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Nb–Zr–Y Systematics and Thermal Regimes of Subcontinental Lithospheric Mantle in the Archaean: Data from Mantle Xenoliths

Authors
  • Nikitina, L. P.1
  • Babushkina, M. S.1
  • 1 Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St.-Petersburg, 199034, Russia , St.-Petersburg (Russia)
Type
Published Article
Journal
Izvestiya, Physics of the Solid Earth
Publisher
Pleiades Publishing
Publication Date
Mar 01, 2021
Volume
57
Issue
2
Pages
217–231
Identifiers
DOI: 10.1134/S1069351321020075
Source
Springer Nature
Keywords
License
Yellow

Abstract

Abstract—The analysis of Nb–Zr–Y systematics and thermal regimes of lithospheric mantle of the Siberian, Kaapvaal, North American, North China, and South China cratons indicates their belonging to two types, one comprising the first three cratons (Siberian, Kaapvaal, and North American) (SKNA) and the other including the last two cratons (NCSC). Peridotites in the mantle of SKNA cratons have a Paleoarchaean age and are characterized by Nb content up to 5–6 ppm which is higher than in the primitive mantle (PM). The Zr and Y content in these peridotites is lower than in PM and, in a significant part of the samples, lower than in CI chondrite; these peridotites have superchondrite Nb/Y ratios (>1.0) and chondrite to superchondrite Zr/Y ratios. Thermal regimes in the mantle of SKNA cratons correspond to the model geotherm with surface heat flux density of 45 mW/m2 and fall in the diamond stability region; they are characterized by low (24.4 to 25.1°C/kb) quasi-thermal gradients (TG) reflecting the change in temperature with pressure increasing by 1 kb. The mantle peridotites of the North China Craton have a predominantly lower Nb content than in PM whereas the Nb content in the South China craton peridotites is somewhat higher than in PM. The Nb/Y ratios (ranging from subchondritic to superchondritic but remaining below 1.0) and the Zr/Y ratios (mainly subchondritic) correspond to those in peridotites of the ophiolite complexes of the South Urals and Northern Tibet. The thermal regimes in the mantle of the North China and South China cratons are characterized by the geotherms close to the model ones with heat flux densities of 55 and 60 mW/m2, respectively, and high average TG ​​(36.7 ± 0.5 and 41.3 ± 1.3°C/kb); these regimes correspond to the graphite stability region. The depth of the petrological boundary between the lithosphere and asthenosphere (lithosphere–asthenoasphere boundary, LAB) in the mantle of the SKNA cratons was ~200 km in the Paleoarchaean and, with the allowance for the geophysical data, has remained at the same level up to present. In the mantle of the North China and South China cratons as well as in the Baikal rift (Central Asian folded belt) and epi-Grenville Svalbard platform, since the end of the Paleoarchaean this boundary has been regularly rising to at least 70–80 km in Late Proterozoic. The difference of the mantle underlying the SKNA and NCSC cratons in terms of Nb–Zr–Y systematics and thermal regimes indicates the existence of different domains in the lithosphere since the end of the Paleoarchaean. The mantle of the first type of cratons is probably the relics of chondrite material that has not been completely reworked by melting and fractionation of the Fe-group elements and Nb with close properties. The mantle of the second type of cratons has geochemical features peculiar to PM.

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