Constraints on subvolcanic magma plumbing system evolution from crystal size distribution analysis of igneous groundmass, Henry Mountains, Utah

Abstract

Shallow magma systems drive surface volcanism and are commonly built through multiple injections of magma. Recognizing separate magma injections can be difficult because differences in resulting rock texture, geochemistry, etc. may be subtle or non-existent. However, differences in injection crystallization histories may be recognizable through analysis of the late-crystallizing groundmass in porphyritic subvolcanic igneous rocks. In igneous bodies built from component magma sheets, early injections generally cool rapidly relative to later injections, resulting in distinct groundmass crystal size distribution (CSD) in the youngest, slowly cooled magma sheets compared to older, faster-cooled sheets. Previous work demonstrates that the ~28 Ma Copper Ridge laccolith (Henry Mountains, Utah) was constructed at a depth of ~2 km from at least two texturally distinct igneous sheets stacked atop one another and suggests these two sheets may themselves include multiple injections of magma. This study tests the hypothesis that the relative timing between intrusive sheets in a laccolith can be constrained using CSD analysis of groundmass texture and that the individual intrusive sheets are comprised of multiple pulses of magma.

To test this, a suite of porphyritic diorite samples was collected from a natural cross-section through the entire 400-m-thick Copper Ridge intrusion, including samples at well-exposed upper and lower contacts of the laccolith with sedimentary host rock, at contacts with an intercalated layer of host rock within the laccolith, and at regular intervals within the upper and lower igneous sheets themselves. CSD analysis was conducted on electron backscattered diffraction (EBSD) mineral phase maps of quartz, anorthite, and orthoclase in the groundmass (crystals <100 microns).

Examining the crystal size population density variation across different elevations of the Copper Ridge Laccolith (CRL) reveals that samples at some elevations have a wider variation of population density with fewer large groundmass crystals, suggesting more rapid crystallization of those samples. The pattern of CSDs at the margins of both upper and lower sheets displays a wider population density variation, suggesting fewer large groundmass crystals and therefore more rapid crystallization at the margins. Additionally, areas within the lower sheet of the CRL at elevations of 2475 and 2575 m exhibit a wider variation of population densities, suggesting more rapid crystallization at these elevations. These regions of fewer large groundmass crystals are interpreted as boundaries between component sheets within the intrusion. Based on these findings, boundaries are interpreted at the top and bottom of both upper (2730 m and 2660 m) and lower (2640 m and 2410 m) sheets. Therefore, the lower sheet is interpreted to contain three component sheets, with boundaries at 2575 m and 2475 m.

Overall, the pattern of CSD for the ~100 m thick upper sheet is consistent with injection of a single magma pulse and shows generally larger groundmass crystals suggesting it cooled relatively slowly and perhaps intruded after the lower sheet. The pattern of CSD for the ~300 m thick lower sheet is more complex and shows a wider variation of crystal sizes but has generally fewer large groundmass crystals, suggesting it was intruded before the upper sheet and is constructed from three or more component pulses of magma, each being approximately 100 m thick.

The results of this study provide insight into the construction history of the CRL, suggesting that it was constructed through multiple pulses of magma, resulting in distinct CSD patterns for the upper and lower sheets. This has broader implications for understanding the construction history of this and other shallow magma systems, where recognizing multiple magma injections through CSD analysis can offer valuable information on the timing and dynamics of intrusive events.

Overall, this study demonstrates the utility of CSD analysis of igneous groundmass in understanding the complex history of shallow magma plumbing systems and provides a framework for identifying and interpreting multiple pulses of magma within igneous intrusions. These findings contribute to a broader understanding of dynamics of shallow magma systems and the processes that drive surface volcanism.