IMAGE ANALYSIS AND 3D RECONSTRUCTION OF MAGMA-MINGLING STRUCTURES IN THE VINALHAVEN IGNEOUS COMPLEX, MAINE

The three-dimensional shapes of magma mingling structures and flow directions are difficult to reconstruct from geologic field data. We therefore apply quantitative fabric analysis methods to this problem using multiple outcrop sections. The Silurian bimodal Vinalhaven Igneous Complex of Coastal Maine contains a ~1 km thick mafic gabbrodiorite unit. This unit contains abundant mafic pillows enclosed in a felsic matrix that formed by the emplacement of mafic magma into the crystallizing granitic magma chamber. Using a DSLR camera, 26 photos were taken of three separate outcrops, one on the east shore of Lane’s Island, and two contiguous outcrops on the west shore. A total of ~750 pillow shapes were traced using Inkscape, and the outlines created for each image were processed using EllipseFit. A moment-equivalent ellipse was generated for each shape, providing coordinates, maximum and minimum radii, orientation (ϕ), and axial ratio (R). The ellipses were plotted on equal-area fabric plots, and a mean ellipse was generated for each image using shape matrix eigenvalues. By combining the average 2D ellipses with the corresponding orientation (strike and dip) of each photo, a best-fit ellipsoid was generated for each outcrop using the Robin method. The southwest outcrop best-fit ellipsoid principal axes and major axis trend and plunge are 1.32 : 1.06 : 0.71 and 126.92°/10.58°. This northwestern flow trend is consistent with field relations suggesting that the southwest outcrop is a large compost dike that sharply cuts the underlying gabbroic unit and forms a distinct layer to the northwest. The northwest outcrop best-fit ellipsoid axes are 1.36 : 0.97 : 0.76 and 056.82°/10.22°. A northeastern flow trend within the northwest outcrop is suggestive of the spreading and sinking of pillows on top of the gabbroic sheet within a felsic medium. The east outcrop best-fit ellipsoid axes are 2.22 : 0.78 : 0.58 and 187.43°/4.64°. This ellipsoid has high residuals, likely due to high variability of flow, however north-south elongation is evident. These results suggest that quantitative fabric analysis can be useful to define 3D magma mingling geometries, however it may be necessary to apply these techniques to individual outcrops rather than larger map regions.