Petrology of a jadeite-quartz/coesite-almandine-phengite fels with retrograde ferro-nyboeite from the Dora-Maira Massif, Western Alps
Petrology of a jadeite-quartz/coesite-almandine-phengite fels with retrograde ferro-nyboeite from the Dora-Maira Massif, Western Alps
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DOI：
10.1127/ejm/5/5/0943
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发表时间：
1993-10
影响因子：
2.1
通讯作者：
T. Hirajima;R. Compagnoni
中科院分区：
文献类型：
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作者：
T. Hirajima;R. Compagnoni
在意大利西阿尔卑斯山Dora-Maira地块南部的超高压Brossasco-Isasca单元（BIU）的含硬玉岩中发现了铁铌铁矿。超高压峰期含硬玉岩主要由硬玉、石榴石、多硅白云母和柯石英组成。在早期阶段的退变质路径，绿辉石，ferro-nyboite到katophorite和钠长石的发展在牺牲的高峰期阶段。随着退变质过程的继续，绿辉石由霓辉石转变为几乎不含硬玉的霓辉石，铁钠闪石和钾镁矾被贫硅的钾镁矾和铁铝榴石镶边。铁铌铁矿和钾镁矾以铁铝榴石周围的反应环和钠长石基质中的自变晶板状颗粒的形式出现。该钙铝榴石的组成比以往报道的任何一种钙铝榴石都更富Fe 2+。石榴子石-绿辉石地质温压法给出早期退变质阶段的温度约为500-570 °C和12-15 kbar。在类似的推断P-T条件下，Schertl等人（1991）报道了在Dora-Maira的高镁岩石中蓝闪石作为退变质相的发展。研究表明，在低温榴辉岩相条件下，铁铌铁矿在局部富Na、Al和Fe的体相组成中是稳定的，而不是铁蓝闪石
Ferro-nyboite is newly found in a jadeite-bearing fels of the ultra high-pressure Brossasco-Isasca unit (BIU), southern Dora-Maira Massif, Western Alps, Italy. At the ultra high-pressure climax, the jadeite-bearing fels was mainly composed of jadeite, garnet, phengite and coesite. During an early stage of the retrograde path, omphacite, ferro-nyboite to katophorite and albite developed at the expense of peak phases. With a continuation of the retrograde process, the omphacite changed its composition through aegirine-augite to almost jadeite-free aegirine, and the ferro-nyboite and katophorite were rimmed by Si-poor katophorite and ferro-alumino-taramite. Ferro-nyboite and katophorite occur as a reaction rim around almandine garnet and as idioblastic tabular grains in the albite matrix. The composition of the nyboite is richer in Fe 2+ than any other nyboite previously reported. Garnet-omphacite geothermobarometry gives about 500-570 °C and 12-15 kbar for the early retrogression stage. Under the similar inferred P-T conditions, Schertl et al. (1991) reported the development of glaucophane as a retrograde phase in highly magnesian rocks from Dora-Maira. This study suggests that ferro-nyboite, instead of ferro-glaucophane, is stable in local Na-, Al- and Fe-rich bulk compositions under low-temperature eclogite facies conditions