Mesozoic lithospheric modification and replacement beneath the Cathaysia Block: Mineral chemistry and water contents of the Daoxian peridotite xenoliths
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Peridotite
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Abstract Geochemical data on Baegryeong Island spinel peridotites found in Miocene alkali basalt provide the information for lithosphere composition, chemical processes, equilibrium pressure and temperature conditions. Spinel peridotite xenoliths, showing transitional textures between protogranular and porpyroclastic textures, were accidentally trapped by the ascending alkali basalt magma. The xenoliths originate at depths from 50 to 70 km with a temperature range from 800 to 1100°C. The variations of modal and mineral compositions of the spinel peridotite xenoliths indicate that the xenoliths have undergone 1–10% fractional melting. The spinel peridotites from Baegryeong Island have undergone cryptic mantle metasomatism subsequent to melt extraction. Metasomatic agent of enriched spinel peridotite xenoliths was carbonatite melt.
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Alkali basalt
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A rare kimberlite mantle xenolith displaying a direct contact relationship between a MARID rock and metasomatised peridotite is described. There is an abrupt textural transition from a MARID (phlogopite-K-richterite-diopside) portion, to a 2 cm rind of peridotite (olivine pseudomorphs-phlogopite-K-richterite-diopside-chromite). Small, but significant differences in mineral composition exist across the boundary, the most important of which are lower Fe/Mg ratios in the peridotite rind. Our interpretation is that MARID magmas crystallised and expelled fluids which metasomatised surrounding peridotite wall-rock, and the xenolith has sampled a cumulate-wall-rock contact of this nature.
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Phlogopite
Diopside
Chromite
Country rock
Mineral redox buffer
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Pseudomorph
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Metasomatism
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Xenolith
Melt inclusions
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The properties of garnets in peridotite xenoliths from kimberlites in China are compared with garnets occurring as megacrysts (disaggregated peridotites) in alkali basalts. The pyropes in xenoliths from kimberlites in the Shandong and Liaoning provinces are distinctly purple, whereas the megacryst pyropes in basalts are considerably darker. Kelyphitic reaction rims are common on pyropes in peridotite xenoliths from kimberlites, but are rare on pyropes in basalts. The classification of pyropes according to the scheme of Dawson and Stephens (1975) demonstrates that pyropes in peridotite xenoliths from kimberlites are mainly Group-9 garnets, whereas those in basalts typically belong to Group 3 (calcic pyrope-almandine) and Group 1 (titanian pyrope). In China, pyropes in peridotite xenoliths of kimberlites, as compared with those in basalts, are richer in Cr2O3, Cr/(Cr + Al), knorringite, uvarovite, and Cr-component, but lower in Fe2O3 + FeO, Fe/(Fe + Mg), grossularite, and almandine molecules. Unlike the situation in basalts, there is a distinct negative correlation between Cr2O3 and Al2O3 contents of the pyropes in peridotite xenoliths from kimberlites. The formation pressures of pyropes in the peridotite xenoliths mainly plot within the diamond stability field (i.e., more than 4GPa). These garnets may be useful as an indicator mineral for diamond exploration.
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The 27 mantle xenoliths have been collected in eastern China.The content of CaO,Au and PGE of 21 peridotite xenoliths were determined.The relationship of gold with CaO for the 27-peridotite xenoliths suggests that the gold content of the xenoliths relates to partial melting degree of mantle.The peridotite xenoliths that their chondrite-normalized PGE patterns are smooth have higher golden contents.The peridotite xenoliths that their chondrite-normalized normalized PGE patterns are decline have lower gold content.
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The crystal chemistry of some Cr-spinels from the lherzolite body of the Ronda peridotite in southern Spain has been investigated. Cell edge spans between 8.1692(2) and 8.2367(1) Å, while the oxygen positional parameter u ranges between 0.26306(7) and 0.26351(7). By using the Princivalle thermometer (1999), an intracrystalline closure temperature between 640 and 840 °C has been calculated. The higher temperatures are very close to the intercrystalline temperatures based on the olivine-spinel thermometer calculated by Woodland et al. (2006) for the Ronda orogenic lherzolites suggesting that the intracrystalline closure occurred soon after the intercrystalline closure. By comparison with Cr-spinels from lherzolite mantle xenoliths, it should be noted that: (1) the oxygen positional parameter can be linearly related to the intracrystalline temperature for both mantle peridotite and mantle xenolith Cr-spinels; (2) the intracrystalline closure temperature is reached faster and is higher in Cr-spinels in mantle xenoliths; and (3) Cr content is linearly related to u in mantle peridotite, but not in mantle xenoliths, suggesting the u value in Cr-spinels from mantle peridotite is driven solely by the chemistry of the spinels.
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Amphibole
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