The geochemical nature of apatites in high Sr/Y two-mica granites from the North Himalayan Gneiss Domes, southern Tibet
藏南北喜马拉雅穹窿高Sr/Y二云母花岗岩中磷灰石地球化学特征及其岩石学意义

Recent investigations in the Yardoi gneiss dome, the easternmost one of the Northern Himalayan Gneiss Domes (NHGD), have identified two suites of high Sr/Y two-mica granites (TMG) formed at ca. 43~44 Ma and ca.18~20 Ma, respectively. Though they differ substantially in the Sr-Nd isotope systematics and the timing of formation, they show similar characteristics in mineral assemblage as well as in element geochemistry (e.g. high CaO and Sr contents, high Na/K and Sr/Y ratios). LA-ICP-MS analyses were carried out on apatite grains from these TMGs to investigate the geochemical behavior of apatite during their magmatic evolution. Analytical results show that: (1) trace element partitioning behavior between apatite and granitic melt are similar in these TMGs; (2) the Eocene TMG contains relict apatite, possibly inherited from its source; (3) relatively large scattering in trace element compositions among individual apatite grains from the same sample is due to chemical variations in local melts in equilibrium with apatite; (4) fractional crystallization of plagioclase rich in anorthite component had played a primary role in regulating the chemical compositions (e.g. Ca, Na, Sr, and LREE) of melts that crystallized apatite. Our investigation demonstrates that combined investigation of the growth textures and chemical compositions of apatite and plagioclase could yield important insights on the petrogenesis of granitoids.