Journal of Petrology | Volume 43 | Number 9 | Pages 1783-1786 | 2002
© Oxford University Press 2002
Cadomian Lower-crustal Contributions to Variscan Granite Petrogenesis (South Bohemian Pluton, Austria): a Reply
1LABORATORY FOR GEOCHRONOLOGY, INSTITUTE OF GEOLOGY, UNIVERSITY OF VIENNA, GEOZENTRUM, ALTHANSTRASSE 14, A-1090 VIENNA, AUSTRIA
2INSTITUTE OF PETROLOGY, UNIVERSITY OF VIENNA, GEOZENTRUM, ALTHANSTRASSE 14, A-1090 VIENNA, AUSTRIA
3GEOLOGICAL SURVEY OF AUSTRIA, RASUMOFSKYGASSE 23, A-1030 VIENNA, AUSTRIA
4INSTITUTE OF GEOLOGY AND PALAEONTOLOGY, UNIVERSITY OF SALZBURG, HELLBRUNNERSTRASSE 34, A-5020 SALZBURG, AUSTRIA
Received February 5, 2002; Revised typescript accepted March 14, 2002
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PREFACE |
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The comment by Finger & Clemens (2002) nicely confirms the statement in the introduction of our paper (Klötzli et al., 2001) on p. 1622: ‘Unravelling the origin of a given granite still requires a major analytical effort and commonly ends in some contradictory results’. Finger & Clemens raise a number of issues, both minor and major. We will discuss all of them, as far as they are relevant to the subject. We will start with the problem of nomenclature. After that we will address the problem of remnants of an old crustal component versus a cumulate origin and the problem of the age of the crustal remnants.
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PETROLOGY |
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Opx- and cpx-bearing Weinsberg type granitoid rocks are widespread in an area of
15–20 km2 surrounding the villages of Sarleinsbach, Sprinzenstein and Arnreit in the Southern Bohemian Massif (Klötzli et al., 2001, figs 1 and 2). This type of rock shows a wide variation in chemical and mineralogical composition. The most conspicuous features are large K-feldspars up to 20 cm in size. As a consequence, the determination of modal mineralogy must take into account the large size of the K-feldspar megacrysts and be carried out on large samples only. We referred to this rock as ‘pyroxene-bearing Weinsberg type granite’ as it is doubtful whether it should be named according to IMA rules in view of its very complex origin. Even if we look at these rocks in a purely descriptive sense, neglecting all genetic aspects, the granitoids range in composition from minor quartz monzodiorites to a majority of quartz monzonites and granites according to the IMA nomenclature. The interpretation of Finger & Clemens (2002) that all the rocks are quartz monzodiorites is an oversimplification and conflicts with the geochemical composition of the pyroxene-bearing rocks, their K contents (4–6 wt % K2O) and the constant high amount of modal K-feldspar. In addition, in all norm calculations the amount of normative K-feldspar has to be corrected according to the measured preserved feldspar composition of Or0·78Ab0·20An0·01Cs0·01 (F. Koller, 2000).
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Finger & Clemens (2002) in their comment accept the formation of the Weinsberg granite sensu lato magma under granulite-facies conditions within the stability field of opx, as was already postulated by Koller & Höck (1993). Finger & Clemens interpret opx and cpx as a product of fractional crystallization of a quartz-monzodioritic magma after partial melting of biotite-bearing lower-crustal rocks. Consequently, in their view the pyroxenes are cumulate phases. In contrast, we interpret the pyroxenes as part of an older magmatic event with a superimposed granulite-facies re-equilibration texture. Our interpretation is based on a careful mineralogical–petrographical, textural and geochemical study.
There is a lot of evidence that suggests that a cumulate origin for the pyroxenes is not likely. Extensive optical and electron microprobe studies of the pyroxenes have shown that most of them are homogeneous in composition. Figure 1 clearly demonstrates the coexistence of the homogeneous opx and cpx. Nevertheless, there are, of course, some examples of exsolution lamellae, mainly in cpx. Not only Fig. 1, but also fig. 3b of Klötzli et al. (2001) demonstrates clearly that cpx is not a replacement product of opx but rather coexists with opx in textural equilibrium. Furthermore, andesine–bytownite, K-fsp and quartz contribute to the older, granulite-facies mineral assemblage of which opx and cpx form inclusions in large feldspar crystals, which in our view are remnants of the granulite-facies mineral assemblage. It has to be noted, in addition, that inclusions and host minerals are in mutual textural equilibrium (Fig. 1). The composition of these inclusions is, within analytical precision, identical to the composition of the pyroxenes outside the feldspars. These clear equilibrium relations allow the application of various geothermobarometry methods. Opx–cpx thermometry (Andersen et al., 1993) and cpx–plagioclase barometry (Newton, 1983) give consistent results for all investigated samples. Equilibrium temperatures and pressures are 755 ± 26°C and 0·75 + 0·10 GPa. In view of the textural evidence and the internal consistency of the results we consider that the calculated P–T conditions are petrologically meaningful. For a cumulate system including opx and cpx these T conditions are definitely too low. We are fully aware ‘that pyroxenes appear near the liquidi of many compositional types of granitic magmas’. A convincing example from the Ballachulish Igneous Complex was described by Weiss & Troll (1989). From experimental and field evidence (Naney, 1983; Clemens, 1984; Clemens et al., 1986; Weiss & Troll, 1989; Troll & Weiss, 1991) the liquidus temperatures and thus the first appearance of opx are significantly (up to 200°C) higher than the calculated P–T conditions from Sarleinsbach. Therefore, in the case of a cumulate nature of the pyroxenes from Sarleinsbach, we would expect that at least the inclusions in the feldspars should still contain some information on their origin close to a liquidus system within an upper-crustal intrusion. This would be, for instance, exsolution phenomena or partly preserved mineral compositions characteristic of higher formation temperatures.
In addition to the fact that the temperature calculations provide a strong argument against a cumulate origin for the pyroxenes, there is completely independent evidence from geochemistry. In contrast to what would be expected from cumulate-forming processes, the abundances of compatible elements (i.e. Cr, Ni, Co; with respect to the pyroxenes) in the pyroxene-bearing rocks are lower than or comparable with those in the pyroxene-free Weinsberg granite, and, vice versa, the incompatible elements (i.e. Zr, Nb, Y, REE; again with respect to both pyroxenes) are high in the pyroxene-bearing rocks and low in the pyroxene-free granites.
Other important features are the common replacement textures, which allow further conclusions on the evolution of the Sarleinsbach rocks:
- Opx is commonly rimmed by quartz and tiny biotite needles as a first replacement reaction. This represents the breakdown of 3opx + 1kfsp + H2O = 1bio + 3qu. Higher amounts of quartz, when compared with biotite, in these reaction textures support this interpretation rather than a reaction involving a melt (i.e. opx + melt + H2O = bio). The latter might remove the reaction products into the melt. These reaction areas are commonly overgrown by large biotite crystals related to the granitic assemblage (compare Haunschmid & Finger, 1994) but without obliterating the quartz–biotite reaction area.
- Opx also can show a normal breakdown reaction to cummingtonite–grunerite and a later replacement to hornblende. This reaction type is rather typical for a magmatic or early subsolidus replacement, commonly also found in all Moldanubian diorites (Koller & Niedermayr, 1981).
- In rare cases cpx shows a reaction zone of quartz and a coronitic ferro-tschermakite, which is also more typical for a solid-state reaction than a magmatic reaction. Also here it is clear that the biotites of the granitic assemblage appear later.
- The most common breakdown of cpx is an uralitic replacement by an amphibole with Fe-hornblende to Fe-actinolite composition. This amphibole replacement forms both idiomorphic and xenomorphic amphiboles and might represent a magmatic to post-magmatic (Weinsberg granite) evolution.
- In some cases cpx is simply overgrown by the large Weinsberg granite biotite generation.
- Around the large K-feldspar crystals a replacement of vermicular quartz and oligoclase is rather common. In some rare cases the large Weinsberg granite biotite generation is part of this myrmekitic reaction zone.
The observed solid–solid reaction textures [particularly those described in points (1) and (3)] imply clearly that the breakdown of both pyroxenes must have taken place before any melting process. Therefore, we conclude that the pyroxenes must have existed before the onset of the Variscan melting.
Further arguments for the old age of the pyroxenes can be drawn from the age of their K-feldspar hosts. Both Pb and Sr isotope systematics support the assumption that at least some of the K-feldspars crystallized before Variscan magmatism. Evidently, if the K-feldspars are pre-Variscan their inclusions must also be of the same age or even older. K-feldspar Pb isotopic compositions favouring a pre-Variscan crystallization age have also been recorded in the Rastenberg pluton (Gerdes et al., 2000; Klötzli, 1999) and the Mecsek Mountains high-K–Mg granitoids of South Hungary (Buda et al., 2000).
Bearing in mind all this evidence, it seems an unavoidable consequence that, combined with the prevalence of the pre-Variscan J4 zircons, the whole assemblage has to be an Early Palaeozoic one. A leap of faith?
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GEOCHRONOLOGY |
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Finger & Clemens (2002) do not believe in our zircon U/Pb and Pb/Pb evaporation data as providing a reliable age estimate for the formation and emplacement of the pyroxene-bearing rocks of Sarleinsbach. Despite the fact that these ages may be important for large parts of the South Bohemian Batholith, if they were to be verified in other parts of it, we restricted the validity of our conclusions only to the immediate surroundings of Sarleinsbach. Finger & Clemens (2002) argue for a post-collisional age of intrusion. The supporting age data referred to are reported only in abstract form by Friedl et al. (1996). We are in complete agreement with Finger & Clemens (2002) that the intrusion of parts of the Weinsberg granite probably occurred around 328 Ma [monazite U/Pb age data of Friedl et al. (1996)]. However, in view of the complex age relations observed in the multiple intrusions forming the Weinsberg granite sensu lato it seems rather daring to use monazite U/Pb ages derived from completely different parts (a minimum 50 km distance to Sarleinsbach) of the batholith to ‘date’ the pyroxene-bearing rocks of Sarleinsbach. We have therefore not ignored the monazite U/Pb age data of Friedl and coworkers, but simply find them not relevant to the genesis of the Sarleinsbach complex.
Thus far, the pyroxene-bearing rocks of Sarleinsbach have not been found to contain any primary monazite. Apart from the abundant zircon and apatite, only large crystals of allanite are found. Therefore it seems even more problematic to compare the U/Pb monazite ages of Friedl et al. (1996) with the age systematics of the Sarleinsbach rocks. It certainly would be interesting to date the granulite-facies mineral assemblage directly. But this has not been feasible, as it was not possible, for example, to separate opx and cpx for Sm/Nd isotopic analyses.
Some minor points raised by Finger & Clemens (2002) involve the questions of (1) magma mingling; (2) the water-undersaturated nature of granitic magmas; (3) the relation of the J4 zircon fraction to the metamorphic grade.
Finger & Clemens (2002) suggest that a small enclave of the inherited mineral assemblage, as shown in fig. 3 of Klötzli et al. (2001), could be better interpreted as a ‘feature due to magma mingling’. In the view of the arguments outlined above, such an interpretation seems unlikely to us. Furthermore, it should not be ‘magma mingling’ sensu stricto but rather a mixture of a magma with cumulate phases according to their interpretation.
Regarding points (2) and (3) it is not appropriate to debate the comments of Finger & Clemens (2002), simply because nowhere in our original paper did we touch on any of these problems.
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FOOTNOTES |
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*Corresponding author. E-mail: urs.kloetzli{at}univie.ac.at.
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REFERENCES |
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