Coexisting clinopyroxene/spinel and amphibole/spinel symplectites in metatroctolites from the Buck Creek ultramafic body, North Carolina Blue Ridge
Coexisting clinopyroxene/spinel and amphibole/spinel symplectites in metatroctolites from the Buck Creek ultramafic body, North Carolina Blue Ridge
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DOI：
10.2138/am-2004-0104
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发表时间：
2004
影响因子：
3.1
通讯作者：
H. M. Lang;Aletha J. Wachter;V. Peterson;J. Ryan
中科院分区：
文献类型：
--
作者：
H. M. Lang;Aletha J. Wachter;V. Peterson;J. Ryan
摘要 巴克溪变滑岩保存了该地区地质历史三个阶段的证据：残余火成岩阶段；高压、麻粒岩相变质阶段；和含水蚀变阶段。在高压麻粒岩相变质作用过程中，邻近的橄榄石和斜长石发生反应，产生复杂的冠岩结构。残橄榄石被放射状、柱状的斜方辉石包围，斜长石的边缘是细粒辛辉石，这些细粒辛辉石由单斜辉石/尖晶石、切尔玛克角闪石/尖晶石辛辉石和少量单斜辉石/蓝宝石辛辉石的单独颗粒组成。对辛辉石背散射电子图像和 X 射线元素图的结构关系的检查表明，角闪石/尖晶石和单斜辉石/尖晶石辛辉石在高温下同时生长，取代了这些岩石中的斜长石。在一些样品中，单斜辉石/蓝宝石辛辉石在斜长石替代的最后阶段生长。样品 BC7 中主要电晕形成反应的成功模型，1.00 Pl + 1.72 Ol + 0.17 H2O = 0.59 Di + 0.17 Amp + 0.68 Spl + 0.79 Opx，仅当包含角闪石时才会产生结果。该模型预测产品单斜辉石、角闪石和尖晶石的相对体积，这些体积通常与成像辛辉石中的比例一致。单斜辉石和角闪石为主的辛辉石的比例似乎受到斜长石中 Na 和 Al 的局部质量平衡和/或 H2O 的可用性的控制。形成内部单斜辉石/蓝宝石日冕的最成功模型，1.00 Pl + 1.47 Ol + 0.17 H2O = 0.59 Di + 0.17 Amp + 0.30 Spr (10 ox.) + 0.57 Opx，还包括与单斜辉石/蓝宝石辛辉石相关的角闪石。形成辛角闪石所需的足够的水必须存在于残余火成闪石的粒间流体中，或者在高 P-T 条件下的变质过程中渗透。
Abstract Buck Creek metatroctolites preserve evidence of three stages in the geologic history of the region: a relict igneous stage; a high-pressure, granulite-facies metamorphic stage; and a hydrous alteration stage. During high-pressure granulite-facies metamorphism, adjacent olivine and plagioclase reacted to produce complex coronitic textures. Relict olivine is surrounded by radial, columnar orthopyroxene, and plagioclase is rimmed by fine-grained symplectite composed of separate grains of clinopyroxene/ spinel, tschermakitic amphibole/spinel symplectite and minor clinopyroxene/sapphirine symplectite. Examination of textural relationships in back-scattered electron images and X-ray element maps of the symplectites demonstrates that amphibole/spinel and clinopyroxene/spinel symplectites grew simultaneously at high temperature to replace plagioclase in these rocks. Clinopyroxene/sapphirine symplectite grew in the latest stages of plagioclase replacement in some samples. A successful model for the main corona-forming reaction in sample BC7, 1.00 Pl + 1.72 Ol + 0.17 H2O = 0.59 Di + 0.17 Amp + 0.68 Spl + 0.79 Opx, results only if amphibole is included. The model predicts relative volumes of product clinopyroxene, amphibole and spinel that are generally consistent with the proportions in imaged symplectites. Proportions of clinopyroxene- and amphibole-hosted symplectite appear to have been governed by local mass-balance of Na and Al in plagioclase and/or availability of H2O. The most successful model for formation of the inner clinopyroxene/sapphirine corona, 1.00 Pl + 1.47 Ol + 0.17 H2O = 0.59 Di + 0.17 Amp + 0.30 Spr (10 ox.) + 0.57 Opx, also includes amphibole, which is present in association with clinopyroxene/sapphirine symplectite. Sufficient H2O for formation of symplectic amphibole must have been present in the intergranular fluid of relict igneous troctolites or have infiltrated during metamorphism at high P-T conditions.