74 How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution (Posters)

This session integrates structural and petrologic views of plutonic systems, exploring the crustal response to pluton formation, and processes that control the physical and chemical characteristics of pluton-host rock systems from initial emplacement to solidification.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Magma flow and differentiation in mafic sills of the Karoo Large Igneous Province, South Africa: Insights from internal fabric and geochemistry

Paper number: 74-0
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Mafic sills generally have well-organized anisotropy of magnetic susceptibility (AMS) fabrics interpreted to result from orientation of magnetic minerals during viscous flow of magma. In some cases, the AMS is surprisingly oblique with respect to the sill walls and require an explanation. The previous hypothesis of viscous drag against the sill margin, classically referred to as fabric imbrication (e.g., Knight and Walker, 1988) appears unfounded due to lack of symmetry between footwall and hanging wall. Instead, I propose that the oblique AMS arises from single domain (SD) titanomagnetite, a mineral known to have inverse fabric, in the chilled hanging wall.

To test this hypothesis, I use well studied mafic sills from the Karoo Large Igneous Province (LIP) of South Africa. I use two representative sills (Country Club Road ~58 m and Ferndale ~27 m thick) to investigate fabrics, internal zonation, and magma flow. Geochemical studies suggest that the Karoo sills have a consistent tholeiitic composition across the entire Karoo Basin, particularly for major elements. The sills also present undeformed magmatic fabrics due to the lack of post emplacement tectonic history. The investigated rocks are also fresh and therefore constitute an ideal material to test the hypothesis.

The AMS is measured on oriented hand specimens collected across vertical profiles. Scalar parameters such as magnetic susceptibility, degree of anisotropy and shape factor show systematic variations with height in the intrusion. The parameter variations are interpreted in terms of magmatic, dynamic, and static processes including thermal convection, gravitational crystal settling or magmatic mush compaction. Directional parameters provide additional constraints on the dynamic vs. static nature of fabric-forming processes.

Petrographic studies indicate that the primary carrier of AMS is titanomagnetite. Accessorily, pyrrhotite and hematite are present, but not in sufficient quantities to account for more than 1% of the magnetic properties. Hydrous phyllosilicates occur in one sample, suggesting some alteration took place along the upper contact of the Country Club Road sill. The magnetic investigations reveal complex magnetic susceptibility and magnetic remanence as a function of stratigraphic height. In addition to magnetic investigations, major and trace elements will be analyzed to assess any vertical geochemical zonation indicative of composite emplacement and late-magmatic to post-solidification fluid-rock interaction.

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

doi: 10.1130/abs/2025AM-414896

© Copyright 2025 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.