Resubduction of lawsonite eclogite within a serpentinite-filled subduction channel

Translating burial and exhumation histories from the petrological and geochronological record of high-pressure assemblages in subduction channels is key to understanding subduction channel processes. Convective return flow, either serpentinite or sediment hosted, has been suggested as a potential mechanism to retrieve rocks from significant depths and exhume them. Numerical modelling predicts that during convective flow, subducted material can be cycled within a serpentinite-filled subduction channel. Geochronological and petrological evidences for such cycling during subduction are preserved in lawsonite eclogite from serpentinite melange in the Southern New England Orogen, eastern Australia. Ar-Ar, Rb-Sr phengite and U-Pb titanite geochronology, supported by phase equilibrium forward modelling and mineral zoning, suggest Cambro-Ordovician eclogite underwent two stages of burial separated by a stage of partial exhumation. The initial subduction of the eclogite at ca. 490 Ma formed porphyroblastic prograde-zoned garnet and lawsonite at approximate P-T conditions of at least 2.9 GPa and 600 °C. Partial exhumation to at least 2.0 GPa and 500 °C is recorded by garnet dissolution. Reburial of the eclogite resulted in growth of new Mg-rich garnet rims, growth of new prograde-zoned phengite and recrystallization of titanite at P-T conditions of approximately 2.7 GPa and 590 °C. U-Pb titanite, and Ar-Ar and Rb-Sr phengite ages constrain the timing of reburial to ca. 450 Ma. This was followed by a second exhumation event at approximately 1.9 GPa and 520 °C. These conditions fall along a cold approximate geotherm of 230 °C/GPa. The inferred changes in pressure suggest the lawsonite eclogite underwent depth cycling within the subduction channel. Geochronological data indicate that partial exhumation and reburial occurred over ca. 50 M y., providing some estimation on the timescales of material convective cycling in the subduction channel.
Author(s): R. Tamblyn [sup.1], M. Hand [sup.1], L. Morrissey [sup.2], T. Zack [sup.1] [sup.3], G. Phillips [sup.4], D. Och [sup.5] Author Affiliations: (1) grid.1010.0, 0000 0004 1936 7304, Department of [...]