Origin of rare earth element variations in clinopyroxene from plutonic and associated volcanic rocks from the Fouldé basin, Northern Kédougou inlier, Sénégal, West Africa

Abstract: The Paleoproterozoic basin of Foulde´, in the northern part of the Ke´dougou inlier (eastern Se´ne´gal), comprises bothvolcanic and plutonic suites belonging to the same K-rich calc-alkaline series. Plutonic and volcanic rocks are coeval and they showstrong compositional similarities in major and trace elements including rare earths (REE). Clinopyroxene is ubiquitous in the maficrocks and has the same major-element (except CaO) and trace-element (except LREE, Th and U) contents in both suites. Theclinopyroxenefoundinbasaltsandbasalticandesiteshasconvexupwardchondrite-normalizedREEpatterns,withaLREEdepletion[(Ce/Yb) N ¼0.64–0.67)].Clinopyroxenefromgabbroshaschondrite-normalizedREEpatternscharacterizedbyaLREEenrichmentwith(Ce/Yb) N ¼1.86–1.66.Thesedifferencesarerelatedtotheemplacementmodeofthetwomagmaticsuites,i.e.tothecoolingrateof the magma. In volcanic rocks, very rapid cooling allows clinopyroxene to retain its primary magmatic composition. In plutonicrocks the lowercooling rate allows late-crystallization andsubsolidus processes, which modify the early trace-element composition.Themagmaticprocessesaffectingthemonzogabbrosareoftwotypes:(i)reactionbetweenthemainigneousphases(clinopyroxeneand plagioclase) and an interstitial melt crystallizing quartz and K-feldspar, and (ii) exsolutions of amphibole, ilmenite andorthopyroxene in the clinopyroxene. The core of monzogabbros clinopyroxene that shows no exsolution has a higher CaO contentthanthatofclinopyroxeneinthevolcanicrocks.WeproposethatexsolutionofCa-and-LREEdepletedorthopyroxeneintheinnerrimofthe clinopyroxeneinmonzogabbrosinduces anincrease oftheCa content ofitscore andmay explain itscompositionaldifference(Ca and LREE) with the clinopyroxene in volcanic rocks.Key-words: clinopyroxene, in situ trace elements, monzogabbros, basalts, exsolution texture, Paleoproterozoic, Se´ne´gal.