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Reply to comment by C. Miller and J. Konzett on “First evidence for ultrahigh‐pressure metamorphism of eclogites in Pohorje, Slovenia: Tracing deep continental subduction in the eastern Alps”

2005; Wiley; Volume: 24; Issue: 6 Linguagem: Inglês

10.1029/2005tc001875

1944-9194

Marian Janák, Nikolaus Froitzheim, Mirijam Vrabec, Erling J. Krogh Ravna,

Geochemistry and Geologic Mapping

[1] Miller and Konzett [2005] comment on our finding of ultrahigh-pressure metamorphism (UHPM) in kyanite eclogites from the Pohorje Mountains (eastern Alps, Slovenia). Their main points are that the microtextural and geothermobarometric evidence for UHP metamorphism in our article [Janák et al., 2004] is only indirect and therefore insufficient. Furthermore, Miller and Konzett doubt the evidence for very high pressure conditions of garnet peridotites in the same area published by Hinterlechner-Ravnik et al. [1991] which was taken as another evidence for UHP metamorphism by Janák et al. [2004]. In addition, Miller and Konzett examine zircon crystals for coesite inclusions but did not find any. In one sample they found an unfractured quartz inclusion in zircon, interpreted to confirm peak pressure conditions just in the quartz stability field. With respect to tectonic implications, Miller and Konzett doubt the pressure gradient of metamorphism in the Austroalpine units from the Koralpe-Saualpe to Pohorje and offer an alternative interpretation according to which the P-T conditions in these areas were not substantially different. In the following, we will reply to these points. [2] In our paper [Janák et al., 2004] we used the following observations as evidence for UHP metamorphism in Pohorje eclogites: (1) polycrystalline quartz inclusions surrounded by radial fractures in garnet, omphacite, and kyanite, interpreted to be pseudomorphs after coesite; (2) quartz rods and needles in omphacite indicating an exsolution from a preexisting supersilicic clinopyroxene; and (3) geothermobarometry on the mineral assemblage garnet + omphacite + kyanite + phengite + quartz/coesite calibrated by Krogh Ravna and Terry [2004]. [3] Regarding the polycrystalline quartz inclusions with associated radial cracks, we discussed several factors of coesite breakdown in our paper, which do not need to be repeated here. Also we agree that rods of SiO2 in clinopyroxene do not necessarily imply UHP conditions [e.g., Page et al., 2005]. In addition to quartz, Miller and Konzett found amphibole in the aggregates and take this as evidence against the breakdown of Ca-Eskola pyroxene according to the reaction 2 Ca0.5AlSi2O6 = CaAl2SiO6 + 3 SiO2. We have not observed the presence of oriented amphibole together with quartz in our samples. In order to reconstruct the composition of primary omphacite we used the integral analysis of omphacite together with SiO2 precipitates under defocused electron beam [Janák et al., 2004] and modal analysis of quartz needles in omphacite using backscattered electron images [Lupták et al., 2004]. Cation deficiency and excess Al on the octahedral site suggest the presence of a Ca-Eskola component (up to 10 mol %) in omphacite. The presence of amphibole may indicate that such clinopyroxene contained enough OH [Katayama and Nakashima, 2003; Terry et al. 2003; Bromiley and Keppler, 2004] to provide that for exsolution of amphibole during decompression. [4] Miller and Konzett recalculated P-T conditions for our samples JV03 and PO6, as well as their own sample CM31/03, using the approach of Brandelik and Massonne [2004] in addition to that of Krogh Ravna and Terry [2004] which we had used in our paper. [5] As a result, Miller and Konzett obtained P-T conditions of 2.9 GPa/719°C for sample JV03 using Brandelik and Massonne's [2004] approach, which is 0.2 GPa less than our result obtained from Krogh Ravna and Terry [2004] but still in the coesite stability field. Despite different thermodynamic data and activity models, both methods indeed yield UHP conditions in our sample. We would like to emphasize that a difference of 0.2 GPa in the thermobarometric results is well within uncertainty. In their own sample CM31/03, taken according to Miller and Konzett from "the same eclogite block as (our sample) JV03," they derived only high-pressure conditions (2.8 GPa/785°C using Krogh Ravna and Terry [2004] and 2.4GPa/760°C using Brandelik and Massonne [2004]) within the stability field of quartz. This only shows that their sample equilibrated at different pressure than ours since mineral compositions, especially that of phengite, are markedly different (3.36 versus 3.48 Si per formula unit). In our experience, phengite most readily breaks down during uplift. The higher Si content of our phengites is certainly reflecting higher pressure. It should be clear that a sample may equilibrate under lower P-T conditions than the peak conditions reached by the rock body from which it was taken, but the opposite is hardly possible. [6] To summarize the P-T conditions, it seems to be obvious that Pohorje eclogites reached higher pressure than 2.5 GPa, in contrast to Sassi et al. [2004]. Miller and Konzett's judgment of the accurateness of the geothermobarometric methods for eclogites is sort of a general comment, but from the recent paper by Krogh Ravna and Terry [2004] it can be seen that this method gives consistent values for most HP/UHP metamorphic terrains examined. [7] Miller and Konzett write the following about the garnet peridotites from Pohorje: "Textures and mineral chemical data … show beyond any reasonable doubt that these rocks are metatroctolites: garnet and olivine are not primary mantle phases since garnet replaces plagioclase. Therefore garnet and olivine need not imply UHP conditions." [8] This statement is very confusing. It suggests that the nature of the protolith (crustal troctolite versus mantle peridotite) is decisive for the question if the rocks experienced UHP metamorphism or not. We do not see how this can be the case. The metamorphic grade depends on the peak assemblage, not on the origin of the protolith. Our study on garnet peridotite from Pohorje yields peak P-T conditions of ∼900°C and 4 GPa, using garnet-olivine and garnet-orthopyroxene Fe-Mg exchange thermometers in combination with the Al-in-orthopyroxene barometer on the assemblage garnet + olivine + orthopyroxene + clinopyroxene + Cr-spinel [Janák et al., 2005]. This supports the previous results by Hinterlechner-Ravnik et al. [1991]. In fact, as inclusions in garnet we found Cr-spinel, Al-rich clinopyropxene and orthopyroxene as lower-pressure assemblage but no plagioclase. Peridotites, regardless of their protolith, were carried down to UHP depth during subduction, along with the eclogites. Garnet peridotites provide an important and additional evidence for UHP metamorphism in the Pohorje Mountains. [9] Miller and Konzett claim that "Cretaceous P-T conditions in Saualpe, Koralpe and Pohorje Mountains are not substantially different". [10] We deduced a southeastward increase in pressure from comparison of our data on Pohorje eclogites (∼3.0 GPa) with those of ∼2.0 GPa reported by Hoinkes et al. [1999] for the Saualpe-Koralpe complex. According to a recent review [Schuster et al., 2004], the highest P-T conditions determined in the Saualpe-Koralpe complex are 680°C/2.0 GPa, obtained from the mica schists [Thöni and Miller, 1996] and 600–650°C/1.8–2.0 GPa from the eclogites [Miller and Thöni, 1997]. In their comment, Miller and Konzett do not present any new geothermobarometric calculations for the Saualpe-Koralpe eclogite facies rocks. Indeed, even their own calculations presented for Pohorje in the comment imply higher P-T there than in Koralpe and Saualpe (2.5 versus 2.0 GPa, a difference that may well exceed uncertainty). [11] Therefore the available data clearly indicate a southeastward increase in metamorphic pressure in the eastern Alps. This is also shown on the new metamorphic map of the Alps [Oberhänsli et al., 2004]. On the other hand, geothermobarometric data on Koralpe and Saualpe eclogites need to be revised if these rocks show microtextural features indicating the breakdown of UHP phases such as in Pohorje. [12] We appreciate the effort of Miller and Konzett to find coesite in zircon although "only one zircon crystal (Pohorje eclogite sample CM27/03, E Vranjek) did contain quartz". We would like to emphasize that from this locality we have not obtained UHP conditions. This locality is ∼9 km west of the UHP eclogites (sample JV3, respectively CM31/03). Miller et al. [2005], using U-Pb zircon and Sm-Nd garnet geochronology, dated the peak metamorphism in the Pohorje eclogites at ca. 90 Ma, which is important information confirming that HP/UHP metamorphism occurred during the Eo-Alpine (Cretaceous) orogeny. This is also crucial for the tectonic model developed in our paper. [13] The pieces of evidence in favor of UHP conditions presented by Janák et al. [2004] are, in our opinion, convincing, although none of them taken alone would be compelling. If one takes microtextural indications and the calculated P-T conditions together, they add up to (at least in our opinion) good enough evidence to assume UHP conditions. [14] We are grateful to Miller and Konzett for their comment, which helped to clarify certain aspects of our paper [Janák et al., 2004]. In conclusion, we have shown that there is ample and growing evidence for UHP metamorphism in the Pohorje Mountains which awaits further studies of this fascinating topic.