Subduction Initiation and Early Subduction Zone Development: Insights From the P–T–t Path of the Bay of Islands Metamorphic Sole

Abstract

The processes involved during subduction initiation (SI) remain poorly constrained, in part due to the rarity of geological archives. Ophiolites and their underlying metamorphic soles are thought to form during subduction zone infancy and may thus provide critical constraints. Metamorphic soles are derived from oceanic crust of the lower plate accreted to the base of the upper plate following SI and are exhumed during upper plate extension and ophiolitic crust generation. Their metamorphic evolution therefore provides insights into the thermal conditions at the nascent subduction plate interface and the trajectory of the lower plate during subduction infancy. In this study, we revisit the Bay of Islands complex (BOIC) metamorphic sole in the Newfoundland Appalachian Orogen and constrain P–T–t paths using phase equilibria modelling, melt reintegration and in situ U–Pb titanite geochronology. Our findings reveal generic SI and early subduction processes that align with geochronologic and metamorphic constraints reported for other soles as well as with the proposed early development of some modern subduction zones. The BOIC sole exhibits an inverted metamorphic gradient, with granulite and amphibolite facies metabasites at upper and middle structural levels, and greenschist facies metasedimentary and meta-igneous rocks in the lowest level. Our results are consistent with burial of garnet–clinopyroxene–amphibolite sole rocks to peak pressure conditions of ~14 kbar and 750°C–850°C, followed by decompression to peak temperature conditions of 880°C–940°C and 10.5–12 kbar. The immediately underlying plagioclase–hornblende amphibolites equilibrated at ~740°C and ~8.5 kbar. Sole metamorphism produced melts and fluids that may have contributed to arc magmatism during early subduction. Titanite geochronology of granulite facies samples yields dates of 512 ± 11 Ma and 501 ± 11 Ma, interpreted to constrain prograde metamorphism. Our results indicate that the sole formed along a clockwise P–T–t path, with initial burial occurring along a relatively cold 18°C/km metamorphic gradient, followed by heating of the subduction interface during decompression and eventually exhumation to a 26°C–27°C/km gradient. Peak temperature conditions in the sole were coeval with c. 484–488 Ma ophiolitic crust formation. We propose that subduction initiated from far-field forces and evolved to a self-sustained state as the slab pull force increased. Upwelling of hot asthenospheric mantle during rollback can explain decompressional heating and ensuing exhumation along an increasing thermal gradient, coeval with upper plate extension.