Apatite geochemical and Sr Nd isotopic insights into granitoid petrogenesis

Apatite geochemical and SrNd isotopic compositions provide robust means for constraining a wide range of geological processes in earth and planetary sciences, including magmatism, metamorphism, metasomatism, planetary geochemistry, and geochronology. Here we extend the utility of these data to fingerprint magmatic evolutionary processes and granitoid petrogenesis. Apatite grains in Early Cretaceous (ca. 130 Ma) granite from northeastern China have distinct occurrences, geochemical features, and SrNd isotopic compositions, indicating their different origins. Geochemically, they can be divided into two groups as follows. Group 1 apatites mainly occur as euhedral to subhedral crystals, and have relatively low initial 87Sr/86Sr ratios (0.7071–0.7110), weakly negative εNd(t) values (−11.5 to −4.9), and variable chemical features such as positive to negative Eu anomalies and continuously variable total rare-earth element, Sr, Y, and Th contents, suggesting crystallization from host magma. Combining with petrographic observations and whole-rock geochemistry, it is indicated that Group 1 magmatic apatite records the history of magma evolution, including the crystallization history of minerals, and possibly fractionation of early crystallized minerals such as plagioclase and titanite. Group 2 apatites occur as anhedral crystals, and display variable initial 87Sr/86Sr ratios (0.7010–0.7506) and strongly negative εNd(t) values (−33.4 to −7.7) at the emplacement age of the host granite, suggesting they did not crystallize from the magma, nor were they inherited from its source. Group 2 apatites have geochemical and SrNd isotopic compositions comparable with those from local Archean and Paleoproterozoic intrusive rocks, suggesting capture from wall rocks and interaction with the host melt. The occurrence of assimilated apatite grains in the host granite suggests a crustal assimilation process involved in granite petrogenesis, which is not easily identified through whole-rock geochemistry. Our results confirm that apatite in situ geochemical and SrNd isotope analyses are powerful tools for elucidating the source and evolution of magma, thus providing compelling evidence for the origin of granitoids.