Neoarchean metamorphic evolution and geochronology of the Miyun metamorphic complex, North China Craton
Neoarchean metamorphic evolution and geochronology of the Miyun metamorphic complex, North China Craton
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DOI：
10.1016/j.precamres.2018.10.015
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发表时间：
2019-01
影响因子：
3.8
通讯作者：
H. C. Zhang;Jiahui Liu;Yichao Chen;Q. Zhang;Van Tho Pham;Tao Peng;Zhen M. G. Li;Chunming Wu-Chunming-W
中科院分区：
文献类型：
--
作者：
H. C. Zhang;Jiahui Liu;Yichao Chen;Q. Zhang;Van Tho Pham;Tao Peng;Zhen M. G. Li;Chunming Wu-Chunming-W
北京密云变质杂岩中的镁铁质和长英质麻粒岩以夹层或透镜体形式产出于晚太古代TTG片麻岩中。在镁铁质麻粒岩中，变质峰组合（M1）以变斑矿物加基质矿物为代表（斜方辉石+单斜辉石+斜长石+石英±钾长石±角闪石±黑云母）及副矿物锆石+磷灰石+钛铁矿±磁铁矿，峰后组合（M2）以“红眼窝”合晶岩为特征（石榴石+单斜辉石+斜长石+石英）镶嵌在斜方辉石或单斜辉石变斑晶及金红石副矿物上，最后的退变质组合（M3）以角闪石-黑云母共生为标志。长英质麻粒岩的峰值组合（M1）以变斑岩和基质矿物为代表（石榴石+斜方辉石+黑云母+斜长石+石英±钾长石）及副矿物（锆石+独居石+磷灰石+黄铁矿），峰后组合（M2）以“红眼窝”合晶岩为特征（石榴石+斜长石+石英），最后的退变质组合（M3）以黑云母-方解石共生为标志。温压计算表明，该麻粒岩M1、M2和M3组合分别形成于800 ~ 860 °C/116 ~ 10 kbar、620 ~ 700 °C/11 ~ 12 kbar和590 ~ 660 °C/4 ~ 6 kbar的P-T条件下，表明这些麻粒岩记录了逆时针的P-T路径，包括最后的近等温减压（ITD）段，造山作用的标志对于长英质麻粒岩，M1和M3组合的变质P-T条件估计分别为680 - 750 °C/6-8 kbar和670 - 680 °C/5 kbar，也包括ITD过程。麻粒岩的广泛穿透性和P-T路径表明，该地区曾发生过冷推覆体逆冲推覆热推覆体或热推覆体被冷推覆体掩埋/俯冲并最终经历快速构造折返的碰撞事件。麻粒岩变质锆石的高精度西姆斯和LA-ICP-MS U-Pb定年结果表明，变质作用发生在距今约2.5Ga。这些数据证实了密云变质杂岩在华北陆块东部经历了一次新太古代碰撞事件。
Mafic and felsic granulite from the Miyun metamorphic complex, northeast Beijing, North China Craton, occur as either interlayers or lenses within the Late Archean TTG gneiss. In the mafic granulite, the metamorphic peak assemblages (M1) are represented by porphyroblast minerals plus matrix minerals (orthopyroxene + clinopyroxene + plagioclase + quartz ± K-feldspar ± hornblende ± biotite) as well as accessory minerals of zircon + apatite + ilmenite ± magnetite, the post-peak assemblages (M2) are characterized by “red-eye socket” symplectites (garnet + clinopyroxene + plagioclase + quartz) rimming the orthopyroxene or clinopyroxene porphyroblast as well as accessory minerals of rutile, and the final retrograde assemblages (M3) are marked by the hornblende-biotite intergrowth. As for the felsic granulite, the peak assemblages (M1) are represented by the porphyroblast and matrix minerals (garnet + orthopyroxene + biotite + plagioclase + quartz ± K-feldspar) as well as accessory minerals in the matrix (zircon + monazite + apatite + pyrite), the post-peak assemblages (M2) are characterized by “red-eye socket” symplectites (garnet + plagioclase + quartz) rimming the orthopyroxene porphyroblast, and the final retrograde assemblages (M3) are marked by the biotite-calcite intergrowth. Geothermobarometric computation suggests that the M1, M2 and M3 assemblages of the mafic granulite were formed underP-Tconditions of ∼800 to 860 °C/∼6 to 10 kbar, ∼620 to 700 °C/11 to 12 kbar and ∼590 to 660 °C/4 to 6 kbar,respectively, suggesting that these granulites record anticlockwiseP-Tpaths including final nearly isothermal decompression (ITD) segments, indicative of orogenic process. For the felsic granulite, metamorphicP-Tconditions of the M1 and M3 assemblages were estimated to be ∼680 to 750 °C/6–8 kbar and ∼670 to 680 °C/5 kbar,respectively, also including ITD process. The widely penetrative gneissosity of the granulite and theseP-Tpaths, suggest a collisional event of cold nappes overthrusting on hot ones or hot nappes being buried/subducted into cold ones and finally experienced fast tectonic exhumation. High-precision SIMS and LA-ICP-MS U-Pb dating of metamorphic zircon of the granulite suggest that the metamorphism occurred at ∼2.5 Ga. These data confirm that the Miyun metamorphic complex experienced a Neoarchean collisional event within the Eastern Block of the North China Craton.