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首页> 外文期刊>Journal of Metamorphic Geology >On the origin of multi-layer coronas between olivine and plagioclase at the gabbro-granulite transition, Valle Fértil-La Huerta Ranges, San Juan Province, Argentina
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On the origin of multi-layer coronas between olivine and plagioclase at the gabbro-granulite transition, Valle Fértil-La Huerta Ranges, San Juan Province, Argentina

机译：在辉长岩-粒石过渡的橄榄石和斜长石之间的多层日冕的起源上，阿根廷圣胡安省的ValleFértil-LaHuerta Ranges

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摘要

Troctolitic gabbros from Valle Fértil and La Huerta Ranges, San Juan Province, NW-Argentina exhibit multi-layer corona textures between cumulus olivine and plagioclase. The corona mineral sequence, which varies in the total thickness from 0.5 to 1mm, comprises either an anhydrous corona type I with olivine|orthopyroxene|clinopyroxene+spinel symplectite|plagioclase or a hydrous corona type II with olivine|orthopyroxene|amphibole|amphibole+spinel symplectite|plagioclase. The anhydrous corona type I formed by metamorphic replacement of primary olivine and plagioclase, in the absence of any fluid/melt phase at <840°C. Diffusion controlled metamorphic solid-state replacement is mainly governed by the chemical potential gradients at the interface of reactant olivine and plagioclase and orthopyroxene and plagioclase. Thus, the thermodynamic incompatibility of the reactant minerals at the gabbro-granulite transition and the phase equilibria of the coronitic assemblage during subsequent cooling were modelled using quantitative μMgO-μCaO phase diagrams. Mineral reaction textures of the anhydrous corona type I indicate an inward migration of orthopyroxene on the expense of olivine, while clinopyroxene+spinel symplectite grows outward to replace plagioclase. Mineral textures of the hydrous corona type II indicate the presence of an interstitial liquid trapped between cumulus olivine and plagioclase that reacts with olivine to produce a rim of peritectic orthopyroxene around olivine. Two amphibole types are distinguished: an inclusion free, brownish amphibole I is enriched in trace elements and REEs relative to green amphibole II. Amphibole I evolves from an intercumulus liquid between peritectic orthopyroxene and plagioclase. Discrete layers of green amphibole II occur as inclusion-free rims and amphibole II+spinel symplectites. Mineral textures and geochemical patterns indicate a metamorphic origin for amphibole II, where orthopyroxene was replaced to form an inner inclusion-free amphibole II layer, while clinopyroxene and plagioclase were replaced to form an outer amphibole+spinel symplectite layer, at <770°C. Calculation of the possible net reactions by considering NCKFMASH components indicates that the layer bulk composition cannot be modelled as a 'closed' system although in all cases the gain and loss of elements within the multi-layer coronas (except H _2O, Na _2O) is very small and the main uncertainties may arise from slight chemical zoning of the respective minerals. Local oxidizing conditions led to the formation of orthopyroxene+magnetite symplectite enveloping and/or replacing olivine. The sequence of corona reaction textures indicates a counter clockwise P-T path at the gabbro-granulite transition at 5-6.5kbar and temperatures below 900°C.
机译：来自阿根廷西北部圣胡安省ValleFértil和La Huerta Ranges的小排石质辉长岩在橄榄石和斜长石之间表现出多层电晕纹理。总厚度从0.5mm到1mm不等的电晕矿物序列由带有橄榄石的斜长石和斜长石斜晶石组成的无水电晕I型或带有斜向石的水合电晕II型组成。斜长石。在<840°C下不存在任何流体/熔体相的情况下，通过变质置换初级橄榄石和斜长石形成的无水电晕I型。扩散控制的变质固态置换主要受反应物橄榄石和斜长石和邻位邻苯二酚和斜长石的界面处的化学势梯度支配。因此，使用定量的μMgO-μCaO相图对在辉长岩-颗粒相转变时反应物矿物的热力学不相容性和随后冷却过程中的皮质族化合物的相平衡进行建模。 I型无水电晕的矿物反应织构表明邻苯二酚向内迁移，而橄榄石为代价，而邻苯二茂铁和尖晶石共沸石向外生长以取代斜长石。 II型含水电晕的矿物质地表明，在橄榄石积云和斜长石之间夹有间隙液，该间隙液与橄榄石反应，在橄榄石周围产生包晶的邻苯二甲rim边缘。区分了两种闪石类型：相对于绿色闪石II，无夹杂的褐色闪石I富含微量元素和REE。闪石I从包晶间的邻苯二茂合物和斜长石之间的积屑间液体演化而来。绿色的角闪石II的离散层以无内含物的边沿和角闪石II +尖晶石的共生体形式出现。矿物质地和地球化学模式表明角闪石II的变质成因，在770°C下，邻苯二甲醚被取代形成无内含物的角闪石II层，而斜cl石和斜长石则被替换以形成外部闪石+尖晶石共混物层。通过考虑NCKFMASH组分计算可能的净反应表明，尽管在所有情况下，多层电晕（H _2O，Na _2O除外）中元素的得失都在模拟中，但不能将层体组成建模为“封闭”系统。很小，主要不确定性可能来自各个矿物的轻微化学分区。局部氧化条件导致包埋和/或取代橄榄石的邻苯二茂铁+磁铁矿共沸石的形成。电晕反应织构的顺序表明，辉长岩-粒晶转变在5-6.5kbar和温度低于900°C时逆时针P-T路径。

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