Mathematics – Logic
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...58s&link_type=abstract
Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 58
Mathematics
Logic
Geology, Igneous Rocks, Lunar Rocks, Mineralogy, Moon, Tectonics, Volcanology, Lunar Evolution, Selenology, Magma, Geochemistry, Precambrian Period
Scientific paper
Important feature of tectonic-magmatic activity on the Moon is it closeness to the Earth's early Paleoproterozoic stage of evolution. The most ancient magmatism of the Moon was begun at highlands from low-Ti melts of the magnesian suite, which were rather close in composition of rocks, their mineralogy and geochemistry to the early Palaeoproterozoic terrestrial magmatism of the siliceous high-Mg (boninite-like) series (SHMS). Around 3.8 Ga, it was changed by basaltic magmatism of maria, where high-Ti varieties were common. This type of magmatism is resembled the oceanic-type magmatism of the Earth (including Fe-Ti basalts of oceanic islands), which firstly appeared in the late Palaeoproterozoic: ca. 2 Ga. On the Moon are lacking both analogs of the Archean type of activity of the Earth, when formation of granite-greenstone terranes occurred, and the Phanerozoic subduction-related magmatism. On Earth, the SHMS rocks of 2.5-2.1 Ga formed large igneous provinces within the Archean granite-greenstone cratons. For example, on the Baltic Shield such province evolved on territory measuring about 1 million square-km and represented by different volcanics (low-Ti picrites, Mg basalts, high-Al basalts, andesites, dacites, and rhyolites, where basalts predominated) in riftlike structures, gabbronorite dyke swarms, and large layered intrusions. The latter consist of dunites, harzburgites, pyroxenites, norites and gabbronorites, including pigeonite varieties, gabbro-anorthosites, Mgt gabbronorites, and diorites. The SHMS rocks on their major, rare, and rare earth elements contents were rather close to the Phanerozoic calc-alkaline series related to subduction zones, but on its geological position they had within-plate tectonic settings and look like the continental flood basalt province. The Epsilon-Nd(T) value of -1 to -2 is characteristic for the studied SHMS-rocks, indicating that the origin of such melts was linked with large-scale assimilation of crustal rocks by high-temperature, mantle-derived magmas during their ascending to the surface. Potassic granites and monzodiorites and K-enriched volcanics, varied from low-Ti picrites and trachybasalts to alkaline andesites and dacites, are not uncommonly associated with the SHMS rocks. According to isotopic data, these rocks also had mixed mantle-crustal origin. Around 2.2-2.0 Ga, practically on the whole Earth simultaneously, the Fe-Ti picrites and basalts of different alkalinity, similar to the Phanerozoic within-plate rocks were appeared, and substituted the SHMS rocks as a main type of within-plate magmatic activity. The lunar magmatism of the highlands was rather close to the early Paleoproterozoic magmatism of the Earth. The earliest (4.45-4.25 Ga) magnesian-suite magmatism, which intruded the primary anorthositic crust, was represented by volcanics and intrusive rocks. Pristine intrusive rocks of this suite are represented by dunites, harzburgites, troctolites, norites, gabbronorites, and anorthosites (ANT-series). Important role in these rocks play orthopyroxene and pigeonite; Cr spine], orthoclase, quartz, apatite, and Ti phases occur in many varieties. On their composition these intrusive rocks rather close to the rocks of the early Paleoproterozoic layered intrusions. As a whole, all these rocks on their major, rare and rare earth elements pattern are rather close to the terrestrial early Palaeoproterozoic SHMS. What is more, the Epsilon-Nd(T) value in the rocks of the lunar magnesian suite is about -1, which correlates with data for the terrestrial SHMS rocks (from -1 to -2). The main difference between these lunar rocks from terrestrial rocks is a lower alkali content and very subordinate role of acid and intermediate melts. According to Snyder et al., the origin of these rocks was linked with contamination of high-temperature mantle-derived magmas by lunar crustal material during their ascension to the surface. Rocks of the 4.3-4.0 Ga KREEP series are the second type of magmatism of the highlands, where they are associated with the rocks of the magnesian suite. They are characterized by elevated K content, REE and P alkalinity, and are represented by volcanics (mainly basalts) and their intrusive analogs "alkaline series" "alkaline" anorthosites, gabbro and gabbronorites, monzodiorites and potassium granites could be finite members of the series. The rocks of these composition and tectonic settings closely resemble the enriched K in the early Paleoproterozoic terrestrial rocks of cratons. Thus, the lunar magmatism is close to only one (the Palaeoproterozoic), stage of terrestrial magmatism evolution. The cause of this could be both in some different primordial composition of the silicate mantles of these planetary bodies, and the different degree of their differentiation during solidification of their planetary magma oceans: it suggests that the depth of the latter in the case of the Moon was about 200-300 km, whereas in the Earth it was about 700 km.
Bogatilkov O. A.
Sharkov E. V.
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