Genesis,evolution and tectonic significance of K-rich volcanics from the Alban Hills (Roman comagmatic region) as inferred from trace element geochemistry |
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| Authors: | A Peccerillo G Poli L Tolomeo |
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| Institution: | (1) Department of Earth Sciences, University of Florence, Via La Pira n. 4, 50100 Firenze, Italy;(2) Institute of Geochemistry, University of Rome, Città Universitaria, Roma, Italy |
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| Abstract: | Trace element data are reported in 21 lava samples from the Alban Hills, one of the most important volcanic complexes of the
Roman comagmatic region. The samples consist mostly of tephritic leucitites with minor phonolitic tephrites and tephritic
phonolites emplaced during two distinct phases of activity, separated by a caldera collapse.
The ferromagnesian element contents are variable (Ni=93-26 ppm; Co=37-20 ppm; Cr=359-5 ppm; Sc=35-6 ppm) and tend to have
higher values in the post-caldera rocks. Rb, Cs, Th, Sr, and LREE are extremely enriched in all the samples analyzed, with
the pre-caldera rocks displaying a lower content of Rb and Cs and a higher abundance of Th, light REE and La/Yb ratio. Ta
and Hf are not so high and are more enriched in the pre-caldera samples. Sr displays comparable values in the two groups of
rocks.
The trace element variation indicates that the rocks from the Alban Hills represent two distinct series of liquids formed
by crystal/liquid fractionation processes starting from two parental magmas. The genesis of the primary melts is hypothesized
as due to a low degree of partial melting of a mantle peridotite enriched in incompatible elements.
All of the studied samples have distribution patterns of incompatible elements normalized against a hypothetical primordial
mantle composition, which are similar to that displayed by the aeolian calc-alkaline and leucite-tephritic products and distinctively
different from those of typical K-rich volcanics from an intraplate rift environment. This strongly supports the hypothesis
that there is a close genetic connection between Roman magmatism and subductionrelated processes. |
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