Oriented crystal attachment in granites

Many fundamental questions about the nature of granitic magmas remain unresolved. Two important topics of recent debate are:

1) Do granite textures faithfully record magmatic processes, or are they mainly dictated by near- or sub-solidus processes?

2) Are granitic magmas fluid enough to allow phenomena such as crystal settling and turbulence, or so viscous that such processes are impossible and the magmas are confined to laminar flow?

We present new microstructural data from K-feldspar megacrysts and their inclusions that are pertinent to both of these questions.

K-feldspar megacrysts are common in granitic rocks and their formation has been variously ascribed to melt-rich, melt-poor or solid-state processes. The megacrysts frequently contain systematically arranged mineral inclusions, particularly of plagioclase, but also of amphibole, quartz, biotite and titanite. We studied samples from porphyritic granodiorite units in the Tuolumne Intrusive Complex, to ascertain how the crystals came to be included in the K-feldspar megacrysts.

We performed CL and EBSD analyses of the plagioclase inclusions. CL imaging reveals that many of the inclusions feature complete, symmetrical, concentric zoning, indicating that they grew freely in melt before becoming included. EBSD analysis demonstrates that the inclusions’ orientations are strongly controlled by the faces of both the plagioclase and the K-feldspar. Within the constraints imposed by faces, there is an additional crystallographic control, with some orientations more common than others.

Taken together, these findings strongly indicate the process of synneusis, whereby crystals drift together in melt and attach to each other, often on their largest faces and in certain low-energy orientations. This process occurs in melt-rich environments and its preservation in K-feldspars indicates that these textures could not have formed near or below the solidus. Synneusis also requires the relative motion of crystals through the melt (for example, due to crystal settling or magma turbulence), so its occurrence implies that the granitic magma was, at least episodically, fluid enough to allow widespread relative motion and rotation of its crystals.