Naslov
Tourmaline nodules occurrence in Moslavačka Gora (Croatia) granite: a snapshot of magmatic processes

Autori
Balen, Dražen

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

Izvornik
8th Workshop on Alpine Geological Studies / - : Akademie der Naturwissenschaften, Schweizerischer Nationalfonds zur Foerderung der Wissenschaftlichen Forschung, 2007, 3-3

Skup
8th Workshop on Alpine Geological Studies

Mjesto i datum
Davos, Švicarska, 10.10.2007. - 12.10.2007

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
Tourmaline nodule; Moslavačka Gora; Croatia; granite; magmatic processes

Sažetak
Tourmaline nodule occurrences around the world have been described by authors as “ bizarre” , “ ” spherical entities” and under different and vague names like clots, clusters, spots, coca(r)des, ovoids and orbic(u)les. Although descriptions mutually differ in age, origin and occurrence of host rocks, features of nodule itself are quite unique and significant for genesis. The aim of this contribution is to rouse the discussion about genetic importance of tourmaline nodule occurrence for Moslavačka Gora granite. Tourmaline nodules are typically spherical, usually 1 to 10 cm in diameter, consisting of tourmaline and quartz (&plusmn ; feldspar) core surrounded by a leucocratic rim (also called bleached zone or halo comprising quartz and feldspar &plusmn ; muscovite). The core and rim of the nodule are placed inside the Cretaceous peraluminous biotite granite (quartz, K-feldspar, plagioclase, biotite, and muscovite) characterized with relatively flat REE chondrite normalized patterns, slight enrichment in the LREE and Eu negative anomaly. Moving towards the top of the granite body number and size of nodules appear to rise. They are often found in the inhomogeneous randomly distributed swarms aligned along streamlines defined by chaotic magmatic flow field. Voids and/or miarolitic cavities are occasionally present inside the nodule cores. Beside the nodules, biotite granite comprise metapelitic xenoliths (biotite, quartz, feldspar, sillimanite &plusmn ; andalusite) and K-feldspar megacrysts up to 10 cm in size. From a morphological point of view, nodules display variable typically spherical morphologies corroborating a magmatic origin. Textural features like rounded shape, physical separation from the groundmass, occurrence of grabbed phenocrysts inside the nodules, differences in grain size and texture, lack of (micro)vein network connecting the nodules indicates that the nodules once formed, behaved as an almost closed system in the course of their crystallization. A plausible genetic model can be summarized as follows: magma is transferred from source (anatectic melting of pelitic source?) to shallow emplacement level as inferred from magmatic andalusite and miarolitic cavities ; decompression along path induces a liquidus undercooling of the granite system and promotes nucleation of K-feldspar phenocrysts together with exsolution of fluid phase ; crystallization of volatile-free minerals further leads to a rise of volatile concentration (B, H2O, ...) in melt and to formation of boiling bubbles. Newly formed boron-rich silicate melt bubbles, due to lower density, start to rise in magmatic column and to accrete into sphere as preferred shape for pockets of immiscible boron-rich fluids to decrease surface tension. With reaching of solidus temperature crystallization of bubble, which did not manage to escape from coexisting host granite magma, while it was still behaving as a melt, produces tourmaline nodule with characteristic morphology.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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