تحليل ژئوشيميايي سنگ هاي بازالتي منطقة لاهرود

Abstract: Compositional analyses of materials from prehistoric contexts have become an important component of most scientifically oriented archaeological projects. Characterizing the elemental signature of materials (obsidian, metals, stones) helps determine provenance and technological production techniques, and therefore is useful for reconstructing trade and interaction between people prehistorically. Simple petrographic descriptions can provide useful information on raw material provenance, but for some lithic materials more sophisticated techniques are required. Geochemistry can provide this sophisticated tool, trace and interpret the bedrock or the source of the ancient lithic materials. Geochemists have determined the geologic, magmatic and/or tectonic affinities of igneous rocks for decades. This project reports on mineral assemblages and whole-rock geochemistry that mafic rocks used to make lithic artifacts and especially building stones in the Ardebil province, northwestern Iran, have two bedrock sources. Searching the sources and bedrocks of the building stones is mainly based on the mineralogy and whole-rock geochemistry. Basalts, andesites, trachyandesites and latites with shoshonitic affinity are the main volcanic rocks of Eocene age in the Lahrud region (Ardabil). Plagioclase, K-feldspar, biotite and amphibole associated with clinopyroxne are the main constituents of trachyandesites (shoshonites) while clinopyroxene, plagioclase and biotite are the rock-forming minerals of basalts (absarokites). The Lahrud volcanic rocks show enrichment in LREE and characterized by enrichment in LILE and depletion in HFSE. Petrographical observations along with geochemistry of rare earth and trace elements of these lavas suggest shoshonitic affinity and derivation from a subduction zone. The geochemical behavior of the Lahrud lavas reveals the role of the trench sediments in the source region and genesis of these volcanic rocks. These lavas exhibit low degree of partial melting from a garnet-spinel lherzolite source. The comparison between the Lahrud volcanic rocks with Plio-Quaternary lavas of Sabalan and Eocene lavas of Hashtjin region highlights the different mantle source and degree of partial melting for the genesis of these volcanic rocks. The formation of these lavas is linked to slab steepening and breakoff in Eastern Anatolian collision zone. Keywords: Lahrud volcanic rocks, HFSE depletion, LREE enrichment, Shoshonite, Subduction zone.