Summary
The crystal-chemistry of clinopyroxene from a suite of Sunda arc volcanic rocks was investigated by X-ray structure refinement and microprobe analysis. Relationships with clinopyroxene from volcanic rocks of the intraplate environment were highlighted.
Sunda clinopyroxenes were distinguished into two groups. The first consists of clinopyroxene from SiO2-saturated rock types and crystal rim from SiO2-undersaturated rock types, the second of crystal core from SiO2-undersaturated rock types. The crystal chemical behaviour of the two groups differs significantly, mainly in the geometrical relationships between M2 and T sites. While in the first group 〈M2-O3〉 and 〈T-O3〉 bond lengths shorten and lengthen respectively due to Ca and AlIV increase, in the second group in which M2 site is almost fully coordinated by Ca, 〈-O3〉 bond lengths are forced to lengthen in spite of low AlIV content, in order to mantain the local charge balance around the O3 oxygens.
It is noteworthy that Sunda clinopyroxene shows strong analogies with that from basaltic rocks from the Ethiopian plateau and K-rich lavas from the Roman Province respectively, reflecting similarities between the composition and mineralogical assemblages of the host rocks, although they all came from different tectonic environments. A relationship between intracrystalline Mg-Fe 2+ disorder and the explosive character of the volcanism is demonstrated.
Zusammenfassung
Die Kristallchemie von Klinopyroxenen aus einer Serie von vulkanischen Gesteinen des Sunda-Bogens wurde mit Röntgen-Strukturverfeinerungen und Mikrosonden-analysen untersucht. Beziehungen zu Klinopyroxenen vulkanischer Gesteine aus intra-Plattenpositionen werden beleuchtet. Die Sunda-Klinopyroxene wurden in zwei Gruppen eingeteilt. Die erste umfaßt Klinopyroxene SiO2-gesättigter Gesteinstypen und Kristallsäume SiO2-untersättigter Gesteinstypen, die zweite Kristallkerne SiO2-untersättigter Gesteinstypen. Das kristallchemische Verhalten der beiden Gruppen unterscheidet sich beträchtlich, hauptsächlich in den geometrischen Beziehungen zwischen den M2 und T Positionen. Während in der ersten Gruppe die 〈M2-O3〉 und 〈T-O3〉 Bindungsldngen durch Zunahme von Ca und AlIV kleiner bzw. größer werden, sind in der zweiten Gruppe die 〈T-O3〉 Bindungslängen trotz niedrigen AlIV Gehaltes zu einer Vergrößerung gezwungen, damit um die O3 Atome der lokale Ladungsausgleich erhalten bleibt.
Es ist bemerkenswert, daß die Sunda-Klinopyroxene große Analogien zu denen aus basaltischen Gesteinen des Abessinischen Plateaus und aus K-reichen Laven sowohl der Römischen Provinz als auch des Westafrikanischen Grabens zeigen. Dies spiegelt Ähnlichkeiten zwischen Zusammensetzung und Mineralbestand der Muttergesteine wider, obwohl sie all aus tektonisch unterschiedlichen Positionen kommen.
Eine Beziehung zwischen intrakristalliner Mg-Fe2+ Unordnung und dem explosiven Charakter des Vulkanismus wird aufgezeigt.
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Salviulo, G., Molin, G.M. Crystal-chemistry of clinopyroxene from Sunda volcanic arc. Mineralogy and Petrology 49, 233–248 (1993). https://doi.org/10.1007/BF01164596
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DOI: https://doi.org/10.1007/BF01164596