| 密云杂岩西段等压冷却P－T－t轨迹确定及地球动力学成因 |
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| 引用本文： | 陈能松,王人镜,单文琅,钟增球.密云杂岩西段等压冷却P－T－t轨迹确定及地球动力学成因[J].地质科学,1994,29(4):355-365. |
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| 作者姓名： | 陈能松 王人镜 单文琅 钟增球 |
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| 作者单位： | 中国地质大学 武汉 430074 |
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| 摘 要： | 非平衡结构代表的变质反应性质和多种矿物地质温压计的研究表明,北京太古宙密云杂岩西段第二期区域变质作用的退变质P-T轨迹具等压冷却特点。Sm-Nd同位素定年显示,区内广泛发育的石榴石冠状体形成于(1717±34)Ma.初步分析认为,P-T-t轨迹的地球动力学成因可与吕梁运动期间华北地台裂谷作用和同构造壳下岩浆增生的演化背景相联系。
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| 关 键 词： | 等压冷却P－T－t轨迹 变质反应 地质温压计 地球动力学 密云杂岩 北京 |
| 收稿时间： | 1993-07-03 |
| 修稿时间： | 1993-07-03; |
ISOBARIC COOLING (IBC) P-T-t PATH FOR THE WESTERN PORTION OF ARCHAEAN MIYUN COMPLEX, BEIJING: CONSTRAINTS AND ITS GEODYNAMIC GENESIS |
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| Chen Nengsong,Wang Renjing,Shan Wenrang,Zhong Zengqiu.ISOBARIC COOLING (IBC) P-T-t PATH FOR THE WESTERN PORTION OF ARCHAEAN MIYUN COMPLEX, BEIJING: CONSTRAINTS AND ITS GEODYNAMIC GENESIS[J].Chinese Journal of Geology,1994,29(4):355-365. |
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| Authors: | Chen Nengsong Wang Renjing Shan Wenrang Zhong Zengqiu |
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| Institution: | Department of Geology, China University of Geoscicnces, Wuhan 430074 |
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| Abstract: | The western portion of Archaean Miyun complex in the northeastern Beijing is mainly composed of garnet-bearing metamafic rocks and metafelsic rocks, with minor metaultramafic rocks and occasional plagioclasite. Garnets dominantly occur as coronas on the rims of pre-existing orthopyroxene and clinopyroxene and sometimes on sphene and apatite. Newly generated neogarnets are locally associated with retrograde hornblende in two-pyroxene hornblendites (metaultramafics). Petrographical evidence indicates that both coronal garnet and neogarnet were formed through the following simplified reactions: Hy1 + Pl1→Grt2+Qtz; Hy1+ Pl1→Grt2+Cpx2+Pl2; Hy1+Pl1+H2O→Hbl2+Grt2; Hy1+Cpx1+Pl1+H2O→Hbl2+ Qtz; Cpx1+ Pl1→Grt2;Cpx1+Hy1+PI1+H2O→Hbl2; Grt2+Cpx1-2+H2O→Hbl2+ Pl2. All the reactions together suggest that a retrograde process of nearly isobaric cooling (IBC) had operated following the peak of Luliang regional metamorphism. The calculated temperature and pressure conditions for the growth of coronal garnet and neogarnet give the results of 668-774℃ and 886-957MPa for the western region and 845℃, 1336MPa for the eastern region. Data from geothermobarometers also record in someextent an IBC retrograde process. The Sm-Nd dating of coronal garnet, clinopyroxene (product of the Fuping episode) and whole rock specimen yielded an excellent isochron, indicating that the coronal garnet grew at aout 1717Ma ago, and that isotopes Sfn and Nd had been homogenized among the different generations of minerals and the whole rock specimen. The two-point Sin-Nd isochronic ages of whole rock specimen with individual clinopyroxene and coronal garnet are 1858Ma and 1717Ma,respectively. This difference might result from a low cooling rate after the metamorphic peak of Luliang episode. The geodynamic process for the IBC P-T-t path might be related to rifting tectonics with concomitant magma under accretion within the North China platform. |
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| Keywords: | IBC P-T-t path metamorphic reactions geothermobarometry geodynamics Archaean miyun complex Beijing |
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