Naslov
Zircon typology in crystalline rocks of Moslavačka Gora (Croatia) – preliminary petrogenetic insight from transmitted light (TL) and scanning electron microscopy (SEM)

Autori
Starijaš, Biljana ; Balen, Dražen ; Tibljaš, Darko ; Finger, Fritz

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Abstracts book / Tomljenović, Bruno ; Balen, Dražen ; Vlahović, Igor - Zagreb : Hrvatski geološki institut, 2005, 89-90

Skup
7th Workshop on Alpine Geological Studies

Mjesto i datum
Opatija, Hrvatska, 29.09.2005. - 01.10.2005

Vrsta sudjelovanja
Poster

Vrsta recenzije
Domaća recenzija

Ključne riječi
zircon ; morphology ; typology ; Moslavačka Gora ; Croatia

Sažetak
The mineral zircon is extremely variable both in terms of external morphology and internal textures. These features reflect the geologic history of the mineral crystallization and depend on many geological factors such as temperature and pressure of crystallization, chemistry, etc. Zircon has become one of the most widely used minerals for the extraction of information on the history and genesis of magmatic, metamorphic and sedimentary rocks. One of the major advantages of zircon is its ability to survive magmatic, metamorphic and erosional processes that destroy most other common minerals (CORFU et al., 2003). Systematic examination of zircon typology has led to the establishment of the widely used “ Pupin diagram” in which zircon crystals are classified according to the relative development of the {; ; 100}; ; vs. {; ; 110}; ; prismatic forms and the {; ; 211}; ; vs. {; ; 101}; ; pyramidal crystal forms (PUPIN, 1980). The typologic study of zircon populations from granitic rocks lead to the proposition of a genetic classification with following divisions: (1) zircon grains from granites of mantle or mainly mantle origin (relatively dry alkaline and tholeiitic series granites) tend to be dominated by {; ; 100}; ; and {; ; 101}; ; forms ; (2) granites of crustal or mainly crustal origin (aluminous to calc-alkaline rocks exhibit various combinations of forms with a prominent presence of {; ; 211}; ; form ; (3) and those from water-rich granites and pegmatites tend to have {; ; 110}; ; and {; ; 101}; ; as their dominant forms (PUPIN, 1980). Here we describe zircons from the Moslavačka Gora granitoids, whose magmatic affinity has already been determined by petrological and geochemical studies (PAMIĆ, 1990). Recently, STARIJAŠ et al. (2004) distinguished two different groups of Cretaceous age fine-grained biotite-granites (biotite contents 4-10 vol.%), i.e. the Pleterac type and the Garić-grad type. The Pleterac type (PT) granites are mostly undeformed, chemically correspond to peraluminous S-type granite and comprise muscovite (2-4 vol.%). The Garić-grad type (GGT) granites often show a slight foliation and their chemistry point to igneous lower crustal sources. Zircon from the two granite types (7 samples, each from different locality) was separated by conventional heavy-mineral separation technique (rock crushing, sieving, wet shaking table, bromoform separation, hand picking) from ca. 2 kg per sample. Zircon grains were analyzed with standard polarizing microscope (transmitted light) and additionally representative grains were analyzed with Tescan SEM. Figure 1. shows the main populations that have been recognized in the samples studied. The euhedral prismatic PT zircons, with euhedral to subhedral mineral and fluid inclusions, are clear without evidence for the presence of older cores. The ratio length/width vary from ~ 1.5 – 3.5. The long-prismatic zircons show length/width ratio 6 - 7. Length of zircons varies from 50 – 100 µ ; m to up to 300 µ ; m for long-prismatic ones. The PT granites show unusual zircon morphology patterns with two maxima, one in the right upper corner of the Pupin diagram {; ; 101}; ; combined with {; ; 110}; ; i.e. I-type morphology, the other one showing steep {; ; 211}; ; pyramides combined with the {; ; 110}; ; prism. Such bimodal patterns may be indicative for felsic I-type granite melts that were chemically modified and driven towards S-type compositions through assimilation of meta-sedimentary material (HUMER et al., 2002). The GGT granites have zircons with large {; ; 100}; ; prisms and a dominant {; ; 101}; ; pyramide, in accordance with their chemical I-type signatures. The zircon occurs as clear, unzoned, euhedral, sometimes fractured and unmetamict crystals with no overgrowth, their homogeneous morphology suggest a monogenic, igneous origin. Tiny and elongate inclusions of apatite are abundant. The GGT zircons have a length/width ratio of ~ 2.5 and a crystal size of ~ 100 μ m. They appear to be of primary, igneous origin, with no evidence of metamorphic reworking. References CORFU, F., HANCHAR, J.M., HOSKIN, P.W.O., KINNY, P. (2003): Atlas of Zircon Textures.- Reviews in Mineralogy and Geochemistry, 53, 469-500. HUMER, B., GERDES, A., FINGER, F. (2002): The Weitra Granite in north-western Lower Austria - a late-stage high-level I-Type intrusion associated with greisen in the Variscan South Bohemian Batolith.- Zbl. Geol. Palaeont. Teil I, 3/4, 213-235, Stuttgart. PAMIĆ, J. (1990): Alpine granites, migmatites and metamorphic rocks from Mt. Moslavačka Gora and the surrounding basement of the Pannonian Basin (Northern Croatia, Yugoslavia).- Rad JAZU 10, 7-121, Zagreb. PUPIN, J.P. (1980): Zircon and Granite Petrology.- Contrib. Mineral. Petrol., 73, 207-220. STARIJAŠ, B., BALEN, D., TIBLJAŠ, D., SCHUSTER, R., HUMER, B., FINGER, F. (2004): The Moslavačka Gora Massif in Croatia: part of a Late Cretaceous high-heat-flow zone in the Alpine-Balkan-Carpathian-Dinaride collision belt.- Ber. Inst. Erdwiss. K.-F.-Univ. Graz, 9, 453-454, Graz.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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