Ion-microprobe determinations of trace-element concentrations in garnets from anatectic assemblages

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doi: 10.1016/0009-2541(92)90101-A
Authors:Harris, N. B. W.; Gravestock, P.; Inger, S.
Author Affiliations:Primary:
Open University, Department of Earth Sciences, Milton Keynes, United Kingdom
Volume Title:Chemical Geology
Source:Chemical Geology, 100(1-2), p.41-49. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541
Publication Date:1992
Note:In English. 31 refs.; illus. incl. 1 table
Summary:Trace-element data for garnet, cordierite, plagioclase, alkali feldspar and tourmaline were obtained by ion-microprobe analysis of assemblages from a pelite-derived leucogranite, a sillimanite- grade anatectic gneiss and a kyanite-grade sub-solidus schist from the High Himalaya. Results confirm that for equilibrium melting residual plagioclase will cause depletion in Sr and Eu in the melt, and residual garnet will result in strong depletion of Y and HREE, consistent with their published partition coefficients. All trace elements of interest are strongly incompatible in cordierite, implying that for equilibrium fractional melting of pelites at shallow depths (< 5 kbar) the stability of cordierite as a restite phase will result in melts that are undepleted in Y and HREE. HREE concentrations in garnet from metasediments are independent of metamorphic grade, but the metamorphic garnets are significantly less enriched in HREE than is the leucogranite garnet. This suggests either that the leucogranite is derived from a source with lower modal abundances of garnet than the analysed metasediments, or that garnet from the anatectic gneiss has not reached chemical equilibrium with a granitic melt. The latter possibility is consistent with sluggish self-diffusion rates for REE at T < 800 degrees C. Disequilibrium between pre-anatectic garnets and pelite-derived melts accounts for the observation that granites derived from crustal anatexis are often undepleted in HREE even though garnet is probably a common residual phase during crustal anatexis at P > 5 kbar. [Authors' abstract] [P.Br.]
Subjects:Alkali feldspar; Anatexis; Cordierite; Feldspar group; Framework silicates; Garnet group; Gneisses; Granites; Igneous rocks; Ion probe data; Leucogranite; Mass spectra; Melting; Metals; Metamorphic rocks; Metapelite; Metasedimentary rocks; Mineral assemblages; Nesosilicates; Orthosilicates; Phase equilibria; Plagioclase; Plutonic rocks; Rare earths; Ring silicates; Schists; Silicates; Spectra; Tourmaline group; Trace elements; Asia; Himalayas
Abstract Numbers:93M/1888
Record ID:1993001208
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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