The different composition of trace elements in clinopyroxenes of alkaline and calc- alkaline magmas: a case study of Piranshahr pluton, NW of Iran

The clinopyroxene composition of two types of alkaline and calc-alkaline gabbros at Piranshahr pluton was investigated in this study. The major oxides variations are similar in all clinopyroxenes of two types of gabbros and are classified as diopside- augite; but trace element compositions are very different in these two groups. The clinopyroxenes of alkaline gabbros are rich in high field strength elements (HFSE) and rare earth elements (ΣREE= 34- 64 ppm) and have high 87Sr/86Sr ratios. In other side, the clinopyroxenes in calc-alkaline samples have high Sr, Rb and Ba and elevated 87Sr/86Sr; but are depleted in HFSE and rare earth elements (ΣREE= 9.4- 21.6 ppm). Calculation of equilibrated melts from trace elements composition of clinopyroxenes using partition coefficients between clinopyroxene and the basaltic melt show that they are formed by different sources which is consistent with whole-rock geochemical data of gabbros.