Diverse apatite geochemical compositions in early Paleozoic granitoids of the North Qinling orogen, China: Insights into their petrogenesis and magma sources

Abstract

Apatite geochemistry in granitoids is controlled primarily by the original melt compositions and partition coefficients. However, additional factors (e.g., post-crystallization effects, mineral crystallization sequences, and crustal assimilation) can also contribute to variations in apatite compositions. In this study, we collected three groups of early Paleozoic granitoids from the North Qinling Orogen and obtained in situ geochemical data for apatite. The granitoids were formed by partial melting of oceanic arc crust (group A), recycling of ancient crust (group B), and melting of the lithospheric mantle and extensive fractionation (group C). Trace element and Nd isotope data show the apatite from the Taiping and Manziying plutons (group A) were affected by post-crystallization processes and crustal assimilation, respectively. In addition, crystallization of plagioclase/allanite and apatite resulted in the variable Eu anomalies and rare earth element contents of apatite in the group A granitoids. In contrast, apatite compositions in group C were controlled primarily by the fractionation of hornblende. However, the decoupling of Eu anomalies of apatite and their host rocks, particularly for group B, cannot be easily explained by the aforementioned factors. As such, the redox state may also have been an important factor. We used a ternary diagram (10 × [Eu/Eu*]_N–V–Ga) to constrain the redox state from the apatite data, and provide insights into the types and petrogenesis of the host granitoids. Combined with the whole-rock and zircon geochemical data, we found that the apatite from the rocks derived from continental crust (group B and cratonic adakites) is generally reduced, whereas apatite from the mafic rocks and high BaSr granitoids is highly oxidized. The highest degree of oxidation was found for the altered apatite from the group A granitoids and unaltered apatite in the orogenic adakites, possibly due to interactions with fluid derived from oceanic crust. Moreover, unaltered group A granitoids exhibit a transitional trend from an oxidized to a reduced state. Apatite geochemistry can be used to constrain the redox state of adakitic rocks and whether they have a continental or oceanic crustal origin. The high Sr contents of high BaSr granitoids are inherited from their mantle source.