Field and petrographical insight into the formation of orbicular granitoids from the Bonney Pluton, southern Victoria Land, Antarctica

Abstract

AbstractThe diversity of orbicule types exposed within granitoids of the Bonney Pluton in southern Victoria Land attests to the dynamic and complex interplay of magmatic processes that were responsible for their formation. Orbicules formed in small pockets of H2O-rich silicate melt that was extracted from the crystallizing and fractionating Bonney Pluton magma and concentrated along the pluton margins. These pockets of melt experienced a superheating event that destroyed almost all pre-existing nuclei and a subsequent delay in crystallization, which led to undercooling conditions that promoted rapid dendritic crystal growth. Superheating was induced by the injection of hot mafic magmas, evidenced by elevated plagioclase XAn, and Mg-Al-Ti-contents in hornblende that point to a higher temperature and a more mafic composition in the melt that the orbicule shells crystallized from. Variation in the type and structure of orbicules (hornblende-rich versus plagioclase-rich shells) were likely due to repeated changes in the composition, H2O-content, temperature and PH2O at the crystallizing orbicule boundary layer in response to pulses in the movement of magma, competition between crystallizing phases, episodic vesiculation, degassing and/or second boiling which dictated the composition, texture and size of individual orbicule shells. Brittle fragmentation of orbicules occurred in response to vesiculation and fragmented orbicules are often found as cores within intact orbicules, indicating that multiple phases of orbicule formation were common. The alignment and compaction of orbicules in ‘pods’ indicates movement of orbicules within the melt-rich pockets occurred, prior to resumption of near-equilibrium crystallization in the host granite.