Fergus Tweedale

The South Mountain Batholith (SMB) of Nova Scotia is a Late Devonian, peraluminous, discordant, granitoid complex, consisting of many plutons that intrude the Meguma Supergroup metasediments. A coastal transect between Aspotogan Point and Portuguese Cove in Halifax County features kilometre-scale SMB outcrop and is the study area for this research. Within the study area, the SMB is host to at least 151 centimetre- to metre-scale ring schlieren. Ring schlieren are alternating melanocratic and leucocratic bands in granites forming open to closed, nested, circular to elliptical, concentric to eccentric, prolate to oblate structures with crosscutting relationships indicating a younging direction toward the centre. The purpose of this investigation is to develop a conceptual model for ring schlieren formation based on macroscopic structural features documented in the field. Of the 151 ring schlieren, 16 are single-ring structures, 79 are multi-ring structures, and 56 are complex-ring structures. Single-, multi-, and complex-ring schlieren average long-axis lengths are 0.54 m, 1.53 m, and 0.96 m, respectively. Average aspect ratios are 1.33, 1.34, and 1.45, respectively. Within the study area, ring schlieren clusters occur in six geographical locations. The traversed outcrop between clusters is barren of ring schlieren. The local disruption of regional-flow foliation in the granitoid-host rock around the clusters suggests that ring schlieren are late magmatic structures, created when the degree of crystallinity of the magma was 55-75%, a condition permitting both deformation of the mush and retention of the deformed state. Three-dimensional outcrop exposures reveal the verticallyoriented and cylindrical shape of ring schlieren clusters. As such, ring schlieren appear to represent vertical fossil pathways, either of solids descending from the roof of the pluton, or of bubbles ascending from late-stage degassing of magma at greater depth. Shear flow at the margins of descending xenoliths or ascending bubble trains can produce flowage differentiation between silicate melt and solids of various sizes. The Bagnold effect may explain the particlesorting textures in ring-schlieren structures. A miarolitic cavity in one multi-ring structure suggests that a rising bubble train may have produced the rings. Natural (i.e., bubbling mudpits) and synthetic (i.e., beer and setting concrete) analogue systems provide qualitative support for a bubble-train model of ring schlieren formation.

Pages: 90
Supervisor: D. Barrie Clarke