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Mineralogy and Petrology

, Volume 45, Issue 1, pp 47–67 | Cite as

Mineralogical relations and magma mixing in calc-alkaline andesites from lake Atitlán, Guatemala

  • S. P. Halsort
  • W. I. Rose
Article

Summary

Quaternary calc-alkaline andesites erupted form three neighboring volcanoes along the Guatemalan volcanic front have mineralogic compositions and textures which show varying degrees of disequilibrium. Basaltic andesites and andesites (SiO2 % = 50−59), erupted from Atitlán volcano located nearer to the trench, have the lowest degree of disequilibrium. These lavas contain an anhydrous phenocryst assemblage of mildly bimodal plagioclase, olivine, augite opx, and magnetite. Orthopyroxene occurs at the expense of olivine with increasing whole rock SiO2. Most pyroxene phenocrysts show a trend of Fe enrichment.

Andesites from Tolimán (SiO2% = 53−62) and San Pedro (Si02% = 54−67) volcanoes, located further away from the trench, show comparatively high and moderate degrees of disequilibrium, respectively. Tolimán andesites have bimodal plagioclase compositions and textures. Olivine persists with increasing whole rock Si02 and lacks clear modal relations with coexisting orthopyroxene and hornblende phenocrysts. When compared to Atitlán andesites, Toliman olivines are more forsteritic and pyroxenes contain higher proportions of Mg-rich rims, though normal zoned phenocrysts occur within the same rock. Tolimán andesite also have lower proportions of phenocrysts to microphenocrysts, more calcic plagioclase groundmass compositions, and higher modal phenocrystic magnetite. San Pedro andesites have disequilibrium assemblages similar to Tolimán andesites but are not as striking.

Magma mixing is proposed as the dominant cause for observed disequilibrium. Disequilibrium features are preserved best in Tolimán and San Pedro andesites because inferred durations between mixing and eruption are shortest, and consequently, these mixed andesites more clearly record mafic and silicic endmember compositions. The mafic component is a relatively high temperature, high-Al basalt containing phenocrysts of Mg-rich olivine (Fo = 78−80), calcic plagioclase (An 70−80), augite and titanomagnetite. The silicic component contains quartz, sodic plagioclase (An 40−50), Fe-rich orthopyroxene and titanomagnetite. Short durations between mixing and eruption produce petrographic features which, in part, mimic the effects of increasing PH20 in a fractionating magma. Inferred mixing durations for Atitlánn andesites are longer and involve a less-silicic composition. The intervolcano disequilibrium relations suggest that as Si02 in a silicic endmember increases, the duration and efficiency of mixing decreases.

Keywords

Olivine Trench Calcic Plagioclase Pyroxene Phenocryst Hornblende Phenocryst 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Mineral-Reaktionen und Magma-Mixing in Kalk-Alkali-Andesiten vom Atitlan See, Guatemala

Zusammenfassung

Quartäre Kalk-Alkali-Andesite von drei benachbarten Vulkanen aus dem Guatemala Vulkan-Gürtel zeigen anhand ihrer mineralogischen Zusammensetzung und ihrer Textur variierende Bedingungen des Ungleichgewichts. Basalt-Andesite und Andesite (SiO2% = 50−59), die aus dem dem Trench am nächsten gelegenen Vulkan Atitlan eruptierten, lassen den niedrigsten Grad an Ungleichgewicht erkennen. Diese Laven führen eine Phenokristall-Assoziation bestehend aus leicht bimodalem Plagioklas, Olivin, Augit, Orthopyroxen und Magnetit. In Gesteinen mit steigendem GesamtgesteinsSiO2, tritt Orthopyroxen auf Kosten des Olivins auf. Die meisten Pyroxene zeigen einen Trend zur Fe-Anreicherung. Andesite vom Toliman (Si02% = 53−62) und vom Vulkan San Pedro (Si02% = 54-67), die beide weiter vom Trench entfernt liegen, zeigen hohen bzw. mittleren Grad an Ungleichgewicht. Die Toliman Andesite sind durch bimodale Zusammensetzung der Plagioklase und Textur gekennzeichnet. Olivin bleibt auch bei steigendem SiO2-Gehalt bestehen, und zeigt hinsichlich seiner Zusammensetzung keine Verbindung mit koexistierenden Phenokristallen von Orthopyroxen und Hornblende. Im Vergleich zu den Atitlan Andesiten, weisen Toliman Olivine höheren Forsteritgehalt auf, die Pyroxene zeigen häufiger Mg-reiche Ränder, obwohl normal zonierte Phenokristalle auch im selben Gestein auftreten. Die Toliman Andesite sind durch ein kleineres Verhältnis von Phenokristalle/Mikrophenokristalle, höheren Ca-Gehalt der Matrix-Plagioklase und höheren Gehalt an Magnetit-Phenokristallen gekennzeichnet. Die San Pedro Andesite zeigen Ungleichgewichts-Paragenesen vergleichbar mit denen der Toliman Andesite, jedoch nicht so auffallend. Es ist zu vermuten, da\ Magma-Mixing vorwiegend für die UngleichgewichtsParagenesen verantwortlich ist. Die Toliman- und San Pedro Andesite zeigen die best erhaltenen Anzeichen für Ungleichgewicht, da vermutlich der Zeitraum zwischen Magma-Mixing und Eruption am kürzesten gewesen ist, weshalb diese Andesite die gemischten mafischen bzw. salischen Endglieder am besten widerspiegeln. Das mafische Endglied ist ein Al-reicher Hoch-Temperatur-Basalt, der sich aus Phenokristallen von Mg-reichem Olivin (Fo = 78−80), Ca-reichem Plagioklas (An 70−80), Fe-reichem Orthopyroxen und Titanomagnetit zusammensetzt. Das salische Endglied besteht aus Quarz, Na-reichem Plagioklas (An 40−50), Fe-reichem Orthopyroxen und Titanomagnetit. Aufgrund des kurzen Zeitabstandes zwischen Magma-Mixing und Eruption entstehen petrographische Strukturen, die den Effekt von steigendem PH20 in einem fraktionierenden Magma widerspiegeln. Es wird vermutet, da\ bei den Atitlan-Andesiten mehr Zeit zwischen Mixing und Eruption vergangen ist; dies manifestiert sich in einem geringeren Anteil der salischen Komponente. Die Zusammenhänge der Ungleichgewichts-Bedingungen innerhalb eines Vulkans lassen vermuten, daß die Dauer und Intensität des Mixing mit zunehmender salischer Komponente abnimmt.

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Copyright information

© Springer-Verlag 1991

Authors and Affiliations

  • S. P. Halsort
    • 1
  • W. I. Rose
    • 1
  1. 1.Department of Geology, Geophysics and Geological engineeringMichigan Technological UniversityHoughtonUSA

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