Summary
The results of microprobe analyses of clinopyroxenes from alkaline melasyenites and layered melagabbros, produced by intra-plate magmatism of Paleocene age at Punta delle Pietre Nere, are here given and discussed.
The analysed pyroxenes range from diopsidic to acmite-rich compositions.
The first crystallized pyroxenes (diopside) show AlVI contents suggesting shallow depths of crystallization. In addition pyroxenes from melasyenite and those from melagabbro display different Cr contents, Al/Ti and Mg/(Mg+Fe2++Fe3+) ratios confirming their crystallization from melts produced by different parental liquids.
Diopsides and salites show an overall trend towards high Al, Ti and Fe3+, suggesting that the crystallization occurred under decreasing SiO2/Al2O3 ratios and under relatively highpH2O−pO2 conditions.
Pyroxenes from the Pietre Nere melasyenite show a progressive variation towards acmite rich compositions at Mg/(Mg+Fe2++Fe3+) lower than 0.5; those from the layered melagabbro, instead, show a continuous enrichment in Ca Fe3+ AlSiO6. This different behaviour is due to the co-crystallization, with the latest pyroxenes, of phases with different K/Na and Si/Al ratios.
Zusammenfassung
Es werden die Ergebnisse der Mikrosonden-Untersuchungen von Klinopyroxenen aus Alkali-Melasyeniten und schichtigen Alkali-Melagabbros, die durch “Intra-plate”-Magmatismus paläozenen Alters auf Punta delle Pietre Nere entstanden sind, beschrieben und erörtert.
Die untersuchten Pyroxene reichen von diopsidischen bis zu Akmit-reichen Zusammensetzungen.
Die zuerst kristallisierten Pyroxene (Diopsid) zeigen AlVI-Gehalte, die auf geringe Tiefe des Kristallisationsvorganges hinweisen. Dazu zeigen die Pyroxene aus dem Melasyenit und aus dem Melagabbro unterschiedliche Cr-Gehalte; die Al/Ti- und Mg/(Mg+Fe2++Fe3+)-Verhältnisse bestätigen deren Kristallisation aus Schmelzen, die aus unterschiedlichen Ursprungsmagmen stammen.
Die Diopside und Salite zeigen eine einheitliche Tendenz zu hohem Al-, Ti- und Fe3+-Gehalt; dies deutet darauf hin, daß die Kristallisation unter abnehmenden SiO2/Al2O3-Verhältnissen und unter relativ hohenpH2O−pO2-Bedingungen stattfand.
Die Pyroxene aus dem Punta delle Pietre Nere-Melasyenit zeigen eine zunehmende Änderung zu Akmit-reichen Zusammensetzungen bei weniger als 0,5 Mg/(Mg+Fe2++Fe3+); die Pyroxene aus dem schichtig differenzierten Melagabbro zeigen dagegen eine allmähliche Zunahme von CaFe3+AlSiO6. Dieses unterschiedliche Verhalten rührt daher, daß Mineralphasen mit unterschiedlichen K/Na- und Si/Al-Verhältnissen zugleich mit den zuletzt gebildeten Pyroxenen kristallisierten.
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De Fino, M., La Volpe, L. & Piccarreta, G. Crystallization trends of pyroxenes from alkaline subvolcanites of Punta delle Pietre Nere (Gargano, Southern Italy). TMPM Tschermaks Petr. Mitt. 29, 177–192 (1982). https://doi.org/10.1007/BF01084641
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DOI: https://doi.org/10.1007/BF01084641