Mineralogy, geochemistry and Sr-isotopes in orendites from South Tuscany, Italy; constraints on their genesis and evolution

The Orciatico (ORC) and Montecatini Val di Cecina (MVC) volcanic rocks are composed of plagioclase-free lamproitic assemblages with abundant phlogopite, sanidine, clinopyroxene, olivine and apatite and minor ilmenite, magnesiochromite and K-richterite. Both suites of rocks are rich in SiO (sub 2) (56-59%), incompatible elements and radiogenic Sr and have high mg values (72-76), Ni (150-250 ppm) and Cr (350-450 ppm); in contrast, they are low in Al (sub 2) O (sub 3) (10-12%), CaO (3-4%) and Na (sub 2) O (1-1.5%). The relatively primitive ORC rocks probably did not undergo any differentiation during their ascent to the surface; the MVC magma was probably derived from the ORC magma by assimilation and fractional crystallization processes. The ORC rocks have high (super 87) Sr/ (super 86) Sr and low (super 143) Nd/ (super 144) Nd ratios as well as very high contents of incompatible elements. The proposed mantle source was depleted in basaltic elements before the enrichment in incompatible elements and radiogenic isotopes. There are similarities with the rocks of the Roman comagmatic province and those from SE Spain. Results of 40 EPMA silicate analyses are tabulated.