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Първи U-Pb цирконови геохроноложки данни на диабазите...
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55 БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2016“ BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2016” First U-Pb zircon geochronology data of the diabases from the Lower Paleozoic section along Iskar and Gabrovnitsa River valleys Първи U-Pb цирконови геохроноложки данни на диабазите от долнопалеозойския разрез по долините на реките Искър и Габровница Stoyan Georgiev 1 , Mitko Popov 1 , Eleonora Balkanska 2 , Ianko Gerdjikov 2 , Dian Vangelov 2 Стоян Георгиев 1 , Митко Попов 1 , Елеонора Балканска 2 , Янко Герджиков 2 , Диан Вангелов 2 1 Geological Institute, Bulgarian Academy of Sciences; E-mails: [email protected]; [email protected] 2 Sofia University “St. Kliment Ohridski”, Dept. of Geology, Paleontology and Fossil Fuels; E-mails: [email protected]; [email protected]; [email protected] Keywords: U-Pb zircon geochronology, diabases, Lower Paleozoic section. The present study is focused on the U-Pb zircon geo- chronology of the mafic lavas (diabases) from the Lower Paleozoic section along Iskar and Gabrovnitsa River valleys. It is important to note, that there is sig- nificant controversies regarding the stratigraphy and the position of the studied rocks. Dimitroff (1929) is the first to describe the diabases along the Iskar River gorge in detail. He distinguished 2 lithostratigraphic units in the Lower Paleozoic section with an uncon- formity between them. According to him the diabases occur in both units, but are more abundant in the upper unit. Ivanov (1972) studued in detail the classical sec- tion of pre-Ordovician rocks between the villages Bov and Lakatnik. The proposed lithostratigraphic scheme was partly opposite to that of Dimitroff (1929). Later the basic magmatic rocks in this section were interpret- ed as olistostromes (Ivanov et al., 1987). Dzhelepov (1983) described diabase lavas and lava breccias at several different levels in the Lower Paleozoic section along Iskar and Gabrovnitsa River valleys. He distin- guished 3 lithostratigraphic units all of them including basic magmatic rocks. Two samples, from the lower parts of the suc- cession, were taken from the diabases in the vicinity of Balkan railway stop (N 43.04025°, E 23.35321°) and “The Seven Thrones” Monastery (N 43.00979°, E 23.47374°). Cathodoluminescence and back-scat- tered electron images were made for distinguishing the internal structure of the zircons. The U-Pb iso- topes were obtained using LA-ICP-MS equipment in the Geological Institute, BAS. The data reduction was proceeded using GLITTER software and the Concordia plots were realized with Isoplot software. Dimitroff (1929) interpreted the mafic magmatic rocks as a result of under-water eruptions in a shallow marine environment. At the Balkan railway stop lo- cality a subaqueous lava flow with hyaloclastite outer part (Fig. 1a) formed due to lava and water interaction, while in the inner parts well-developed pillow lavas (Fig. 1b, c) are observed. At “The Seven Thrones” Monastery locality the sampled diabases are massive (Fig. 1d). At both localities the rocks are metamor- phosed in green-schist facies and crosscut by at least two generation of veins. The age of the basic magmatic rocks is assumed to be not younger that Cambrian based on acritarch fos- sils (Kalvacheva, Shishkova-Simova, 1968) or Lower Ordovician (Ivanov et al., 1987). In both samples the zircons are a few and give well-constrained, similar Concordia ages around 520 Ma (Fig. 1e, f). A few xenocrystic grains give a hint for assimilated material in the intervals of 580, 610 and 800 Ma. The obtained ages prove the Lower Cambrian age of the mafic magmatic event in the lowermost unit of the Lower Paleozoic succession along the Iskar River gorge, sup- posedly continuous to the Ordovician–Devonian sec- tion. Interestingly, these data overlap with the young- est ages of basic rocks from the Frolosh Unit in SW Bulgaria. Yet, while the Frolosh Unit is assumed to be a piece of the Protothetys crust (summarized by Zagorchev et al., 2015 and references therein), we suggest that the studied rocks originated in an exten- sional regime at the northern margin of Gondwana that marked the onset of the supercontinent break-up (as proposed for similar rocks from the Variscan orogeny from the Bohemian Massif, Soejono et al., 2010).
Transcript of Първи U-Pb цирконови геохроноложки данни на диабазите...
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БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2016“BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2016”
First U-Pb zircon geochronology data of the diabases from the Lower Paleozoic section along Iskar and Gabrovnitsa River valleysПърви U-Pb цирконови геохроноложки данни на диабазите от долнопалеозойския разрез по долините на реките Искър и ГабровницаStoyan Georgiev1, Mitko Popov1, Eleonora Balkanska2, Ianko Gerdjikov2, Dian Vangelov2
Стоян Георгиев1, Митко Попов1, Елеонора Балканска2, Янко Герджиков2, Диан Вангелов2
1 Geological Institute, Bulgarian Academy of Sciences; E-mails: [email protected]; [email protected] 2 Sofia University “St. Kliment Ohridski”, Dept. of Geology, Paleontology and Fossil Fuels; E-mails: [email protected]; [email protected]; [email protected]
Keywords: U-Pb zircon geochronology, diabases, Lower Paleozoic section.
The present study is focused on the U-Pb zircon geo-chronology of the mafic lavas (diabases) from the Lower Paleozoic section along Iskar and Gabrovnitsa River valleys. It is important to note, that there is sig-nificant controversies regarding the stratigraphy and the position of the studied rocks. Dimitroff (1929) is the first to describe the diabases along the Iskar River gorge in detail. He distinguished 2 lithostratigraphic units in the Lower Paleozoic section with an uncon-formity between them. According to him the diabases occur in both units, but are more abundant in the upper unit. Ivanov (1972) studued in detail the classical sec-tion of pre-Ordovician rocks between the villages Bov and Lakatnik. The proposed lithostratigraphic scheme was partly opposite to that of Dimitroff (1929). Later the basic magmatic rocks in this section were interpret-ed as olistostromes (Ivanov et al., 1987). Dzhelepov (1983) described diabase lavas and lava breccias at several different levels in the Lower Paleozoic section along Iskar and Gabrovnitsa River valleys. He distin-guished 3 lithostratigraphic units all of them including basic magmatic rocks.
Two samples, from the lower parts of the suc-cession, were taken from the diabases in the vicinity of Balkan railway stop (N 43.04025°, E 23.35321°) and “The Seven Thrones” Monastery (N 43.00979°, E 23.47374°). Cathodoluminescence and back-scat-tered electron images were made for distinguishing the internal structure of the zircons. The U-Pb iso-topes were obtained using LA-ICP-MS equipment in the Geological Institute, BAS. The data reduction was proceeded using GLITTER software and the Concordia plots were realized with Isoplot software.
Dimitroff (1929) interpreted the mafic magmatic rocks as a result of under-water eruptions in a shallow marine environment. At the Balkan railway stop lo-cality a subaqueous lava flow with hyaloclastite outer part (Fig. 1a) formed due to lava and water interaction, while in the inner parts well-developed pillow lavas (Fig. 1b, c) are observed. At “The Seven Thrones” Monastery locality the sampled diabases are massive (Fig. 1d). At both localities the rocks are metamor-phosed in green-schist facies and crosscut by at least two generation of veins.
The age of the basic magmatic rocks is assumed to be not younger that Cambrian based on acritarch fos-sils (Kalvacheva, Shishkova-Simova, 1968) or Lower Ordovician (Ivanov et al., 1987). In both samples the zircons are a few and give well-constrained, similar Concordia ages around 520 Ma (Fig. 1e, f). A few xenocrystic grains give a hint for assimilated material in the intervals of 580, 610 and 800 Ma. The obtained ages prove the Lower Cambrian age of the mafic magmatic event in the lowermost unit of the Lower Paleozoic succession along the Iskar River gorge, sup-posedly continuous to the Ordovician–Devonian sec-tion. Interestingly, these data overlap with the young-est ages of basic rocks from the Frolosh Unit in SW Bulgaria. Yet, while the Frolosh Unit is assumed to be a piece of the Protothetys crust (summarized by Zagorchev et al., 2015 and references therein), we suggest that the studied rocks originated in an exten-sional regime at the northern margin of Gondwana that marked the onset of the supercontinent break-up (as proposed for similar rocks from the Variscan orogeny from the Bohemian Massif, Soejono et al., 2010).
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ReferencesDimitroff, S. 1929. Die diabasgesteine im Iskerdurchbruche
zwischen den eisenbahnhaltestellen Bow und Lakatnik. – Ann. Univ. de Sofia, 25, 3–Sci. natur., 175–237 (in Bulgarian with a German abstract).
Dzhelepov, T. 1983. Lithostratigraphic features of the Diabase-phyllitoid complex from the region of Iskar gorge. – In: Proceeding of the Scientific Conference “30 years University of Mining and Geology”, 181–189 (in Bulgarian).
Kalvacheva, R., F. Shishkova-Simova. 1968. Acritarchs from the Diabase-phyllitoid formation along Iskar gorge. – C. R. Acad. Bulg. Sci., 21, 11, 1205–1207 (in Russian).
Ivanov, Z. 1972. Structure of the old Paleozoic rocks between
Fig. 1. Characteristic features of the Lower Paleozoic subaqueous diabase lava flows along Iskar and Gabrovnitsa River valleys – Balkan railway stop locality: a, hyaloclastites at the outer parts of the subaqueous lava flow; b, c, pillow lavas outlined with well-developed quenched margins; “The Seven Thrones” Monastery locality: d, diabases crossed by two generations of quartz veins. Zircon LA-ICP-MS geochronology Concordia diagrams for diabases from: e, “The Seven Thrones” Monastery locality; f, Balkan railway stop locality; g, probability density plot for the xenocrys-tic material
the railway stations Bov and Lakatnik. – Ann. Univ. de Sofia, 64, 1–Géol., 83–95 (in Bulgarian).
Ivanov, Z., K. Kolcheva, S. Moskovski, D. Dimov. 1987. Des particularités et du caractère de la “formation diabaso-phyl-litoide”. – Rev. Bulg. Geol. Soc., 48, 2, 1–24 (in Bulgarian with a French abstract).
Soejono, I., E. Zackova, V. Janousek, M. Machek, J. Kosler. 2010. Vestige of an Early Cambrian incipient oceanic crust incorporated in the Variscan orogen: Letovice Complex, Bohemian Massif. – J. Geol. Soc., 167, 1113–1130.
Zagorchev, I., C. Balica, E. Kozhoukharova, I. C. Balintoni, S. Gavril, E. Negulescu. 2015. Cadomian and post-cadomian tectonics west of the Rhodope Massif – the Frolosh green-stone belt and the Ograzhdenian metamorphic supercom-plex. – Geologica Macedonica, 132, 2, 101–132.
