Early activity of the largest Cenozoic shield volcano in the circum-Mediterranean area: Mt. Karacadag, SE Turkey

Abstract

Volcanic activity at Mt. Karacadag, SE Turkey, developed between ~11 and ~0.01 Ma. In this paper we investigate the oldest products (older than 2.6 Ma) that created a large volcanic plateau and a N-S aligned volcanic edifice in the form of a shield volcano. These igneous rocks are mildly alkaline to transitional olivine-clinopyroxene phyric basalts with minor hawaiites, basanites and very rare differentiated lithologies (mugearites and benmoreites). The poor correlation of major elements with MgO in these lavas is qualitatively consistent with polybaric depths of magma production, variable degrees of partial melting (from ~2 to ~10 %), heterogeneous mantle sources and differences in the fractionating crystal assemblage. Primitive mantle-normalized patterns resemble typical anorogenic magma compositions, with peaks at HFSE (Nb, Ta, Hf, Zr) and high HFSE/LILE ratios. REE contents are compatible with derivation of the basanites from a mixed garnet-spinel facies peridotite after ~2 % partial melting. Alkali basalts are compatible with higher degrees of melting (between 5 and 10 %) from the same type of source. Initial 87 Sr/ 86 Sr ratios range from 0.70349 to 0.70522 while those of 143 Nd/ 144 Nd range from 0.512853 to 0.512659. Early-stage lavas show higher 87 Sr/ 86 Sr and lower 143 Nd/ 144 Nd compared to plateau-stage lavas. The Sr-Nd isotopic variations and their relation with major and trace elements cannot be explained by AFC-like (Assimilation and Fractional Crystallization) processes involving average crustal lithologies. More likely, the Sr-Nd isotopic ratios are related to the existence of heterogeneous mantle sources with only minor involvement of AFC-like processes. The Cenozoic lavas in a 200 x 800 km area between the Karasu Valley and the Syria-Iraq-Turkey border in south-eastern Anatolia form a distinct igneous province which can be characterised on the basis of Sr-isotope signatures. The lithospheric mantle beneath this area is characterized by anomalously enriched 87 Sr/ 86 Sr compositions (up to 0.7055) as well as more isotopically depleted compositi o ns ( 87 Sr/ 86 Sr down to 0.7030).