The study of geochemical behavior of major and rare earth elements of garnet in the metamorphic rocks at Boroujerd area (Sanandaj-Sirjan Zone)

Garnet–mica schist and hornfels rock units are exposed in the east and southeast of Boroujerd. These rocks consist primarily of quartz, K-feldspar, plagioclase, garnet (almandine–spessartine), chlorite, cordierite, andalusite, sillimanite, biotite, muscovite, and minor amounts of apatite, iron oxides (ilmenite and magnetite), and zircon. Whole-rock geochemical analyses reveal that the dominant protoliths are pelitic rocks. Major and trace element compositions suggest that the Boroujerd pelites were deposited along an active continental margin. Garnet porphryblasts in some hornfels samples are compositionally homogeneous with respect to major, trace and rare earth elements; this is attributed to the diffusional re-equilibration at high temperatures (>600 ºC). Garnet in schists and some hornfels samples show reverse compositional zoning with increasing Mn and decreasing Fe and Mg from core to rim. Higher concentrations of Mn in garnet rims are attributed to resorption during retrogression. The presence of chlorite around garnet porphryblasts in these schists also supports resorption during retrogression. In schists, concentrations of HREE and Y in garnet decrease from core to rim. These zoning patterns are interpreted to record garnet growth in a closed system (i.e., Rayleigh fractionation of compatible elements). Core–rim variations in the concentrations of trace elements and rare earth elements in garnet in the hornfels samples is negligible. The lack of prominent zoning of these elements in garnet from hornfels is interpreted as minimal fractionation due to rapid garnet growth.