Autochthonous development of Superior Province greenstone belts?

Abstract

Models for Superior Province greenstone belt development have changed over the last 30 years from ensimatic, generally autochthonous models to suspect allochthonous terrane models based on the plate tectonic paradigm. Models intermediate between these end-members such as autochthonous terrane models have not been prominent. Within the Superior Province, there is a secular variation from early ca. 3 Ga platformal assemblages with overlying rift-related komatiite–tholeiite units representing breakup of ca. 3 Ga continental fragments to late (ca. 2.7 Ga) pull-apart basins. Autochthonous terranes should demonstrate some sort of logical order such as that described above, whereas allochthonous terranes will be largely oceanic in character and should be essentially random in their occurrence rather than exhibiting secular order. In the face of this situation, this paper reviews evidence from throughout the Superior Province that suggests many, but not all greenstone assemblages developed autochthonously or parautochthonously. The earliest greenstone assemblages in the Superior Province are generally quartzite and carbonate-bearing platform sequences in the North Caribou terrane and the Marmion terrane deposited unconformably upon granitoid or greenstone basement. The evidence for autochthonous/parautochthonous development includes the presence of unconformities between Keewatin type greenstone units such as: (1) basal unconformities between ca. 3 Ga granitoid or volcanic basement and slightly younger platformal assemblages; (2) between ca. 2.9 Ga platformal assemblages and younger platformal and Andean arc assemblages. Stratigraphic patterns such as adjacent greenstone belt assemblages representing sharply differing geodynamic settings with some indication of palinspastic order or progression in the northern, central and southern Superior Province are considered suggestive of orderly, autochthonous progression from platforms through rifting of continental fragments and late assembly during the Kenoran orogeny. Petrogenetic data indicating involvement of older granitoid crust in genesis of greenstone assemblages of a variety of ages throughout the Superior Province suggests the presence of older basement, implying autochthonous or parautochthonous development of many assemblages. Younger greenstone assemblages in the Abitibi–Wawa subprovince and the North Caribou terrane contain xenocrystic zircons similar in age to stratigraphically underlying greenstone assemblages. This pattern has been interpreted to represent autochthonous greenstone development. The current above enumerated evidence for autochthonous development of greenstone assemblages is generally indirect; few actual unconformities between Keewatin type greenstone assemblages have been recognized in the Superior Province. To aid in future recognition of unconformities in Keewatin type greenstone assemblages, examples of unconformities of varying degrees of subtlety in Archean sequences are reviewed. Archean unconformities ranging from sharp angular discordances in the northern Superior Province to subtle stratigraphic patterns, to weathering-related geochemical mobilization along inter-formational contacts are described. These styles of unconformity in Archean sequences suggest previously unrecognized major disconformities and unconformities may well exist between Keewatin type assemblages in the Superior Province generally and the Abitibi–Wawa subprovince specifically. The most important disconformity suggested in the Abitibi–Wawa subprovince is between the ca. 2726 Ma Deloro and the ca. 2708 Ma Tisdale assemblages. The presence of unconformities and disconformities coupled with the indirect lines of evidence cited above suggest an ‘autochthonous’ origin for many Superior Province greenstone belts. This ‘autochthonous’ origin involves either autochthonous or parautochthonous evolution of assemblages at least in part in place. That is, development of ca. 3 Ga platform sequences in selected locales followed by in place evolution of a succession of plume, oceanic and arc assemblages culminating in late pull-apart basin sequences. Implications of the autochthonous model for greenstone belts of the Superior Province, particularly the Abitibi–Wawa subprovince, include the future ability to recognize unconformity bounded sequences, potentially improving regional scale correlation.