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Quaternary marginal basin volcanism in the Bransfield Strait as a modern analogue of the southern Chilean ophiolites

Abstract The Bransfield Strait is an actively-spreading, 65 km wide, Quaternary marginal basin at the northern tip of the Antarctic Peninsula. Across the Drake Passage from the Bransfield Strait are the Tortuga and Sarmiento ophiolites of southern Chile, which have been interpreted as uplifted, Jurassic-Cretaceous, marginal-basin curst that formed in a setting similar to the present Bransfield Strait. We present a comparison between the Bransfield Strait and the Chilean ophiolites, emphasizing new petrological and geochemical data for volcanic rocks dredged from two seamounts at the Bransfield Strait rift axis and published analyses of volcanic rocks from the marginal basin now preserved as the Chilean ophiolites. Volcanic rocks from the seamounts are about 100 000 years old, and major element abundances in these rocks are broadly similar to enriched mid-ocean ridge basalt (E-MORB). The rocks are slightly enriched in light rare earth elements (LREE) relative to heavy rare earth elements (HREE), as in E-MORB, but they are moderately enriched in alkali, and alkali earth elements, as in arc basalts, and concentrations and ratios of diagnostic trace elements show island arc affinities. Geochemical discrimination diagrams suggest that Bransfield Strait lavas represent the initial stages of a petrogenetic transition linked to the tectonic transition from rifting in an ensialic arc to rifting in a marginal basin. Published rare earth element data from dykes and lavas in the Chilean ophiolites range from mildey depleted in LREE in the Tortuga ophiolite (Ce/Yb = 0.5 to 0.8, normalized to chondrites) to mildly enriched in LREE in the Sarmiento ophiolite (normalized Ce/Yb = 1.6 to 2.5). The latter are strikingly similar to data from the seamounts in the Bransfield Strait both in LREE enrichment (normalized Ce/Yb = 2 to 2.5) and total REE concentrations (20–45 times chondrite). We believe these chemical similarities, as well as other geological similarities pointed out by other authors, suggest that the Bransfield Strait marginal basin is at the same stage of development as was the marginal basin now preserved as the Sarmiento ophiolite.