Problems in the study of old Archaean volcanic complexes. Essential problems in the evolution of magmatism in the Earth’s history
Institute of Geology, Karelian Research Centre, RAS, Petrozavodsk
Reconstruction of the conditions of dynamic evolution of the Archaean lithosphere from 4 to 2.5 Ga, responsible for the basic geological-structural characteristics of the modern crust, is one of the most complex problems in modern geology.
The Archaean geological evolution of the Earth is most clearly reflected in two basic types of structures: granite-greenstone and granulite-gneiss structures.
Greenstone belts are most informative for geological reconstructions, because they retain relics of primary rock units, whose ensembles have often suffered a relatively low degree of metamorphic alteration. As the structural and textural characteristics of the rocks have remained unchanged, integrated studies can be carried out. The palaeovolcanological, lithogeochemical and petrological study of Archaean rock assemblages has provided the opportunity to reconstruct the conditions of formation of magmatic systems, the evolution of the Archaean crust, the thermal regime of the upper mantle and crust-mantle interaction mechanisms during this period of geological history.
The study of komatiite-basalt-series rock assemblages, identified in most Archaean greenstone structures, makes it possible to reconstruct the evolution of mantle magma composition and heat generation rate, to calculate the geothermal gradient in Archaean time and to assess the Earth’s general thermal history, and integrated analysis of all existing rock ensembles, represented in relics of Upper Archaean greenstone units, provides data for reconstructing the evolution dynamics of the asthenospheric-lithospheric system and describing geodynamic regimes at early stages in the Earth’s evolution (Arndt et al., 2008). An interest in the study of greenstone structures is due to the fact that some big and unique nickel, gold and platinum deposits are confined to them. Therefore, reconstruction of the conditions of formation of igneous assemblages provides a better understanding of the pattern of ore mineralogenic processes.
The study of andesite-series volcanics from Archaean greenstone structures provides unique evidence for the evolution of the crust-mantle system, which is used to reconstruct the conditions of generation of convergent regimes in Archaean time (Polat& Kerrich, 2006; Svetov, 2009).
It should be emphasized that detailed studies, conducted in the past few years, have shown that most Archaean granite-greenstone terrains in old cratons of the world, such as Superior, Wyoming, Yilgarn etc., are fragments of oceanic plateaus, island-arc systems, back-arc basins and continental-margin volcanic belts that accreted to the continental basement (Polat& Kerrich, 2006; Kusky & Polat, 1999), suggesting the existence of plate-tectonic interactions in Archaean time.
At the same time, there are some essential problems in the study of Archaean volcanic complexes. The basic problem is evaluation of the degree of preservation and representativeness of the units studied. As we commonly deal with reduced units that only occur as preserved relics of earlier rock assemblages, the real volumes and types of volcanogenic rocks are hard to estimate. Opposite phenomena, e.g. thick complexes, produced by slider tectonics and composed of coeval slabs, are also encountered, visually increasing the real volumes of rock assemblages to a great extent (Svetov, 2005; 2009).
As Archaean complexes in both greenstone belts and granite-gneiss areas are highly metamorphosed, matter is studied primarily by geochemical and isotopic-geochemical methods (assuming that the preservation of geochemical systems at the mineral-rock level is studied in detail), and the data obtained are then interpreted petrologically.
Direct parallels between Phanerozoic and Archaean geological processes should be used with regard for existing differences such as a higher thermal regime of the Archaean mantle, responsible for the different composition of the oceanic and continental crust, probably more complex geodynamic processes during this period and the different compositions of the atmosphere and hydrosphere.
However, there are some clear parallels between the Archaean and the Phanerozoic. Detailed geochemical study of Archaean intermediate to felsic volcanics has shown that they are most similar to Phanerozoic island-arc and continental-margin assemblages, as evidenced by finds of adakitic, high-Mg andesitic (baijaitic), high-Nb and other assemblages, dated at ca.3.0 Ga, on the Canadian (Polat& Kerrich, 2006), Fennoscandian (Svetov, 2009) and other shields. The similarity of Archaean ÒÒÃ-series and Phanerozoic adakites (Martin, 1999, Martin et al., 2005), suggests the existence of gently dipping and hot subduction regimes in Archaean time.
The study of Archaean volcanogenic complexes is now approached in many promising ways, e.g. by detailed geochemical typization of Archaean magmatic series and correlating them with Phanerozoic analogues; by reconstructing the PT-regimes on the Earth in Archaean time; by revealing periods of formation of large-scale geochemically unique rock assemblages that mark global events and changes in the Earth’s geodynamics (for example, the interval 2.7-2.75 Ga and sanukitoid formation (Bibikova et al., 2005)); searching for signs of early life in the rocks formed in Archaean time in sub-aqueous environments (for example, the discovery of bacterial mats in the marginal zones of pillow lava from the Barberton greenstone belt, South Africa (Westall, 2009), etc.
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