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| Titel |
Generating contrasting granitic melts from the same source: the ca. 3.1 Ga Heerenveen and Mpuluzi batholiths, South Africa |
| VerfasserIn |
Jean-François Moyen, Simon Carrouée, Michel Cuney, Armin Zeh |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250090282
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| Publikation (Nr.) |
 EGU/EGU2014-4506.pdf |
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| Zusammenfassung |
| The Mpuluzi and Heerenveen batholiths of the Barberton Granite—Greenstone Terrain (BGGT),
South Africa, belong to the post-tectonic, ca. 3.1 Ga GMS (granite—monzogranite—syenite)
suite. They are complex batholiths formed by successive intrusion phases; and they
include a range of granitoid phases, defining at least three main magmatic series: (1)
leucogranites and granites; (2) quartz-monzonites and syenogranites; (3) tonalites and
relatively mafic granodiorites. Despite their close spatial and temporal association,
each series is very distinct geochemically, demonstrating the coexistence of various
magma types. The emplacement sequence starts with leucogranites, moving on to
(slightly porphyritic) granites forming the bulk of the batholiths; to late leucogranites,
syenogranites/monzonites and dark granodiorites emplacing in late, syn-magmatic shear
zones.
However, despite the clear emplacement sequence and the well identified, successive
emplacement phases, the isotopic characteristics of the GMS batholiths are strikingly
homogeneous. Zircons from 18 samples of the Mpuluzi and Heerenveen batholiths,
representative of all phases and magmatic series, were dated and analysed for Hf isotopes.
Regardless of the relative age and petrological type, they demonstrate emplacement ages
identical within error (3106 ± 8 Ma); and, perhaps more surprinsingly, similar Hf isotopic
values (εHf(T) = -1.5 ± 0.7, corresponding to model ages of ca. 3520 Ma). This would
suggest that a diverse range of granitoids, ranging from leucogranites to syenites,
including both peraluminous and metaluminous compositions, and spanning both
sodic and potassic compositions (0.9 < Na2O-K2O < 2.5), were all generated
concomitantly (or in short succession) from the same source, or from isotopically similar
sources.
The lack of mafic components, associated with the crustal signature of all granitoids,
implies a crustal source for the GMS granites. Regionally, both TTG plutonic rocks
(Steynsdorp pluton and component of the Ancient Gneiss Complex of Swaziland, ACG), and
felsic volcaniclastics and amphibolites of the Theespruit formation emplaced at ca. 3.52 Ga,
and represent potential sources. In all geochemical diagrams, the different rock types do not
define single trends; rather, each magmatic series define its own, individual trend,
precluding the different facies to be represent a single differentiation (or mixing)
series.
We suggest that the various component of the GMS granitoids mirror fast melting of a
composite crust. Melting of the Theespruit volcaniclastics resulted in the generation of
leucogranites and granites. Melting of amphibolitic portions of the Theespruit group yielded
granodiorites and tonalites. Finally, the syenogranites are the result of melting of dry
lithologies – either a dry tonalitic basement (ACG), or melt-depleted portions of
Theespruit volcaniclastics, that lost melt due to the extraction of the leucogranites and
granites. |
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