Resolving the “megacryst paradox”: Feldspar orientation relationships record crystal mobility in granites

Abstract

K-feldspar megacrysts are common in silicic plutons but there is a long-running debate around how they form and what their presence tells us about magmatic systems. Field, textural and geochemical evidence supports growth in a melt-rich environment, but experimental evidence and phase-equilibria modelling indicate that K-feldspar grows late in the crystallization sequence, when the magma is highly crystalline. We provide a new perspective on this problem by examining the arrangement of plagioclase inclusions within megacrysts, to test whether they exhibit the systematic low-energy crystallographic relationships expected from attachment by synneusis in melt-rich environments where crystals have space to rotate. We use electron backscatter diffraction to quantify the crystal orientations, and find that the megacrysts’ plagioclase inclusions do occupy these preferred orientations and therefore were incorporated in a melt-rich environment. K-feldspar is also present as an interstitial network, but plagioclase crystals hosted within this network have non-systematic orientations. This transition from systematic to non-systematic plagioclase orientations marks the point at which the crystals formed a rigid, interconnected framework that impeded rotation into low-energy orientations. Phase-equilibria modelling indicates that this transition occurred when the magma was ~55% crystalline. The remaining ~45% melt crystallized at the eutectic, forming the interstitial phases. Thus we resolve the “megacryst paradox”; the megacrysts grew freely in melt and the groundmass K-feldspar formed after crystal lock-up. Megacrysts therefore provide a detailed textural and chemical record of a critical period in the system’s evolution: the transition from a mobile and potentially eruptible magma to an immobile mush.