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- Title
Evidence for Modal Metasomatism in the Orogenic Spinel Lherzolite Body from Caussou (Northeastern Pyrenees, France).
- Authors
FABRIÈS, JACQUES; BODINIER, JEAN-LOUIS; DUPUY, CLAUDE; LORAND, JEAN-PIERRE; BENKERROU, CONSTANCE
- Abstract
Metasomatic mineral-bearing and/or trace element-enriched ultramafic assemblages have been reported from very few Alpine-type massifs. The small ultramafic body from Caussou (Ariège, northeastern Pyrenees) compared with other north Pyrenean ultramafic complexes shows distinctive features which are similar to those of modally metasomatized mantle xenoliths found in alkali basalts. It is mainly composed of clinopyroxene-rich spinel lherzolites (cpx/opx ratios ≧1), with subordinate titanian pargasite-rich peridotites, both greatly depleted in orthopyroxene. Moreover the Caussou peridotites differ from other Ariége peridotites in the presence of ilmenite, the abundance of sulfide inclusions in pyroxenes and amphiboles, higher Al, Ca, Na, K, Ti, and lower Mg contents, and enrichment in incompatible trace elements (ITE). Such mineralogical and geochemical features are interpreted as resulting from modal metasomatism produced by influxes of silicate melt into the peridotites. At Caussou, the metasomatic assemblage comprises Ti-pargasite+Ti-bearing clinopyr oxene+ilmenite+Ti-phlogopite+sulphide+fluid, suggesting that K, Ti, Na, ITE (including S, HO CO and possibly Fe and Ca, were introduced by the metasomatizing agent. This metasomatism was probably imposed on an ultramafic association dominated by LREE-depleted peridotites similar to the north Pyrenean spinel lherzolites. These features indicate that, under upper lithospheric mantle conditions, a mafic melt locally infiltrated lherzolites by a grain-boundary percolation process and reacted with the original mineral assemblage. The infiltration of alkali-basaltic liquids into spinel peridotite led to: (1) partial dissolution of orthopyroxene and, locally, spinel; (2) crystallization of clinopyroxene directly from introduced melts; and (3) re-crystallization/equilibration of pre-existing clinopyroxene with these magmatic liquids. In the last stage of the metasomatism, segregation of more fractionated silicate liquids, coexisting with a (CO+HO) fluid phase, may have been responsible for the crystallization of titanian pargasite, possibly by means of hydro-fracturing mechanism. The pervasive modal metasomatism at Caussou was contemporaneous with the segregation of amphibole-bearing dykes in the Lherz-Freychinède bodies (northeast Pyrenees) (101–103 Ma). They represent two manifestations of the same magmatic event in the lithospheric mantle, probably related the Middle Cretaceous alkaline magmatisrn of the Pyrenees.
- Publication
Journal of Petrology, 1989, Vol 30, Issue 1, p199
- ISSN
0022-3530
- Publication type
Article