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首页> 外文期刊>Contributions to Mineralogy and Petrology >Prograde destruction and formation of monazite and allanite during contact and regional metamorphism of pelites: petrology and geochronology
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Prograde destruction and formation of monazite and allanite during contact and regional metamorphism of pelites: petrology and geochronology

机译：珍珠岩的接触和区域变质过程中，独居石和尿石的深度破坏和形成：岩石学和地球年代学

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摘要

The conditions at which monazite and allanite were produced and destroyed during prograde metamorphism of pelitic rocks were determined in a Buchan and a Barrovian regional terrain and in a contact aureole, all from northern New England, USA. Pelites from the chlorite zone of each area contain monazite that has an inclusion-free core surrounded by a highly irregular, inclusion-rich rim. Textures and 208Pb/232Th dates of these monazites in the Buchan terrain, obtained by ion microprobe, suggest that they are composite grains with detrital cores and very low-grade metamorphic overgrowths. At exactly the biotite isograd in the regional terrains, composite monazite disappears from most rocks and is replaced by euhedral metamorphic allanite. At precisely the andalusite or kyanite isograd in all three areas, allanite, in turn, disappears from most rocks and is replaced by subhedral, chemically unzoned monazite neoblasts. Allanite failed to develop at the biotite isograd in pelites with lower than normal Ca and/or Al contents, and composite monazite survived at higher grades in these rocks with modified texture, chemical composition, and Th–Pb age. Pelites with elevated Ca and/or Al contents retained allanite in the andalusite or kyanite zone. The best estimate of the time of peak metamorphism at the andalusite or kyanite isograd is the mean Th–Pb age of metamorphic monazite neoblasts that have not been affected by retrograde metamorphism: 364.3±3.5 Ma in the Buchan terrain, 352.9±8.9 Ma in the Barrovian terrain, and 403.4±5.9 Ma in the contact aureole. Some metamorphic monazites from the Buchan terrain have ages partially to completely reset during an episode of retrograde metamorphism at 343.1±9.1 Ma. Interpretation of Th–Pb ages of individual composite monazite grains is complicated by the occurrence of subgrain domains of detrital material intergrown with domains of material formed or recrystallized during prograde and retrograde metamorphism.
机译：在Buchan和Barrovian区域地形以及接触金针中确定了独生石在钙质岩的正变质过程中产生和破坏独居石和尿囊石的条件，这些条件都来自美国新英格兰北部。来自每个区域的亚氯酸盐区域的珍珠岩都含有独居石，独居石的核心是无规则的，高度不规则的，富含内含物的边缘。通过离子微探针获得的布坎地区这些独居石的织构和208 Pb / 232 Th日期表明，它们是具有碎屑核和非常低等级变质过度生长的复合颗粒。正好在该地区地形的黑云母等速线中，复合独居石从大多数岩石中消失，并被原形变质的尿囊石代替。恰恰在这三个区域中的红柱石或蓝晶石等梯度中，尿囊石依次从大多数岩石中消失，并由化学性质未分区的亚面体独居石成球细胞取代。钙铁和/或铝含量低于正常值的球母岩中，黑云母未能在黑云母等速线上发育，复合独居石在这些岩石中具有较高的品位，且质地，化学成分和Th-Pb年龄均发生了变化。 Ca和/或Al含量升高的珍珠岩将红柱石保留在红柱石或蓝晶石带中。最佳的红柱石或蓝晶石等速线变质时间的估计值是不受逆向变质影响的变质独居石成因的平均Th–Pb年龄：布坎地区为364.3±3.5 Ma，布坎地区为352.9±8.9 Ma。 Barrovian地形，并且在接触光环中为403.4±5.9 Ma。在布坎地区的一些变质独居石在逆行变质事件发生期间（343.1±9.1 Ma），部分年龄会完全重置。单个复合独居石晶粒的Th–Pb年龄的解释由于碎屑物质的亚晶粒区域与在逆行和逆行变质过程中形成或再结晶的物质的区域混杂在一起而发生。

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  • 来源
    《Contributions to Mineralogy and Petrology》 |2003年第2期|228-250|共23页
  • 作者

    Boswell A. Wing; John M. Ferry; T. Mark Harrison;

  • 作者单位

    Department of Earth and Planetary Sciences Johns Hopkins UniversityEarth System Science Interdisciplinary Center University of Maryland;

    Department of Earth and Planetary Sciences Johns Hopkins University;

    Research School of Earth Sciences The Australian National University;

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