242-6 Boogardie Orbicular Granite Reveals Rapid Pluton-scale Convection

Session: Crustal Petrology, Part I

Presenting Author:

Chris Spencer

Authors:

Spencer, Chris¹, Walsh-Estabrooks, Talia², Joy, Brian³, Layton-Matthews, Daniel⁴, Leybourne, Matthew⁵, Leduc, Evelyne⁶, Pluister, Emma⁷, Scanlan, Emma⁸, Voinot, Alex⁹

(1) Queen's University, Kingston, ON, Canada, (2) Queen's University, Kingston, Ontario, Canada, (3) Queen's University, Kingston, Ontario, Canada, (4) Queen's University, Kingston, Ontario, Canada, (5) Queen's University, Kingston, ON, Canada, (6) Queen's, Kingston, Ontario, Canada, (7) Queen's University, Kingston, Ontario, Canada, (8) Queen's University, Kingston, Ontario, Canada, (9) Geological Survey of Canada, Ottawa, Ontario, Canada,

Abstract:

Orbicular granitoids are unusual magmatic rocks that provide insight to the dynamic processes of pluton formation. Despite extensive documentation of their mineralogical and structural features, the mechanisms responsible for their formation remain debated, particularly regarding the role of magma mixing, magma chamber dynamics, and crystallization rates. The Archean Boogardie orbicular granite near Mount Magnet in Western Australia presents a unique opportunity to interrogate these processes, given its well-preserved and undeformed orbicules and sharp compositional boundaries. This study evaluates the petrogenesis of the Boogardie orbicular granite by integrating amphibole thermobarometry, plagioclase compositions, and in situ hydrogen and strontium isotopic analyses to investigate the magma source(s) and pressure-temperature evolution during orbicule growth. Here we show that individual orbicules record substantial pressure differences between core and rim (~1.5 km in depth) within a granitic pluton that is otherwise isotopically and mineralogically homogeneous. If the observed pressure variations are real and representative, they imply transient vertical motions of orbicules during growth, which are consistent with magmatic convection at rates of 2 to 8 m/hr. These findings suggest that convection and internal recycling within mid-crustal magma chambers can occur at rates fast enough to imprint pressure variations within a single growing orbicule.

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

doi: 10.1130/abs/2025AM-8392

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