Petrogenesis of the late Archean Quetico alkaline suite intrusions, Western Superior Province, Canada.

Title: Petrogenesis of the late Archean Quetico alkaline suite intrusions, Western Superior Province, Canada.
Authors: Lassen, Birgitte.
Date: 2004
Abstract: A suite of 13 late Archean subalkaline to alkaline intrusions were studied by major and trace element geochemistry and neodymium, hafnium and lead isotopic methods. The intrusions (2683 - 2678 Ma) are located in the Quetico metasedimentary belt, western Superior Province, and include pyroxene hornblendite, diorite, monzonite, syenite and carbonatite. The samples display arc-like trace element patterns with high Cs, Ba, Sr and light rare earth element abundances and low Zr, Hf, Nb, Ta and Ti. Neodymium and hafnium isotope data reflect derivation from a depleted mantle source with minor contribution from an enriched source. Lead isotope ratios from K-feldspar separates are dominated by a slab fluid or crustal component. Quantitative modeling of trace element and isotope ratios illustrates that a metasomatic event shortly before melting produced the main geochemical signatures, but a contribution from an older crustal source is also required to explain the range in data. Melting models based on rare earth element variations suggest that melting occurred over a range of pressures or took place in the garnet spinel transition zone. The intrusive complexes can be subdivided into two groups on the basis of high field strength element distribution, which appears to reflect mantle heterogeneities resulting from differences in metasomatic processes. The most abundant group exhibits super-chondritic Nb/Ta and excess Nb relative to magmas produced by melting of a fluid metasomatized mantle. These characteristics are in accord with metasomatism by silicic melts that have left rutile in the residue and the metasomatic agent is thought to be a mixture of slab-derived fluids and melts. The trace element chemistry of the remaining intrusions reflects a source affected by fluid metasomatism. Carbonatite constitutes a minor part of the Beaverhouse Lake intrusion. Qualitative major and trace element modeling suggests that the carbonatite formed by carbonate-silicate liquid immiscibility. The carbonatite is characterized by highly depleted hafnium isotopic signatures, which are interpreted to result from a previous episode of carbonate metasomatism. Modeling of Lu-Hf partitioning during silicate-carbonate liquid immiscibility suggests that anomalously high hafnium isotopic ratios should develop with time in the carbonate phase, which has potentially important implications for carbonate metasomatism.
CollectionTh├Ęses, 1910 - 2005 // Theses, 1910 - 2005
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