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

Abstract

The Orciatico (ORC) and Montecatini Val di Cecina (MVC) volcanics are composed by plagioclase-free lamproitic assemblages, containing abundant phlogopite, sanidine, clinopyroxene, olivine and apatite, as well as minor ilmenite, magnesio-chromite, K-richterite. A few differentiated samples contain thorite, zircon and perierite. The ORC and MVC rocks are characterized by enrichment in silica (56-59%), incompatible elements and radiogenic Sr as well as high Mg-values (72-76), Ni (150-250 ppm) and Cr (350-450 ppm) abundances. By contrast, they show relatively low Al 2 O 3 (10-12%), CaO (3-4%) and Na 2 O (1-1/5%) contents. Based on the chemical composition of both minerals and rocks, the ORC products were crystallized from mantle-derived magmas, which appear to be more primitive than the MVC volcanics. Moreover, the most primitive ORC magmatic rocks probably did not undergo any differentiative processes during their uprise to the surface from the source region. The MVC parental magma was likely derived from the ORC magma by means of Assimilation+Fractional Crystallization (AFC) processes. After its formation and initial crystallization, the MVC parental magma rose in the form of a crystal mush. The origin of the leucocratic veins and ocelli can be explained by rifting of the lower portion of the MVC crystal mush, and squeezing out of the residual melt that migrated into the early formed cracks. This process was probably triggered by the subsequent emplacement to shallow depths of the MVC crystal mush. The presence of winding leucocratic veins and ocelli in the higher portion of the body is probably due to the lower viscosity of this portion of the crystal mush because of its enrichment in volatiles. The most primitive member of the ultrapotassic magmatic suite in South Tuscany (i.e. ORC) nevertheless have high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd isotopic ratios as well as very high incompatible elements contents. It is suggested that magma genesis occurred in a mantle source enriched in incompatible elements and radiogenic isotopes. The proposed mantle source composition was depleted in basaltic components some time before the occurrence of the enrichment. The close similarity between the incompatible element abundance pattern of these rocks and that of the most primitive rocks of the Roman Comagmatic Province and Spanish regions, suggests a common origin for the metasomatizing agents that affected the source regions of the Western Mediterranean alkaline magmatic province