Geology and Petrology of the Intraplate Cenozoic Continental Basalts in the Transcaucasian Intermountain Area (Georgia)

Abstract

ABSTRACT The Transcaucasian intermountain area is part of the Caucasus segment of the Alpine-Mediterranean mountain belt. The continental intraplate basalts of the study area range in age from 6.10 ± 0.20 to 6.40 ± 0.20 Ma. The basalt erupted from monogenetic volcanoes are formed by lava flows and their pyroclastic equivalents. They are generally characterized by low volumes, are predominantly subalkalic with minor alkaline composition. The ultramafic xenoliths have not been identified in the basalts. The basalts may be subdivided into porphyritic and oligophyric groups. Fractional crystallization plays an important role in the petrogenesis of basalts. Almost all the studied samples showed different degrees of fractionation of olivine ± plagioclase ± clinopyroxene. No significant contamination of basalts with upper continental crustal material was confirmed by Rb/Sr and Rb/Ba ratios or by Sr, Nd isotopic and geochemical composition (87Sr/86Sr = 0.703683-0.704531±2; 143Nd/144Nd = 0.512788-0.512848 ±10; 147Sm/144Nd = 0.1036-0.1144 ±2-3). The studied basalts display, compared to heavy rare earth elements (HREE), highly fractionated light rare earth elements (LREE) with La/Yb=9.25-24.00. This makes them similar to ocean island basalts (OIB), which is also evidenced by Ce/Pb, La/Nb, Zr/Nb, Zr/Y ratios. The Dy/Yb-La/Yb and Yb-La/Yb and 87Sr/86Sr-143Nd/144Nd ratios indicating a “mixed” evolution of basalt-forming magmas. The basalt feeding magma chambers of the Transcaucasian inter-mountain area seem to be formed from a mixture of partial melting of Normal-MORB (Mid-Ocean Ridge Basalt) type upper mantle (garnet and spinel lherzolite) and EMII type components with strong ocean island basalts (OIB)-like signature.