Mineral chemistry and zoning in eclogite inclusions from Colorado Plateau diatremes
Mineral chemistry and zoning in eclogite inclusions from Colorado Plateau diatremes
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科罗拉多高原榴辉岩包裹体的矿物化学和分区
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发表时间：
1979
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通讯作者：
M. Zientek
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作者：
Douglas J. Smith;M. Zientek
科罗拉多高原金伯利岩斜长岩的榴辉岩包裹体含有复杂的石榴石和辉石，其结构不寻常。斜辉石-石榴石-辉石辉石-辉石辉石-金红石组合的详细电子探针穿越显示石榴石从Alm69Gr21Py10（核心）到Alm61Gr13Py26（边缘）的分带，辉石从Jd50（核心）到Jd77到Jd55（边缘）的分带。辉石芯在另一种岩石中富含铬。石榴石和辉石中保存着明显的成分不连续和分带反转。在10-20 μm尺度上，两相均发生振荡带化，石榴石中Py的变化高达6%，辉石中Jd的变化高达15%。辉长石无分带，端部含有74%的青瓷。在一些岩石中常见的是骨架状、辉石质填充的石榴石晶体，而在另一些岩石中的石榴石显然是以贝壳状晶体开始生长的。有些岩石含有粗糙的棱柱状辉石域，其中含有粒度非常细的间质硅酸镁。这种结构似乎是在流体相存在的情况下结晶的结果，推断在结晶过程中水压等于总压力。高水压下榴辉岩的形成可能反映了地壳下的结晶。误差传播分析表明，从电子探针数据计算亚铁对这些翡翠辉石没有意义，并且无法记录核心和边缘结晶之间的温差。石榴石结构和振荡带在变质岩中是不常见的，它们表明在超越反应边界后结晶不平衡。所有数据都符合玄武岩岩脉在冷却和交代过程中侵入冷上地幔形成榴辉岩的模型，但这里的结果并不排除其他成因，如俯冲带变质作用。
Eclogite inclusions from kimberlitic diatremes on the Colorado Plateau contain intricately zoned garnet and pyroxene and unusual textures. Detailed electron microprobe traverses for a clinopyroxene-garnet-phengite-lawsonite-rutile assemblage show garnet zoning from Alm69Gr21Py10 (core) to Alm61Gr13Py26 (rim) and pyroxene zoning from Jd50 (core) through Jd77 to Jd55 (rim). Pyroxene cores are Cr-rich in another rock. Sharp compositional discontinuities and zoning reversals are preserved in garnet and pyroxene. Oscillatory zoning occurs in both phases on a 10–20 μm scale, with variations of up to 6% Py in garnet and 15% Jd in pyroxene. Phengite is unzoned and contains 74% celadonite endmember.Skeletal, pyroxene-filled garnet crystals are common in some rocks, and garnets in other rocks clearly began growth as shell-like crystals. Some rocks contain domains of coarse, prismatic pyroxene with very fine-grained, interstitial magnesium silicates. The texture appears to have resulted from crystallization in the presence of a fluid phase, and water pressure is inferred to have equalled total pressure during crystallization. Eclogite formation at high water pressure may reflect subcrustal crystallization.An analysis of error propagation shows that ferrous iron calculations from electron probe data are not meaningful for these jadeitic pyroxenes, and temperature differences between core and rim crystallization cannot be documented. The garnet textures and oscillatory zoning are unusual for metamorphic rocks, and they suggest disequilibrium crystallization after overstepping of reaction boundaries. All data fit a model of eclogite formation during cooling and metasomatism of basaltic dikes intruded into a cool upper mantle, but the results here do not preclude other origins, such as subduction zone metamorphism.