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Petrology of the Boundary Intrusions and Associated Brecciation in Creighton, Saskatchewan and Flin Flon, Manitoba



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Detailed mapping, petrographical and geochemical data for the Green Street and Louis Lake Boundary Intrusion breccias are presented, as well as petrographical and geochemical data for the Phantom Beach intrusion breccia and the Club Lake intrusion, located in Creighton, Saskatchewan and Flin Flon, Manitoba. The Green Street Boundary intrusion was emplaced in volcanic rocks of the Flin Flon formation as a dyke with the most northerly section containing a largely polymictic, chaotic-textured, matrix-supported intrusion breccia. The margins of the dyke also exhibit evidence of hydraulic brecciation through mosaic or jigsaw textures. The Louis Lake intrusion is a chaotic-textured, matrix-supported breccia and contains no exposed contacts with the Louis formation volcanic host rock, though the presence of large layered blocks indicates the intrusion may have been emplaced as a sill. The Phantom Beach intrusion breccia is also chaotic-textured and ranges from fragment- to matrix-supported. Intrusion morphology and breccia textures suggest possible processes for fragment generation include dyke crack-tip propagation, hydraulic fracturing, mechanical and thermal delamination, and stoping. Fragment populations for all intrusion breccias include mafic to ultramafic intrusives and locally derived volcanics, ranging from angular to rounded to disaggregated, with the intrusive fragments exhibiting greater degrees of rounding and disaggregation. This may suggest the intrusive fragments have had longer residence times or traveled greater distances within the magma than the volcanic xenoliths. Fragment size can reach up to 2 m in diameter with an average diameter typically between 10 – 30 cm. Settling rates of fragments within the magmas, or the magma ascent rate, of the mapped sites are calculated using Stoke’s Law (for a Newtonian fluid) and calculations by Shaw (1969) (for a Bingham fluid). Resulting magma ascent velocities range from 0.01 m/hr to 1.84 km/hr for the Green Street intrusion, 0.01 m/hr to 872.16 m/hr for the Louis Lake intrusion, and 1.63 m/yr to 0.33 m/hr for the dominantly clast-supported breccia intruding the Louis Lake breccia. Petrographical analysis of mafic to ultramafic intrusive fragments within the Green Street intrusion exhibit textures which may suggest different stages of disaggregation and incorporation into the magma matrix. Petrographical comparisons of whole, intact fragments to fragments with irregular boundaries may demonstrate various stages of disaggregation taking place during emplacement of the intrusion. In addition, comparisons of floating actinolite crysts in the intrusion matrix to known phenocrysts and crystals of actinolite in fragments may further indicate previous disaggregation of intrusive fragments. Whole rock and trace element geochemistry for the Boundary intrusions and their mafic to ultramafic intrusive fragments show the samples display island arc signatures and are overall calc-alkaline, though select fragment samples from the Phantom Beach area show some tholeiitic tendencies. High field strength element (HFSE) patterns suggest an enriched and complex mantle source for all samples, with the resulting magma possibly affected by magma mixing. HFSE ratios may also indicate contamination by crustal assimilation for the Green Street intrusion. Low mid-ocean ridge basalt (MORB)-normalized TiO2/Y ratios (<1) suggests that the magma source for the Boundary intrusions was at shallow depths with higher water contents (<50km) where garnet is not a factor in retaining yttrium relative to titanium. Similarities in trace element patterns and higher Zr/TiO2 ratios suggest fractionation within the sample suite for the Green Street area.



Flin Flon, Creighton, Boundary intrusion, intrusion, breccia, brecciation, fragment, ascent rate, mantle mixing, contamination, fragmentation processes, disaggregation



Master of Science (M.Sc.)


Geological Sciences




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