ORIGIN AND EVOLUTION OF OSCILLATORY ZONED GARNET FROM KASVA SKARN, NORTHEAST TAFRESH, IRAN

摘要

Within the central Urumieh-Dokhtar Magmatic Belt of northern Iran, the Kasva skarn deposit (KSD) formed by metasomatic alteration of Eocene intercalated carbonate and volcaniclastic sediments in response to Oligo-Miocene intrusion by granitoid porphyries. The KSD contains abundant oscillatory-zoned garnet crystals, which are characterized by isotropic cores of nearly pure andradite that are rimmed by anisotropic grossular-andradite (grandite). The Fe-rich andraditic cores are enriched in U and LREE, with positive Eu anomalies, whereas the Al-rich granditic rims are enriched in Ti, Nb, Zr, Hf, and HREE, without Eu anomalies. Variation in optical and chemical features in Fe- and Al-rich garnet are controlled by external factors such as (1) infiltration of compositionally distinct fluids, (2) incorporation of LREE and U at the {X} site in association with substitution of Fe~(3+) for Al~(3+) at [Y] within the crystal structure of andraditic garnet, and (3) substitution of Ti and HFSE for Al in granditic garnet.