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Journal of Petrology | Volume 43 | Number 9 | Pages 1779-1781 | 2002
© Oxford University Press 2002
Cadomian Lower-crustal Contributions to Variscan Granite Petrogenesis (South Bohemian Batholith, Austria): a Comment
1INSTITUT FÜR MINERALOGIE, UNIVERSITÄT SALZBURG, HELLBRUNNERSTRASSE 34, A-5020 SALZBURG, AUSTRIA
2SCHOOL OF EARTH SCIENCES AND GEOGRAPHY, CEESR, KINGSTON UNIVERSITY, PENRHYN ROAD, KINGSTON-UPON-THAMES KT1 2EE, UK
Received December 19, 2001; Revised typescript accepted January 11, 2002
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Klötzli et al. (2001) present a genetic model for a particular granitic rock unit within the Variscan (Carboniferous) South Bohemian Batholith. We studied and published on these same rocks some years previously (Finger & Clemens, 1995). The interpretations of Klötzli et al. (2001) contrast so significantly with our own that we consider it necessary to comment briefly on their paper.
The rock unit in question is the so-called Sarleinsbach quartz monzodiorite (SQMD), first described by Frasl & Finger (1988). This unit is an opx-bearing facies of the Weinsberg granite—a very coarse-grained biotite granite and the most prominent rock type in the batholith. As with all granitic units assigned to the Weinsberg type, the SQMD contains large K-feldspar phenocrysts. On chemical variation diagrams, the data for the SQMD join smoothly with the other Weinsberg points, at the more mafic end of the compositional spectrum. Although such a variation is by no means definitive, this and the field and textural evidence suggest that the SQMD is part of the Weinsberg granite. This has long been held to be the case.
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ORIGIN OF PYROXENES IN THE SQMD |
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We (Finger & Clemens, 1995) considered that the Weinsberg magma (as a whole) originated under granulite-facies conditions, i.e. within the P–T stability field of opx, through partial melting of heterogeneous, high-K, biotite-bearing, lower-crustal source rocks. We interpreted the orthopyroxenes in the SQMD rocks as having crystallized early, and probably at somewhat higher P than the final emplacement of the magma. In the course of protracted cooling and magmatic crystallization, in a crustal section already heated by regional metamorphism, the orthopyroxenes were affected by various exsolution and recrystallization processes. They reacted with the residual melts and became marginally overgrown and replaced by clinopyroxene, amphibole and biotite–quartz symplectites (Haunschmid & Finger, 1994). Such magmatic overgrowths are to be expected because aH2O and aKAlSi3O8 both increase in the residual liquids as crystallization proceeds toward the solidus.
In contrast with this, Klötzli et al. (2001) hypothesize that the pyroxenes are xenocrystal in origin, derived from a Cadomian magmatic rock that crystallized some 200 Myr earlier, and accidentally incorporated into the SQMD magma. Klötzli et al. arrive at this conclusion mainly through a geochronological study of the zircons of the SQMD. This work reveals the existence of a population of zircons with Variscan ages (interpreted as dating the formation and intrusion of the granite magma), and morphologically distinct (so-called J4-type) zircons with Cadomian ages (
520 Ma), clearly pre-dating the Variscan magmatic cycle. They observed the relationship that pyroxene-rich variants of SQMD have a systematically greater proportion of J4-type zircons than pyroxene-poor variants. They therefore concluded that the pyroxenes must also be of Cadomian age.
Such a conclusion represents a considerable leap of faith. A similar age–concentration relationship would exist in the case of magmatic pyroxene growth, if the pyroxenes were early magmatic phases and had accumulated, together with Cadomian restitic zircons, in the more mafic differentiates of an evolving Weinsberg magma. It should be noted that there is firm experimental evidence that pyroxenes appear near the liquidi of many compositional types of granitic magmas under conditions inferred to represent early crystallization (e.g. Clemens & Wall, 1981; Thompson, 1983; Clemens et al., 1986; Johnston & Wyllie, 1988a, 1988b; Whitney, 1988; Johnston & Rutherford, 1989). Furthermore, it has long been recognized that granitic magmas are generally H2O undersaturated when they begin crystallization [e.g. data summarized by Clemens (1984)]. These points have been widely appreciated for decades, and Klötzli et al. adopt contrary points of view, though their paper does not debate such matters.
Apart from the large orthopyroxenes, which we consider to be part of the granitic mineral assemblage, the SQMD locally contains fine-grained, pyroxene-bearing enclaves—from their morphologies and compositions best interpreted as former globules of contemporaneous intermediate to mafic magmas. Figure 3 of Klötzli et al. (2001) appears to show a fragment of just such a microgranodioritic enclave. However, whereas they interpret this as an inherited paragenesis of a Cadomian rock, we would suggest that this is a feature caused by magma mingling. The presence of small pyroxene crystals in the fine-grained enclave and larger pyroxenes in the host granite is permissive evidence for a magmatic origin of these minerals.
Notwithstanding what we have just written, we cannot exclude the possibility that some of the pyroxene may be restitic. Such residual crystals would have formed as products of melting reactions that involved biotite breakdown in the source rocks and become entrained in the granitic melt as it ascended. However, even in this case, the crystallization ages of the pyroxenes would be Variscan, and not Cadomian. The only pre-Variscan minerals in the SQMD are probably the inherited zircons recorded by Klötzli et al. Their ages (Cadomian and Proterozoic) show that a large proportion of the SQMD magma formed through partial melting of pre-existing crust. However, the presence of such inherited zircons (common in such types of granitic rocks) cannot be used to infer the metamorphic grade of the crustal source rocks before the magma-forming event. Whether the lower crust was already granulitic during the Cadomian orogeny, as the title of the Klötzli et al. paper suggests, is a matter of debate. We would expect that the source rocks of the Weinsberg granite were not already granulitic. Otherwise, they would have been quite infertile during the Variscan orogeny (Clemens & Vielzeuf, 1987), i.e. incapable of producing large volumes of granitic partial melts.
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ORIGIN AND TECTONIC SETTING OF THE SQMD MAGMA |
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On the basis of the relatively mafic composition and the unusually high contents of Ba, Sr and Zr, we (Finger & Clemens, 1995) suggested that the SQMD crystallized from a cumulate magma rather than a true melt. We continue to hold this view, and do not share the conclusion of Klötzli et al. (p. 1640) that ‘textural evidence, zircon typology and geochemical fractionation trends rule out the possibility that the pyroxene bearing mineral assemblage found in the Weinsberg granite varieties of Sarleinsbach is of cumulate origin’. In fact, this ‘conclusion’ is merely a contention, totally unsupported by data. Klötzli et al. (p. 1624) refer to ‘details’ given by Koller (1994a, 1994b) and Klötzli & Koller (1998). However, all these works are short conference abstracts, and none contains any information additional to that in the paper under discussion.
Another problematic point in the paper of Klötzli et al. (2001) is their hypothesis that the Weinsberg granite melt formed at 355 ± 9 Ma and intruded at 345 ± 5 Ma, i.e. accompanying Variscan thrust tectonics. This runs counter to almost all recent geological work in the Bohemian Massif, where a post-collisional setting of the South Bohemian Batholith is generally supported (e.g. Büttner & Kruhl, 1997; Büttner, 1999; Franke, 2000; Gerdes et al., 2000). Furthermore, the post-collisional timing of the intrusion of the Weinsberg granite has already been demonstrated through U–Pb monazite dating (Friedl et al., 1996; Friedl, 1997), although this work was completely ignored by Klötzli et al. Finally, it seems debatable whether the geochronological data that they present, relying mainly on zircon evaporation ages, can provide any precise answer to the question of the timing of granite-forming processes in the South Bohemian Batholith.
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FOOTNOTES |
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*Corresponding author. E-mail: Friedrich.Finger{at}sbg.ac.at
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