Lithospheric mantle evolution: Evidences from ultramafic xenoliths in the Lessinian volcanics (northern Italy)

F. Siena*, M. Coltorti

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

28 Citations (Scopus)

Abstract

The Lower-Middle Eocene volcanic rocks from the Monti Lessini area, Veneto region, carry a suite of inclusions of mostly lherzolitic composition. The magmatic activity represents the most important example of alkaline within-continental plate volcanism occurred within the South Alpine Domain during post-collisional age. The lavas, including xenoliths, range from alkali-olivine basalts to basanites. Their anorogenic character is evidenced by the absence of any negative anomalies for Nb and Ti. The high mg-values, the Ni content, and the forsteritic nature of the olivine indicate that these lavas may be considered undifferentiated or very slightly differentiated mantle-derived magmas. The great majority of xenoliths are spinel lherzolites, plus minor harzburgites and rare websterites and olivine-websterites. Protogranular textures are common to all xenoliths; pyrometamorphic textures, with SiO2/1bNa2O/1bK2O/1brich glassy patches or blebs, are widespread and have been related to local incipient melting, possibly favoured by the uprising of the mantle materials incorporated in the mafic lavas. The bulk-rock and mineral chemistry of the nodules outlines a depletion sequence from the nearly primitive to the most depleted mantle material. However, variable and anomalously high contents of the most incompatible elements such as La, Ce, Rb, Sr, Nb, Zr, K, and to a lesser extent, Na, indicate for most nodules a more complicated petrogenetic history. A petrological model based on incompatible elements at mantle level suggests that peridotite inclusions have to be considered as variably depleted mantle material, accidentally incorporated into the host magma during its ascent. The proposed sequence of events is: (a) partial melting episodes responsible for the depletion of mantle materials; (b) partial equilibration of mantle materials caused by their upwelling to shallower levels and subsequent long residence time periods, possibly in connection with the rifting stage which affected the Southern Alps during Triassic time; (c) contamination events by alkaline fluids with variable H2O CO2 ratio, which occurred after the mantle depletion, but before and/or during the incorporation of xenoliths in the alkaline host magma; and (d) local incipient melting episodes during the ascent and subsequent decompression of the nodules, resulting in their pyrometamorphic textures.

Original languageEnglish
Pages (from-to)347-364
Number of pages18
JournalChemical Geology
Volume77
Issue number3-4
DOIs
Publication statusPublished - 15 Nov 1989
Externally publishedYes

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