Volatiles and subduction-recycled lithologies in the petrogenesis of Cenozoic alkaline magmatism in the West Antarctic Rift System

Abstract:

In the West Antarctic Rift System (WARS), the failure of both passive and active models of decompression melting to explain unusually voluminous Cenozoic volcanism has prompted debate about the roles of thermal plume-related melting and ancient subduction-related flux melting. The latter is supported by ~500 Ma of subduction along the paleo-Pacific margin of Gondwana[1], a process capable of generating easily fusible, volatile-rich lithologies and producing the broad seismic low-velocity anomaly imaged beneath the Southern Ocean[2]. We present new geochemical information from submarine lavas in the Ross Sea and subaerial lavas from Franklin Island, Beaufort Island, and Mt. Melbourne in Northern Victoria Land (NVL) supportive of an evolving fluxed mantle source. Lavas exhibit ocean island basalt (OIB)-like trace element signatures and isotopic affinities for the C/FOZO mantle endmember consistent with subduction processing of recycled ocean lithosphere. Lava major-oxide compositions suggest multiple recycled source components, including pyroxenite (associated with older lavas), amphibole-rich metasomes, and volatilized peridotite (associated with the youngest lavas). In-situ analyses of olivine-hosted melt inclusions (MIs) from a subset of host lavas confirm high H₂O and CO₂ ranging up to 2.94 wt % and 4657 ppm, respectively. MIs exhibit OIB-like trace element compositions and Ba/Rb and Rb/Sr consistent with melting in the presence of hydrated, amphibole-bearing lithosphere[3,4]. We interpret these observations as evidence that ongoing tectonomagmatic activity in the WARS is facilitated by melting of subduction-modified mantle generated by 550 – 100 Ma Gondwana subduction. Following radiogenic ingrowth in high-µ (U/Pb) domains, Cenozoic extension triggered decompression melting of easily fusible, hydrated metasomes and volatilized mantle. This multistage magma model attempts to reconcile geochemical observations with increasing geophysical evidence that the seismic anomaly imaged beneath West Antarctica and the Southern Ocean may be in part a compositional structure related to previous active margin tectonics.

[1] Mukasa & Dalziel, 2000, GSA Bulletin, 112(4)

[2] Finn et al., 2005, G³, 6(2)

[3] Furman & Graham, 1999, Lithos, 48

[4] Pilet et al., 2008, Science, 320(5878)