HR: 16:00h
AN: V12I-01    [PDF]
TI: Combined High-Precision Garnet Ages and Garnet Ionprobe Data from Granulite-Facies Migmatites and Granites -Implications for Element Redistribution During High-Grade Metamorphism and Melting
AU: * Jung, S
EM: sjung@mpch-mainz.mpg.de
AF: Max-Planck Insitut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Hellebrand, E
EM: ehelle@mpch-mainz.mpg.de
AF: Max-Planck Insitut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AB: Migmatitic and metapelitic garnet from the granulite-facies (5-6 kbar/750oC) part of the Damara orogen (Namibia) has been used to date high-grade regional metamorphism and yielded high-precision Sm-Nd grt-whole rock ages that range from 523 to 509 Ma similar to the U-Pb monazite ages (531 - 517 Ma) from the same rock. All garnet species show significant variation in Pb isotope composition, Nd/U-Th/U and U/Nd-Sm/Nd space suggesting a significant contribution of minute acessory phases (i.e. monazite and zircon). Trace element variations in the metapelite garnet show no core-rim REE variation. The negative Eu anomaly is less pronounced than in migmatite garnets. These garnets show no pronounced core-rim variations in Zr (9-6 ppm) and Sr (0.12 ppm) and moderate concentrations of Ti (30 ppm), V (40 ppm) and Cr (100 ppm) suggesting variable contributions of zircon, monazite and ilmenite during garnet growth. Garnets from in-situ migmatites show REE, Sr, Zr, Ti and Cr depletion and increasing negative Eu anomalies from core to rim suggesting the presence of a REE and HFSE-enriched granitic melt during melting. Garnets from intrusion-related migmatites show REE, Zr and Sr depletion from core to rim similar to garnet from in-situ migmatites. The decreasing negative Eu anomaly from core to rim is a result of decreasing LREE concentrations relative to HREE concentrations at constant Eu. These garnets have overall low concentrations of V, Cr and Sr similar to igneous garnets. These features suggest initial growth of garnet during in-situ melting with plagioclase present followed by growth of garnet in an intrusive trace element-depleted granitic melt. Igneous garnet is one order of magnitude less enriched in REE than the other garnet species but shows enrichment in HREE and depletion of LREE in the core relative to the rim. This igneous garnet is depleted in Sr, Zr, V and Cr compatible with the trace element-depleted nature of the host leucogranite. All garnet species show distinct (Sm/Gd)N and Eu/Eu* covariations which are discussed together with previously published trace element systematics of amphibolite-facies, granulite-facies and igneous garnets.
DE: 1020 Composition of the crust
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting