Nina Buchanan HBSc Thesis Abstract

The Saturday Night Intrusion (SNI) was identified due to its remnantly polarized magnetic anomaly, similar to mafic-ultramafic intrusions in the area related to the early/plateau stage of the Midcontinent Rift (MCR). A single hole (SN-16-001) was drilled through the SNI to a depth of 601m. Different units of the SNI were identified: leucogabbro, gabbro, peridotite, and melagabbro/gabbro. This study provides the first report of the mineralogy and petrology of the SNI.

 

Leucogabbro is the dominant lithology within the SNI composed of medium-grained mesocumulate plagioclase, interstitial quartz, and hornblende altered to chlorite and associated fine-grained clay alteration. The gabbro unit is mainly medium-grained orthocumulate

clinopyroxne with interstitial plagioclase and olivine altered to chlorite and associated finegrained clay alteration. The peridotite is dominantly mesocumulate olivine pseudomorphed into serpentine, talc, calcite and magnetite with interstitial clinopyroxene, orthopyroxene and plagioclase. Melagabbro/gabbro is the basal unit, dominantly composed of medium-grained anhedral/subhedral plagioclase with clinopyroxene altered to chlorite. The SNI was emplaced within the Trout Lake granites of the Dog Lake Granitoid Chain. Two phases of the Trout Lake granite are present; the first phase is dominantly plagioclase, orthoclase, and biotite altered to chlorite present within the hanging wall; and, the second phase is dominantly plagioclase, quartz, chlorite, with microcline megacrysts found in the footwall. An important aspect of the SNI and surrounding Trout Lake granites is the extensive hydrothermal alteration. The alteration assemblage consists of chlorite, epidote, calcite, clay and iron oxide staining. 

 

The SNI is a mafic-ultramafic intrusion related to the early/plateau stage of the MCR. The majority of the Saturday Night Intrusion has a trace element and REE geochemical signature of enriched LREE compared to depleted HREE, enriched Th, and depleted Nb. Depleted HREE signature is indicative of a partial melt from depth. Enriched Th and depleted Nb is characteristic of crustal contamination. A source of the crustal contamination may be the Trout Lake granites. The magma source of the SNI is between N-MORB and OIB with some degree of crustal contamination. The trace element and REE geochemical trends are similar to other maficultramafic intrusions related to the early/plateau phase of the MCR.