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Amadine garnet3/22/2023 ![]() ![]() Garnet can preferentially concentrate the HREE compared to a co-existing melt and this feature has been used extensively to elucidate the source lithologies of igneous and metamorphic rocks, such as basaltic volcanics ( Davidson et al., 2013) and granitoids ( Halla, 2018). Its high density is important for generating the “slab pull” in subduction as oceanic crust transforms to eclogite (but note that the “majorites” recorded in many mantle xenoliths may not actually be majorite according to the strict definition ( Grew et al., 2013)). It therefore seems likely that certain horizons of the mantle consist of majorite-rich garnet. Laboratory experiments have revealed that with increasing temperatures and pressures pyroxene becomes unstable and converts to majorite. Garnet is a major mineral phase in granulites and pyroxenites of the lower crust and upper mantle and is important in the transformations that take place there ( Wood et al., 2013). The Al-free garnet majorite has also been found in meteorites which have been subjected to shock metamorphism. These types of garnets are also found in certain ultrabasic rocks such as the mica peridotites and diamondiferous kimberlites (Cr-rich pyrope), and also occur in peraluminous volcanic rocks, granites, and pegmatites. Typically the garnet here is almandine-rich and an indicator of intermediate grades of metamorphism of argillaceous sediments, roughly equivalent to amphibolite facies. Indeed, garnet constitutes an index mineral in the classic Barrovian metamorphic zones. The pyrope– almandine series of garnet occurs in metamorphic rocks of a variety of types, of which the garnet-mica schists of regional metamorphism are typical. ![]() David Alderton, in Encyclopedia of Geology (Second Edition), 2021 Occurrence ![]()
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