Lithospheric Mantle Evolution during Continental Break-Up: The West Iberia Non-Volcanic Passive Margin

Abstract

Ultramafic (lherzolites, metasomatized peridotites, harzburgites, websterites and clinopyroxenites) and mafic igneous (basalts, dolerites, diorites and gabbros) rocks exposed at the sea-floor along the West Iberia continental margin represent a rare opportunity to study the transition zone between continental and oceanic lithosphere. The igneous rocks are enriched in LREE, unlike North Atlantic MORB. A correlation between their 143Nd/144Nd isotopic composition and Ce/Yb ratio suggests that they originate from mixing between partial melts of a depleted mantle source similar to DMM and of an enriched mantle source which may reside within the continental lithosphere. Clinopyroxenes and amphiboles in the ultramafic rocks are LREE depleted and have flat HREE patterns with concentrations higher than those of abyssal peridotites. Clinopyroxenes in the harzburgites are less LREE depleted but have lower HREE concentrations. The clinopyroxenes in the Galicia Bank (GB) lherzolites have radiogenic Nd (143Nd/144Nd ranging from 0·512937 to 0·513402) and unradiogenic Sr (87Sr/86Sr ranging from 0·702100 to 0·702311) isotopic ratios similar to, or higher than, DMM (Depleted MORB Mantle) whereas the clinopyroxenes in the Iberia Abyssal Plain websterites have low-Nd isotopic compositions (143Nd/144Nd ranging from 0·512283 to 0·512553) with high-Sr isotopic ratios (87Sr/86Sr ranging from 0·704170 to 0·705919). Amphiboles in Galicia Bank lherzolites and diorites have Nd–Sr isotopic compositions (143Nd/144Nd from 0·512804 to 0·512938 and 87Sr/86Sr from 0·703243 to 0·703887) intermediate between those of the clinopyroxenes from the Galicia Bank and the Iberia Abyssal Plain, but similar to the clinopyroxenes in the 5100 Hill harzburgite (143Nd/144Nd = 0·512865 and 87Sr/86Sr = 0·703591) and to the igneous rocks (143Nd/144Nd ranging from 0·512729 to 0·513121 and 87Sr/86Sr ranging from 0·702255 to 0·705109). The major and trace element compositions of cpx in the Galicia Bank spinel lherzolites provide evidence for large-scale refertilization of the lithospheric upper mantle by MORB-like tholeiitic melts. The associated harzburgites did not undergo partial melting during the rifting stage, but, in earlier times, probably during, or even before, the Hercynian orogeny. Iberia Abyssal Plain websterites are interpreted as high-pressure cumulates formed in the mantle. Their high Sm/Nd ratios (from 0·43 to 0·67) coupled with very low-Nd isotopic compositions are best explained by a two-stage history: formation of the cumulates from the percolation of enriched melts long before the rifting, followed by low-degree partial melting of the pyroxenites, accounting for their LREE depletion. This last event probably occurs during the rifting episode, 122 Myr ago. The isotopic heterogeneities observed in the ultramafic rocks of the Iberia margin were already present at the time of the rifting event. They reflect a long and complex history of depletion and enrichment events in an old part of the mantle, and provide strong arguments for a sub-continental origin of this part of the upper mantle.