Eclogite facies relies in metabasites from the Sierra de Guadarrama (Spanish Central System): P-T estimations and implications for the Hercynian evolution

Relies of HP-MT eclogitic assemblages related to the first metamorphic stag e of the Hercynian orogeny in the Sierra de Guadarrama (Spanish Central Sys tem, SCS) are preserved as boudins of pre-Ordovician metabasites enclosed b y felsic gneisses. Textures indicate a multi-stage metamorphic history star ting in the MT eclogite facies (as deduced from the presence of omphacite a nd rutile included in garnet) and continuing through medium to low pressure granulite and retrograde amphibolite-greenschist facies. Thermobarometric calculations in the eclogitic paragenesis yield pressures of similar to 14 kbar for temperatures in the range 725-775 degrees C. Thermobarometry for t he subsequent granulitic stage indicates a significant drop in pressure (P < 10 kbar) for similar temperatures of similar to 750 degrees C. Metabasite s vary from gabbro to leucotonalites showing the typical Fe enrichment of t he tholeiitic series. Chemical characteristics indicate a derivation from l ow-pressure crystallization of tholeiitic melts more enriched than typical MORB compositions. Their original location far from continental margins as evidenced by the absence of ophiolitic material in the area and their assoc iation with platform sediments suggests that eclogitization was related to intracontinental crustal subduction and thickening. The P-T conditions esti mated in the metabasites for the first metamorphic stage are similar to one s deduced for the surrounding metasediments and suggest that the Hercynian crust could have reached a thickness of similar to 70-80 km, which is more than the double the present thickness.