Geochemistry and mineral chemistry of pyroxenite xenoliths and host volcanic alkaline rocks from north west of Marand (NW Iran)
Résumé
The Plio-Quaternary alkaline volcanic rocks from the northwest of Marand (NW Iran) consist of trachy-andesites, trachy-basaltic andesites, leucite-tephrites and tephrites. They display a distinct LILE and LREE enrichment, a HFSE depletion (Ta, Ti, and Nb) and high Ba/Ta and Ba/Nb ratios, which are among the characteristics of subduction-derived magmatic rocks. The investigated clinopyroxenite xenoliths mostly occur within the trachy-andesites and more rarely within the trachy-basaltic andesite rocks. These xenoliths, which have a cumulate texture, are classified into four groups based on their mineralogical and chemical features. Group 1 contains clinopyroxene and amphibole as the main minerals. This group does not indicate a clear enrichment in incompatible trace elements in contrast with the Groups 2 and 4, where Ba, Th and U are enriched. Major element contents of clinopyroxenes and amphiboles of Group 1 xenoliths are similar to those of their counterparts of intermediate volcanic rocks. In addition, their contents of compatible elements such as Cr and Ni (whole rock) are also the same, implying a similar magmatic origin. Group 2 contains clinopyroxene and phlogopite as the main minerals. This group, similarly to potassic and ultrapotassic volcanic rocks, is enriched in LREE compared to HREE and unlike the intermediate volcanic rocks, does not contain amphibole. Their 143Nd/144Nd and 86Sr/87Sr ratios are also different. Given the Cr and Ni contents, the REE pattern shape and the chemical composition of clinopyroxene and phlogopite, it seems that the parental melt of this group is similar to the one of potassic and ultrapotassic volcanic rocks. Group 3 contains clinopyroxene and biotite as the main minerals. The REE pattern of this group, unlike those of Groups 1, 2 and 4, has a relatively flat slope. In addition, the content of compatible elements, such as Cr and Ni, also differ as well as the chemical composition of clinopyroxene and mica, which are also different from those of volcanic rocks. Group 4 clinopyroxenites contain clinopyroxene, phlogopite and amphibole as the main minerals. The LREE-enriched patterns and the Ba, U and Pb enrichment of this group are similar to those of the Group 2. The samples of this group on the other hand differ from those of the Group 2 clinopyroxenites in terms of crystal size, crystallization depth, Cr and Ni contents, and chemical composition of clinopyroxene. The parental melt of this group is similar to that of the intermediate volcanic rocks.