Apatite Geochemistry from Mafic Rocks in the Northeastern North China Craton: New Insights into Petrogenesis

Apatite is widely utilized to reflect the character and petrogenesis of the parental felsic magma. The compositional variation of apatite from mafic rocks and its applicability to the record of whole-rock are still not well understood. In this study, we present the in situ geochemical analysis of apatite from two types of mafic rocks in the northeastern North China Craton (amphibole-rich mafic rocks for group 1, and pyroxene-rich mafic rocks for group 2). All apatite samples display typically right-sloping rare earth element (REE) patterns, with enrichment in light REE relative to heavy REE, and homogeneous Nd isotopic compositions, suggesting a magmatic origin. Higher apatite REE contents in mafic rocks of group 2 than those of group 1 show a different crystallization sequence in apatite. Extensive apatite crystallization in amphibole-rich mafic rocks, occurs earlier than in pyroxene-rich mafic rocks. The magma differentiation is responsible for the low apatite REE concentrations in group 1 mafic rocks. Combined with previous apatite data, both apatite REE patterns and compositional variations in mafic rocks are mainly controlled by the composition of the original melt. The crystallization of titanite before apatite impoverishes melts in the apatite-compatible trace elements (e.g., middle REE), whereas the crystallization of plagioclase concurrently with apatite can cause some subtle compositional variations of Sr and light REE. Apatite in mafic rocks exhibits higher Sr concentrations than in felsic rocks, which is most likely attributed to the weakening extent of the plagioclase crystallization.