Evidence for Modal Metasomatism in the Orogenic Spinel Lherzolite Body from Caussou (Northeastern Pyrenees, France)

Metasomatic mineral-bearing and/or trace element-enriched ultramaficassemblages have been reported from very few Alpine-type massifs.The small ultramafic body from Caussou (Ari?ge, northeasternPyrenees) compared with other north Pyrenean ultramafic complexesshows distinctive features which are similar to those of modallymetasomatized mantle xenoliths found in alkali basalts. It ismainly composed of clinopyroxene-rich spinel lherzolites (cpx/opxratios 1), with subordinate titanian pargasite-rich peridotites,both greatly depleted in orthopyroxene. Moreover the Caussouperidotites differ from other Ari?ge peridotites in the presenceof ilmenite, the abundance of sulfide inclusions in pyroxenesand amphiboles, higher Al, Ca, Na, K, Ti, and lower Mg contents,and enrichment in incompatible trace elements (ITE). Such mineralogicaland geochemical features are interpreted as resulting from modalmetasomatism produced by influxes of silicate melt into theperidotites. At Caussou, the metasomatic assemblage comprisesTi-pargasite+Ti-bearing clinopyr oxene+ilmenite+Ti-phlogopite+sulphide+fluid,suggesting that K, Ti, Na, ITE (including S, H₂O CO₂ and possiblyFe and Ca, were introduced by the metasomatizing agent. Thismetasomatism was probably imposed on an ultramafic associationdominated by LREE-depleted peridotites similar to the northPyrenean spinel lherzolites. These features indicate that, underupper lithospheric mantle conditions, a mafic melt locally infiltratedlherzolites by a grain-boundary percolation process and reactedwith the original mineral assemblage. The infiltration of alkali-basalticliquids into spinel peridotite led to: (1) partial dissolutionof orthopyroxene and, locally, spinel; (2) crystallization ofclinopyroxene directly from introduced melts; and (3) re-crystallization/equilibrationof pre-existing clinopyroxene with these magmatic liquids. Inthe last stage of the metasomatism, segregation of more fractionatedsilicate liquids, coexisting with a (CO₂+H₂O) fluid phase, mayhave been responsible for the crystallization of titanian pargasite,possibly by means of hydro-fracturing mechanism. The pervasive modal metasomatism at Caussou was contemporaneouswith the segregation of amphibole-bearing dykes in the Lherz-Freychin?debodies (northeast Pyrenees) (101–103 Ma). They representtwo manifestations of the same magmatic event in the lithosphericmantle, probably related the Middle Cretaceous alkaline magmatisrnof the Pyrenees.