Trace element fractionation during high-grade metamorphism and crustal melting-constraints from ion microprobe data of metapelitic,migmatitic and igneous garnets and implications for Sm-Nd garnet chronology

摘要

Rare earth element (REE)and other trace element (Y,Sr,Ti,Cr,V,Na)abundances in garnet from a garnet-bearing metapelite,a pelitic migmatite,a syn-tectonic granite and a post-tectonic leucogranite were measured by secondary ion mass spectrometry (SIMS)in order to identify the effective variables on the trace element distribution between garnet and the host rock.Garnet from the garnet-bearing metapelite,the pelitic migmatite and the syn-tectonic granite is zoned with respect to REE.The cores are enriched by a factor of 2-3 relative to the rims.For the garnets from the garnet-bearing metapelite equilibrium distribution following a simple Rayleigh fractionation is responsible for the decreasing concentrations in REE from core to rim.Garnet from the pelitic migmatite shows a more complex trace element pattern following distinct enrichment and depletion patterns for Ti,V,Cr and REE from core to rim.These features suggest disequilibrium between garnet and the associated melt in which the enrichment of trace elements probably correspond to a period of open-system behaviour in these rocks at a time when the garnet,originally nucleated in the metamorphic environment was incorporated into the melt.The garnet from the syn-tectonic granite shows stepwise decreasing concentrations in REE from core to rim:a REE-rich core can be distinguished from a broad REE-depleted rim.Notably,from core to rim an inflection of the Yb/Er and Yb/Dy ratios is visible.Whereas the decrease of HREE abundance in the core region of the garnet from the syn-tectonic granite may arise from equilibrium partitioning during garnet growth,the inflection can be interpreted as a result of partial melting.Garnet cores with high Yb/Er and Yb/Dy> 1 nucleated in the metamorphic environment without the presence of a melt whereas the rims with lower Yb/Er and Yb/Dy < 1 crystallized in the presence of a melt.Garnet from the leucogranite has lower REE abundances and is considered to be of igneous origin.In contrast to garnet from the other samples,its core has low trace element abundances,whereas its rim is significantly enriched in REE but depleted in Ti.These features suggest that only the outermost rim was in equilibrium with the melt.For this garnet,liquid diffusion controlled partitioning is more likely to explain the extreme trace element variation.An evaluation of Sm and Nd concentrations in garnet and a comparison of Sm-Nd and U-Pb gamet ages and U-Pb monazite ages form the terrane indicate that the observed LREE systematics in the different garnet species are a primary feature and are not homogenized by volume diffusion during high grade amphibolite facies conditions.