Clues from the microstructural features of the K-Feldspar megacrysts growth mechanism in granites.

 

Alkali feldspar megacrysts (commonly named K-feldspar) are a distinctive feature of many plutonic rocks. Yet, there is still no consensus on their formation mechanism. Two main contrasting explanations have been proposed so far: (i) the megacrysts form late during the crystallization history of the plutonic rock in which they occur and attain their size in a melt-poor - even subsolidus- environment, or (ii) the megacrysts crystallize at an early stage and grow in the presence of large volumes of melt. In this research we address this two contrasting hypotheses looking at the microstructural features within the megacrysts from the granitic Castellaccio Pluton of the Asinara Island (NW Sardinia, Italy). Combining the petrographic observations, EMPA elemental maps and phase maps elaborations we observe a systematic occurrence of included and corroded relict of  K-bearing mineral (muscovite and biotite) and plagioclase always mantled by a rim of anorthoclase and K-rich albite. The temperatures calculated from the Ti-in-Kfs thermometer indicate that the megacrysts have crystallized in the T range of 745-860 °C data. The strong positive europium anomaly and enrichment in barium (~ 4000 ppm) with respect to the whole-rock compositions (~350 ppm) exclude a former significant plagioclase fractionation, suggesting that K-feldspar megacrysts formed at an early stage of the crystallization of the melt. In this framework, the nucleation of the K-Feldspar is likely triggered by the assimilation of K and Al from biotite and muscovite. The presence of these two relict phases locally provides the necessary stoichiometry to start the crystallization of the anorthoclase rim and then the crystallization of the K-feldspar itself. The uptake of Ba from the melts by the K-Feldspar extends its thermal stability field beyond the thermal range predicted by the pseudosection models, allowing its euhedral growth till a megacrystic size (up to 20 cm).