COMPLEX PT(t) PATHS DEDUCED FROM HP METABASIC GRANULITES IN THE HAUT ALLIER (french Massif Central) GEODYNAMIC IMPLICATIONS
C. NICOLLET and M. LAHLAFI
(Dpt de Géologie, URA 10, C71ermont Ferrand)
In the European Variscan belt, the allochtonous "complexes leptynoamphibolitiques" (CLA), of continental and oceanic origins, comprise granulitic relics enclosed in IP amphibolite facies rocks. These relics give evidence of an early HP event (step 1 : 1,5-2 GPa ; = 800°C), later equilibrated in the IP granulite facies (step 2 : = l GPa ; 750° C), before retrogression, by hydration and cooling, into the amphibolite facies conditions (step 3 : 0.6 GPa ; 700 °C).These constraints allowed to interpret the Variscan belt as the result of the convergence of Baltica and Africa blocks, after closing of 2 oceanic domain separated by a " median block ". In this model, the HP granulites record the end of oceanic subduction below the median block and the early stage of collision, whereas amphibolitic conditions occured during final exhumation.
Thie circumstantial study by Lasnier (1977) of Ht-Allier granulites, played a great part in the élaboration of this model. We have focused our attention on the mafic granulites described by this author. The HP granulites show remarquable coronas indicating different stages of the uplift. The main isograd réactions are : Cpx+Gt+Q--Opx (and/or Hb)+ PI (step 1 to 2) and Opx+Cpx+PI=Hb (2-->3). Coronitic 0l metagabbros are devoid of the HP assemblage and the polymineral coronas surrounding 0l imply the reaction Ol+Opx+Pl =Hb+Gt and/or Sp (2-->3). In addition to the 3 aforementioned steps, a fourth stage (4) in granulite facies has been documented : indeed, backscattered electron images reveal that the amphibolitic paragenesis is replaced by very fîne-scale symplectites involving the following reactions : Hb+Gt+Q=Opx+Pl and Hb=Opx+Cpx+Pl in the HP granulites and Hb+Gt=Opx+PI+Sp in the 0l metagabbros. Consequently, we propose that the uplift began with a décompression (1-->2) followed by hydration and cooling (low P/T slope of the rétrograde path : 2-->3) ; the transition 3 to 4 implied either (i) an increase of the P/T slope during cooling or (ii) heating while P decreased. Depending on wether (i) or (ii) implied, the following geodynamic interprétation of the Variscan collision may be considered : in case (i), step 4 could result from the underthrust of the Africa block beneath the CLA (=relics of oceanic domain), inducing an efficient screen effect ; in case (ii), step 4 would be contemporaneous of the LP regional metamorphism associated with the post-collisional thinning of the orogen, well known in other parts of the Variscan belt.