Profiles of REE+Y zoning in garnet and their genetic significance (a case study of metapelites from the Nothern Ladoga Region)
Gulbin Yu. L.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2015. V. 144. N 3. P. 31-44
The study is aimed to understand the REE and Y behavior during the garnet porphyroblast formation in staurolite-bearing schists in the Late Paleoproterozoic metapelites of Ladoga Complex. The MnNCKFMASH P—T pseudosection for a single sample and Grt—Bt thermometry indicate that growth of the garnet core occurred at 520 °C and 7.0—7.2 kbar in the Grt—Bt—Pl—Chl—Ms—Zo field, whereas the garnet rim was equilibrated at 590—600 °C and 3.5—4.0 kbar. Measured zoning profiles show strong depletions of REE + Y in the garnet core containing high concentrations of Mn and Ca. The intermediate zone of garnet is enriched in La, Ce, Pr, Nd (the inner LREE + Nd annulus), as well as in Dy, Er, Yb, Lu, and Y (the outer HREE + Y + Dy annulus). According to pseudosection analysis, these peaks were probably produced by the breakdown of epidote-group minerals (allanite, REE-rich epidote) at T < 535 °C and P > 6.5 kbar. Towards the rim the HREE + Y contents decrease monotonically, with MREE (Sm, Eu, Gd) displaying the inverse trend. Also, the rim exhibits a negative Eu anomaly. The former tendency reflects an increase of temperature during the garnet crystallization and partitioning of elements between garnet and monazite while the latter one suggests an additional source of REE and involvement of plagioclase in Eu redistribution in the late stages of porphyroblast growth in connection with increasing concentration of Eu2+ in the intergranular medium at the staurolite isograd.
Key words: garnet, REE, Y, zoning, porphyroblast growth, plagioclase, accessory minerals, metapelites, Northern Ladoga region.