EARLY STAGES OF THE TECTONIC AND MAGMATIC DEVELOPMENT OF THE EARTH AND MOON: SIMILARITIES AND DIFFERENCES

Abstract

The main peculiarity of the tectonic and magmatic development of the Moon is its resemblance to the Paleoproterozoic stage of the Earth development. The Moon shows no analogues of both the ancient (Archean) terrestrial magmatism, which produced tonalite series granitoids and minor komatiite and basalt, and Phanerozoic magmatism related to active plate boundaries. The earliest (4.45-4.25 Ga) highland magmatism of the Moon is represented by magnesian series rocks cutting the primary anorthositic crust. These rocks are chemically similar to the terrestrial early Paleoproterozoic igneous rocks (2.5-2.2 Ga) that compose the siliceous highly magnesian series (SHMS), but the lunar melts are more reduced. Similar to the Earth, their intrusive analogues form layered complexes of mafic and ultramafic rocks (ANT series). Starting from an age of 4.34 Ga, the magnesian series associated with rocks enriched in K, REE, and P (KREEP series) including potassium granites. The second stage (3.9-3.2 Ga) of the tectonic and magmatic development of the Moon was characterized by extensive generation of mare basalts, which filled the depressions of newly formed lunar maria. Similar to the oceanic and continental flood basalt provinces of the Earth, two varieties of mare basalts are distinguished on the basis of chemical composition, low-titanium and high-titanium. The former are similar to MORB, and the latter are close to geochemically enriched Fe-Ti picrites and basalts, which first appeared on the Earth in considerable amounts only at 2.2-2.0 Ga, simultaneously with the onset of plate tectonics. Similar to the Earth, the mare magmatism is believed to be related to the ascent of mantle plumes of the second generation from the boundary between the liquid metallic core, which existed then, and the silicate mantle. The spreading of plume heads was probably responsible for the formation of large mare depressions with reduced crust thickness. It is proposed that the formation of the Earth and Moon occurred simultaneously and proceeded in two stages: (1) the formation of their iron cores from a protoplanetary nebula around the Sun and (2) the subsequent accumulation of silicate chondritic material. However, the proximity of the Earth, which more efficiently "scavenged" volatile components (especially, H2O) from the surrounding space owing to its greater mass, resulted in the depletion of the Moon and especially its core in these components. The differences in the evolution of tectonic and magmatic processes in the Earth and Moon are probably related to different energy capacities of these planetary bodies.