Early crystallization of K-feldspar in granitic systems. Implications on the origin of magmatic fabrics in granitic rocks

J. Díaz-Alvarado

Abstract


One of the most outstanding characteristics of some granodioritic to granitic rocks is the presence of K-feldspar megacrysts. Granodiorites and monzogranites of the Central System batholith present variable amounts of large (up to 10 cm in length) euhedral K-feldspar crystals. The porphyritic textures, the euhedral shape, the alignment of plagioclase and biotite inclusions and the magmatic fabrics point to a magmatic origin for these megacrysts.

This work presents the study of the phase equilibria in the high-K2O granodioritic system. A series of experiments were conducted to study the crystallization sequence at the emplacement conditions, i.e. 4 H2O wt% and 0.4 GPa. Experimental results show that orthopiroxene is the liquidus phase at 1010 ºC, which reacts with the H2O-rich melt to stabilize biotite between 980 and 940 ºC. Plagioclase crystallizes at around 910 ºC, and K-feldspar crystallizes in the matrix between 750 and 700 ºC when the crystal fraction is around 0.5. However, at 850 ºC, a pelite-doped experiment shows euhedral K-feldspar (»5 vol%) in both the reactive xenolith domain together with cordierite and the granodioritic domain, where the K2O wt% rise from 4.5 in the normal experiment to 5.9 in the doped experiment. These results suggest that the bulk-assimilation process promotes the bulk and heterogeneous K2O enrichment in a huge granodioritic magma volume, which evolves with the early crystallization of K-feldspar megacrysts. Therefore, magmatic fabrics defined by K-feldspar megacrysts are formed during and after the emplacement processes and are highly influenced by tectonic kinematics.

Keywords


K-feldspar megacrysts; experimental petrology; granodiorites; magmatic fabrics

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DOI: http://dx.doi.org/10.1344/%25x

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Geologica Acta (ISSN-1695-6133)
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