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The geothermobarometry of Variscan medium-grade metamorphic rocks from the Kutjevačka Rijeka (Slavonian Mts., Croatia)

A disrupted Variscan belt that extends southeastwards of the Bohemian Massif through the Carpathians to the Caucasus also occurs south of the western Carpathians and builds the crystalline basement beneath the southern parts of the Tertiary Pannonian Basin (Tisia Unit). Tisia is commonly regarded as a lithosphere fragment that was broken off from the margin of Eurasia and after a complex drifting and rotation, occupies its present tectonic position. In the South Tisia, the largest and best outcrops of Variscan crystalline rocks occur in Slavonian Mts: Psunj, Papuk and Krndija located between the Sava and Drava rivers (Croatia). The Variscan crystalline complex of Slavonia, though tectonically disturbed, builds up domal structure with S-type granites in the core that are winded by migmatitic zones and regionally metamorphosed sequences at the flanks. Mesozoic (mainly Triassic) sediments and Tertiary sediments of the South Pannonian Basin disconformably overlie the Variscan domal structure. All these Variscan crystalline rocks also make up the basement of the South Pannonian Basin. The aim of this preliminary communication is to present the geothermobarometry of Variscan medium-grade metamorphic rocks from poorly known sector of the Variscan orogen. The Kutjevačka Rijeka north-south striking profile is one of the best outcrop of the progressively metamorphosed Barrow-type metamorphic sequences, which contain in its medium-grade parts mica schists and paragneisses with orthoamphibolite intercalations. In the mica schists, two different garnets are present: large (up to 1 mm) pre- to syn-tectonic, hipidiomorphic, discontinuously zoned garnets (Fig. 1A) generally have Mn-rich cores (XAlm=0.690, XSps=0.102, XPrp=0.117, XGrs=0.091) and Ca-rich rims (XAlm=0.717, XSps=0.008, XPrp=0.094, XGrs=0.181) whereas smaller syn-tectonic garnets (up to 0.5 mm) with composition corresponding to rim of the larger garnets. Apatite, ilmenite and quartz are inclusions in cracked core of large garnets. Apart from these, peak mineral assemblage comprises biotite, muscovite, plagioclase (~18% An) and quartz. Distribution of chemical elements in profile show same trend but with rapid "Ca jump" in core-rim transition area (Fig. 1B). The large garnet rims and small garnets show continuously decreasing XGrs, XSps, and Fe/(Fe+Mg) ratio and increasing XPrp and XAlm toward garnet rim. Such pattern suggests prograde metamorphic conditions during the garnet rim crystallization. The same, but less prominent, prograde pattern is also present in the core region up to "Ca jump". The rapid change of Ca composition and discontinuous character of garnet zoning probably involve breakdown and consummation of earlier-formed Ca-bearing phase (apatite?). It may imply a complex evolution of these rocks with two different metamorphic stages or a change in the reaction assemblage during prograde evolution. The distribution of Mn shows undisturbed "bell-shaped" pattern throughout whole grain that supports the latter assumption. Paragneiss assemblage comprises syn-tectonic garnets with slightly different core (XAlm=0.659, XSps=0.077, XPrp=0.128, XGrs=0.136) and rim composition (XAlm=0.699, XSps=0.044, XPrp=0.133, XGrs=0.124), suggesting prograde character. Peak assemblage includes biotite, amphibole (Si 6.20, Mg 1.95, Fe2+ 1.23 p.f.u.), plagioclase (~28 %An) and quartz. Apatite, ilmenite, zircon, amphibole (Si 6.97, Mg 3.24, Fe2+ 0.60 p.f.u.) and quartz are inclusions in garnets. Amphibolites also contain syn-tectonic garnets with core compositon of XAlm=0.595, XSps=0.121, XPrp=0.081, XGrs=0.203 and rim with XAlm=0.677, XSps=0.065, XPrp=0.122, XGrs=0.145. Cation distribution suggests prograde character. Peak assemblage also includes amphibole (Si 6.06-6.82, Mg 1.58-2.50, Fe2+ 1.27-1.79 p.f.u.), plagioclase (20-30 %An) and quartz, with minor ilmenite, apatite, titanite, epidote-clinozoisite. Thermobarometric calculations are performed using Holland & Powell's THERMOCALC and Gerya & Perchuk's GeoPath software, and individual calibrations of Graham & Powel (1984) for Grt+Hbl geothermometry, Kohn & Spear (1990) for Grt+Hbl+Pl geobarometry, Holland & Blundy (1994) for Hbl+Pl geothermometry. In the mica schists, Grt+Bt thermometry yielded 572&plusmn ; 14 °C and Grt+Ms+Bt+Qtz barometry yielded P values of 7.0&plusmn ; 0.5 kbar using GeoPath software, while with THERMOCALC we obtained 600-630 °C and 9-11 kbar. In the case of gneisses we yielded 600-630 °C and 6.5-7.5 kbar with THERMOCALC, while results for Grt+Hbl+Pl assemblage are 500-580 °C and 8-10 kbar. Additionally, using GeoPath, temperature values of 591&plusmn ; 11° C (Grt+Bt), 595&plusmn ; 29 °C (Grt+Hbl) and 640&plusmn ; 12 °C (Hbl+Pl) were obtained. In the amphibolites calculated T values are 619&plusmn ; 37 °C for edenite-tremolite and 588&plusmn ; 45 °C for edenite-richterite reaction using Holland & Blundy (1994) calibrations. In addition, Grt+Hbl thermometry yielded 490-570 °C, while using Grt+Hbl+Pl barometry we obtained 7-10 kbar. The presented peak metamorphic data together with additional microtextural, paragenetic and mineral chemical data can be correlated with similar results obtained in nearby parts of the South Tisia in Hungary, which support an idea of a complex evolution path in sense of a model (Variscan-Variscan or pre-Variscan-Variscan) with two different stages of metamorphism.

geothermobarometry; Variscan; metamorphism; Kutjevačka Rijeka; Slavonian Mts.; Croatia

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