Geochemistry of sphene from granodiorites surrounding the Hutti-Maski schist belt: Significance to rare earth element (REE) modeling

A study of the distribution of REE in sphene from three granodiorite samples around NNW-SSE trending Hutti-Maski schist belt has been described here. Two distinct sets of sphene-melt K-d values for REE have been observed among the three sphene-whole rock pairs studied. The chondrite. normalized REE pattern of sphene from east (Gajalagatta) and northeast (Uti) granodiorites show LREE enriched and HREE depleted patterns with slight or no negative Eu anomaly. These REE patterns are parallel to the respective whole rock samples. But the chondrite normalized REE pattern of sphene from the sample to west of the schist belt (Ankasdoddi) has markedly depleted Ce and Nd with slight or no negative Eu anomaly and steeply fractionated HREE, resulting in a convex-up pattern. The variation of REE K-d values of sphenes is not due to differences in the composition of host magmas (because they are all granodiorites with identical major element abundances), but could be a function of temperature of crystallization. Small degrees of fractional crystallization of sphene from granodiorite magmas would result in residual magmas having lower REE abundances either with parallel REE patterns or HREE depleted fan shaped REE patterns, depending on the K-d values. Partial melting of granitoid rocks, leaving minor amounts of sphene in the residue gives rise to melts with lower REE abundances than the source. The REE patterns of the melts are parallel to the source if one set of REE K-d sphene-melt values are considered. If the other set of K-d values are used, the partial melts have (fan shaped) REE patterns with more depletion for heavy REEs.