The relationship between alkaline magmatism, lithospheric extension and slab window formation along continental destructive plate margins
Authors: Hole M. J. , Saunders A. D. , Rogers G. , Sykes M. A. 
Published: April 2008

Abstract

Two distinct groups of Late Cenozoic (15-<0.1 Ma), OIB-like basalts are recognized along the Antarctic Peninsula. Small volumes of undersaturated basanites, tephrites, alkali and olivine basalts and ‘within-plate’ tholeiites, which post-date the cessation of subduction as a result of ridge crest-trench interactions by 40 to <10 Ma, are scattered along much of the southern part of the peninsula. At James Ross Island, in the north of the peninsula, alkali basalts, hawaiites and rare mugearites were erupted synchronously with subduction at the South Shetland Islands trench to the west. Both groups of basalts are remarkably similar in terms of their geochemical and isotopic characteristics, although they apparently owe their origin to two distinct combinations of tectonic processes. The south Antarctic Peninsula basalts are causally related to the cessation of subduction, the formation of slab windows and upwelling and decompressional melting of sub-slab asthenosphere. Correlated trace element-isotope systematics demonstrate that the slab window-related basalts exhibit little evidence for interaction with subduction-enriched mantle or continental lithosphere and must have been derived from sub-slab mantle that had not recently been affected by subduction; LILE/HFSE ratios (e.g. Th/Ta 1.0–2.5, Rb/Nb, 0.25–1.5, Ba/Nb 2.5–7.0) and Sr- and Nd-isotope ratios (
87
Sr/
86
Sr 0.7027–0.7035,
143
Nd/
144
Nd 0.51286–0.51296) are well within the range for ocean island basalts. The slab window-related basalts underwent rapid uprise from the mantle to the crust accompanied by limited fractional crystallization.

The syn-subduction alkalic basalts of James Ross Island were generated during slab roll-back, probably related to the formation of the extensive slab window to the south. The James Ross Island Volcanic Group (JRIVG) exhibits evidence of polybaric fractionation including ponding at the base of the crust. Again, there is little evidence for interaction of these asthenospheric magmas with subduction-enriched mantle or continental lithosphere, although in detail, there are recognizable differences between the JRIVG and the slab windowrelated basalts. Melting at James Ross Island was facilitated by slab roll-back and associated lateral and vertical asthenospheric migration into the locus of a pre-existing area of attenuated lithosphere (‘thinspot’). This phase of extensional tectonism was probably originally initiated during the Late Jurassic, but other periods of extension may have taken place in latest Cretaceous times.
Neither of these suites of basalts were generated as a result of significant lithospheric extension and passive asthenospheric upwelling on a regional scale. In addition, there is no evidence for the existence of a mantle hotspot beneath the region. The commonly held assumption that alkaline volcanism along consuming plate margins results from periods of significant inter-arc extensional tectonism is, therefore, not necessarily valid. The unique tectono-magmatic regime resulting from the formation of slab windows is probably the only setting in which small degree melts of MORB source asthenosphere not associated with a plume are generated and preserved in the geological record.