PETROGENETIC EVOLUTION OF OROGENIC LHERZOLITE MASSIFS IN THE CENTRAL AND WESTERN PYRENEES

A petrological and geothermobarometric study of several tens of repres entative spinel peridotite and co-existing pyroxenite samples indicate s that orogenic lherzolite massifs of the central-western Pyrenees (CW P) display significant differences from those of the eastern Pyrenees (EP). They are characterized by a predominance of clinopyroxene-rich s pinel lherzolites, an abundance of coarse-granular textures, the absen ce of thick harzburgitic bands, lower proportions of pyroxenites, the absence of high-pressure amphibole-pyroxenite dykes, and a greater dev elopment of high-stress deformation textures which define 100m-scale m ylonitic shear zones. The highly fertile compositions of the CWP perid otites can be accounted for by accretion of asthenospheric protoliths to the lithosphere via passive cooling or very limited degrees of adia batic melting. This accretion event was followed by a thermal relaxati on, resulting in the steady-state equilibrium stage (1050 degrees C an d 15-18 kbar) recorded by the garnet pyroxenites and granular lherzoli tes. Compared to the single decompression and cooling step identified in the EP Iherzolite massifs, the CWP massifs record a two-step, non-a diabatic uplift. The earliest event is a nearly isothermal (1050-950 d egrees C) decompression from 60 lan up to 25 km depth caused by a lith ospheric thinning event, possibly related to the late Hercynian extens ion. The further uplift step from 25 lan to 15 lan depth was accompani ed by cooling down to 600 degrees C and high-stress mylonitic deformat ion, resulting in tectonic denudation and emplacement of the western P yrenean massifs onto the floor of small Albo-Aptian pull-apart basins via large-scale shear zones. Geospeedometric considerations yield esti mates around 5-10 Ma for the duration of cooling and mylonitic deforma tion, coeval with the mid-Cretaceous anticlockwise motion of the Iberi an and European plates. So, the western Pyrenean peridotite massifs le ft the mantle between 109 and 117 Ma and were uplifted to shallower le vels (15 km) than the EP massifs. (C) 1998 Elsevier Science B.V. All r ights reserved.