Spatial and temporal variations in the geochemistry of komatiites and komatiitic basalts in the Abitibi greenstone belt

Komatiites and komatiitic basalts in the Abitibi greenstone belt exhibit pronounced spatial and temporal variations in major and trace element geochemistry that provide important constraints on their petrogenesis and tectonic setting. Ti-depleted komatiites exhibit high Al2O3/TiO2 (25-35) and low [Gd/Yb](MN) (similar to0.6-0.8), suggesting derivation by (1) dynamic melting of a refractory source leaving a refractory harzburgite residue, similar to that which produced Gorgona picrites, or (2) relatively high degree melting of a garnet-rich source leaving a peridotite residue. Al-depleted-Ti-enriched komatiites exhibit low Al2O3/TiO2 (6-14) and high [Gd/Yb](MN) (similar to1.2-2.0), suggesting derivation by partial melting of a garnet peridotite source leaving a garnet-bearing residue. Al-undepleted komatiites exhibit moderate Al2O3/TiO2 (15-25) and moderate [Gd/Yb](MN) (similar to0.8-1.2), suggesting derivation by shallower or higher-degree partial melting of a garnet peridotite source leaving a garnet-free residue. Those in the 2750-2735 Ma Pacaud assemblage are all Ti-depleted komatiites, those in the 2725-2720 Ma Stoughton-Roquemaure assemblage are predominantly Al-depleted-Ti-enriched komatiites with lesser Al-undepleted komatiites, those in the 2718-2710 Ma Kidd-Munro assemblage are predominantly Al-undepleted komatiites with lesser Al-depleted-Ti-enriched komatiites, and those in the 2710-2703 Ma Tisdale assemblage are predominantly Al-undepleted komatiites. Some of the komatiites in the younger assemblages (Kidd-Munro and Tisdale) are enriched in highly incompatible lithophile elements, exhibit low [Nb/Th](MN), and felsic-intermediate rocks in these assemblages contain inherited zircons, suggesting contamination by upper crustal rocks or sediments derived from upper crustal rocks. The high Mg contents of the komatiites suggest that they formed in mantle-derived plumes. The number of plumes required to generate the komatiitic magmatism in the Abitibi greenstone belt is not clear; it may be possible to derive all of the different magma types from different parts of a single plume at different times, but the long duration of komatiitic magmatism (greater than or equal to47 My) suggests that multiple plumes are more likely. In any case, the temporal changes in composition are consistent with a decreasing influence of garnet in the source region of the plume(s) with time. There are several possible mechanisms to account of these variations, none of which are mutually exclusive: (1) a decrease in the depth of melt extraction as the plume(s) ascended: (2) it decrease in the amount of magma derived from deeper levels in the plume relative to that derived from shallower levels in the Plume. possibly representing it change in plume morphology with time. for example as it was dragged by the overlying plated (3) a decrease in the rate of plume ascent due to interaction with subducting slabs: or (4) it decrease in the thickness of the lithosphere during plume ascent. perhaps related to lithospheric thinning by the rising plume(s), The restriction of contamination to individual flow units or individual parts of flow units in the Kidd-Munro and Tisdale assemblages suggests that crustal contamination occurred locally, via assimilation of country rocks during emplacement. (C) 2002 Elsevier Science B.V. All rights reserved.