Evidence for Miocene subduction beneath the Alboran Sea (Western Mediterranean) from 40Ar/39Ar age dating and the geochemistry of volcanic rocks from holes 977A and 978A
Volcanic pebbles in gravels from Sites 977 and 978 in the Alboran Sea (western Mediterranean) were dated (using the 40Ar/
39Ar single-crystal laser technique) and analyzed for their major- and trace-element compositions (determined by X-ray fluorescence
and inductively coupled plasma-mass spectrometry). The samples range from basalts to rhyolites, and belong to the
tholeiitic, calc-alkaline, and shoshonitic series. Single-crystal and step-heating laser 40Ar/39Ar analyses of plagioclase, sanidine,
biotite, and amphibole phenocrysts from basaltic to rhyolitic samples indicate that eruptions occurred between 6.1 and 12.1 Ma.
The age data conform to the stratigraphy and agree with microfossil ages, when available. The major-element and compatible
trace-element data of samples with H2O < 4 wt% show systematic variations, consistent with fractionation of the observed phenocryst
phases (plagioclase, olivine, clinopyroxene, magnetite, hornblende, quartz, and biotite). The incompatible-element patterns
formed by normalizing to primitive mantle for all samples show spiked patterns with peaks generally at mobile elements
and troughs at immobile elements, in particular Nb and Ta. The calc-alkaline affinities and the incompatible-element systematics
are characteristic of subduction zone volcanism, which indicates that subduction occurred beneath the eastern Alboran from
6 to at least 12 Ma. We propose that the change in chemistry from calc-alkaline and potassic to sodic compositions between 5–
6 Ma reflects detachment of the subducting slab. Uplift of the Strait of Gibraltar, associated with this detachment, could have
caused the Messinian Salinity Crises
