| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Journal of Petrology 45(7) © Oxford University Press 2004; all rights reserved
Complex Growth Textures in a Polymetamorphic Metabasite from the Kraubath Massif (Eastern Alps)
1 INSTITUTE OF PETROLOGY AND STRUCTURAL GEOLOGY, CHARLES UNIVERSITY, PRAGUE, CZECH REPUBLIC
2 INSTITUTE OF MINERALOGY AND PETROLOGY, KARL-FRANZENS-UNIVERSITY, GRAZ, AUSTRIA
RECEIVED NOVEMBER 20, 2002; ACCEPTED JANUARY 15, 2004
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONGEOLOGY AND METAMORPHIC HISTORYPETROGRAPHY AND MINERALOGY OF...P-T CONDITIONS OF METAMORPHISMDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Zoned garnet and amphibole occur in metabasites of the Kraubath Massif, Eastern Alps, that contain relic magmatic clinopyroxene. The amphibole composition gradually changes from core (XMg = 0·83) to rim (XMg = 0·6–0·7). A number of compositional varieties of garnet occur in the metabasite. An older porphyroblastic garnet (Py23–27, Alm41–43, Grs29–33) has two different compositional domains, one relatively rich in Mg (Py27–30) and the other rich in Ca (Grs35–38) with a low Mg (Py20–25) content. The youngest variety, which forms rims on, or microveins in, the porphyroblastic garnet, has high Ca and low Mg (Grs40–57, Py2–7, Alm46–51). The amphibole cores and garnet porphyroblasts are interpreted to represent minerals formed during Variscan regional metamorphism under amphibolite-facies conditions. Alpine metamorphism is represented by the most recent Ca-rich and Mg-poor variety of garnet that coexists with the amphibole rims, epidote and chlorite. Fracturing in the porphyroblastic garnet probably originated during retrogression of the Variscan amphibolite-facies assemblages. Textural relations suggest that the garnet in the microveins formed by dehydration of hydrous phases during an Alpine metamorphic overprint that reached P–T conditions of 550–583°C at 1·0 GPa.
KEY WORDS: microveins; garnet; metabasites; Kraubath Massif; Eastern Alps
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGY AND METAMORPHIC HISTORYPETROGRAPHY AND MINERALOGY OF...P-T CONDITIONS OF METAMORPHISMDISCUSSIONCONCLUSIONSREFERENCES |
|---|
The overgrowth of a mineral on itself is a well-documented petrographic feature in metamorphic rocks. Especially for garnets, there are many reported examples in which a second metamorphic event led to the overgrowth of garnet of a different composition on a pre-existing garnet crystal (e.g. Albee, 1968
; Brown, 1969; Rumble & Finnerty, 1974; Erambert & Austrheim, 1993). In most cases, the younger garnet rims the older crystals; however, it is not always clear if the rim of the garnet formed during a later metamorphic event or resulted from continuous changes in chemical and thermodynamic parameters of the system during a single metamorphic event. Detailed textural observations along with petrogenetic analysis and geothermobarometry are needed to address this problem. Particularly important information on the metamorphic history can be provided by veins or microveins of mineral(s), which are not uncommon in eclogite- and amphibolite-facies rocks and minerals (e.g. Holland, 1979; Heinrich, 1986; Selverstone et al., 1992; Hames & Menard, 1993; Perchuk, 2002).
Veins containing hydrous and anhydrous phases in minerals or rocks may have different origins. In addition to their formation during earlier metamorphic episodes in polymetamorphic rocks, they can originate during dehydration (e.g. of eclogite, Becker et al., 1999), or during exhumation of rocks from deeper parts of the crust (Chopin et al., 1997). Metabasites from the Kraubath mafic and ultramafic massif preserve a relict igneous mineralogy despite medium- to high-pressure metamorphism during several tectonic events. They contain chemically zoned garnet, amphibole and pyroxene, and at least four compositional varieties of garnet. The new garnet forms rims on, or microveins in, pre-existing garnet porphyroblasts and also occurs in intergranular spaces between the other minerals. The aim of this study is to discriminate equilibrium mineral compositions and hence the estimation of P–T conditions for peak metamorphic minerals and for the subsequent metamorphic episodes that led to the formation of microveins. The metamorphic re-equilibration of older amphibolite-facies minerals during a younger metamorphic overprint is also described.
|
GEOLOGY AND METAMORPHIC HISTORY |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGY AND METAMORPHIC HISTORYPETROGRAPHY AND MINERALOGY OF...P-T CONDITIONS OF METAMORPHISMDISCUSSIONCONCLUSIONSREFERENCES |
|---|
The Kraubath mafic and ultramafic body is part of the Speik Complex, interpreted as a fragment of Devonian oceanic lithosphere subducted northwards beneath the European continent (Neubauer & Frisch, 1993
). In the area east of the Tauern Window, the Speik Complex is exposed mostly along boundaries between the Wölz Complex and Seckau Complex (Fig. 1). All three complexes belong to the Middle Austroalpine basement, which shows a polymetamorphic evolution. Based on mineral assemblages and/or geochronological dating it is possible to distinguish at least four metamorphic events in the Middle Austroalpine units, east of the Tauern Window, as follows.
- An early/pre-Variscan eclogite-facies metamorphism (397 Ma) in the Speik Complex rocks from the Hochgrößen Massif was dated by the Ar–Ar method on amphibole coexisting with omphacite and garnet (Faryad et al., 2002).
- A regional Variscan medium-pressure amphibolite-facies metamorphism (c. 330 Ma) followed by granite magmatism was dated in different parts of the Austroalpine units east of the Tauern Window (Frank et al., 1987; Schermaier et al., 1997).
- Permian low-pressure metamorphism was confirmed by a combination of igneous activities (pegmatites and gabbros) and Sm–Nd ages of 246 Ma for garnet in the Koralpe, Saualpe and Wölz complexes (Schuster & Thöni, 1996; Miller & Thöni, 1997; Schuster & Frank, 1999).
- An Eo-Alpine overprint (
90 Ma, Frank et al., 1987; Neubauer et al., 1995; Thöni & Miller, 1996) grading southwards from epidote–amphibolite-facies conditions in the Speik and Seckau complexes, through amphibolite facies in the Wölz Complex to eclogite facies in the Koralpe Complex (Hoinkes et al., 1999). In the Wölz Complex, peak metamorphism around 1·1 GPa and 650°C was followed by nearly isothermal decompression to 0·5 GPa and 600°C, and then by rapid cooling and decompression (Faryad & Hoinkes, 2003).
|
Except for eclogite-facies rocks in the Hochgrößen Massif, there is little evidence of Pre-Alpine metamorphism in the Speik Complex. Most of the Pre-Alpine minerals were transformed or re-equilibrated during Eo-Alpine metamorphism that affected the entire basement rocks to different degrees. This is probably the explanation for the assumption by Meisel et al. (1997)
of a Mesozoic age for the magmatic crystallization of ultramafic rocks based on 187Os/188Os data from chromite in the Kraubath and Hochgrößen massifs. Puhl (2000) obtained an 40Ar–39Ar age of 175·3 ± 8·2 Ma for amphibole in the metabasite investigated in this study, and K–Ar ages of 116·8 and 78·7 Ma for amphibole in two other metabasite samples from the Kraubath Massif. However, the age spectra were strongly disturbed, increasing from c. 100 Ma for the first low-temperature steps to maximum values of c. 203 Ma for the high-temperature steps.
The mafic and ultramafic rocks of the Kraubath Massif are represented by serpentinites and amphibolites that form an east–west-trending body of 10 km x 5 km scale. The serpentinites contain bodies of up to 80–100 m size of well-preserved orthopyroxenite, dunite, harzburgite and websterite (El Ageed, 1979; Puhl, 2000). These contain enstatite, forsterite and diopside as relic igneous phases. The most common metamorphic minerals are antigorite, chlorite, tremolite, talc, carbonates and sulphides. Based on the geochemical and isotopic studies of Meisel et al. (1997), the Kraubath and Hochgrößen mafic and ultramafic rocks are assumed to represent fragments of oceanic lithosphere.
|
PETROGRAPHY AND MINERALOGY OF THE METABASITES |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGY AND METAMORPHIC HISTORYPETROGRAPHY AND MINERALOGY OF...P-T CONDITIONS OF METAMORPHISMDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Metabasites from the Kraubath Massif have mostly amphibolite-facies mineral assemblages with plagioclase + amphibole ± garnet and retrograde epidote, titanite, albite and chlorite. Some plagioclase grains are replaced by clinozoisite, albite, phengite and paragonite. Plagioclase-free metabasite with relic clinopyroxene occurs locally. This variety (sample Kr-25) contains amphibole (70 vol. %), epidote (20%), clinopyroxene (10%), garnet (5%) and accessory amounts of chlorite, titanite, rutile and ilmenite. According to the major oxide content (Table 1), this metabasite corresponds to a basalt with normative plagioclase (33% anorthite + albite), olivine (25·8%), diopside (31·9%) and nepheline (2·6%). The weak foliation of the rock is defined by amphibole and epidote showing preferred orientation. Relic clinopyroxene forms grains of up to 1 mm irregularly distributed on the thin-section scale. Garnet forms porphyroblasts (1–2·5 mm in size), which are overgrown by amphibole and epidote. Some large (1 mm) grains of epidote with inclusions of chlorite seem to be pseudomorphs after garnet.
|
All minerals were analysed with a JEOL 6310 scanning electron microscope with wavelength- and energy-dispersive spectrometers at the Institute for Mineralogy and Petrology in Graz. Standards were pyrope (Mg, Al), adularia (K), rutile (Ti), tephroite (Mn), jadeite (Na, Si) and andradite (Fe, Ca). The accelerating voltage was 15 kV. Beam currents were 10 or 15 nA with 20 s counting time. Mineral formulae were calculated on the basis of 6 oxygens, 4 cations (pyroxene), 23 oxygens (amphibole), 12 oxygens, 8 cations (garnet), 28 oxygens, 20 cations (chlorite), and 12 oxygens + 1 OH (epidote). Ferric and ferrous iron ratios in amphibole were calculated on the basis of averaged Fe3+ from 13 and 15 cations. Representative mineral analyses are summarized in Table 2.
|
Pyroxene
Back-scattered electron images allowed identification of different composition domains in single crystals of clinopyroxene, amphibole, and especially garnet. Clinopyroxene grains have two compositional domains: darker cores (Cpx1) and lighter rims and along cracks (Cpx2) (Fig. 2a and b). These two domains have different Fe and Mg contents: Cpx1 has higher XMg (0·89–0·91) relative to Cpx2 (0·80–0·83) (Table 2). Both varieties are very low in Na (<0·03 a.p.f.u.).
|
Amphibole
Amphibole is pargasite with Si = 6·1–6·6 a.p.f.u., showing two or even three zones in the back-scattered electron (BSE) image (Fig. 2a and b) that gradually change from a dark core (Amph1), through grey and finally to light rims (Amph2). Amph2 also occurs along cracks in amphibole grains and as idioblastic crystals in association with epidote and chlorite. The dark cores are higher in Mg (XMg = 0·83, Table 2) and slightly higher in Altot (2·221 a.p.f.u.) than the light rims (XMg = 0·6–0·7, Altot = 2·111 a.p.f.u.). The grey zones have an intermediate composition between Amph1 and Amph2. The highest content of Altot = 2·9 a.p.f.u. occurs in amphibole enclosed in garnet Gr3 (see below).
Garnet
At least four compositional varieties of garnet can be observed in back-scattered images (Fig. 2c–g). The first three varieties (Gr1, Gr2, Gr3) form porphyroblasts, whereas the last variety (Gr4) forms overgrowths and microveins in garnet porphyroblasts. Figure 2c–g shows a light grey garnet (Gr2), which contains two types of darker domains. The first type (Gr1) occurs mostly in inner parts of the grains (Fig. 2c and d), and compared with Gr2 (Alm41–43, Py23–27, Grs29–33) has slightly higher Mg (Py27–30) and Ca (Grs33–36) and lower Fe (Alm36–40, Table 2, Fig. 3a and b). The Gr1 domains are penetrated by narrow dendritic veins of Gr4 (Figs 2d and 3a and b). The second type of darker domains (Gr3) forms concentric and irregularly zoned individual areas that form amoeba-like (spots) textures with light grey cores and darker rims (Fig. 2e and f). In some cases, Gr3 occurs in tabular forms (Fig. 2g), suggesting its formation by transformation of a pre-existing tabular phase. Compared with Gr1, the Gr3 domains contain relatively wide microveins of Gr4. The light core of the amoeba-like texture has low Mg (Py20–27) and high Ca (Grs35–38) relative to Gr2 (Fig. 4). The Fe content (Alm34–40) is slightly low but close to that of Gr2. Gr3 differs from Gr1 mainly by low Mg and high Fe contents (Table 2). There is a gradual change in Mg, Ca and Fe contents at contacts between Gr2 and Gr3. All these garnet varieties (Gr1, Gr2 and Gr3) are low in spessartine (Sps0–1).
|
|
Gr4 forms overgrowths or microveins within Gr1–3 and occurs within the intergranular space between other minerals (Fig. 2b–i). It is rich in Ca (Grs40–57) and poor in Mg (Py0·2–0·5) with Alm46–51. Compared with older garnet varieties, Gr4 has a relatively high Mn content (Sps1–5, Table 2, Figs 3 and 4). Gr4 may enclose Amph2, having sharp contacts with chlorite, epidote and idioblastic crystals of Amph2 (Fig. 2h and i) that suggest textural equilibrium among Gr4, Amph2, chlorite and epidote.
Other minerals
Epidote has low Fe3+ with Cz = (Al – 2)/(Al – 2 + Fe3+) = 0·63 and XAl = Al/(Al + Fe3+) = 0·87. Chlorite is homogeneous with XMg = 0·7. Chlorite or epidote can be observed in microveins with Gr4.
|
P–T CONDITIONS OF METAMORPHISM |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGY AND METAMORPHIC HISTORYPETROGRAPHY AND MINERALOGY OF...P-T CONDITIONS OF METAMORPHISMDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Textural relations and mineral compositions in metabasite sample Kr-25 indicate an igneous and several metamorphic growth stages of minerals.
- The magmatic stage is preserved by relic igneous clinopyroxene. Different compositions of clinopyroxene in cores (Cpx1) and at rims (Cpx2) of grains suggest partial re-equilibrium of clinopyroxene during metamorphism. Exchange thermometry gives low temperatures of 638–680°C for Cpx1 and Gr1 and unrealistically high temperatures of >850°C for Cpx2 and Gr1. Only Cpx2 at contacts with Gr2 yields consistent temperatures of 701–769°C by different calibrations (Table 3). These temperatures, mainly those calculated by the method of Krogh (1988) and Yang (1994), are close to and consistent with the temperatures obtained for eclogite-facies metamorphism (700°C) for the Speik Complex metabasites in the Hochgrößen Massif (Faryad et al., 2002).
- A temperature of 610 ± 10°C (Table 4) was obtained using amphibole–garnet thermometry (Graham & Powell, 1984) for Amph1 and Gr2. A higher temperature of 713 ± 7°C was calculated using this method applied to Amph1 and Gr1. However, these two phases are not in direct contact. Decreases in XMg and Ca from Gr1 to Gr2 (Fig. 3b) may reflect the decrease in pressure and fall in temperature during retrogression.
- A new metamorphic event is assumed to have occurred as a result of the presence of Gr4 indicating textural equilibrium with amph2, epidote and chlorite. Amphibole/garnet thermometry (Graham & Powell, 1984) gave a temperature of 583 ± 7°C for this assemblage (Table 4). A lower temperature (550°C at 1·0 GPa) was obtained using the TWQ program (Berman, 1996), by the intersection of five end-member reactions (Fig. 5).
|
|
|
P–T conditions of the last metamorphic event are close to those (510–530°C at 0·7–0·9 GPa) obtained for Alpine metamorphism in the Permian chloritoid phyllite from the Hochgrößen Massif (Faryad & Hoinkes, 2001
) and in the Seckau and Gleinalm complexes (unpublished data of the authors). The 175·3 ± 8·2 40Ar–39Ar age obtained for amphibole (Puhl, 2000), can be interpreted as a mixed age of Pre-Alpine (Amph1) and Alpine (Amph2) amphiboles, distinguished in this study. |
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGY AND METAMORPHIC HISTORYPETROGRAPHY AND MINERALOGY OF...P-T CONDITIONS OF METAMORPHISMDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Timing of metamorphic reactions
Major element chemical composition and normative mineral contents indicate that the primary igneous phases in the studied metabasites were olivine, plagioclase, and clinopyroxene. Considering normative mineral contents, the plagioclase was rich in Ca (An84; or An78, if all Na was confined to plagioclase). Different compositions of pyroxene on rims, but mainly along cracks in large grains, can be interpreted as the results of kinetic and material transport related to incomplete reaction. Straume & Austheim (1999)
assumed that transport along fractures results from advective movement of fluid or diffusive transport through the fluid. The change in Ca and XMg contents between Gr1 and Gr2 (Fig. 3a and b) may be explained as a result of (1) chemical change (consumption of Ca-rich plagioclase and olivine) during the early stage of recrystallization or (2) a retrogression leading to decrease in XMg and Ca towards the rim of garnet grains. An argument supporting the first alternative is the presence of igneous clinopyroxene, suggesting that total homogenization was not achieved in the rock during the metamorphism. The interpretation of retrograde zoning of garnet from Gr1 to Gr2 may be due to the presence of Pre-Alpine amphibolized eclogite in the Speik Complex (Faryad et al., 2002).
The results of thermobarometric calculations in combination with geochronological data indicate that Gr4 originated during the Alpine metamorphic event. Equilibrium conditions of Gr4 with epidote, Amph2 and chlorite, and the presence of one or more of these phases in microveins, suggest that Gr4 was derived from dehydration reactions of hydrous phases that had already been formed during retrogression of amphibolite-facies metamorphic assemblages. Calculations of mass balance (Si, Al, Mg + Fetot and Ca) among Gr4, epidote and chlorite indicate reaction of epidote and chlorite to form garnet via the reaction
 | (6) |
However, more Fe-rich chlorite and epidote are needed to balance Fetot and Mg contents in this reaction. Mn contents in Gr4 are generally 3–4 times higher than in older garnet varieties (Figs 3 and 4). Because Mn preferentially enters garnet, being the only Mn-bearing phase in the rock, at least 4 vol. % of older garnet (Gr1–3) must have been transformed to the hydrous phases that gave rise to Gr4 during the Eo-Alpine metamorphism.
Origin of fractures in garnet
Fracturing and formation of microveins in minerals has been described in many eclogite- and amphibolite-facies metamorphic rocks (Hames & Menard, 1993; Chopin et al., 1997; Straume & Austrheim, 1999). The fractures are interpreted as stress induced, caused by volume increase during the retrogression along an exhumation path when temperature falls. In this situation, when the retrogression is driven or accompanied by hydration reactions, strong gradients in fluid composition may occur. Straume & Austrheim (1999) proposed that deformation is important not only for the opening of fractures, along which fluids and solute could travel, but also for chemical change in minerals which may have resulted directly from the propagation of fractures. A fluid phase is obviously present during fracturing. Based on studies of the Holt–Tyssedalvatnet metagabbro, Engvik et al. (2001) considered that fractures filled with garnet could have formed during eclogitization. The possible mechanisms inferred to produce fractures were (1) high fluid pressure, (2) density changes, (3) volume decrease and (4) external stress, related to tectonic processes.
Detailed observations in the Kraubath metabasite reveal two varieties of fractures in garnets. The most common microfactures are characterized by microveins of Gr4, which have sharp contacts with the host garnet. They show an abrupt increase in Ca and decrease in Mg compared with the original garnets (Gr1, Gr2, Gr3). The second type consists of narrow, dendritic veins visible in Gr1 (Fig. 2c, d and g) that also indicate an increase in Ca and decrease in Mg, although not as strong as in fractures filled with Gr3. Figure 3a and b shows that some of these bright dendritic veins extend into the narrow white veins of Gr4. Such fractures in garnet were described by Straume & Austrheim (1999) and Putnis (2002), and interpreted as a result of dissolution–precipitation processes. Fractures filled with Gr4 could have originated in similar manner to that described above, by deformation under low-grade conditions prior to the Alpine HP/LT metamorphism.
The amoeba-like spots of Gr3 indicate a continuous change in Mg, Ca and Fe contents towards Gr2, but sharp contacts with Gr4, where Mg abruptly decreases and Ca increases. Diffusive boundaries between Gr2 and Gr3 can be interpreted as a result of partial homogenization and formation of both garnets prior to Gr4. Concentric changes in Mg, Fe and Ca contents observed in the amoeba-like textures (Fig. 4) or tabular forms of Gr3 in Gr2 suggest that these elements achieved a close approach to local equilibrium during garnet growth. In this case, both Gr2 and Gr3 could have formed simultaneously during the Pre-Alpine amphibolite-facies metamorphism, and the zoned amoeba-like forms may have resulted from the transformation of existing material, which lost some Ca (e.g. plagioclase) during garnet formation. According to Engvik et al. (2001), such a mechanism could be ascribed to the volume change during transformation of phases enclosed in Gr2. When considering a closed system, the formation of Gr2 (Alm46Grs22 Py20; 3·9 g/cm3) from plagioclase (2·7 g/cm3) and amphibole (3·0–3·2 g/cm3) would have led to a volume decrease. The resulting open space would become preferred fluid channels. Abundant microveins of Gr4 within these darker domains of Gr3 (Fig. 2e and f) suggest reopening of the older fracture system, and consequently formation of the new garnet (Gr4).
Tectonic implications
Mafic and ultramafic rocks of the Speik Complex are interpreted to be parts of a dismembered ophiolite sequence, the protoliths of which formed during the break-up of the Gondwana shelf during Cambrian times (Neubauer & Frisch, 1993). Early Variscan subduction and formation of eclogites in the Speik Complex rocks were confirmed by Faryad et al. (2002) in the Hochgrößen Massif, which occurs c. 50 km west of the Kraubath Massif (Fig. 1). Variscan regional metamorphism of late Early Carboniferous age in the Eastern Alps is deduced from geochronological dating and mineral and textural relations in basement rocks (Frank et al., 1987; Schermaier et al., 1997). According to Neubauer & Frisch (1993), this metamorphism occurred during continental plate collision, which subsequently resulted in granite intrusions. The three compositional varieties of Gr1–3 from the Kraubath Massif may reflect changes in P–T conditions during Pre-Alpine metamorphism from early Variscan high-pressure(?) amphibolite-facies (700°C) to Variscan relatively low-grade metamorphism of 620°C (Fig. 5). Preservation of igenous pyroxene and of early Variscan garnet suggests that both metamorphic processes were fairly rapid.
The Eo-Alpine (90 Ma) high-pressure amphibolite- and eclogite-facies metamorphic rocks in the Austroalpine units are interpreted as products of subduction and crustal thickening (Miller & Thöni, 1997). Metamorphic rocks exhumed from different portions of the subducted slab are now tectonically stacked between southward-dipping thrusts. Metamorphic conditions obtained for the Kraubath Massif fit well within the south-grading metamorphic scheme constrained for the Austroalpine basement unit east of the Tauern Window (Faryad & Hoinkes, 2003). Formation of Gr4 and coexisting amphibole, epidote and chlorite in the Kraubath Massif indicate that peak P–T conditions were reached during this event. Similar P–T conditions (510–530°C at 0·7–0·9 GPa) for the Eo-Alpine metamorphism were obtained for the Permian chloritoid-bearing phyllites in the Hochgrößen Massif (Faryad & Hoinkes, 2001). The retrograde part of this metamorphism was characterized by nearly isothermal decompression, which led to partial re-equilibration of high-pressure mineral assemblages.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGY AND METAMORPHIC HISTORYPETROGRAPHY AND MINERALOGY OF...P-T CONDITIONS OF METAMORPHISMDISCUSSIONCONCLUSIONSREFERENCES |
|---|
- Metabasites from the Kraubath mafic and ultramafic massif preserve igneous pyroxene and various metamorphic minerals formed during different tectonic events.
- Compositional varieties of garnet Gr1–3 and Amph1 originated during Pre-Alpine metamorphism, indicating two stages of amphibolite-facies conditions of >700°C and 620°C for the Kraubath Massif.
- Amoeba-like and tabular forms of Gr3 could have been formed by transformation and local equilibrium of existing minerals during the second metamorphic stage.
- Microveins of Gr4 appear to represent older fractures filled with low-grade hydrous phases that were transformed to garnet later during the Alpine peak P–T conditions. The high Mn content in G4 (3–4 times higher than in Gr1–3) suggests that a significant portion of Gr1–3 was converted into hydrous phases that later gave rise to Gr4.
- The relatively large number of fractures in Gr3, filled with Gr4, was probably the results of volume change during the transformation of the former phase to garnet.
- Ar/Ar ages of 175 Ma, obtained for the studied metabasite from the Kraubath Massif (Puhl, 2002) resulted from partial re-equilibrium and preservation of Pre-Alpine cores (Amph1) in amphibole.
- In addition to the preservation of igneous and metamorphic phases, the relatively sharp contact, with no or weak diffusion zones, between different varieties of garnet suggests that both the Pre-Alpine and Eo-Alpine metamorphic events occurred during short time intervals and/or at relatively low temperatures.
|
ACKNOWLEDGEMENTS |
|---|
This work was supported by the Institute of Mineralogy and Petrology, University of Graz. J. Puhl and F. Melcher kindly provided metabasite samples from the Kraubath Massif. Thanks are due to H. Austrheim, P. Philippot and A. Proyer for valuable discussions and comments on the early version of the manuscript. The authors are also grateful to S. L. Harley for helpful editorial comments.
|
FOOTNOTES |
|---|
* Corresponding author. E-mail: faryad{at}natur.cuni.cz
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGY AND METAMORPHIC HISTORYPETROGRAPHY AND MINERALOGY OF...P-T CONDITIONS OF METAMORPHISMDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Albee, A. L. (1968). Metamorphic zones in northern Vermont. In: Zen, E-A., White, W. S., Hardley, J. B. & Thompson, J. B. (eds) Studies of Appalachian Geology, Northern Vermont and Maritime. New York: Wiley, pp. 329–341.
Becker, H., Jochum, K. P. & Carlson, R. W. (1999). Constraints from high-pressure veins in eclogites on the composition of hydrous fluids in subduction zones. Chemical Geology 160, 291–308.[CrossRef][Web of Science]
Berman, R. G. (1996). TWEEQU (version 202). Thermobarometry with estimation of equilibration state. http://www.gis.nrcan.gc.ca/twq.html/.
Brown, E. H. (1969). Some zoned garnets from the greenschist facies. American Mineralogist 54, 1662–1677.[Web of Science]
Chopin, C., Simon, G. & Schenk, V. (1997). Granulite facies overprint in ultrahigh-pressure rocks (Dora Maira Massif): evidence for low-temperature granulite? Terra Nova 9, Abstract Supplement 1, 6.
El Ageed, A. I. (1979). The Hochgrößen ultramafic–mafic association, its associated mineralisation and petrogenetic significance. Ph.D. thesis, University of Cologne, 248 pp.
Ellis, D. J. & Green, D. H. (1979). An experimental study of the effect of Ca upon garnet–clinopyroxene Fe–Mg exchange equilibria. Contributions to Mineralogy and Petrology 71, 13–22.[CrossRef][Web of Science]
Engvik, A. K., Austrheim, H. & Erambert, M. (2001). Interaction between fluid flow, fracturing and mineral growth during eclogitization, an example from the Sunnfjord area, Western Gneiss Region, Norway. Lithos 57, 111–141.[CrossRef][Web of Science]
Erambert, M. & Austrheim, H. (1993). The effect of fluid and deformation on zoning and inclusion patterns in poly-metamorphic garnets. Contributions to Mineralogy and Petrology 115, 204–214.[CrossRef][Web of Science]
Faryad, S. W. & Hoinkes, G. (2001). Alpine chloritoid and garnet from the Hochgrößen massif (Eastern Alps). Mitteilungen der Österreichischen Mineralogischen Gesellschaft 146, 387–396.
Faryad, S. W. & Hoinkes, G. (2003). P–T gradient of Eo-Alpine metamorphism within the Austroalpine basement units east of the Tauern Window (Austria). Mineralogy and Petrology 77, 129–159.[CrossRef][Web of Science]
Faryad, S. W., Hoinkes, G., Melcher, F., Puhl, J., Meisel, T. & Frank, W. (2002). Relics of eclogite facies metamorphism in the Austroalpine basement, Hochgrößen (Speik Complex), Austria. Mineralogy and Petrology 74, 49–73.[CrossRef][Web of Science]
Flügel, H. W. & Neubauer, F. (1984). Erläuterungen zur Geologischen Karte der Steiermark 1:200 000. Vienna: Geologie Bundesanstalt, 127 pp.
Frank, W., Kralik, M., Scharbert, S. & Thöni, M. (1987). Geochronological data from the Eastern Alps. In: Flügel, H. W. & Faupl, P. (eds) Geodynamics of Eastern Alps. Vienna: Franz Deuticke, pp. 272–281.
Ganguly, J., Cheng, W. & Tirone, M. (1996). Thermodynamics of aluminosilicate garnet solid solution: new experimental data, an optimized model, and thermodynamic application. Contributions to Mineralogy and Petrology 126, 137–151.[CrossRef][Web of Science]
Graham, C. M. & Powell, R. (1984). A garnet–hornblende geothermometer, calibration, testing and application to the Pelona Schist, Southern California. Journal of Metamorphic Geology 2, 13–31.[Web of Science]
Hames, W. E. & Menard, T. (1993). Fluid associated modification of garnet composition along rims, cracks, and mineral inclusion boundaries in samples of amphibolite facies schists. American Mineralogist 78, 338–344.[Abstract]
Heinrich, C. A. (1986). Eclogite facies regional metamorphism of hydrous mafic rocks in the central alpine Adula nappe. Journal of Petrology 27, 123–154.
Hoinkes, G., Koller, F., Rantitsch, G., Dachs, E., Höck, V., Neubauer, F. & Schuster, R. (1999). Alpine metamorphism of the Eastern Alps. Schweizerische Mineralogische und Petrographische Mitteilungen 79, 155–181.[Web of Science]
Holland, T. J. B. (1979). Experimental determination of the reaction paragonite = jadeite + kyanite + H2O, and internally consistent thermodynamic data for part of the system Na2O–Al2O3–SiO2–H2O, with applications to eclogite and bluschists. Contributions to Mineralogy and Petrology 68, 293–301.[CrossRef][Web of Science]
Krogh, E. J. (1988). The garnet–clinopyroxene Fe–Mg geothermometer—a reinterpretation of existing experimental data. Contributions to Mineralogy and Petrology 99, 44–48.[CrossRef][Web of Science]
Meisel, T., Melcher, F., Tomascak, P. B., Dingeldey, C. & Koller, F. (1997). Re–Os isotopes in orogenic peridotite massifs in the Eastern Alps, Austria. Chemical Geology 143, 217–229.[CrossRef][Web of Science]
Miller, Ch. & Thöni, M. (1997). Eo-Alpine eclogitisation of Permian MORB-type gabbros in the Koralpe (Eastern Alps, Austria)—new geochronological, geochemical and petrological data. Chemical Geology 137, 283–310.[CrossRef][Web of Science]
Neubauer, F. & Frisch, W. (1993). The Austro-Alpine metamorphic basement east of the Tauern Window. In: von Raumer, J. & Neubauer, F. (eds) Pre-Mesozoic Geology in the Alps. Berlin: Springer, pp. 515–536.
Neubauer, F., Dallmeyer, R. D., Dunkl, I. & Schirnik, D. (1995). Late Cretaceous exhumation of the metamorphic Gleinalm dome, Eastern Alps: kinematics, cooling history and sedimentary response in a sinistral wrench corridor. Tectonophysics 242, 79–98.[CrossRef][Web of Science]
Perchuk, A. L. (2002). Eclogites of the Bergen Arcs complex, Norway: petrology and mineral chronometry. Petrology 10, 99–118.[Web of Science]
Powell, R. (1985). Regression diagnostics and robust regression in geothermometer/geobarometer calibration: the garnet–clinopyroxene geothermometer revisited. Journal of Metamorphic Geology 3, 231–243.[Web of Science]
Puhl, J. (2000). Vergleichende Geochemie und Petrologie von ultramafischen Massiven der Ostalpen. Ph.D. thesis, University of Leoben, Austria, 332 pp.
Putnis, A. (2002). Mineral replacement reactions: from macroscopic observations to microscopic mechanisms. Mineralogical Magazine 66, 689–708.
Rumble, D. & Finnerty, T. A. (1974). Devonian grossular–spessartine overgrowths on Ordovician almandine from eastern Vermont. American Mineralogist 59, 558–562.[Web of Science]
Schermaier, A., Hauschmid, B. & Finger, F. (1997). Distribution of Variscan I- and S-type granites in the Eastern Alps: a possible clue to unravel pre-Alpine basement structures. Tectonophysics 272, 315–333.[CrossRef][Web of Science]
Schuster, R. & Frank, W. (1999). Metamorphic evolution of the Austroalpine units east of the Tauern Window: indications for Jurassic strike slip tectonics. Mitteilungen der Gesellschaft der Geologie- und Bergbaustudenten in Wien, 42, 37–58.
Schuster, R. & Thöni, M. (1996). Permian garnet: indications for a regional Permian metamorphism in the southern parts of the Eastern Alps. In: Aman, G., Handler, R., Kurz, W. & Steyrer, H. P. (eds) 6 Symposium Tektonik–Strukturgeologie. Salzburg: Universtät Salzburg, pp. 389–391.
Selverstone, J., Spear, F. S., Franz, G., Thomas, S. & Getty, S. (1992). Fluid variability in 2 GPa eclogites as an indicator of fluid behavior during subduction. Contributions to Mineralogy and Petrology 112, 341–357.[CrossRef]
Straume, A. K. & Austrheim, H. (1999). Importance of fracturing during retro-metamorphism of eclogite. Journal of Metamorphic Geology 17, 637–652.[CrossRef][Web of Science]
Thöni, M. & Miller, Ch. (1996). Garnet Sm–Nd data from the Saualpe and the Koralpe (Eastern Alps, Austria): chronological and P–T constraints on the thermal and tectonic history. Journal of Metamorphic Geology 14, 453–466.[CrossRef][Web of Science]
Yang, A. (1994). A version of the garnet–clinopyroxene Fe2+–Mg exchange geothermometer. Contributions to Mineralogy and Petrology 115, 467–473.[CrossRef]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||