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P-T conditions and age of medium-grade metamorphism in the micaschists from the Kutjevačka Rijeka transect (Slavonian Mts., Croatia) (CROSBI ID 508978)

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Balen, Dražen ; Horváth, Peter ; Tomljenović, Bruno ; Finger, Friedrich ; Humer, Bernhard ; Pamić, Jakob ; Árkai, Peter P-T conditions and age of medium-grade metamorphism in the micaschists from the Kutjevačka Rijeka transect (Slavonian Mts., Croatia) // Abstracts Book / Velić, Ivo ; Vlahović, Igor ; Biondić, Ranko (ur.). Zagreb: Hrvatski geološki institut, 2005. str. 9-11-x

Podaci o odgovornosti

Balen, Dražen ; Horváth, Peter ; Tomljenović, Bruno ; Finger, Friedrich ; Humer, Bernhard ; Pamić, Jakob ; Árkai, Peter

engleski

P-T conditions and age of medium-grade metamorphism in the micaschists from the Kutjevačka Rijeka transect (Slavonian Mts., Croatia)

In this abstract we provide new data on P-T conditions and metamorphic age of the medium-grade part of progressively metamorphosed sequence exposed along the Kutjevačka Rijeka transect in the eastern part of the Slavonian Mts. obtained by means of microstructural data, garnet compositional zoning and electron-microprobe age dating on monazite grains of garnet-bearing micaschists. The N-S striking transect in the Slavonian Mts. with a zoned distribution of index minerals along the Kutjevačka Rijeka valley is a section where the prograde metamorphic sequence can be studied in great detail. Along this transect heading north, the prograde metamorphic sequence is firstly represented by its medium-grade (amphibolite facies) part largely composed of garnet-bearing micaschists and paragneisses with subordinate orthoamphibolite intercalations and granitoid intrusions. Further north, this grades into greenschist facies schists comprising the low-grade part (chlorite zone) of the prograde metamorphic sequence, which is firstly covered by sub-greenschist facies rocks of the Radlovac metamorphic complex and then by a non-metamorphic clastic-carbonate succession of Permian and Triassic age (for details see JAMIČIĆ, 1983, 1988 ; JAMIČIĆ & BRKIĆ, 1987 ; JAMIČIĆ et al., 1987). Mesoscopic observations in the prograde metamorphic sequence reveal evidence of two foliations that predate Alpine deformation. An older (S1) foliation is only locally recognized and is marked by a metamorphic layering characterized by a cm-scale alternation of micaschist, paragneiss and amphibolite, all containing mm-scale garnet trails parallel to the metamorphic layering. This relationship and the mineral assemblage indicate that amphibolite facies conditions took place during formation of S1 foliation. By passing from F2 fold hinges into strongly attenuated limbs S1 becomes sub-parallel and parallel to a younger (S2) planar fabric, which is the most evident mesoscopic foliation in these rocks. This foliation is either parallel or show consistent geometrical relationship with the axial planes of predominantly E-W trending, isoclinal F2 folds. Hence, it is interpreted as to represent an axial plane cleavage of F2 folds related to D2 deformational event, which resulted in greenschist facies retrogression of the prograde metamorphic sequence. Sporadically along the transect, S2 is found to dip in opposite directions, which is interpreted as a result of a subsequent post-D2 folding event attributed to Alpine deformation. Micaschists have a well preserved metamorphic fabric (S1) with a peak metamorphic assemblage of garnet, biotite, muscovite, plagioclase and quartz corresponding to amphibolite facies conditions. This foliation is predominantly marked by preferentially oriented biotite, muscovite and abundant garnet trails. Garnets are hypidioblastic and partly fractured, and typically surrounded by asymmetric pressure shadows filled with chlorite, biotite, muscovite, epidote and quartz. Regarding the grain size garnets form two distinct groups comprising relatively larger- and smaller-sized populations, respectively. Large-sized (~1 mm) garnet population clearly preserves a complex growth history. Here, garnet cores are characterized by: (1) inclusion trails of ilmenite, apatite and quartz, which are characteristically oblique to external foliation and restricted to garnet cores, and (2) fracture sets, some of which not extending into the garnet rims. Hence, both features indicate that growth of garnet rims postdates some earlier cataclastic deformations preserved only within the garnet cores. Smaller garnets do not show such cataclastic deformations. S2 is marked by extensive chloritization of biotite and garnet, pointing to a retrograde process. Thus, the formation of S2 ductile fabric is linked to the younger deformational event that took place after the amphibolite facies PT conditions. Large-sized garnet population in micaschists is represented by relatively large hypidioblastic, discontinuously zoned garnets, which have Mn-rich cores (here referred to as garnet I: XAlm=0.690, XSps=0.102, XPrp=0.117, XGrs=0.091) and Ca-rich rims (here referred to as garnet II: XAlm=0.717, XSps=0.008, XPrp=0.094, XGrs=0.181). The “ textural” garnet core and rim correspond to the “ chemical” core and rim. The second, small-sized garnet population (up to 0.5 mm in size) has the same chemical composition as observed in the rims of the large garnets. Complex compositional zoning of garnet can be well observed in back scattered electron images. In their cores garnet I usually exhibit prograde (saddle) shaped compositional profile and are rich in Mn, Mg and poor in Ca compared to the rims composition (garnet II). There is an abrupt change in chemical composition between garnet I and II, indicating discontinuous zoning. The distribution of chemical elements in the garnet profile shows the same trend but with a rapid "Ca jump" in the core-rim transitional area. The small garnets and the rims of large garnets show continuously decreasing XGrs, XSps, and Fe/(Fe+Mg) ratio and increasing XPrp and XAlm toward garnet rim. Such patterns suggest prograde metamorphic conditions during the crystallization of garnet rims. The same, but less prominent, prograde pattern is also present in the core region up to the "Ca jump". The rapid change of calcium content and the discontinuous character of garnet zoning probably involve a breakdown and a consummation of earlier-formed calcium-bearing phase (apatite, An-rich plagioclase, calcite?). Thermobarometric calculations are performed on the peak assemblages with garnet II (rims and small-grained garnets) using THERMOCALC yielded 600-630 °C and 9-11 kbar. In an attempt to constrain the age of the medium-grade metamorphism in the Kutjevačka Rijeka transect by means of electron-microprobe based monazite dating two micaschists containing a few small monazite grains were analyzed. The monazites occurred isolated in the matrix and showed quite unusual shapes, forming elongate crystals with ~ 2-3 &micro ; ; m width and ~ 10-15 &micro ; ; m length. Nineteen such monazites have been analyzed. The measured Th-U-Pb concentrations imply, surprisingly, a formation of the monazites in the Silurian and not during the Variscan orogeny. The single point ages are all consistent within error and cluster around weighted averages of 428 &plusmn ; ; 25 and 444 &plusmn ; ; 19 Ma in the two sections. There is no evidence for a polygenetic nature of the monazite populations. The most plausible interpretation is that the obtained average ages date the medium-grade metamorphism in these rocks. This interpretation is additionally supported by the partly very high yttrium contents of the monazites (almost 3 wt.% Y2O3 !). Such high yttrium contents are indicative of monazite growth at middle or upper amphibolite facies conditions, and accord with the PT estimates derived for the host rocks by means of geothermobarometric calculations. In such way of interpretation some of Variscan mica and amphibole ages for Barrovian-type metamorphic sequences in Slavonian Mts. (PAMIĆ & JURKOVIĆ, 2002) can be possibly attributed to the D2 event. The presented peak metamorphic data together with additional microstructural, paragenetic, mineral chemical and age data can not be directly correlated with results obtained in the nearby parts of the Tisia in Hungary. At the moment, available data all around the Tisia are too scarce to be used for reliable geodynamic interpretation of the Silurian medium-grade metamorphism in the area. References JAMIČIĆ, D. (1983): Strukturni sklop metamorfnih stijena Krndije i južnih padina Papuka (Structural fabric of the metamorphosed rocks of Mt. Krndija and the eastern part of Mt. Papuk).- Geol. vjesnik, 36, 51-72 (in Croatian). JAMIČIĆ, D. (1988): Strukturni sklop slavonskih planina (sjeverni Psunj, Papuk, Krndija) (Structural pattern of the Slavonian Mountains (Krndija, Papuk and Psunj Mountains)).- PhD Thesis, University of Zagreb, 152p. (in Croatian). JAMIČIĆ, D. & BRKIĆ, M. (1987): Osnovna geološka karta SFRJ 1:100000. List Orahovica L 33-96. (Basic Geological Map of Yugoslavia in scale 1:100.000, sheet Orahovica).- Geol. zavod Zagreb (1971-1986), Sav. geol. zavod Beograd. JAMIČIĆ, D., BRKIĆ, M., CRNKO, J., VRAGOVIĆ, M. (1987): Osnovna geološka karta SFRJ 1:100000. Tumač za list Orahovica L 33-96 (Basic Geological Map of Yugoslavia – Explanatory notes for sheet Orahovica).- Geol. zavod Zagreb (1986), Sav. geol. zavod Beograd, 72 p. (in Croatian). PAMIĆ, J. & JURKOVIĆ, I. (2002): Paleozoic tectonostratigraphic units in the northwest and central Dinarides and the adjoining South Tisia.- Intern. J. Earth Sci., 91, 538-554.

zoned garnet; geothermobarometry; monazite dating; Silurian; Tisia; Croatia

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Podaci o prilogu

9-11-x.

2005.

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objavljeno

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Abstracts Book

Velić, Ivo ; Vlahović, Igor ; Biondić, Ranko

Zagreb: Hrvatski geološki institut

Podaci o skupu

3. hrvatski geološki kongres

predavanje

29.09.2005-01.10.2005

Opatija, Hrvatska

Povezanost rada

Geologija, Rudarstvo, nafta i geološko inženjerstvo