In situ geochemical composition of apatite in granitoids from the eastern Central Asian Orogenic Belt: A window into petrogenesis

The compositional record of accessory minerals can provide insights into the character and petrogenesis of the parental magma. We demonstrate apatite's applicability to petrogenetic studies through in situ trace element analysis of apatite in 60 granitoid plutons from the eastern Central Asian Orogenic Belt (CAOB). Rare earth element (REE) patterns of apatites usually mimic the trend of the host rock but are slightly enriched in MREE (middle REE), which is predominantly controlled by parental melt composition and REE partitioning. However, the REE patterns of apatites that are decoupled from those of host rocks, such as the depletions of LREE (light REE) and MREE, as well as the HREE (heavy REE) enrichment, result from the early crystallization of other REE-bearing minerals and late-stage metamorphism. Our data displays no significant relationship between apatite trace elements and highly fractionated granitoids in the eastern CAOB, highlighting the limitation of assessing the degree of magma differentiation through apatite geochemistry. More significantly, the variation of Sr in apatite shows a close correlation with the early crystallization of plagioclase, and the linear correlations of Sr_Pl-Sr_WR, Sr_Pl-Sr_Ap, and Sr_Ap-Sr_WR imply constant Sr partition coefficients of plagioclase and apatite. Furthermore, a partition coefficient of Sr between apatite and bulk rock is proposed (D_{Apatite/bulkrock}^Sr=0.69). Our results also imply that apatite crystallizing from adakites inherits the typical trace element signatures of the adakitic melt and is characterized by high Sr, low HREY (Gd ∼ Lu + Y) contents and lack of Eu anomaly. In conclusion, apatite provides a vital bond to understand the petrogenesis of granitoids and shows the potential to track granitic sources in detrital grains.