Ring schlieren: Description and interpretation of field relations in the Halifax Pluton, South Mountain Batholith, Nova Scotia

A ∼35 km coastal transect of the Halifax Pluton in South Mountain Batholith (SMB) reveals 96 m-scale ring schlieren, clustered in four locations. Ring schlieren are alternating melanocratic gradational to leucocratic bands in granites forming open to closed, nested, circular to elliptical, concentric to eccentric structures with centres aligned along the major axis or minor axis of the outer ellipse, with a cylindrical form in three dimensions, and in which cross-cutting relations suggest a younging direction towards the centre. We use detailed geometric measurements of these ring schlieren to interpret the process(es) of their formation, and to indicate the relative timing of emplacement of the SMB. The dimensions and orientations of the rings are highly variable: major-axis lengths (1.39 ± 1.43 m), minor-axis lengths (1.01 ± 0.79 m), ellipticities (1.34 ± 0.25), and major-axis orientations (008°±54°). Ring schlieren appear to be the youngest structures in this part of the batholith. Modal-abundance and grain-size variation in the rings suggests that they result from shear flow. In three dimensions, the rings are nested vertical cylinders, and probably represent vertical fossil pathways, either of descending xenoliths or of ascending supercritical vapour bubbles. Such processes can produce, and preserve, a track in a crystal-melt mush having a degree of crystallinity between 55 and 75%. Two miarolitic cavities within one multi-ring structure suggest that a rising bubble train may have produced the rings.

► We provide detailed observations on 96 ring schlieren in the South Mountain Batholith.
► We propose a comprehensive new definition of ring schlieren.
► Modal-abundance and grain-size sorting in the rings suggests an origin by shear flow.
► Vertically ascending supercritical vapour bubbles are probably the agents of shear flow.
► Aspect ratios and vertical orientations of the rings show that the batholith is post-tectonic.