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首页> 外文期刊>Contributions to Mineralogy and Petrology >Geochronology and geochemistry of leucosomes in the North Dabie Terrane, East China: implication for post-UHPM crustal melting during exhumation
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Geochronology and geochemistry of leucosomes in the North Dabie Terrane, East China: implication for post-UHPM crustal melting during exhumation

机译：华东北部大别山地带白质体的年代学和地球化学：UHPM后发掘过程中地壳融化的意义

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摘要

Migmatites are widespread in the North Dabie ultrahigh-pressure metamorphic terrane (NDT) of Dabie orogen, East China. Idiomorphic and poikilitic amphibole grains in both leucosome and melanosome contain inclusions of plagioclase, quartz and biotite, suggesting formation of leucosome by fluid-present melting of biotite + plagioclase + quartz-bearing protoliths at P = 5-7 kbar, T = 700-800 °C. Precise SIMS zircon U-Pb dating indicates that migmatization of Dabie orogen initiated at ~ 140 Ma and lasted for ~ 10 Ma, coeval with the formation of low-Mg~# adakitic intrusions in Dabie orogen. Based on mineralogical, petrographic and geochemical data, leucosomes in NDT can be subdivided into three groups. (1) High La/Yb(N)-Medium Sr/Y group (Group I), whose high Dy/Yb(N) but medium Sr/Y ratios are caused by amphibole and plagioclase residual during partial melting of dioritic to granodioritic gneisses. (2) Low La/Yb(N)-Low Sr/Y group (Group II), whose flat HREE patterns are produced by entrainment of peritectic amphiboles into melts derived from partial melting of dioritic gneiss. (3) High La/Yb(N)-High Sr/Y and Eu~# group (Group 111), whose extremely high Sr and Eu but low other REE concentrations are caused by accumulation of plagioclase and quartz. Although Group I and III fall in the adakitic fields on La/Yb(N)-Yb(N) and Sr/Y-Y diagrams, they are chemically distinct from contemporary high-pressure adakitic intrusions in Dabie orogen in a series of geochemical indexes, for example, lower Dy/Yb(N) and/or Sr/Y ratios at given La/Yb(N) ratio, lower Sr/CaO ratios, lower Rb concentration but higher K/Rb ratios. Therefore, leucosomes are produced by anatexis of the exhumed ultrahigh-pressure metamorphic rocks at middle crustal level, instead of partial melting of thickened lower crust with garnet-rich and plagioclase-poor residual. The coeval occurrence of migmatites and high-pressure adakitic intrusions in Dabie orogen indicates large-scale partial melting of middle to thickened lower crustal column in the early Cretaceous. The required heat source may be the mantle heat conducting through the lithospheric mantle whose lower parts have been convectively removed.
机译：在华东大别造山带的北大别超高压变质地层（NDT）中，广泛存在着斑岩。白质体和黑素体中的同质和多角质闪石颗粒均包含斜长石，石英和黑云母的内含物，这表明黑云母+斜长石+含石英的原生石在P = 5-7 kbar，T = 700-800时通过流体存在的熔融形成了亮白体。 ℃。精确的SIMS锆石U-Pb测年表明，大别造山带的迁移开始于〜140 Ma并持续了〜10 Ma，与大别造山带中低Mg〜＃Adakitic侵入的形成同时发生。根据矿物学，岩石学和地球化学数据，无损检测中的白质可分为三类。 （1）高La / Yb（N）-Sr / Y中等组（I组），其高Dy / Yb（N）但中等Sr / Y比是由闪长岩和斜长岩片麻岩部分熔融过程中的闪石和斜长石残留引起的。 （2）低La / Yb（N）-低Sr / Y组（II组），其平坦的HREE模式是通过包晶包晶的闪石夹带到由二方片麻岩的部分熔融而产生的熔体中产生的。 （3）高La / Yb（N）-高Sr / Y和Eu〜＃组（111组），其Sr和Eu极高，但其他REE浓度低是由于斜长石和石英的堆积引起的。尽管第一组和第三组属于La / Yb（N）-Yb（N）和Sr / YY图上的adakitic领域，但在一系列地球化学指标上，它们在化学上不同于当代大别造山带的高压adakitic侵入，例如，在给定的La / Yb（N）比下，较低的Dy / Yb（N）和/或Sr / Y比，较低的Sr / CaO比，较低的Rb浓度，但较高的K / Rb比。因此，白垩纪是由中地壳水平上发掘出的超高压变质岩的无足动物所产生的，而不是由于富含石榴石和斜长石的残留物使下层地壳部分融化。大别造山带同时发生的辉铁矿和高压阿德克奇侵入岩的同时出现表明白垩纪早期中地壳至下地壳柱的大规模局部熔融。所需的热源可以是通过岩石圈地幔传导的地幔热量，其下部已被对流移除。

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