Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (2) Citing Articles via Google Scholar Google Scholar Articles by BRAGA, R. Articles by ZANETTI, A. Search for Related Content GeoRef GeoRef Citation

Origin of a mica megacryst in an alkaline dyke from the Veneto Volcanic Province, Italy

¹ Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Piazza di Porta S. Donato 1, I-40126 Bologna, Italy
² Istituto di Geoscienze e Georisorse - C.N.R., Sezione di Pavia, Via Ferrata 1, I-27100 Pavia, Italy

^* Corresponding author, e-mail: braga{at}geomin.unibo.it

We report the first finding of a mica megacryst included in an alkaline basic dyke belonging to the Tertiary Veneto Volcanic Province (northern Italy). The megacryst is surrounded by a corona with a mineral assemblage consisting of fine-grained mica flakes + Ti-magnetite. The megacryst is a biotite with a mg# of 0.75, containing 1.35 wt. % TiO₂, 1041 ppm Ba and 3.8 wt. % H₂O. The mica flakes have a mg# of 0.79 and contain 4.33 wt. % TiO₂,21 968 ppm Ba and 3.0 wt. % H₂O. Both mica types show high concentrations of LILE and Nb (461–135 ppm). REE contents are typically below the analytical detection limit. Fractional crystallization modelling shows that the mica megacryst crystallized at depths in the mantle consistent with the spinel-facies stability field. The unusually high Nb_N/Sr_N and Sr_N/Zr_N < 1 observed in the megacryst can be explained by a hydrous K-Fe-rich parent liquid that has already undergone fractional crystallization of Sr-bearing phases such as apatite and plagioclase.

The calculated structural formulae based on 24(O+F+Cl) indicate that full occupancy of the tetrahedral site can only be attained by allowing the entry of a small fraction of Fe³⁺ or Ti⁴⁺. In addition, Ti-oxy (likely coupled with Ba-oxy) substitution occurs to provide charge balance with the excess negative charge at the O4 position. The occurrence of such exchange mechanisms can enhance the thermal stability of the mica flakes that most likely crystallized at shallower depths under high temperature and high f(O₂) conditions.

Key-words: megacryst, mica crystal chemistry, fractional crystallization modelling, ion microprobe, Veneto Volcanic Province.

This article has been cited by other articles:

				V. Hurai, M. Huraiova, P. Konecny, and R. Thomas Mineral-melt-fluid composition of carbonate-bearing cumulate xenoliths in Tertiary alkali basalts of southern Slovakia Mineralogical Magazine, February 1, 2007; 71(1): 63 - 79. [Abstract] [Full Text] [PDF]