International Geologiical Congress - Oslo 2008

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MRD-06 Granitic magmatism and related mineralizations

 

Origin and microstructure of K-feldspar megacrysts from Cretaceous granite (Pannonian Basin, Croatia)

 

Dražen Balen, Faculty of science, university of Zagreb (Croatia)
Viktória Kovács Kis, Hungarian academy of sciences, research institute for technical physics and materials science (Hungary)
Nenad Toma?iĉ, Faculty of science, university of Zagreb (Croatia)
Marija Horvat, Croatian seological survey (Croatia)
 

 

The Moslavacka Gora (MG) granite body located in the southwestern part of the Pannonian Basin (Croatia) shows marks of a complex evolution history during Cretaceous time. Field, petrographic and chemical studies reveal existence of several types of granitic rocks (andalusite-, sillimanite- and tourmaline- bearing granite, leucogranite, biotite granite, monzogranite, granodiorite). These granites penecontemporaneously occur together with medium-grade metamorphic rocks (amphibolite, marble, metapelite) and often comprise metapelitic xenoliths (biotite, quartz, feldspar, sillimanite ± andalusite).
K-feldspars megacrysts, up to 10 cm in size, locally occur inside the Cretaceous peraluminous biotite granite (quartz, plagioclase, biotite, K-feldspar and muscovite) from Srednja Rijeka locality together with peculiar tourmaline nodules (see Balen & Broska, this volume). Chemical analysis of the hosting granite yields 73.9 wt.% SiO2, 14.2 wt.% Al2O3, low concentrations of Fe2O3 (0.93 wt.%), MgO (0.25 wt.%), CaO (0.48 wt.%) and TiO2 (0.14 wt.%), and relatively high K2O (6.01 wt.%) and Na2O (2.71 wt.%). Strong peraluminosity (ASI=1.2) and high SiO2 content are coupled with low concentrations of ferromagnesian elements together with high LIL trace elements Ba (330 ppm), Sr (70 ppm), Cs (26ppm), Rb (277 ppm). The host granite is characterized with moderately flat and fractionated REE chondrite normalized patterns, slight enrichment in the LREE and strongly negative Eu anomaly ((La/Yb)N = 4.3; Eu/Eu* = 0.4; low ΣREE=70.3 ppm). The X-ray powder diffraction (XRPD) patterns together with selected area electron diffraction (SAED) patterns of K-feldspar megacrysts obtained on b* and <20-1> directions and b* and c* directions indicate monoclinic symmetry of K-feldspar, which defines it as orthoclase. However, slight continuous streaking parallel to b* on some SAED patterns reveals that the orthoclase structure is modulated on the nano scale. If reflections are splitted parallel to b*, they show mirror symmetry which is proper of albite twinning. The thickness of the twin lamellae varies between 30 and 200 nm.
K-feldspar megacrysts, chemical and petrographic features of hosting peraluminous biotite granite including existence of tourmaline nodules point to temperature, pressure and compositional fluctuations in the magma during crystallisation. The megacrysts nucleated and grew in an environment of increasing undercooling, probably during the ascent of the magma. The low population density of the megacrysts has been ascribed to nucleation difficulties i.e. they develop at conditions of unusually low nucleation to growth ratio in granitic melt until the temperature is close to the solidus. Though K-feldspar is commonly among the last minerals crystallizing in granitic magmas, abundant melt is still present at that stage. Igneous microstructures of the megacrysts as crystal shape, simple twinning, zonal growth (compositional zoning) are consistent with growth from melt.

 

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