1.57-Ga magmatism in the South Carpathians: Implications for the pre-Alpine basement and evolution of the mantle under the European continent

Chemical and isotopic analyses of amphibolites and gneisses from the oldest lithostratigraphic group in the South Carpathians produce new insights reg arding the chronology and evolution of the mobile Proterozoic crust in cent ral-eastern Europe. These results date an episode of magmatism related to o ceanic crust consumption, reveal a period of continental stabilization, and put constraints on the underlying mantle composition. Amphibolites from th e Cumpana Group have their origin in basalts, chemically similar to island are tholeiites. A whole-rock isochron age of 1.57 +/- 0.09 Ga records the o ldest magmatic event found in the Carpathians. It corresponds to the develo pment of a primitive island are and reflects the most recent isotopic equil ibrium, achieved during magmatic crystallization and solidification in pre- Grenvillian time. The depleted source, implied by an initial value of epsil on(Nd) = +6.4 at 1.57 Ga, is comparable to that of the 0.8 Ga Dragsan metat uffs, located approximately 100 km to the southwest of the Cumpana Group. I t is assumed that a Dalslandian event could have isolated these structures in the Precambrian. A period of almost 800 m.yr. accounts for the homogenei ty of a dynamic and depleted mantle, which was probably enriched around 800 Ma, precluding a stable pre-Variscan central-eastern European continent. T he Sm-Nd mineral isochron ages, between 323 and 358 Ma, show Variscan recry stallization ages of the metatholeiites from the Cumpana Group. Their prese nt whole-rock Sr-87/Sr-86 values vary widely between 0.7039 and 0.7079 with out any isochron trend, suggesting Rb mobilization. The Rb-Sr mineral isoch rons show partial isotopic equilibration during younger tectonothermal even ts, probably reflecting the Alpine orogeny. The K-Ar mineral ages of amphib olites are also Variscan, with some exceptions due to Ar loss caused by ear ly Alpine events. We therefore recommend a reevaluation of the Precambrian cores from the Carpathians on the basis of Nd radiometric methods, which ar e able to overcome the problems of isotopic partial reequilibration during multiple metamorphisms of the original rocks.