MINERAL VARIATIONS FROM MEDITERRANEAN LAMPROITES: MAJOR ELEMENT COMPOSITIONS AND FIRST INDICATIONS FROM TRACE ELEMENTS IN PHLOGOPITES, OLIVINES AND CLINOPYROXENES

Lamproites are the most extreme melting products derived from a multiple metasomatised mantle source. Oligocene to Pleistocene ultrapotassic volcanism in the post-collisional environment of the Mediterranean Region often demonstrates clear lamproite affinity. To illuminate the major puzzling features of lamproitic rocks, samples from the Murcia-Almeria Province (Spain), the Vardar Suture Zone (Serbia) and the Menderes Massif (Turkey) were studied. Major and trace element geochemistry of phlogopite, olivine and clinopyroxene grains was determined using electron microprobe and LA-ICPMS. Samples from Spain are represented by madupitic lamproites from Jumilla and phlogopite lamproites from Fortuna, Serbian samples are covered by olivine-dominated leucite lamproites from Borac and Turkish rocks by phlogopite lamproites from Ilicasu and leucite lamproites from Buçak.

Major element analyses of phlogopites from all localities exhibit Al₂O₃ concentrations below 14 wt. % and evolutionary trends of decreasing Al with increasing Ti and Fe, which are typical for lamproites. Extreme contents of Cr and F indicate the presence of mantle-derived xenocrysts in Serbian and Turkish samples. Trace element data reveal interesting variations among the genetic populations, e.g. for Rb, Sr and Zr, and between the discrete lamproite localities, e.g. for Li, Nb and Ta; furthermore, they allow the distinction of lamproitic phlogopites from those occurring in kimberlites and carbonatites.

Olivines are abundant in Mediterranean lamproitic rocks. They occur as strongly resorbed phenocrysts and mantle xenocrysts. Phenocrysts and xenocryst rims bear inclusions of ultra-refractory Mg-chromite and in some cases of apatite. Contents of Ca are often very low in the xenocrysts, while phenocrysts frequently exhibit high Mg# of more than 94 and NiO concentrations up to 0,8 wt. %. Analyses of trace elements reveal equilibration of relatively primitive olivine with a lamproitic melt rich in incompatible trace elements. Additionally, the received data allow the discrimination of lamproitic olivines from olivines occurring in kimberlites.

Late stage crystallisation of diopside took place ubiquitously. Green-core clinopyroxenes were found in several samples. All of the analysed mineral grains exhibit low Al₂O₃ and TiO₂ concentrations, down to 0,4 wt. % and 0,3 wt. %, respectively, indicating their crystallisation in a lamproitic melt. Trace element analyses display constantly strong enrichment in LREE and distinguish these clinopyroxenes from those in kimberlites and other rock types.

Our study demonstrates that compositional variations of early crystallising minerals such as phlogopite and olivine provide an excellent archive for the discrimination of asthenospheric versus crustal influx into the lithospheric mantle. Moreover, mineral compositions of the early phases in lamproites may support understanding of the melting interactions between a refractory mantle and a heterogeneous lithosphere.

Mantle derived F-phlogopite xenocrysts should be the focus of further research, since they incorporate plenty of trace elements and have the ability to control these at high temperatures and pressures. Therefore, dissolution of mantle xenocrystic phlogopites as well as the dissolution of other refractory mantle generated minerals, e.g. K-richterite, are supposed to represent an integral part of lamproite genesis.