Multi‐Stage Evolution of Subducted Oceanic Mantle: The Marlborough Ultramafic Suite (New England Orogen, Australia)

Abstract

Oceanic lithosphere is continuously destroyed by subduction, challenging our ability to develop tectonic reconstructions. One way to tackle this problem is to examine relics of once‐consumed oceanic domains preserved along convergent margins. In the New England Orogen (eastern Australia), disrupted exposures of ultramafic rocks loosely define a continuous trend. The exposures may represent vestiges of ancient oceanic lithosphere that existed along the eastern Gondwanan margin. To investigate the origin and evolution of this ultramafic belt, we obtained new petrological and geochemical data from the Marlborough Ultramafic Suite, the largest ultramafic massif in eastern Australia. It comprises harzburgite, serpentinite, and high‐grade serpentinite. Major and rare earth element systematics resemble residual abyssal peridotites from mid‐ocean ridges. The rocks preserve early lizardite serpentinization and evidence of high‐temperature fluid–rock interaction that took place at or near the ocean floor. High bulk alkali/U ratios indicate a shift into subduction, supported by the formation of prograde antigorite and talc. Magnesian olivine (Fo ₉₅ ) in high‐grade serpentinite is attributed to subduction‐related prograde metamorphism. Based on the petrological signatures, we suggest that abyssal peridotites from the Paleo‐Pacific Ocean were subducted beneath eastern Gondwana. Slivers of the slab were detached and mixed in the subduction channel before being exhumed and accreted onto the accretionary complex. Our results provide new constraints on the evolution of a once‐consumed oceanic domain along the eastern Gondwanan margin, offering insight into the fate of the oceanic lithosphere upon subduction.