Transport of Magma in Granitic Mush Systems; an Example From the Götemar Pluton, Sweden

Abstract

Granitic magma bodies form in the ephemeral part of magma mush systems and are emplaced by a variety of mechanisms in different tectonic settings. This study investigates how granitic magma emplacement processes and tectonomagmatic interactions assert control over the architecture of mush state pluton-scale magma transport pathways. The 1.45 Ga shallow-crustal Götemar pluton is a 4.5 km diameter circular pluton that consists of three granite units: a coarse-grained red granite, a medium-grained pale to red granite, and fine-grained pale microgranite sheets. We employed geological mapping supported by Anisotropy of Magnetic Susceptibility (AMS) to examine the magmatic and regional tectonic controls on late-stage magma transport in the Götemar granitic magma mush system. Multiple parallel arcuate subhorizontal microgranite and medium-grained granite sheets (from 0.1 to 10s of meters thick) were mapped within the pluton. The arcuate sheets pinch out from the northern part of the pluton toward the SE inferring magma propagation direction. A dominant set of vertical granitic sheets within the granite body strikes NW-SE. The AMS fabrics are contact-parallel in the main medium-grained granite body and indicate inflation. Within the microgranite sheets, the AMS fabrics are parallel to the sheet strike and support a sheet propagation direction to the SE. The Götemar pluton displays a clear link between arcuate (concentric) magma-transporting sheets and concentric strain-partitioning related to the intrusion of medium-grained granite magma. The vertical magma sheet orientations are consistent with an NE-SW extensional stress field that is associated with the extensional back-arc stress regime of the contemporary Hallandian Orogen.