Por su alcance

1_{Annamária Kis}#

, 1Tamás G. Weiszburg,

2

Petr Gadas, 1Tamás Váczi, 1György Buda

1

Department of Mineralogy, Eötvös L. University, Budapest, Hungary, # [email protected]

2 _{Department of Geological Sciences, Masaryk University, Brno, Czech Republic}

Key words: durbachite, Mórágy, Rastenberg, zircon, morphology and texture of zircon, EPMA, SEM-BSE/CL, Raman spectroscopy

INTRODUCTION

High-potassic mafic enclaves are

widespread within the magnesio-potassic granitoids (local names: durbachite) of the Variscan collision belt in Europe (Massif Central, Vosges, Schwarzwald, Central and South Bohemian Plutonic Complexes (CSBPC)).

From petrographical and geochemical

viewpoint, these mafic enclaves

correspond to lamprophyre-derived rocks

and a late/postmagmatic overprinting effect, connected possibly to the potassium enrichment of three rock-types: mafic enclaves, hybrid rocks and the hosting microcline megacryst-bearing granitoids. The origin (restitic or in situ unmixing) of mafic enclaves in these Variscan granitods has been debated widely, based mainly on geochemical constraints. Comparative age determination could provide independent arguments to that debate, but previous age determination trials left open basic questions, mainly because of the ambiguity caused by the uncontrolled heterogeneity of the zircon crystals studied.

Our zircon samples were separated from two granitoid bodies at the easternmost margin of the Western-

Central-European Variscan collision zone.

Mórágy, Hungary is the easternmost, tectonically detached continuation of the Moldanubian zone, while Rastenberg, Austria connects directly to the South Bohemian Batholith.

At the moment no local, texture related (core vs. rim; zoning within core and/or rim) LA-ICP-MS zircon age data are

available for these two magmatic

complexes. Költzli at al. (2004) reported zircon age data, based mainly on evaporation Pb/Pb analyses, where the core and the rim ages of the crystals could not be separated on a controlled way, for the hosting granitoids. For the two other rock types no data is available.

Taken into account the limited size and the typical, frequent, complex zoning of both the Mórágy and the Rastenberg zircon crystals SHRIMP and/or LA-ICP-MS methods may give good U-Pb, Th-Pb age data, provided we can prepare zircon crystals for these measurements careful enough.

PRE-EXAMINATION OF ZIRCONS Our methodology consists of four steps for the pre-examination of zircon crystals:

64 1. determination of morphology-types of zircon (Pupin 1980); 2. determination of internal texture-types (example: zoning) of zircon; 3. determination of the structural state (Nasdala et al. 2006) and chemical composition of zircons zones (Finch and Hanchar 2003); 4. identification of mineral inclusions in zircon. These four types of information are then evaluated together zone by zone in order to define those spots in zircon from where the most undisturbed age data characteristic for the geological processes can be expected. The particular texture features of these spots give the same time the maximum size limits of local probe to be applied.

Zircons of all rock the three types from both localities were processed by that pre- examination methodology. Three major

zircon morphology types were

distinguished in all samples: normal prismatic (S24, S25); flat prismatic (AB5)

and elongated prismatic (P5).Zircons show both primary (growth/sector zoning ± xenocryst core) and secondary (convolute) zoning features on SEM-CL and SEM- BSE images. Raman spectroscopy was used for determining the structural state (FWHM) of the individual zircon zones found by SEM-CL/SEM-BSE imaging. Normal and flat zircon crystals show zones of all structural states (well crystallized, intermediate, metamict), while the elongated zircon crystals are not metamict. Chemical composition of the zones was measured by EMPA. The data confirm that SEM-BSE contrast may predominantly depend on structural state of the zircon zones (Nasdala et al. 2009), if strong differences in structural state are present, however if that variation is limited (within ± 1–2 cm-1

FWHM) the SEM-BSE contrast depends mainly on changes in chemical composition (e.g. elevated U, Y, P, REE concentrations).

The studied zircons are rich in both single phase and multiphase mineral inclusions. The latter type, consisting Na free K­feldspar, albite and quartz (found most commonly in elongated zircons),

indicates the lowest crystallization temperature for Si­rich granite melt, confirming that zircon was crystallizing continuously in a wide temperature range during the consolidation of the granitoid magma.

CONCLUSION

Based on these measurements about 190 well characterized, undisturbed measuring areas (varying in diameter between several ten and one micrometer) in 96 crystals have been selected for future SHRIMP and LA-ICP-MS studies. These areas cover all the three expected geological processes and give a potential for determining the 1) age of inherited zircon of crustal origin (xenocrystic core), 2) crystallization age of rock (growth zoning), 3) the age of the overprinting effect (convolute zoning).

REFERENCES

FINCH, J. R., HANCHAR, J. M. (2003): Structure and chemistry of zircon and zircon-group minerals. In HANCHAR, J. M., HOSKIN, P. W. O. (Eds.), Zircon. Rev. Mineral. Geochem, 53, 1–25.

KLÖTZLI, U., BUDA, GY., SKIÖLD, T. (2004): Zircon typology, geochronology and whole rock Sr-Nd isotope systematics of the Mecsek Mountain granitoids in the Tisia Terrane (Hungary). Mineralogy and

Petrology, 81, 113–134.

NASDALA,L.,KRONZ,A.,HANCHAR,J.M., TICHOMIROWA, M., DAVIS, D. D., HOFMEISTER,W. (2006): Effects of natural radiation damage on back-scattered electron images of single-crystals of minerals. Am. Mineral. 91, 1738–1746. NASDALA L.,KRONZ A., WIRTH R.,VÁCZI T.,PÉREZ-SOBA C.,WILLNER A.,KENNEDY A.K. (2009): The phenomenon of deficient electron microprobe totals in radiation- damaged and altered zircon. Geochimica

et. Cosmochimica Acta, 73, 1637–1650.

PUPIN, J. P. (1980): Zircon and granite petrology. Contrib. Mineral. Petr. 73, 207– 220.

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