Garnet zoning in high pressure granulite-facies metapelites, Mozambique belt, SE-Kenya: constraints on the cooling history
Garnet zoning in high pressure granulite-facies metapelites, Mozambique belt, SE-Kenya: constraints on the cooling history
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肯尼亚东南部莫桑比克带高压麻粒岩相变泥岩中石榴石分带：对冷却历史的限制
DOI：
10.1127/0935-1221/2005/0017-0043
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发表时间：
2005
影响因子：
2.1
通讯作者：
K. Stüwe
中科院分区：
文献类型：
--
作者：
C. Hauzenberger;J. Robl;K. Stüwe
三个变质样品从麻粒岩相Taita山，新元古代莫桑比克带东南肯尼亚的一部分，包含几乎纯的铁铝榴石-镁铝榴石石榴石。这些石榴石与黑云母接触时，其边缘在200-500 μm范围内表现出XFe = Fe/（Fe+Mg）的扩散分带。石榴子石-黑云母Fe-Mg交换测温法得出闭合温度在530-735°C之间。石榴石中的扩散分区配置文件被用来估计使用数值模型的冷却速率。计算得到镁铁质麻粒岩的变质峰温和峰压分别为820°C和1.15GPa。数值模拟和观察到的分区轮廓的匹配表明冷却速率在1-3°C/my之间。通过与Ehlers & Powell（1994）分析方法估算的冷却速率以及地质年代学推导的冷却速率进行比较，发现在闭合过程中黑云母与石榴石的体积比约为0.5。这与在薄片中观察到的体积比是一致的，但与微探针分析不一致，微探针分析表明，只有黑云母在石榴石附近与石榴石平衡。相反，只有在与黑云母接触的地方才会出现明显的石榴石环带。我们认为，这些不一致性可以用冷却过程中晶界过程的变化来解释：在735°C左右的闭合温度以上的热演化中，应用了快速晶界模型，使得薄切片中的所有黑云母与石榴石平衡。在较低的温度下，局部分区的发展，但没有影响的石榴石晶粒中心的组成。晶界过程由快扩散到慢扩散的变化可能与岩石的固相线温度有关。
Three metapelitic samples from the granulite facies Taita Hills, part of the Neoproterozoic Mozambique belt in SE-Kenya, contain nearly pure almandine-pyrope garnets. These garnets show a diffusional zoning of X Fe = Fe/(Fe+Mg) at the rim over a distance of ∼200-500 μm if in contact with biotite. Garnet-biotite Fe-Mg exchange thermometry yields closure temperatures between 530–735°C. Diffusion zoning profiles in garnets are used to estimate cooling rates using a numerical model. For the calculations a metamorphic peak temperature and pressure of 820°C and 1.15 GPa are obtained from mafic granulites. Matching of numerically modelled and observed zoning profiles indicates cooling rates between 1–3°C/my. Comparison with cooling rates estimated with the analytical approach of Ehlers & Powell (1994) and with geochronologically derived cooling rates shows that the volumetric ratio of biotite to garnet was about 0.5 during closure. This is consistent with the volumetric ratio observed in thin section, but inconsistent with microprobe analyses that indicate that only biotite in the immediate vicinity of garnet equilibrated with garnet. Conversely, significant garnet zoning only occurs where in contact with biotite. We suggest that these inconsistencies can be explained with changes in the grain boundary processes during cooling: in the thermal evolution above the closure temperature around 735°C a fast grain boundary model applied so that all biotite in the thin section equilibrated with garnet. At lower temperatures local zoning developed, but did not influence the composition of the garnet grain centers. The change in grain boundary process from fast to slow diffusing grain boundaries may correlate with the solidus temperature of the rock.