Chemical and isotopic compositions of rocks and minerals from the Palenyi Island, Por'ya Guba (Bay), White Sea

Hypersthene-garnet-sillimanite-quartz enclaves were studied in orthopyroxene-plagioclase and orthopyroxene-clinopyroxene crystalline schists and gneisses from shear zones exposed in the Palenyi Island within the Early Proterozoic Belomorian Mobile Belt. Qualitative analysis of mineral assemblages indicates that these rocks were metamorphosed to the granulite facies (approximately 900°C and 10-11 kbar). Oxygen isotopic composition was determined in rock-forming minerals composing zones of the enclaves of various mineral and chemical composition. Closure temperatures of the isotopic systems obtained by methods of oxygen isotopic thermometry are close to values obtained with mineralogical geothermometers (garnet-orthopyroxene and garnet-biotite) and correspond to the high-temperature granulite facies (860-900°C). Identified systematic variations in d18O values were determined in the same minerals from zones of different mineral composition. Inasmuch as these zones are practically in contact with one another, these variations in d18O cannot be explained by primary isotopic heterogeneity of the protolith. Model calculations of the extent and trend of d18O variations in minerals suggest that fluid-rock interaction at various integral fluid/rock ratios in discrete zones was the only mechanism that could generate the zoning. This demonstrates that focused fluid flux could occur in lower crustal shear zones. Preservation of high-temperature isotopic equilibria of minerals testifies that the episode of fluid activity at the peak of metamorphism was very brief.

针对古元古代（Early Proterozoic）别洛莫尔活动带（Belomorian Mobile Belt）内帕伦伊岛（Palenyi Island）出露的剪切带（shear zones）中的斜方辉石（orthopyroxene）-斜长石（plagioclase）结晶片岩与斜方辉石（orthopyroxene）-单斜辉石（clinopyroxene）片麻岩（gneisses），本文对其中的紫苏辉石（hypersthene）-石榴子石（garnet）-矽线石（sillimanite）-石英（quartz）包体开展了研究。对矿物组合的定性分析显示，此类岩石曾经历麻粒岩相（granulite facies）变质作用，变质温压条件约为900℃、10~11 kbar。针对矿物组成与化学组成各异的包体带内的造岩矿物（rock-forming minerals），测定了其氧同位素组成（oxygen isotopic composition），且发现在来自不同矿物组成带的同种矿物中，δ¹⁸O（d18O）值存在系统性变化。通过氧同位素测温法（oxygen isotopic thermometry）得到的同位素体系（isotopic systems）封闭温度，与矿物地质温度计（mineralogical geothermometers，石榴子石-斜方辉石（garnet-orthopyroxene）及石榴子石-黑云母（garnet-biotite））的测算结果相近，对应高温麻粒岩相（high-temperature granulite facies，860~900℃）。鉴于这些包体带之间几乎直接接触，这类δ¹⁸O的变化无法通过原岩（protolith）的初始同位素不均一性得到解释。对矿物中δ¹⁸O变化的幅度与趋势开展的模型计算显示，离散带（discrete zones）内不同积分流体/岩石比（fluid/rock ratios）条件下的流体-岩石相互作用（fluid-rock interaction），是形成此类分带结构的唯一机制。这一结果证实，下地壳剪切带（lower crustal shear zones）中可发生集中流体通量（focused fluid flux）作用。矿物的高温同位素平衡状态得以保留，表明变质峰期（peak of metamorphism）的流体活动持续时间极短。