Two types of garnets in the Cenozoic granites from the Himayalan Orogenic Belt: Geochemical characteristics and implications for crustal anatexis
喜马拉雅造山带新生代花岗岩中两类石榴石的地球化学特征及其在地壳深熔作用中的意义

In the Himalayan Orogenic Belt, garnets occur as one of the key constituent phases in metapelites as well as one of the important accessory phases in leucogranites or leucosomes in migmatites. They have preserved critical information with regard to the geochemical nature of crustal anatexis and could yield important insights on the physical and chemical processes of middle to lower crustal rocks in large collisional orogenic belts. Along the Himalayan orogenic belt, two types of garnets, magmatic and metamorphic, occur in Cenozoic granitic rocks (leucogranite and leucosome). Magmatic garnets are euhedral to subhedral and commonly free of inclusions, whereas those derived from the source rocks show embayment texture and have been subjected to various degrees of melt-garnet reactions. Chemical analyses show that magmatic garnets are characterized by (1) typical oscillatory growth zonation; (2) enrichment in HREE, but highly depletion in LREE, and rim-ward decreases in Hf, Y and HREE; (3) pronounced negative Eu anomalies; (4) elevated Mn and Zn concentrations relative to those in metamorphic garnets derived from the source rocks. Petrographic and geochemical characteristics of garnets from leucogranites suggests that a large portion of Zn was incorporated in garnet and fractional crystallization of plagioclase and zircon is the major factor that regulate the magnitude of negative Eu anomalies and the abundance and rim-ward decreasing pattern of Hf, Y and HREE in magmatic garnet. Our data indicate that fine-scale chemical variations in magmatic garnet could serve as another important revenue to unravel the magmatic processes during the crystallization of leucogranitic melts.