Nd, Sr, Pb isotope study of the Western Carpathians: Implications for Palaeozoic evolution

Schweizerische Mineralogisch-Petrologische Mitteilungen, v.81, 159-174 (2001) 01/2001; 81(2).
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Nd, Sr and Pb-Pb whole rock isotope data from the Variscan basement of the Western Carpathians are presented in order to define the eastern continuation of the Variscides in Central Europe. Isotope analyses of migmatites, orthogneisses and granitoids from the Tatra Mountains (Upper unit) and two neighbouring areas (Velká Fatra and Lower Tatra Mountains) have been examined. The granites, migmatites and orthogneisses show mainly crustal characteristics with εNd(330) values ranging from -2 to -9 and εSr(330) values between 10 and 387. These results are in good agreement with the Pb-Pb whole rock data and document a crustal origin for the investigated rocks with only minor influence of an enriched mafic component such as recycled oceanic crust. REE and geochemical data are consistent with a collisional environment for the emplacement of the granitoids and the precursor of the orthogneisses. The isotope data confirm that the Western Carpathians own a quite similar isotopic composition than other Variscan regions in Europe. On the basis of the data presented, in combination with our previous U-Pb zircon dating, we propose the following geological evolution for the Tatra Mountains. In Early Devonian time (~406 Ma) subduction-related magmatism led to the formation of the earliest granitoids, the precursors of the present-day orthogneisses. In Late Devonian and Carboniferous time (360-350 Ma), younger granitoids intruded, most probably as a result of collisional thickening. During this event the earlier granitoids were recrystallised and deformed, resulting in the formation of the orthogneiss fabric. The youngest granitic magmatism in the Carboniferous (315 Ma) is related to the Late Variscan orogenic collapse.

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Available from: Milan Kohut, May 20, 2014
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    • "The basement consists of Variscan granitoids (zircon U/Pb ages: 343 ± 4 and 330 ± 10 Ma; Poller et al. 2001; Putiš et al. 2003) and Variscan metamorphic rocks (mostly Fig. 1 a Tectonic sketch map of the Western Carpathians with exposures of Variscan crystalline complexes belonging to three principal units and occurrences of Paleogene sediments and Neogene volcanic rocks. Inset map shows the location a; WC Western Carpathians, EA Eastern Alps. "
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    ABSTRACT: A combination of four thermochronometers [zircon fission track (ZFT), zircon (U–Th)/He (ZHe), apatite fission track (AFT) and apatite (U–Th–[Sm])/He (AHe) dating methods] applied to a valley to ridge transect is used to resolve the issues of metamorphic, exhumation and topographic evolution of the Nízke Tatry Mts. in the Western Carpathians. The ZFT ages of 132.1 ± 8.3, 155.1 ± 12.9, 146.8 ± 8.6 and 144.9 ± 11.0 Ma show that Variscan crystalline basement of the Nízke Tatry Mts. was heated to temperatures >210°C during the Mesozoic and experienced a low-grade Alpine metamorphic overprint. ZHe and AFT ages, clustering at ~55–40 and ~45–40 Ma, respectively, revealed a rapid Eocene cooling event, documenting erosional and/or tectonic exhumation related to the collapse of the Carpathian orogenic wedge. This is the first evidence that exhumation of crystalline cores in the Western Carpathians took place in the Eocene and not in the Cretaceous as traditionally believed. Bimodal AFT length distributions, Early Miocene AHe ages and thermal modelling results suggest that the samples were heated to temperatures of ~55–90°C during Oligocene–Miocene times. This thermal event may be related either to the Oligocene/Miocene sedimentary burial, or Miocene magmatic activity and increased heat flow. This finding supports the concept of thermal instability of the Carpathian crystalline bodies during the post-Eocene period.
    Full-text · Article · Oct 2011 · Swiss Journal of Geosciences
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    • "The main magmatic activity occurred at 368–355 Ma (Poller et al. 2000; Gawe ˛da 2008, 2009; Burda et al. 2011). Further pulses have been dated at 347–341 Ma (Gawe ˛da 1995; Poller et al. 2001; Gawe ˛da 2008). Pegmatites, locally rich in tourmaline and molybdenite, are present both in the metamorphic envelope and inside the granite. "
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    ABSTRACT: The High Tatra granite intrusion is an example of a Variscan syn-tectonic, tongue-shaped intrusion. In some portions of the intrusion, structures occur which appear to be of sedimentary origin. These include structures similar to graded bedding, cross-bedding, troughs and flame structures, K-feldspar-rich cumulates and magmatic breccias. Formation of these structures might be related to changing magma properties, including crystal fraction, development of a crystal mush and a decrease in magma viscosity, stimulated by influx of mafic magma and high volatile content. The suggested processes in operation are: gravity-controlled separation, magma flow segregation, deposition on the magma-chamber floor, filter pressing and density currents stimulated by tectonic activity.%The formation of the sedimentary structures was also aided by the presence of large numbers of xenoliths that acted as a heat sink and influenced the thermal field in the intrusion, stimulating rapid cooling and crystal nucleation. Sinking xenoliths deformed the layering and, to some extent, protected the unconsolidated crystal mush from erosion by magma flowing past.%Areas with well-developed sedimentary magmatic structures can be viewed as having involved magma rich in crystals locally forming closely-packed networks from which residual melt was extracted by filter pressing, and preserved in leucocratic pods and dykes. Interleaved, non-layered granite may be interpreted to have formed from the magma with initially low crystal fractions.%It is suggested that the intrusion was formed from numerous magma injections representing different stages in the mixing and mingling of felsic and mafic sources. It solidified by gravitation-driven crystal accumulation and flow sorting on the magma chamber floor and on the surfaces of large numbers of xenoliths. Shear stress acting during intrusion might have influenced the formation of magmatic structures.
    Full-text · Article · Jun 2011 · Earth and Environmental Science Transactions of the Royal Society of Edinburgh
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    • "The Western Tatra granitoids were previously interpreted as typical S - type granites ( Poller et al . 2001a ) . The textures found in the analysed rocks advocate that mixing and mingling of magmas of different chemistry occurred during the granitoid magma emplacement . The presence of mafic clots and associated micrographic intergrowths at the border zone between the mafic and felsic portions suggest the clots have originated in mafic magma a"
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    ABSTRACT: In the Variscan Western Tatra granites hybridization phenomena such as mixing and mingling can be observed at the contact of mafic precursors of dioritic composition and more felsic granitic host rocks. The textural evidence of hybridization include: plagioclase–K-feldspar–sphene ocelli, hornblende- and biotite-rimmed quartz ocelli, plagioclase with Ca-rich spike zonation, inversely zoned K-feldspar crystals, mafic clots, poikilitic plagioclase and quartz crystals, mixed apatite morphologies, zoned K-feldspar phenocrysts. The apparent pressure range of the magma hybridization event was calculated at 6.1kbar to 4.6kbar, while the temperature, calculated by independent methods, is in the range of 810°C−770°C. U-Pb age data of the hybrid rocks were obtained by in-situ LA-MC-ICP-MS analysis of zircon. The oscillatory zoned zircon crystals yield a concordia age of 368 ± 8Ma (MSWD = 1.1), interpreted as the age of magma hybridization and timing of formation of the magmatic precursors. It is the oldest Variscan magmatic event in that part of the Tatra Mountains.
    Full-text · Article · May 2011 · Mineralogy and Petrology
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