First report of felsic whiteschist in the ultrahigh-pressure metamorphic belt of Dabie Shan, China
First report of felsic whiteschist in the ultrahigh-pressure metamorphic belt of Dabie Shan, China
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DOI：
10.1127/0935-1221/2000/0012-0883
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发表时间：
2000-07
影响因子：
2.1
通讯作者：
F. Rolfo;R. Compagnoni;Shutong Xu;Laili Jiang
中科院分区：
文献类型：
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作者：
F. Rolfo;R. Compagnoni;Shutong Xu;Laili Jiang
在中国大别山超高压长英质变质岩中首次发现了典型的白片岩滑石-蓝晶石组合。白片岩组合出现在厚不到半米的白质层中，与含硅榴辉岩横切。其地质环境和地球化学特征表明，原岩为长英质岩脉，其中某些元素，特别是碱的损失较大。白片岩由石英、少量绿帘石、蓝晶石、滑石和辉长石组成。常见的配件有金红石、磷灰石和锆石。绿帘石呈分带状，岩心含石英假晶，含稀土高达5 wt%。蓝晶石包括鲜辉石，翡翠含量最高（Jd≤0.60）。主榴辉岩由辉长石（X Jd从岩心到边缘= 0.44 ~ 0.37）、石榴石、石英、蓝晶石、绿帘石、辉长石（Si≤3.47 a.p.f.u）和角闪石组成，辅有金红石、磷灰石、锆石和钛铁矿。鲜辉石产于绿帘石中，其多晶石英伪晶包括在辉石和蓝晶石中。白片岩与寄主榴辉岩具有相同的构造变质演化，呈现3个变质阶段：(1)P≥2.6 ~ 2.7 GPa、T = 710±20℃的榴辉岩—榴辉岩相阶段；（II）早期减压阶段，P≥1.5 ~ 2.0 GPa, T = 650±30℃；（III）后期绝热减压至P = 0.7 GPa, T = 670±40℃。
We report the first occurrence of a talc-kyanite assemblage typical of whiteschists in felsic ultrahigh-pressure metamorphic rocks of Dabie Shan, China. The whiteschist assemblage occurs in a leucocratic layer, less than half a meter thick, which crosscuts a coesite-bearing eclogite. Both its geologic setting and geochemistry suggest that the protolith was a felsic dyke, which suffered loss of some elements, in particular alkalies. The whiteschist consists of quartz, minor epidote, kyanite, talc and omphacite. Common accessories are rutile, apatite and zircon. Epidote is zoned, and the core includes quartz pseudomorphs after coesite and contains up to 5 wt% REE. Kyanite includes fresh omphacite with the highest jadeite content ( Jd ≤ 0.60). The host eclogite consists of omphacite (X Jd = 0.44 to 0.37 from core to rim), garnet, quartz, kyanite, epidote, phengite (Si ≤ 3.47 a.p.f.u.) and amphibole, and accessory rutile, apatite, zircon and ilmenite. Fresh coesite occurs in epidote, and its polycrystalline quartz pseudomorphs are included in both omphacite and kyanite. The whiteschist and the host eclogite share the same tectonometamorphic evolution, and show three metamorphic stages: (I) coesite-eclogite-facies stage at P ≥ 2.6–2.7 GPa and T = 710 ± 20°C; (II) early decompression stage at P ≥ 1.5–2.0 GPa and T = 650 ± 30°C; (III) late adiabatic decompression to P = 0.7 GPa and T = 670 ± 40°C.