Krista L. McCuish

The MacEachern - Ponsford Memorial Award - 1999

Dynamic interactions between crystals and melt can form schlieren as primary features during the various stages of the crystallization of magmas. The restricted conditions necessary for the formation and preservation of schlieren account for their relatively rare occurrence, and thus the schlieren can yield information about the chemical and physical processes in the magma prior to its solidification. This thesis presents a comparison of schlieren from four areas in the granites of southern Nova Scotia. At Prospect, schlieren features include: perturbation of the regional flow foliation, development of size-sorted biotite laminae, randomly oriented clusters of K-feldspar megacrysts, and deformation of quartz. At Peggy's Cove, schlieren features include: occurrence of subhorizontal, reversely-graded bands with cuspate biotite margins and pockets of coarse-grained felsics, zoned adcumulate crystals, and myrmekitic textures. At Jackies Island, the major schlieren features are: symmetrical straight schlieren banding, zones of strong biotite and feldspar foliation with and without segregation banding, and the presence of feldspar and mica deformation microstructures. At St.Catherines River Bay, schlieren features include: spatially associated schlieren bands and metasedimentary xenolith slab swarm in host tonalite, hornfelsic texture in xenolith cores, foliated biotite aggregates on xenolith rims, and limited variation in biotite FeO-MgO-TiO² compositions. Schlieren at all four study localities show distinctive mineralogical, textural, and structural features indicating different physical-chemical processes of formation at each location. The inferred processes for schlieren formation are: Prospect - size sorting and shear flow in the wake of a rising vapour bubble or falling xenolith block; Peggy's Cove - repeated magma injection with gravity settling; Jackies Island - synmagmatic shearing in a shear zone with development of strong foliation and segregation banding; and St. Catherines River Bay - partial melting and assimilation of metasedimentary xenoliths. These examples demonstrate that a wide range of physical and chemical processes are responsible for the formation of schlieren in felsic magmatic systems.

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Pages: 80
Supervisor: Barrie Clarke