A mineralogical perspective on the granite alkali feldspar megacrysts paradox

Alkali feldspar megacrysts (commonly named K-feldspar) are a distinctive feature of many plutonic rocks. Yet, there is still no consensus on their origin. 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 large 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 article we address the applicability of these models to the alkali feldspar megacrysts of the Castellaccio Pluton (NW Sardinia, Italy). Petrographic observations and microstructural analysis of the megacrysts highlight the systematic occurrence of relict K-bearing mineral inclusions – biotite, muscovite and oligoclase – rimmed by anorthoclase and K-rich albite. We interpret these microstructures as evidence of reactions between partially assimilated K-bearing phases, likely from wall-rock, and the incoming melt. The chemical assimilation of K-bearing phases is proposed as the mechanism of K and Al enrichment necessary for the megacrysts growth. The temperatures of the megacrysts crystallization, calculated by Ti-in-K-feldspar thermometer, are estimated at c. 820 ± 60 or 780 ± 60 °C, depending on the calculated TiO2 activity in the whole rock (0.47–0.62). Finally, the strong positive europium anomaly and enrichment in barium in the megacrysts with respect to the whole-rock compositions exclude significant plagioclase fractionation, thus supporting megacrysts formation at the early stage of the cooling history. All the results of our work point toward a magmatic origin and evolution of the K-feldspar megacrysts in the Castellaccio granite pluton.