Dear Mr. Perumal,

the Eu anomaly is hard to be linked to a rock type especially in case of a rock type which highly variable as a gneiss which can have igneous or sedimentary sources.Terekhov and Shcherbakova ( 2006) found acid rocks with positive Eu anomalies in the eastern part of the Baltic Shield. The REE pattern of pegmatites show sometimes

marked positive Eu anomaly and strong LREE/HREE fractionation, which is interpreted

in terms of a strong wall rock – pegmatite interaction and hydrothermal alteration

of the entire pegmatite system. Even these two example may demonstrate that it is difficult to link such a REE anomaly to a definite debris or clast type. High-grade regionally metamorphose gneisses close to anatexis often have leucosomes and paleosomes which need to be dealt with separately. If the entire system is subjected to reworking as it is the case in your sedimentary study area, the interpretation is faced with such aggravating circumstances that any interpretation becomes a matter of conjecture.

Best regards

H.G.Dill

Hi Gangatharan,

Studies of the REE distribution patterns in clastic sediments and their potential igneous and metamorphic source terranes during the early 1970s quickly identified a first order-contrast in Eu anomaly signatures between upper continental crust and lower continental crust, and between Archean-aged terranes and Phanerozoic cover successions. High-grade lower crustal granulites typically exhibit flat or slightly positive Eu anomalies (for example the Archean Lewisian intermediate-to-felsic gneisses of NW Scotland), while upper continental crustal granitoids and the (post-Archean) sediments derived from them commonly show negative anomalies. Clastic sediments, particularly those which are texturally immature, which are derived from exhumed high grade Archean terranes can and do inherit some aspects of their parent REE distributions, including Eu abundance anomalies.

There are numerous exceptions to these generalizations, and Harald is quite right to emphasize that the detailed study of REE abundances in high grade metamorphic rocks of all ages throws up many local examples of both positive and negative Eu anomalies, driven by variations in protolith origin, primary and metamorphic mineralogy, and metasomatic/fluid history during high grade regional metamorphism.

Regards,

Graham

Dear Perumal,

The positive Eu anomalies indicate strongly weathered gneisses which are high-grade regionally metamorphose granitic rocks. As you know, granites form plagioclase, alkalifeldispar and quartz, and plagioclase make up Ca, Na AlSi3O8, Eu is similar to Ca in therms of ionic characterizations such as Sr,  and Eu takes place of Ca  as diadohy in the crystal structures in the early crystalization stage. This simplified ion exchange process and exhibits diadohy. But in the surface environment, Eu concentrations increase within the sedimentary rocks  by weathered of the Eu rich rocks. This is linking to groundwater/ meteoricwater circulation because of Eh-pH conditions, see attachment files, graphs.

Leyla

Thank you so much Prof. H.G.Dill, Mr. Graham and Mr. Leyla for your precise answers. 

I do not have experience with sedimentary rocks, but, based on REE geochemistry I would expect that if the clastic rock contains abundant feldspars, positive Eu anomalies would be presente. This is because feldspars concentrate Eu relative to other REE. Have a look on the mineral/magma partition coeficiente for REE in rock forming minerals. Good luck! I hope to have helped! Lauro V Stoll Nardi.