115-6 Formation Mechanisms of Glomerocrysts from High-Threat Volcanoes in the Cascades

Session: Petrology, Volcanology, and Mantle Plumes across the Solar System (Posters)

Presenting Author:

Authors:

Abstract:

Glomerocrysts, or crystal clusters, are common in both intrusive and extrusive igneous rocks, spanning mafic to felsic compositions. A variety of credible glomerocryst formation mechanisms have been proposed, including: mush disaggregation; heterogeneous nucleation or epitaxy; the textural maturation of dendrites; and synneusis (the attachment of independently formed crystals in melt). Each mechanism has distinct petrogenetic implications, therefore discerning between them is essential for correctly interpreting the magmatic processes recorded by glomerocrysts.

We survey glomerocrysts from a selection of high-threat volcanoes in the Cascades (e.g., Mt. Shasta, Three Sisters), focusing mainly on andesites and dacites. The storage and migration pathways of these magmas are relatively poorly understood, so identifying the origin of the near-ubiquitous glomerocrysts would provide valuable information.

We use electron backscatter diffraction (EBSD) to quantify the crystallographic orientation relationships present within the glomerocrysts. Some mechanisms, such as synneusis and epitaxy, have previously been demonstrated to produce characteristic orientation relationships between neighboring crystals. We combine orientation measurements with detailed petrographic and textural observations to interrogate the glomerocryst formation.

The Cascades glomerocrysts contain evidence of mush disaggregation and synneusis. We also find evidence of an unexpected glomerocryst formation mechanism: textural re-equilibration of reaction rims, resulting in unusual glomerocrysts containing remnant symplectites. We discuss the petrogenetic implications of the glomerocryst formation mechanisms identified in the Cascades. We also outline a methodological approach for categorizing glomerocryst origins, applicable to a wide variety of minerals and magmatic systems.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11191

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.