英国剑桥大学Charlotte Gordon课题组近日的研究解决了“巨晶悖论”这一难题，研究通过长石晶体的取向关系记录花岗岩中晶体的活动性。相关论文于2024年7月3日发表在《地质学》杂志上。

课题组人员通过检查巨晶中斜长石包裹体的排列，给这个问题提供了一个新的视角，以测试它们是否表现出系统的低能晶体学关系，这种关系是在晶体有旋转空间的富含熔体的环境中通过共生现象附着而产生的。研究人员利用电子背向散射衍射来量化晶体取向，发现巨晶的斜长石包裹体确实占据了这些首选取向，因此被纳入了富熔体环境中。钾长石也以间隙网络的形式存在，但在该网络中的斜长石晶体具有非系统的取向。

这种从系统斜长石取向到非系统斜长石取向的转变标志着晶体形成一个刚性的、相互连接的框架，阻止了旋转到低能取向。相平衡模型表明，这种转变发生在岩浆结晶率约为55%的时候。剩余的45%熔体在共晶处结晶，形成间隙相。由此，课题组人员解决了“巨晶悖论”，巨晶在熔体中自由生长，结晶体锁定后形成基质钾长石。因此，巨晶提供了系统演化一个关键时期的详细结构和化学记录——从流动、可能喷发的岩浆转变为静止的岩浆。

据悉，钾长石巨晶在硅质侵入岩体中很常见，但关于它们是如何形成的，以及它们的存在对岩浆系统的影响，一直存在着长期的争论。野外、结构和地球化学证据支持钾长石生长在富含熔体的环境中，但实验证据和相平衡模型表明，钾长石生长在结晶序列的晚期，此时岩浆是高度结晶的。

附：英文原文

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

Author: Charlotte Gordon, David Wallis

Issue&Volume: 2024-07-03

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 modeling 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 modeling 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.

DOI: 10.1130/G52045.1

Source: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G52045.1/645290/Resolving-the-megacryst-paradox-FeldsparredirectedFrom=fulltext