| 从变质作用观看板块构造何时在华北克拉通开始 |
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| 引用本文： | 赵国春.从变质作用观看板块构造何时在华北克拉通开始[J].地学前缘,2007,14(1):19-32. |
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| 作者姓名： | 赵国春 |
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| 作者单位： | 香港大学地球科学系 |
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| 基金项目： | 香港PGC项目;国家重点实验室基金 |
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| 摘 要： | 了解板块构造在地球上何时和怎样开始的是地球科学领域还没有解决的重要问题之一。作为板块运动的最终结果,大陆碰撞造山带是识别地球历史演化中板块构造机制起主导作用的重要标志。大陆碰撞带变质作用一般以顺时针p-T轨迹演化为特征,尤其伴有峰期变质之后的等温减压过程。这样,具有峰后等温减压过程的顺时针p-T轨迹是识别地球早期的板块构造作用的重要标志之一。作为世界上最古老的克拉通陆块之一,华北克拉通基底岩石变质作用p-T演化在过去几年已得到广泛深入的研究,使得该克拉通可能成为应用大规模变质作用p-T轨迹途径来探讨构造环境和构造演化过程的最佳场所。构造上,华北克拉通可划分为三个小的陆块(东部陆块、阴山陆块和鄂尔多斯陆块)和三个古元古代活动带(华北中部碰撞带、孔兹岩带和胶—辽—吉带)。东部陆块和阴山陆块新太古代基底岩石变质作用具有等压冷却型逆时针p-T演化轨迹特征,反映变质作用热源与大量地幔岩浆底板垫托或侵位有关。尽管理论上这样大规模的地幔岩浆可形成在大陆岩浆弧、地幔柱或大陆裂谷环境,只有地幔柱模式才能合理地解释东部陆块和阴山陆块新太古代基底岩石时空分布、岩石组合和构造特征。这样,地幔柱可能是主导东部陆块和阴山陆块新太古代地壳形成和演化的主要构造机制,而板块构造在晚太古宙并不是其主要的构造机制。古元古代孔兹岩带和华北中部碰撞带基底岩石变质作用均具有等温减压型顺时针p-T演化特征,反映两造山带都经历地壳加厚和随后的隆升剥蚀构造过程。这样的构造过程是板块构造体制下的碰撞造山带的典型标志。古元古代胶—辽—吉带可进一步划分为南部带和北部带,其中南部带基底岩石具有逆时针p-T演化特征,而北部带基地岩石具有顺时针p-T演化特征,也反映板块构造机制下的产物。现代规模的板块构造在华北克拉通上的启动时间可以由三个活动带中最老的与板块俯冲有关的新生地壳形成时间来大致标定。目前,华北克拉通内部三个活动带中可识别出来的最老的与板块俯冲有关的新生地壳是华北中部碰撞带2·56Ga五台花岗岩,它们的形成可能大致标志着现代样式的板块构造在华北克拉通大规模作用的开始。
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| 关 键 词： | 太古宙 华北克拉通 板块构造 古元古代 变质作用 p-T轨迹 |
| 文章编号： | 1005-2321（2007）01-0019-14 |
| 收稿时间： | 2006-12-06 |
| 修稿时间： | 2006-12-20 |
WHEN DID PLATE TECTONICS BEGIN ON THE NORTH CHINA CRATON? INSIGHTS FROM METAMORPHISM |
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| Guochun Zhao.WHEN DID PLATE TECTONICS BEGIN ON THE NORTH CHINA CRATON? INSIGHTS FROM METAMORPHISM[J].Earth Science Frontiers,2007,14(1):19-32. |
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| Authors: | Guochun Zhao |
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| Institution: | Department of Earth Sciences, The University of Hong Kong , Pok f ulam Road, Hong Kong , China |
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| Abstract: | Understanding when and why plate tectonics began on Earth is one of the most important unresolved problems in earth sciences. As a final result of plate tectonics, continental collisional orogens are formed, and the identification of such orogens is regarded as a milestone in recognizing the operation of plate tectonics in Earths history. Metamorphism of continental collisional belts is generally characterized by clockwise p - T paths, especially for those involving isothermal decompression following the peak metamorphism. Therefore, clockwise p - T paths involving isothermal decompression are considered one of the important indicators for recognition of plate tectonics, especially in the early history of the earth. As one of the oldest continental blocks in the world, the North China Craton is considered a promising area for applying the large-scale synthesis approach of metamorphic p - T paths to understand tectonic settings or processes since extensive investigations have been undertaken on the p - T - t evolution of the basement rocks over the craton. Tectonically, the North China Craton is composed of three small continental blocks (Eastern Block, Yinshan Block, and Ordos Block) and three Paleoproterozoic mobile belts (Trans-North China Orogen, Khondalite Belt, and Jiao - Liao - Ji Belt). Metamorphism of Late Archean basement rocks in the Eastern and Yinshan Blocks is characterized by anticlockwise p - T paths, mostly involving isobaric cooling, suggesting that the heating source of the metamorphism was related to the underplating or intrusion of large volumes of mantle-derived magmas. Although theoretically such mantle-derived magmas form under continental magmatic arc, mantle plume or continental rift environments, only a mantle plume model can reasonably explain spatial and temporal distribution, lithological association, and structural features of the Late Archean basement rocks in the Eastern and Yinshan Blocks. Therefore, mantle plumes may be the major tectonic mechanism that governed the formation and evolution of the Late Archean crust in the Eastern and Yinshan Blocks, and plate tectonics may not have operated in the two blocks during Late Archean period. Metamorphism of the basement rocks in the Paleoproterozoic Khondalite Belt and the Trans-North China Orogen is both characterized by clockwise p - T paths involving isothermal decompression following peak metamorphism, which suggests that the two orogens underwent initial crustal thickening followed by rapid exhumation/uplift tectonic processes. Such tectonic processes are typical indicators of collisional orogens that accommodate plate tectonics. The Paleoproterozoic Jiao-Liao-Ji Belt can be divided into the southern and northern zones, of which the former is characterized by anticlockwise p - T paths, whereas the latter is characterized by clockwise p - T paths, also indicating a regime of plate tectonics. The time of the initiation of plate tectonics in the North China Craton can be approximated by the age of the earliest juvenile crustal components in the three Paleoproterozoic tectonic belts. So far, the convincing oldest, subduction-related, juvenile crustal component in the North China Craton is the 2.56 Ga Wutai granitoids in the Trans-North China Orogen, which can be used to approximate the timing of the onset of plate tectonics in the North China Craton. |
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| Keywords: | Archean North China Craton plate tectonics Paleoproterozoic metamorphism p-T paths |
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