Journal Article

Mafic and Felsic Magma Interaction in Granites: the Hercynian Karkonosze Pluton (Sudetes, Bohemian Massif)

E. Słaby and H. Martin

in Journal of Petrology

Volume 49, issue 2, pages 353-391
Published in print February 2008 | ISSN: 0022-3530
Published online January 2008 | e-ISSN: 1460-2415 | DOI: https://dx.doi.org/10.1093/petrology/egm085
Mafic and Felsic Magma Interaction in Granites: the Hercynian Karkonosze Pluton (Sudetes, Bohemian Massif)

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The Hercynian, post-collisional Karkonosze pluton contains several lithologies: equigranular and porphyritic granites, hybrid quartz diorites and granodiorites, microgranular magmatic enclaves, and composite and lamprophyre dykes. Field relationships, mineralogy and major- and trace-element geochemistry show that: (1) the equigranular granite is differentiated and evolved by small degrees of fractional crystallization and that it is free of contamination by mafic magma; (2) all other components are affected by mixing. The end-members of the mixing process were a porphyritic granite and a mafic lamprophyre. The degree of mixing varied widely depending on both place and time. All of the processes involved are assessed quantitatively with the following conclusions. Most of the pluton was affected by mixing, implying that huge volumes (>75 km3) of mafic magma were available. This mafic magma probably supplied the additional heat necessary to initiate crustal melting; part of this heat could have also been released as latent heat of crystallization. Only a very small part of the Karkonosze granite escaped interaction with mafic magma, specifically the equigranular granite and a subordinate part of the porphyritic granite. Minerals from these facies are compositionally homogeneous and/or normally zoned, which, together with geochemical modelling, indicates that they evolved by small degrees of fractional crystallization (<20%). Accessory minerals played an important role during magmatic differentiation and, thus, the fractional crystallization history is better recorded by trace rather than by major elements. The interactions between mafic and felsic magmas reflect their viscosity contrast. With increasing viscosity contrast, the magmatic relationships change from homogeneous, hybrid quartz diorites–granodiorites, to rounded magmatic enclaves, to composite dykes and finally to dykes with chilled margins. These relationships indicate that injection of mafic magma into the granite took place over the whole crystallization history. Consequently, a long-lived mafic source coexisted together with the granite magma. Mafic magmas were derived either directly from the mantle or via one or more crustal storage reservoirs. Compatible element abundances (e.g. Ni) show that the mafic magmas that interacted with the granite were progressively poorer in Ni in the order hybrid quartz diorites—granodiorites—enclaves—composite dykes. This indicates that the felsic and mafic magmas evolved independently, which, in the case of the Karkonosze granite, favours a deep-seated magma chamber rather than a continuous flux from mantle. Two magma sources (mantle and crust) coexisted, and melted almost contemporaneously; the two reservoirs evolved independently by fractional crystallization. However, mafic magma was continuously being intruded into the crystallizing granite, with more or less complete mixing. Several lines of evidence (e.g. magmatic flux structures, incorporation of granite feldspars into mafic magma, feldspar zoning with fluctuating trace element patterns reflecting rapid changes in magma composition) indicate that, during its emplacement and crystallization, the granite body was affected by strong internal movements. These would favour more complete and efficient mixing. The systematic spatial–temporal association of lamprophyres with crustal magmas is interpreted as indicating that their mantle source is a fertile peridotite, possibly enriched (metasomatized) by earlier subduction processes.

Keywords: Bohemian Massif; fractional crystallization; geochemical modelling; hybridization; Karkonosze

Journal Article.  18125 words.  Illustrated.

Subjects: Petrology

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