Origin of spilitic rocks from the southern slope of the Greater Caucasus

Abstract

A suite of 39 spilitic basalts and andesites of Bajocian age was analyzed for the major elements and Li, Rb, Sr, Ba, Sc, V, Cr, Co, Ni and seven REE, Hf, Ta and Th were analyzed in 20 samples, Sr- and O-isotope ratios have been determined for selected whole-rock samples. The occurrence of biotite and actinolite indicates maximum temperatures of hydrothermal reactions slightly above 400°C. Prehnite, pumpellyite and epidote formed at lower temperatures. Chlorite and sodic plagioclase are abundant products of hydrothermal seawater alteration. A comparison between weakly and intensively altered basalts demonstrates that the largest proportion of Al, V, Cr, Co, Ni and the REE was immobile during spilitization. Enrichment in 18O is correlated with the gain of H 2O, CO 2, Na 2O and Li, which exceeds pre-spilitic levels. Initial 87 Sr 86 Sr ratios for 10 whole-rock samples range from 0.7040 to 0.7049. Relics of fresh clinopyroxene phenocrysts from the spilitic basalts and andesites have initial 87 Sr 86 Sr ratios between 0.7037 and 0.7044 and δ 18O-values between + 4.5 and + 5.6‰ (SMOW). Whole-rock samples have higher 87 Sr 86 Sr init ratios and higher δ 18O-values when compared to their clinopyroxene phenocrysts, which is explained as a result of alteration by seawater. Average δ 18O-values of + 11‰ (SMOW), along with the enrichment of Li, suggest that the major alteration occurred at temperatures below 200°C. The spilitic basalts have higher average Sc, V, Cr, Co and Ni concentrations relative to the spilitic andesites. The abundances of these elements in the spilitic andesites are quite similar to those in typical orogenic andesites. The correlation of Cr and Ni in both the spilitic basalts and the spilitic andesites is explained by various degrees of fractionation of olivine and clinopyroxene. Although Ta and Th may have been mobilized to minor extents, the majority of the analyzed samples has HfTaTh relations which are typical of subduction-related volcanic rocks. Assuming an olivine tholeiite as the parental melt from which the basaltic and andesitic magmas were derived, the average La and Ce concentrations in these rocks require a (metasomatically) enriched peridotite source with 5 times chondritic La and Ce abundances. The coexistence of andesites and basalts at the southern slope of the Greater Caucasus along with the occurrence of ophiolites in the Lesser Caucasus suggests that both could be related to a Middle Jurassic subduction zone in this region.