Summary
Chrome-bearing spinels in 21 ultramafic nodules contained in a basanite from San Giovanni Ilarione (SGI), Veneto Region, Italy, have been grouped from a textural and chemical standpoint into five types: (1) Interstitial, surrounded by silicates, homogeneous, with a Cr/(Cr + Al) ratio averaging 0.11; (2) Within alteration pods and veinlets, in contact with either silicates or alteration products, showing zonation with Mg/(Mg + Fe2+), Cr, and Ti increasing and Al decreasing from core to rim; (3) In contact with the basalt or surrounded by basalt but distinct from ground-mass grains. The former are strongly zoned while the latter, referred to as xenocrysts, are relatively homogeneous. Both belong to a trend distinct from that of (2); (4) Symplectic, intergrown with pyroxenes and homogeneous, or in contact with plagioclase and/or reaction products and showing more variable composition than the former. Both are generally richer in Cr than all other spinet types and define a trend parallel to and with higher Cr/(Cr + Al) and Cr values than that of (3); and (5) groundmass grains within the host basalt, showing lower Mg/(Mg + Fe2+) and Cr/(Cr + Al), and higher Fe values than for all other, types.
Group 1 spinels and the cores of group 2 and group 3 spinets display very similar compositions, suggesting a common origin, modified by later events such as partial melting, solid-liquid reactions, and subsolidus reactions which occurred prior to, during, and subsequent to interaction with the host basalt. Group 2 and 3 spinels define trends diverging from a common composition—i.e., the same origin. Group 4 spinels are probably due to exsolution which occurred after formation of the original spinels, while group 5 spinels are not directly related to types 1–4.
Zusammenfassung
Chromhaltige Spinelle 21 ultramafischer Knollen aus einem Basanit von San Giovanni Ilarione (SGI), Region Veneto, Italien, wurden nach texturellen und chemischen Gesichtspunkten in fünf Gruppen eingeteilt: 1. Interstitiell, von Silikaten umgeben, mit einem durchschnittlichen Cr/(Cr + Al)-Verhältnis von 0,11.2. In Umwandlungstaschen und -gängchen, in Kontakt entweder mit Silikaten oder mit Umwandlungsprodukten, zoniert mit Mg/(Mg + Fe2+), Cr und Ti vom Kern zum Rand zunehmend, Al aber abnehmend. 3. Im Kontakt zum Basalt oder vom Basalt umgeben, aber von den Körnern der Grundmasse verschieden. Die ersteren sind stark zonar, während die Letzteren, die als Xenokrysten angesehen werden, relativ homogen sind. Beide gehören zu einem Trend, welcher von jenem der Gruppe 2 verschieden ist. 4. Simplektitisch, mit Pyroxenen verwachen und homogen, oder im Kontakt zu Plagioklas und/oder Reaktionsprodukten und von variablerer Zusammensetzung ab die ersteren. Beide sind allgemein reicher an Cr als alle anderen Spinelltypen und sie definieren einen Trend parallel zu und mit höheren Cr/(Cr + Al)- und Cr- Werten als jene von Gruppe 3. 5. Körner der Grundmasse des Wirtsbasaltes, die niedrigere Mg/(Mg + Fe2+)- und Cr/(Cr + Al)- Werte und höhere Fe-Werte zeigen als alle anderen Typen.
Die Spinelle der Gruppe 1 und die Kerne der Spinelle der Gruppen 2 und 3 zeigen sehr ähnliche Zusammensetzungen, die auf einen gemeinsamen Ursprung hinweisen, der später modifiziert wurde durch Ereignisse wie partielles Schmelzen, fest-flüssig-Reaktionen und subsolidus-Reaktionen, die vor, wärend und nach der Wechselwirkung mit dem Wirtsbasalt stattfanden. Die Spinelle der Gruppen 2 und 3 definieren Trends, die von einer gemeinsamen Zusammensetzung und damit von einem gemeinsamen Ursprung her auseinanderlaufen Spinelle der Gruppe 4 entstanden wahrscheinlich durch Entmischung, die nach Bildung der Ursprungsspinele eintrat, während die Spinelle der Gruppe 5 zu denen der Gruppen 1 bis 4 in keiner direkten Beziehung stehen.
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Morten, L., Taylor, L.A. & Durazzo, A. Spinels in Harzburgite and Lherzolite inclusions from the San Giovanni Ilarione Quarry, Lessini Mountains, Veneto Region, Italy. Mineralogy and Petrology 40, 73–89 (1989). https://doi.org/10.1007/BF01162470
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DOI: https://doi.org/10.1007/BF01162470