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Journal of Petrology | Volume 45 | Number 4 | Pages 759-792 | 2004
Journal of Petrology 45(4) © Oxford University Press 2004; all rights reserved.
Origin of Minette by Mixing of Lamproite and Dacite Magmas in Veliki Majdan, Serbia
1,2,*2
1 INSTITUTE OF GEOLOGICAL SCIENCES, UNIVERSITY OF GREIFSWALD, JAHNSTRASSE 17A, D-17489 GREIFSWALD, GERMANY
2 FACULTY OF MINING AND GEOLOGY, UNIVERSITY OF BELGRADE, ðU
INA 7, 11000 BELGRADE, YUGOSLAVIA
3 GEOFORSCHUNGSZENTRUM POTSDAM, TELEGRAFENBERG, D-14473 POTSDAM, GERMANY
* Corresponding author. Present address: Faculty of Mining and Geology, University of Belgrade, ðu
ina 7, 11000 Belgrade, Yugoslavia. Telephone and fax: +381-(0)11-630020. E-mail: prelev{at}uni-greifswald.de
Composite dykes consisting of leucominette and dacite as well as discrete dykes and flows of minette and lamproite composition, occur in the Veliki Majdan area, western Serbia. This area is part of the Serbian Tertiary magmatic province, which consists of numerous small occurrences of ultrapotassic igneous rocks. The composite dykes have leucominette margins (up to 150 cm thick) enclosing a central part of dacite up to 100 m in width. Between these two lithologies, a decimetre-sized transition zone may occur. Petrography, mineral chemistry and bulk-rock geochemistry, including Sr, Nd and Pb isotopes, provide evidence that the minettes and leucominettes formed by hybridization between a felsic magma similar in composition to dacite and a mantle-derived lamproitic magma. The leucominettes and minettes contain all phenocryst types (biotite, plagioclase, quartz) present in the dacites, but in partly resorbed and reacted form. The mica displays a great diversity of resorption textures as a result of partial dissolution, incipient melting and phlogopitization, suggesting superheating of the felsic melt during hybridization; the mineral modes and mineral compositions of the leucominettes and minettes resemble those in the lamproites. A model for the modification of lamproite melt towards minette is presented in which minette is formed by mixing of lamproite and <30% felsic magma. The lack of any significant correlation between Pb isotopic ratios and some of the ‘mixing-indices’ (SiO2, Zr, Zr/Nb, 143Nd/144Ndi) recognized in the hybridization model for the Veliki Majdan dykes may be a result of similarity of the Pb-isotopic signature in the two end-members. Highly phlogopitized biotite xenocrysts in the minettes are ascribed to the retention of volatile components after magma mixing and crystallization of a new generation of phlogopite from the hybridized magma. The magma-mixing model explains the reverse zoning and resorption features of phlogopite macrocrysts commonly recognized in calcalkaline lamprophyres elsewhere. Therefore, this mixing mechanism may be globally applicable for the origin of minettes associated with calcalkaline granitic plutonism in post-orogenic settings.
KEY WORDS: Serbia; lamproites; micas; phlogopitization; calcalkaline lamprophyres; superheating; magma mixing
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