Mineral chemistry and origin of chromium in uvarovite-bearing garnets from Baba Nazar (NE Takab)

The garnet calc-silicates at Baba-Nazar are located in the NE of Takab, SanandajSirjan zone. Anhedral to euhedral green colored garnets is a characteristic feature of these Calcsilicates. Mineral chemistry shows that garnets are Andradite- Grossular and contain more than 13% uvarovite. The high chromium content garnet, Cr-Spinel that have relatively Cr rich core than the rim, sulfide opaque minerals such as pyrite and chalcopyrite in this calc-silicate unit are regarded as evidence of the ultramafic protolith which can be considered the source of chromium for these garnets. The similarity in the geochemical composition of whole- rock between these garnet calc-silicates and ultramafic rocks is also consistent with ultramafic protolith. These rocks were affected by hydrothermal alteration that has caused high Ca and low Mg contents in these garnets and the host rocks compare to those from mantle source and ultramafic rocks. The introduction of CO2 rich fluids caused the carbonatization of ultramafic protolith and preferential removal of Mg. The faults, as channels for circulation of the hydrothermal fluids, play a key role during the process of hydrothermal alteration.