Volcanic Rocks from Central Italy: An Oxygen Isotopic Microanalytical and Geochemical Study
by Peter Barnekow
Date of Examination:2000-10-30
Date of issue:2001-02-22
Advisor:Prof. Dr. Jochen Hoefs
Referee:Prof. Dr. Jochen Hoefs
Referee:Prof. Dr. Stephen Foley
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http://hdl.handle.net/11858/00-1735-0000-0006-B35E-D
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Abstract
English
The aim of this work is an investigation on the origin of the Tertiary and Quarternary volcanic rocks in central Italy. The volcanism in this region is characterised by potassic to ultrapotassic rocks. In central Italy two different magmatic provinces overlap each other, the Tuscan and the Roman Magmatic Province, where within a small region one can observe a large diversity of different igneous rocks, i.e. rock types varying from potassic trachybasalts to rhyolites. A common feature of these rocks is their crustal signature characterised by both a high enrichment in large ion lithophile elements (LILE) as well as light rare earth elements (LREE) and high 87Sr / 86Sr ratios as well as high ?18O-values. These characteristics are even typical for the less evolved mantle derived rocks. In general the uncommon crustal signature of the volcanic rocks may have been generated by two different processes: I) a metasomatic overprint of the crustal source itself, II) assimilation of crustal material by the mantle melts. In addition a petrogenetic model must also describe the high diversity of igenous rocks in central Italy.The oxygen isotope composition of volcanic rocks can be used to distinguish normal mantle material from crustal material. Generally, oxygen isotope ratios of normal mantle material are between 5 and 6 whereas crustal material typically lies above 8 or even 10 . In order to analyse the oxygen isotope ratio a new laser ablation mass spectrometry technique was applied. The investigation focussed on phenocrystals. Analysing the first crystallising phases from a melt the primary oxygen isotope composition of the melt prior to a possible contamination of the melt, i.e. by fluids, can be estimated. The laser ablation technique has two advantages in comparison with the standard technique: i) refractive minerals, like olivine and garnet, can be analysed with high precision. ii) In situ investigations can be carried out using a UV laser. Using this method the spatial distribution of isotope heterogeneities in a rock section or in a mineral can be measured. iii) Small samples, i.e. minerals of about 1 mm diameter, can be analyzed. A special sample pretreatment method was developed to apply for the analysis of volcanic rocks.Besides oxygen isotope analysis rock samples and phenocrystals were chemically analysed by ICP-MS including Laser Ablation ICP-MS.Beneath central Italy two different mantle domains exists, the one beneath the Tuscan Province and one beneath the Roman Province. The geochemical characteristics of the primitive rocks from the Tuscan and the Roman Province indicate an enriched mantle. The enrichment results in high concentrations of LILE and LREE as well as high ?18O-values. Prior to the enrichment the Tuscan mantle was refractive, characterised by low concentrations of Al, Na and Ca, whereas the Roman mantle was fertile, containing high concentrations of Al, Na and Ca. Element ratios of the primitive rocks also indicate different metasomatic agents of the two mantle parts. Nevertheless both mantle domains show the same enrichment of 18O. Thus the primitive rocks from the Tuscan and Roman Province are not at all or only slightly influenced by crustal assimilation. They represent an enriched mantle. In contrast to the primitive rocks the highly evolved rocks are clearly influenced by crustal assimilation.Between the mantle melts of the Tuscan and the Roman Province transitional rocks occur. This is an indication that in some parts of Tuscany the Tuscan mantle overlaps the Roman mantle.Crustal anatectic rocks are another common genetic rock type in Tuscany. The enrichment of some LILE is too high for typical anatectic rocks. Therefore a fluid phase had to be involved. The origin of this fluid might have been the enriched mantle source beneath central Italy. Some crustal anatectic rocks were also influenced by mixing with a mantle derived melt. This influence is preserved in oxygen isotope heterogeneities among the minerals of the rock samples.On the results of geochemical and oxygen isotope analyses s further hybrid rock type can be found, which was generated by mixing of mantle and crustal melts.The petrogenetic model of the volcanic rocks from central Italy was implemented into the geodynamic model described by Peccerillo & Panza (1999) of the central Mediterranean region. The Tuscan lithospheric mantle domain represent an old mantle contaminated by a subduction event during Alpine age. In contrast to the Tuscan mantle domain the fertile Roman mantle domain has been contaminated recently above the Appenine subduction front.The results of the project presented herewith agree with a petrogenetic model of a metasomatically enriched mantle beneath central Italy.
Keywords: oxygen isotopes; microanalysis; ultrapotassic rocks; mantle metasomatism; Roman Province; Tuscan Province