At Monte Cavaloro near Bologna, outcrops a small olistolith of the ophiolite sequences of the Northern Apeninnes. This small outcrop is known in the geological literature as Bombicci (1868) find out a new type of rock which he named "oligoclasite". Cappellini (1878) studying the same outcrop, named the rock "Cavalorite". These two terms are still present (although obsclete) in the geological nomenclature. The same outcrop was studied by various authors which gave contrasting description of the mineralogy and recognized different rock types, from oligoclase bearing gabbro to quartzdiorite. The last study was by Gazzi (1961) who recognized the presence of stilpnomelane and classified the rock as stilpnomelane-oligoclase bearing gabbro. The present work lead us to the finding of several litothypes in the Monte Cavaloro olistolith: Fe-gabbros, Fe-gabbrobrodiorites, Fe-Ti-P diorites, Fe-Ti-P meladiorites, quartzdiorites and amphibole bearing albitite dikes. The Fe-Ti-P diorites are made up by oligoclase, ferroaugite, ferrohortonolite, ferrosilite, apatite, titanomagnetite and Fe-edenite and appear to be the product of cumulus processes from Fe-rich andesitic magmas which concentrate Fe-Ti oxides when apatite appeared as a liquidus phase. The other associated rocks are genetically linked through flow differentiation processes which generated the Fe-Ti-P meladiorites and the zircon rich quartzdiorites. The differentiation processes, took place in a closed system under low oxygen fugacity; during the late magmatic stages the high volatile content promoted the formation of hydrous minerals (mainly amphiboles) and the peculiar association biotite, cummingtonite-grunerite, stilpnomelane, chlorite. Stilpnomelane is variable in composition and in spite of its chemical features, it may correspond to that find in late stage granophyre of the Skaergaard pluton. This fact is a clear indication of the highly evoluted nature of our stilpnomelane bearing rock in keeping with the presence of stilpnomelane relics in the quartzdiorites. REE and trace elements abundances and composition of apatites and titano-magnetites agree with this genetic interpretation. Liquid immiscibility, as responsible of Fe-Ti-P rock, would seem to be not a viable genetic mechanism. The amphibole bearing albitite dikes represent a late injection of a slightly evolved gabbric magma.
Simboli G., Gasparotto G., Messino J. (2000). Monte Cavaloro: Small differences in outcrop lithology (Fe-Ti-P diorite) of a complex olistolith (Apennines Bologna). MINERALOGICA ET PETROGRAPHICA ACTA, 43, 27-47.
Monte Cavaloro: Small differences in outcrop lithology (Fe-Ti-P diorite) of a complex olistolith (Apennines Bologna)
Simboli G.;Gasparotto G.;
2000
Abstract
At Monte Cavaloro near Bologna, outcrops a small olistolith of the ophiolite sequences of the Northern Apeninnes. This small outcrop is known in the geological literature as Bombicci (1868) find out a new type of rock which he named "oligoclasite". Cappellini (1878) studying the same outcrop, named the rock "Cavalorite". These two terms are still present (although obsclete) in the geological nomenclature. The same outcrop was studied by various authors which gave contrasting description of the mineralogy and recognized different rock types, from oligoclase bearing gabbro to quartzdiorite. The last study was by Gazzi (1961) who recognized the presence of stilpnomelane and classified the rock as stilpnomelane-oligoclase bearing gabbro. The present work lead us to the finding of several litothypes in the Monte Cavaloro olistolith: Fe-gabbros, Fe-gabbrobrodiorites, Fe-Ti-P diorites, Fe-Ti-P meladiorites, quartzdiorites and amphibole bearing albitite dikes. The Fe-Ti-P diorites are made up by oligoclase, ferroaugite, ferrohortonolite, ferrosilite, apatite, titanomagnetite and Fe-edenite and appear to be the product of cumulus processes from Fe-rich andesitic magmas which concentrate Fe-Ti oxides when apatite appeared as a liquidus phase. The other associated rocks are genetically linked through flow differentiation processes which generated the Fe-Ti-P meladiorites and the zircon rich quartzdiorites. The differentiation processes, took place in a closed system under low oxygen fugacity; during the late magmatic stages the high volatile content promoted the formation of hydrous minerals (mainly amphiboles) and the peculiar association biotite, cummingtonite-grunerite, stilpnomelane, chlorite. Stilpnomelane is variable in composition and in spite of its chemical features, it may correspond to that find in late stage granophyre of the Skaergaard pluton. This fact is a clear indication of the highly evoluted nature of our stilpnomelane bearing rock in keeping with the presence of stilpnomelane relics in the quartzdiorites. REE and trace elements abundances and composition of apatites and titano-magnetites agree with this genetic interpretation. Liquid immiscibility, as responsible of Fe-Ti-P rock, would seem to be not a viable genetic mechanism. The amphibole bearing albitite dikes represent a late injection of a slightly evolved gabbric magma.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
