Petrochemistry of the Woodburn Lake Group komatiitic suite, Amer Lake, N.W.T., Canada.

• Main Author: Annesley, Irvine R.
• Format: Others
• Published: University of Ottawa (Canada) 2009
• Subjects: Geology.
• Online Access: http://hdl.handle.net/10393/5942; http://dx.doi.org/10.20381/ruor-14613

Several metavolcanic-metasedimentary belts containing komatiites have been found in the Baker Lake region, N.W.T. In particular, a sequence of flows of komatiitic composition within the Woodburn Lake Group (WLG) show well preserved flow tops, polyhedral jointing, and spinifex textures, confirming their volcanic origin. The Woodburn Lake Group consists of a lower 2.8-3.0(?)Ga predominantly volcanic greenstone sequence and an upper platformal quartzite sequence. The lowermost rocks of the volcanic sequence comprise an essentially bimodal assemblage of komatiites-komatiitic basalts and dacites-rhyolites; intermediate rocks are rare. The uppermost rocks reflect a change in volcanic activity from komatiitic-tholeiitic to calc-alkaline in nature. This change is accompanied by an increase in sedimentation. The supracrustal succession is presently preserved as narrow linear belts within Late Archean (2.6 Ga) granitic to granodiorite plutons. Although there is no recognized pre-greenstone basement, the upper quartzite sequence suggests the presence of older sialic crust nearby. The komatiites and komatiitic basalts (including high-Mg tholeiites) have been altered by greenschist grade regional metamorphism with complete replacement of primary mineralogy, but with excellent preservation of primary textures. The typical mineralogy of the komatiites is tremolite - antigorite - chlorite - Cr-magnetite and of the komatiitic basalts is amphibole - chlorite - magnetite - plagioclase - epidote - quartz. The grade of metamorphism increases toward the margins of the WLG belts to amphibolite grade as a result of contact metamorphism by younger granitoids. Olivine-spinel geothermometry gives a temperature of 625$\sp\circ$C for contact metamorphism of the komatiites. Chemical variation diagrams of the komatiitic suites reflect variable degrees of element remobilization during alteration. Al, Ti, V, Cr, Ni, and Sc were immobile, whereas Na, K, Ca, Rb, Sr, and LREE exhibit variable mobility. Most ratios of incompatible elements (i.e. Zr, and LREE) show moderate to strong depletion of these elements in the komatiites and strong enrichment in the komatiitic basalts. The WLG komatiites are typical of the aluminum-undepleted type that characterizes Late Archean terrains. The WLG komatiitic basalts are geochemically similar to those from Kambalda (Australia), the Abitibi Greenstone Belt, and modern-day volcanic arcs. Fractional crystallization explains the compositional variations within individual volcanic units, but does not explain adequately the differences between komatiites and komatiitic basalts. The Kambalda komatiitic basalts have been interpreted as resulting from crustal contamination and fractional crystallization of komatiitic magmas within an intra-continental setting. Quantitative modelling suggests derivation of the WLG komatiitic basalts by 20-40% crustal contamination of komatiitic magmas. Alternatively, the geochemical similarity to modern-day arc volcanics suggests an arc-related origin for the komatiitic basalts. REE patterns and inter-element ratios indicate that the mantle source region of the WLG komatiites underwent an early melting event, leaving it depleted in Zr and LREE. 30 to 40% partial melting of this source (assumed to be garnet lherzolite in composition) generated the komatiitic liquids. The WLG is compositionally and lithologically similar to the Prince Albert Group and hence probably correlative. Origin of the komatiitic suite in a continental margin arc-type environment is the favoured interpretation but an ensialic rifting model cannot be ruled out.