Combining zircon texture, REE patterns and U-Pb-Hf isotopes to decipher the formation process of orbicular rocks: A case study from Huangling orbicular granodiorite, Yangtze craton, China

Abstract

The petrogenesis of orbicular rocks is still enigmatic because of the unique texture and variable compositions. In this study, we focus on the textural characteristics and mineral compositions from Huangling Orbicular Granodiorite (HOG), where orbicules with rhythmic growth texture and radial growth texture exist in the same outcrop. Textural and isotopic studies of zircons from different parts of the representative orbicules show three age groups of 1961 ± 19 Ma, 890 ± 6 Ma and 852 ± 6 Ma, corresponding to the ages of protolith crystallization, melt emplacement, and input of a new magma, respectively. Combined with field relationship, Ti-zircon crystallization temperature, trace element modelling and Hf isotopic data, the HOG may be derived from high-temperature contact process that involved the partial melting of granodiorites in contact and interaction with a quartz-bearing dioritic intrusion.Textural features and mineral compositions of the studied orbicules show that different textures are related to different states of crystallization. Orbicules with a rhythmic growth texture have disequilibrium crystallization zone with feldspar An contents of 34–46 and a “W”-shape pattern (forming rhythmic layers), and near-equilibrium crystallization zone with An contents of 42–44 (forming the inner part). Orbicules with a radial growth texture was crystallization in disequilibrium, as An contents show approximate linear variations of 32–44 to form radial growth layer, and with An varies 26–46 to form the inner part.Our observations support the magma quenching hypothesis, proposed as hot orbicule melts underwent rapid cooling with inward crystallization. Moreover, we firstly suggest that the amphibole-plagioclase disequilibrium crystallization could explain different textures of orbicules. Rhythmic growth texture and radial growth texture are formed at undercooling state with fast and slow cooling rates, separately. The numbers of rhythmic layers are controlled by the duration of undercooling state, while the width of each layer is controlled by the cooling rate. If excess amphibole components left in the center of orbicule melt, the “core” will be mafic. Otherwise, the “core” will be felsic.