华北东、西板块的古元古代拼合
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摘要
华北中央造山带是一条陆-陆碰撞造山带,由华北东部和西部两个陆块在古元古代碰撞形成,并沿华北中央造山带拼合形成华北克拉通的古老基底。然而,迄今为止,华北东、西陆块拼合的时代仍然没有得到精确的限定。因此我们在华北中央造山带北段的怀安地区,选择六块代表性岩石样品进行了锆石U-Pb年龄和锆石微区微量元素的分析,其中包括五块高级变质岩样品和一块伟晶岩脉样品。TTG片麻岩样品和高压基性麻粒岩样品中原岩岩浆锆石核的分析点给出的207Pb/206Pb年龄分别为2503±17和1964±38 Ma,应分别代表了TTG片麻岩和高压基性麻粒岩的原岩形成年龄。两块变沉积岩样品中岩浆继承锆石核的分析点给出的年龄在2683±22到1964±13 Ma之间变化,表明变沉积岩原岩物质除少部分来源于华北克拉通的太古宙陆壳岩石,大部分来自于华北中央造山带。TTG片麻岩样品和两块变沉积岩样品中的变质锆石重稀土相对亏损,在球粒陨石标准化图解上呈现从中稀土到重稀土分异较小的稀土配分模式,并可见明显的Eu负异常,表明此类锆石应形成于与石榴石和长石共生的麻粒岩相变质阶段。高压基性麻粒岩样品和TTG片麻岩淡色体样品中,变质锆石重稀土含量也相对较低,从中稀土到重稀土没有明显分异,表明这些锆石也形成于麻粒岩相条件下,但它们的Eu负异常较弱或无Eu负异常,可能与其寄主岩石的原岩组成有关。锆石Ti温度计显示所有这些变质锆石的形成温度在702到749℃之间,与区域内高压麻粒岩的峰期变质温度相近。这些锆石的U-Pb年龄在误差范围内一致,约为1845 Ma,此年龄应代表了怀安杂岩中麻粒岩相峰期变质作用时代的最佳估计值。这一结果也进一步证实了前人提出的华北东部、西部两个陆块在古元古代沿华北中央造山带发生碰撞拼合的模型。此外,在TTG片麻岩淡色体样品中,有部分变质锆石的阴极发光图像显示出面状到扇状分带特征,U,Th含量和Th/U比值较高,REE含量高,在球粒陨石标准化图解上呈现从中稀土到重稀土逐步富集明显的变化趋势,表明这些锆石可能形成于折返过程中有大量石榴石分解和部分熔融发生的阶段,此类锆石给出的加权平均年龄结果为1819±13 Ma,这可能代表了该地区退变质作用发生的时代。结合前人已经发表的有关华北中央造山带高级变质岩中变质锆石的年龄数据分析,明显可见三个不同的年龄峰值,分别为1876±6,1849±2,和1814±4 Ma,这一发现表明在华北中央造山带可能存在一个长期的多阶段变质事件,或者存在至少三期不同的变质作用。此外,穿插高压基性麻粒岩叶理的伟晶岩脉样品中岩浆锆石分析点给出的年龄为1806±15 Ma,此年龄应代表了伟晶岩脉的侵位年龄,表明在怀安地区,华北东部、西部两个陆块的碰撞拼合至此时已经结束。
The Trans-North China Orogen (TNCO) is interpreted as a continent-continent collisional belt along which the discrete Archean Eastern and Western Blocks amalgamated to form the basement of the North China Craton. The timing of its formation is still inconclusive. Six samples, including five high-grade metamorphic rocks and a pegmatite dyke, from the Huai'an complex in the northern segment of the TNCO were selected for zircon U-Pb dating and trace element analysis. A TTG gneiss sample and a HP mafic granulite sample contain magmatic zircon cores, yielding protolith formation ages of 2503±17 and 1964±38 Ma, respectively. Detrital zircon of a magmatic origin in two metasedimentary rocks show concordant or moderately discordant U-Pb ages of 1964±13 to 2683±22 Ma, indicating that they were mainly derived from the TNCO with few from the Archean basement of the North China Craton. Metamorphic zircons in the TTG gneiss and two metasedimentary rocks exhibit low HREE contents, flat HREE pattern and a distinct Eu anomaly, implying their formation coeval with garnet and feldspar, and thus under grannulite-facies conditions. Metamorphic zircons in a HP mafic granulite and a leucosome sample also have depleted HREE contents, flat HREE patterns, but a slight negative Eu anomaly, which may also Were formed under granulite-facies condition, whereas the insignificant Eu anomaly may be due to the whole-rock composition. All these metamorphic zircons have formation temperatures of 702 to 749℃, approaching the peak metamorphic temperatures of HP granulite rocks in this area. They have identical～1845 Ma (2σ) within analytical errors, which is the best age estimate age for HP granulite-facies metamorphism of the Huai'an complex. These results confirm previous models proposing late Paleoproterozoic collision between the Eastern and Western Blocks of the North China Craton along the TNCO. On the other hand, some metamorphic zircons in the leucosome sample show planar or polygonal zoning, high Th, U contents and high Th/U ratios, high REE contents and a steep HREE patterns, indicating that they crystallized during partial melting with abundant garnet decomposition. They yielded a weighted mean age of 1819±13 Ma, which we interpret to register the time of exhumation. Combined with previously published data, there are three age peaks at 1876±6, 1849±2, and, 1814±4 Ma for metamorphic zircons from high-grade metamorphic rocks in the TNCO, which may correspond to prograde, peak, and retrograde metamorphism, respectively. The age of 1849±2 Ma may be regarded as the best estimate for peak metamorphism in the TNCO. A pegmatite dyke cutting the foliation of a mafic granulite boudin was dated at 1806±15 Ma, which signifies termination of collision between the Eastern and Western Blocks.
The Trans-North China Orogen (TNCO) is interpreted as a continent-continent collisional belt along which the discrete Archean Eastern and Western Blocks amalgamated to form the basement of the North China Craton. The timing of its formation is still inconclusive. Six samples, including five high-grade metamorphic rocks and a pegmatite dyke, from the Huai'an complex in the northern segment of the TNCO were selected for zircon U-Pb dating and trace element analysis. A TTG gneiss sample and a HP mafic granulite sample contain magmatic zircon cores, yielding protolith formation ages of 2503±17 and 1964±38 Ma, respectively. Detrital zircon of a magmatic origin in two metasedimentary rocks show concordant or moderately discordant U-Pb ages of 1964±13 to 2683±22 Ma, indicating that they were mainly derived from the TNCO with few from the Archean basement of the North China Craton. Metamorphic zircons in the TTG gneiss and two metasedimentary rocks exhibit low HREE contents, flat HREE pattern and a distinct Eu anomaly, implying their formation coeval with garnet and feldspar, and thus under grannulite-facies conditions. Metamorphic zircons in a HP mafic granulite and a leucosome sample also have depleted HREE contents, flat HREE patterns, but a slight negative Eu anomaly, which may also Were formed under granulite-facies condition, whereas the insignificant Eu anomaly may be due to the whole-rock composition. All these metamorphic zircons have formation temperatures of 702 to 749℃, approaching the peak metamorphic temperatures of HP granulite rocks in this area. They have identical～1845 Ma (2σ) within analytical errors, which is the best age estimate age for HP granulite-facies metamorphism of the Huai'an complex. These results confirm previous models proposing late Paleoproterozoic collision between the Eastern and Western Blocks of the North China Craton along the TNCO. On the other hand, some metamorphic zircons in the leucosome sample show planar or polygonal zoning, high Th, U contents and high Th/U ratios, high REE contents and a steep HREE patterns, indicating that they crystallized during partial melting with abundant garnet decomposition. They yielded a weighted mean age of 1819±13 Ma, which we interpret to register the time of exhumation. Combined with previously published data, there are three age peaks at 1876±6, 1849±2, and, 1814±4 Ma for metamorphic zircons from high-grade metamorphic rocks in the TNCO, which may correspond to prograde, peak, and retrograde metamorphism, respectively. The age of 1849±2 Ma may be regarded as the best estimate for peak metamorphism in the TNCO. A pegmatite dyke cutting the foliation of a mafic granulite boudin was dated at 1806±15 Ma, which signifies termination of collision between the Eastern and Western Blocks.
引文
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