同位素的综合分析来测试。
The intermediate-mafic granulite xenoliths from Tertiary Hannuoba alkali basalts at the northern margin of the North China Craton have been commonly considered to be formed by Mesozoic basaltic underplating due to their abundant Mesozoic zircons and compositionally higher Mg# (molecular Mg/(Mg+Fe)) than rocks of the adjourning Archean granulite terrain. However, such a Mesozoic underplating model cannot account for the presence of Precambrian zircons in the xenoliths and their evolved Sr–Nd isotopic compositions which are dissimilar to expected basaltic melts from both the lithospheric and asthenospheric mantle. Instead, their Sr–Nd isotopic compositions resemble those of the granulite terrain and of the Mesozoic low Mg# granitoids. Here newly acquired data and published data are combined to assess the Mesozoic basaltic underplating model and to discuss the origin of the Hannuoba intermediate-mafic granulite xenoliths in terms of their relationships with the Mesozoic magmatism and the granulite terrain. Zircons from a mafic granulite xenolith (JN0724), which has geochemical and Sr–Nd isotopic compositions indistinguishable from other mafic granulite xenoliths, yield ∼2.5Ga U–Pb ages, providing unambiguous evidence that the mafic xenoliths represent portions of ancient lower crust. Combined data reveals that the pyroxene-rich granulite xenoliths can be best explained as restites left after partial melting of the ancient lower crust to produce the Mesozoic low Mg# granitoids. Hf isotope analysis on Mesozoic zircons from the Hannuoba feldspar-rich granulite xenoliths demonstrates that the zircons crystallized from Mesozoic granitic magmas that were derived from ancient lower crust rather than from Mesozoic underplated magmas. The proposed scenarios successfully explain the age and compositional differences between granulite xenoliths and the granulite terrain and also provide new insights into derivation and evolution of granitic magmas. It is concluded that most of the Hannuoba intermediate-mafic granulite xenoliths and the Mesozoic granitoids reflect reworking of ancient lower crust rather than juvenile crust addition. It provides strong evidence that crustal anatexis may be one of the major processes controlling the chemical differentiation of the continental crust. Our data imply that whether younger zircon ages in granulite xenoliths from many other Archean cratons register post-Archean basaltic underplating could be tested by integrated analysis of U–Pb age and Hf isotopes.