Summary
Phlogopite and biotite coexist in the ultrapotassic rocks from Cabezo Negro de Zeneta (SE Spain). The compositional range of the early crystallizing phlogopite is comparable to other Spanish lamproitic occurrences, except that it is higher in Al2O3, probably reflecting the higher Al2O3 and/or different oxygen fugacity of the Zeneta magma. Magmatic Al-rich and metamorphic Al-poor biotites also occur in these rocks. The magmatic biotite probably crystallised from intermediate to silicic peraluminous magma(s), whereas the metamorphic type comes from crustal relics of metapelitic rocks entrained and dismembered into the lamproitic melt. It is concluded that the melt of Zeneta was generated through the mixing of a Mg-rich lamproitic component, quantitatively dominant, with a crustal-derived anatectic component, both already partially crystallised before mixing. The “mixed” melt attained chemical homogenization as suggested by the development of late overgrowths of similar composition on the two micas.
Zusammenfassung
Phlogopit und Biotit koexistieren in den ultrapotassischen Gesteinen von Cabézo Negro de Zeneta (Südost-Spanien). Die Zusammensetzung des frühkristallisierenden Phlogopits ist mit der anderer spanischer Lamproit-Lokalitäten vergleichbar, mit der einen Ausnahme, daß die Al2O3-Gehalte höher sind. Letzteres geht wahrscheinlich auf den höheren Al2O3-Gehalt, und/oder auf verschiedene Sauerstoff-Fugazität des Zeneta-Magmas zurück. Magmatische Al-reiche und metamorphe Al-arme Biotite kommen auch in diesen Gesteinen vor. Der magmatische Biotit kristallisierte wahrscheinlich aus intermediären bis sauren Al-reichen Magmen, während der metamorphe Typ auf krustale Relikte metapelitischer Gesteine in der lamproitischen Schmelze zurückgeht. So ergibt sich die SchluBfolgerung, daß die Schmelze von Zeneta durch Mischung eines Mg-reichen lamproitischen, und quantitativ dominierenden Magmas, mit einer anatektischen Komponente von Krusten-Herkunft entstanden ist. Beide dürften vor der Mischung bereits teilweise kristallisiert gewesen sein. Die „gemischte” Schmelze erreichte chemische Homogenisierung wie durch die Entwicklung späterer Überwachsungszonen von ähnlicher Zusammensetzung auf beiden Glimmertypen gezeigt wird.
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Toscani, L., Contini, S. & Ferrarini, M. Lamproitic rocks from Cabezo Negro de Zeneta: Brown micas as a record of magma mixing. Mineralogy and Petrology 55, 281–292 (1995). https://doi.org/10.1007/BF01165122
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DOI: https://doi.org/10.1007/BF01165122