Garnet as a major carrier of the Y and REE in the granitic rocks: An example from the layered anorogenic granite in the Brno Batholith, Czech Republic
Abstrakt
Garnet and other rock-forming minerals from A-type granite dikes in the Pre-Variscan Brno Batholith were analyzed to determine relative contributions of individual minerals to whole-rock Y and REE budget and to assess incorporation mechanisms of these elements in garnet. Minor to accessory garnet (<2 vol%) is the essential reservoir for Y+REE in the Hlina granite accounting similar to 84% Y and 61% REE of the total whole-rock budget.
Zircon is another important carrier of REE with similar to 13% Y and similar to 11% REE. At least similar to 21% REE and 1% Y were probably hosted by Th- and U-rich monazite that has been completely altered to a mixture of secondary REE-bearing phases.
The contribution of major rock-forming minerals (quartz and feldspars) is low (similar to 1% Y; 10% LREE; similar to 1% HREE) excluding Eu, which is hosted predominantly by feldspars (similar to 90%). Minor to accessory muscovite and magnetite incorporate similar to 1% Y and similar to 2% REE of the whole-rock budget.
Magmatic garnet Sps(41-46)Alm(28-44)And(0-13)Grs(6-12)Prp(0-1) is Y- and HREE-rich (up 1.54 wt% Y; up similar to 1 wt% Sigma REE), and the Y+REE enter the garnet structure via the menzerite-(Y) substitution. The Y and REE show complex zoning patterns and represent sensitive indicator of garnet evolution, in contrast to a homogeneous distribution of major divalent cations.
General outward decrease of Y+REE is a common feature due to the strong partitioning of Y+HREE in the garnet relative to the other phases. REE underwent significant fractionation during growth of early garnet I; the Yb-N/Nd-N ratio generally decreases from the core to rim of garnet I.
Higher Mn and Al, lower Ca, and Y+REE contents, as well as higher YbN/NdN ratio and more negative Eu anomaly in garnet II overgrowths indicate its crystallization from a more evolved melt.