Rb [sbnd]Sr isotopic evidence in the origin of potash-rich lavas of Western Italy

Abstract

The 87Sr/ 86Sr ratio in the potassic lavas of the Roman province has been found to vary from 0.706 to 0.711, with the ratio rather constant for each volcano, but generally increasing from south to north. The increase in 87Sr/ 86Sr is found to be correlated roughly with total Rb and Sr, both of which reach unusually high levels of concentration. It is concluded that the continental potassic volcanic association in general (leucite-basalt to potash-trachyte), by analogy with this volcanic province, could not have derived its unusually high Sr from carbonatites or the normal types of basalt, or from the assimilation of limestone. The Sr isotopic evidence suggests that these unusual magmas were derived from the refusion or anatexis of ancient continental sialic rocks. The concentrations of Sr and other trace elements suggest that the sialic rocks were deeply depressed into the high pressure regime where garnet and clinopyroxene are dominant residual phases.