Precambrian History of the Pacific Mantle Domain: New Constraints from Woodsreef and Port Macquarie Serpentinized Spinel Harzburgites of the New England Orogen, Australia

The present-day mantle is divided into the African and Pacific domains by the circum-Pacific subduction girdle. Very little is known about the mantle composition of the Pacific Domain before 120 Ma due to the scarcity of the oceanic record, having mostly been destroyed by subduction processes. Accreted oceanic lithosphere (ophiolites) in orogens along the Paleo-Pacific margins provide rare opportunities to partially fill this knowledge gap. The early Cambrian (530-505 Ma) Weraerai ophiolite melange in the New England Orogen in Eastern Australia represents fragments of the now-consumed Paleo-Pacific oceanic plate, predecessor of the Panthalassa and Pacific oceanic plates, accreted to east Gondwana during c. 410-277 Ma. Early work revealed the presence of an accreted volcanic island(s) of possible mantle plume origin. However, due to their heavy alteration and weathering, the geochemical signature of the mafic rocks in the Weraerai ophiolite cannot be used to certify their plume origin with confidence. Therefore, mantle rocks found in the ophiolitic belt offer an alternative way to decipher the origin of the oceanic lithospheric fragments in the Weraerai ophiolite melange. Here, we report the petrographic, major and trace elements composition, including highly siderophile elements (HSE), and Re-Os isotope composition for 16 serpentinized spinel harzburgite samples from Woodsreef and Port Macquarie. The observed spinel-orthopyroxene symplectite intergrowth textures are interpreted as garnet-breakdown textures due to cooling from an initial high temperature of >1200 degrees C. Silicon and Al contents and Mg# of serpentinized spinel harzburgite, as well as heavy rare earth element modeling results, suggest a high-degree of melt extraction of 20-30% in the garnet stability field and in an anhydrous environment, probably in ocean island or oceanic plateau environments. The samples in this study have HSE concentrations interpreted to have resulted from post-melting processes rather than a melt extraction feature, indicating that their Re-Os model ages need to be interpreted with caution. Nevertheless, the melt depletion ages obtained from the Woodsreef and Port Macquaries samples range between 1.4 and 1.1 Ga, consistent with previous studies on other Pacific-rim ophiolitic rocks. We argue that these ages might be related to a major depletion event during the transition between supercontinents Nuna and Rodinia. Such depletion events affected a large proportion of this section of the mantle before the incorporation of the peridotites into the oceanic lithosphere in the Paleo-Pacific associated with rapid ascent of mantle plumes. This interpretation is consistent with the occurrence of accreted volcanic islands in the Weraerai ophiolite melange, as shown by the OIB chemical signatures of some of the mafic rocks, and their association with shallow water limestones that formed in near-equatorial latitudes. The proposed c. 530-510 Ma Paleo-Pacific Ocean mantle plume event coincides with a global peak of oceanic mantle plume events that may record the legacy of a circum-Rodinia subduction girdle driving antipodal mantle superplume episodes.

Automated summary

The present-day mantle is divided into the African and Pacific domains. Accreted oceanic lithosphere in orogens along the Paleo-Pacific margins provides opportunities to understand the mantle composition of the Pacific Domain before 120 Ma. The Weraerai ophiolite melange in the New England Orogen represents fragments of the Paleo-Pacific oceanic plate accreted to east Gondwana, and the mantle rocks found in the ophiolitic belt offer information about the origin of these oceanic lithospheric fragments.