Zusammenfassung
Der retrograde P-T-t Pfad von intermediären und hochtemperierten Eklogiten (T ≥ 500 ±50°C) ist charakterisiert durch die Umformung von Omphazit in symplektitische Verwachsungen von Klinopyroxen und Plagioklas-Lamellen. Häufig zeigt dieser Symplektit eine zweiphasige mikrostrukturelle Entwicklung, das heißt, ein Symplektit mit breiten Lamellen »S1« wird gefolgt von einem Symplektit mit dünneren Lamellen »2«. Jeder Symplektittyp kann sich schließlich selbständig zu einer kugelförmigen MikroStruktur »SG« entwikkeln.
Auf Grund von metallurgischen Vorstellungen wir die Phasenumwandlung Omphazit zu Klinopyroxen + Plagioklas als diskontinuierliche Ausfällungsreaktion klassifiziert. In diskontinuierlichen Ausfällungsreaktionen hängt der Lamellenabstand (L) von der Temperatur (T) nach dem Wachstumsgesetz log (L) = A-B/T ab, in dem A und B Konstanten sind. Daher zeigen dünne Symplektite niedrige Temperaturen an, während breite Symplektite höhere Temperaturen anzeigen.
Um die thermische Entwicklung von kontinentalen Kollisionszonen zu spezifizieren mußte die Zuververlässigkeit des diskontinuierlichen Ausfällungswachstumsgesetz an Eklogiten getestet werden, die unterschiedlich retrograde P-T-t Geschichten aus verschieden Umgebungen zeigen; das heißt aus alpinen, kaledonischen und variszischen Gebirgszügen. Tatsächlich offenbaren unsere Bemühungen, daß der Klinopyroxen/Plagioklas Symplektit als potentielles Geothermobarometer in Eklogiten benutzt werden kann, die Temperaturen über 500°C ± 50°C erfahren haben. Die Temperatur kann man aus dem Lamellenabstand erhalten; den Druck durch den Jadeitgehalt des symplektitischen Pyroxens, in Anwesenheit von Plagioklas und Quarz.
Abstract
The retrograde P-T-t path of intermediate and high-temperature eclogites (T ≥ 500 ± 50°C) is characterized by the transformation of omphacite into a symplectitic intergrowth of clinopyroxene and plagioclase lamellae. Often, this symplectite shows a two stage microstructural evolution, i.e. a coarse lamellar width symplectite, »S1«, followed by a thinner lamellar width symplectite, »S2«. Each symplectite may also evolve independently into a final globular microstructure, »SG«.
According to metallurgical concepts, the phase transition »omphacite → clinopyroxene + plagioclase« is classified as a discontinuous precipitation reaction. In discontinuous precipitation reactions, the lamellar spacing (L) depends on temperature (T) according to the growth law log (L) = A-B/T, in which A and B are constants. Therefore thin symplectites characterize low temperatures, where-as coarse symplectites characterize higher temperatures.
In order of specify the thermal evolution of continental collision zones, the reliability of the discontinuous precipitation growth law has been tested on eclogites showing contrasted retrograde P-T-t histories in different geological environments, i.e. Alpine, Caledonian and Variscan Belts. Indeed, our investigations reveal that the clinopyroxene/plagioclase symplectite can be used as a potential geothermobarometer in eclogites recording temperatures above 500 ± 50°C. The temperature is obtained by the lamellar spacing and pressure by the Jadeite content of the symplectite clinopyroxene in the presence of plagioclase and quartz.
Résumé
La déstabilisation de l'omphacite, en une symplectite à alternance de lamelles de clinopyroxène et de plagioclase, caractérise l'initiation du trajet rétrograde P-T-t pour les éclogites de température ≥ 500 ± 50°C. Cette symplectite présente parfois une évolution microstructurale depuis une symplectite grossière à alternance de lamelles larges, »S1«, relayée par une symplectite à alternance de lamelles fines, »S2«. Chaque symplectite peut aussi évoluer indépendemment en une microstructure globulaire tardive, »Sg«.
En accord avec les concepts établis par les métallurgistes, la transformation »omphacite → clinopyroxène + plagioclase« est une précipitation discontinue. Pour ce type de transformation, l'espace interlamellaire (L) dépend de la température (T) selon la loi de croissance Log (L) =A-B/T (*), où A et B sont des constantes. Ainsi, une symplectite fine (de type S2) caractérise des températures relativement plus basses qu'une symplectite grossière (de type S1).
Afin de préciser l'évolution thermique des zones de collision continentale, nous avons testé la loi de croissance (*) sur des éclogites échantillonnées dans divers contextes géologiques (chaînes alpine, calédonide et varisque), présentant des trajets rétrogrades P-T-t contrastés. Les résultats montrent que la symplectite à clinopyroxène/plagioclase représente un géothermobaromètre potentiel de la rétromorphose des éclogites: la taille de l'espace interlamellaire dépend de la température et la teneur en jadéite des lamelles de clinopyroxène, en présence de quartz et à l'équilibre avec le plagioclase, dépend de la pression.
Краткое содержание
Ретроградное термоб арометрическое разв итие средних и высокотемп ературных эклогитов характеризуется пре образованием омфаци та в симплектитических с ращениях клинопирок сена и пластинок плагиокл аза. Этот симплектит проявляет чаще всего двуфазовое развитие микроструктуры, т.е. за симплектитом, имеющи м широкие пластины «S 1», с ледуют симплектиты из тонки х пластинок «S 2». Кажда я их этих типов может пр еобразовываться сам остоятельно в шарообразные микро структуры «SG».
С точки зрения металл ургических представ лений это фазовое преобраз ование омфацита в клинопироксен и пла гиоклаз рассматрива ют, как прерывистую реак цию осаждения. При так их реакциях расстояние между пластинками (L) зависит от температу ры (Т) — согласно закону роста кристаллов — log (L) = А- В/Т, причем А и В являются к онстантными величин ами. Поэтому тонкие сипле ктиты указывают на низкую температур у, а широкие на высокую. Чтобы специфицировать тер мическое развитие зон коллизи й материков, необходи мо точно проверить зако н прерывистой реакци и осаждения эклогитов, которые в различном окружении имеют и раз личную термобаромет рическую историю, напр.: в альпий ских, каледонских и варисских горных це пях. Действительно, при этих исследовани ях установили, что симплектиты из кл инопироксена/плагио клаза можно рассматривать, как потенциальные геотермобарометры в эклогитах, претерпев ших влияние температур с выше 500°С + 50°С.
Температуру можно оп ределить по расстоян ию между пластинками, да вление — по содержани ю жадеита в симплектит ных пироксенах, где присутствуют еще плагиоклаз и кварц.
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Joanny, V., van Roermund, H. & Lardeaux, J.M. The clinopyroxene/plagioclase symplectite in retrograde eclogites: A potential geothermobarometer. Geol Rundsch 80, 303–320 (1991). https://doi.org/10.1007/BF01829368
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DOI: https://doi.org/10.1007/BF01829368