REE partition among zircon, orthopyroxene, amphibole and garnet in a high-grade metabasic system
REE partition among zircon, orthopyroxene, amphibole and garnet in a high-grade metabasic system
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DOI：
10.1017/s001675681700067x
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发表时间：
2017-08
影响因子：
2.3
通讯作者：
A. Fornelli;A. Langone;F. Micheletti;G. Piccarreta
中科院分区：
文献类型：
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作者：
A. Fornelli;A. Langone;F. Micheletti;G. Piccarreta
本文研究了含镁铁质角闪石麻粒岩和变斑石榴子石的变质演化过程中稀土元素在石榴子石、锆石、斜方辉石和角闪石之间的分配关系，探讨了稀土元素在石榴子石重吸收边缘沿着裂片和弯曲处的分布意义;（3）以石榴子石为参照物，反映化学平衡的稀土元素分配系数（DREE）。并与不含角闪石的含变斑石榴石的中间麻粒岩的结果进行了比较。两个样品的变斑石榴石边缘的连续冠峰后减压过程中形成的特点是稀土富集的叶片和稀土贫湾。日冕角闪石的稀土元素丰度不同，反映了石榴子石原边溶解速率的不同。由于石榴石溶解速率缓慢，晶内扩散导致新石榴石边缘稀土元素浓度升高，从而导致日冕中贫稀土角闪石的形成。随后，在地热梯度和流体存在的条件下，石榴石的溶解速度更快，决定了bights的形成和稀土元素向日冕的转移。用锆石U-Pb年龄验证了石榴石的生长和溶解时间。两种岩石中年龄为339 ~ 303 Ma的锆石与石榴石域（核、外核、边缘）的相对重稀土元素（DHREEzrn/grt）的分布模式相似，显示出化学平衡。锆石年代为C。300马没有出现在平衡与稀土丰富的石榴石裂片，和年轻的锆石（278马）显示了一个新的平衡与稀土贫石榴石湾。在此基础上，DHREEamph/grt值在特定的纹理网站可能会被解释为暗示麻粒岩条件下的平衡。
Abstract A mafic amphibole-bearing granulite with porphyroblastic garnet was investigated to evaluate: (1) the rare earth element (REE) partition among garnet, zircon, orthopyroxene and amphibole during the metamorphic evolution; (2) the significance of the REE distribution along lobes and bights of reabsorbed garnet rim; and (3) REE distribution coefficient values (DREE) suggestive of chemical equilibrium, assuming garnet as a reference. The results have been compared with those deriving from an intermediate granulite containing porphyroblastic garnet, without amphibole. Porphyroblastic garnet from both samples is rimmed by a continuous corona formed during post-peak decompression characterized by REE-enriched lobes and REE-poor bights. The amphiboles from corona have various REE abundances, reflecting a different dissolution rate of original garnet rim. The initial slow rate of garnet dissolution caused high REE concentration in the new garnet rim due to intra-crystalline diffusion, leading to the formation of REE-poorer amphiboles in corona. Subsequently, under an increasing geothermal gradient and fluid-present conditions, the faster dissolution of garnet determined the formation of bights and the transfer of REEs towards the corona. The timing of garnet growth and its dissolution were checked by U–Pb zircon ages. The zircons dated from 339 Ma to 303 Ma in two rock types combined with the garnet domains (core, outer core, rim) show similar distribution of patterns relative to heavy rare earth elements for zircon and garnet (DHREEzrn/grt), suggesting chemical equilibrium. Zircons dated at c. 300 Ma do not appear in equilibrium with REE-rich garnet lobes, and younger zircons (278 Ma) show a new equilibrium with REE-poor garnet bights. On this basis, the DHREEamph/grt values obtained in specific textural sites might be interpreted as suggestive of equilibrium under granulite conditions.