Phenocryst zonation records magma mixing in generation of the Neoproterozoic adakitic dacite porphyries from the Kongling area, Yangtze Craton

Abstract

Adakites and adakitic rocks are pivotal in understanding the formation and evolution of the early continental crust. However, there remain controversies about the origin of these rocks. This paper reports petrological, mineralogical and geochemical data for the newly identified Neoproterozoic (ca. 819 Ma) adakitic dacite porphyries in the Kongling area, Yangtze Craton, with the aim of constraining their petrogenesis and elucidating the role of magmatic processes in generating the adakitic melts. The dacite porphyries are characterized by high SiO2 (62.2–64.8 wt%), Mg# (52–60), Sr/Y and (La/Yb)N ratios, as well as depleted Nd-isotope compositions (ƐNd (819 Ma) = 0.1–1.6), geochemically comparable to high-Mg# adakitic rocks. Core-rim zoned amphibole and plagioclase phenocrysts are common. Overall, the amphibole cores show lower MgO, FeO, Sr, but higher Al2O3 concentrations than the rims. Similarly, the plagioclase cores exhibit lower An [Ca/(Ca + Na + K), atomic ratios] but more enriched 87Sr/86Sr(i) than the rims. Geochemical modeling demonstrates that the melts in equilibrium with the amphibole cores have high SiO2, low MgO, CaO, Fe2O3T and TiO2 as well as flat REE patterns and low Sr/Y ratios. Such felsic melts were likely generated by low-degree dehydration melting of the Paleo- to Meso-proterozoic amphibolites of the Kongling Complex. On the contrary, the melts in equilibrium with the amphibole rims possess lower SiO2 and higher MgO with elevated (La/Yb)N and Sr/Y ratios, suggesting involvement of more mafic melt components. In combination with regional geology, we propose that this mafic endmember possibly formed through fractional crystallization of original subduction-related basaltic magmas that are geochemically equivalent to the Neoproterozoic Panxi-Hannan arc-type basaltic rocks. Mass-balance calculation further reveals that the Kongling dacite porphyries could form by mixing of the above two geochemically distinct endmembers in ratios of ca.50: 50. Importantly, the adakitic signature of these rocks (i.e., high Sr/Y and (La/Yb)N ratios) was inherited from the fractionated arc-type mafic melts, highlighting the significant role of magma mixing in generating the adakitic melts. We propose that this mechanism may have been much more common than as previously thought and contributes significantly to continental growth and evolution.