Petrogeochemistry and genesis of listwaenite xenoliths in the alkali basalts of the southern Tien Shan

Abstract

Mesozoic alkaline (subalkaline) mafic rocks from the southern Tien Shan contain numerous listwaenite xenoliths (on average, 50–60% of all xenoliths) and less abundant serpentinites, which develop after nodules of mantle spinel lherzolites and, more rarely, pyroxenites. Various models of listwaenite genesis and the problem of a “listwaenite layer” were considered. The listwaenites consist of three heterogeneous mineral (element) assemblages of different ages: (a) mantle ultrabasic protolith (relics of spinels, pyroxenes, and other minerals); (b) products of shallow-depth serpentinization; and (c) minerals (elements) of hydrothermal-metasomatic carbonation (listwaenitization), which occurred under near-surface conditions but was related to mantle fluids (carbon isotope signatures of carbonates, introduction of Sr, its correlation with CO2, etc.). It cannot be ruled out that the CO2 degassing of the mantle occurred not only in the Mesozoic but also in the Cenozoic. This process was accompanied by the formation of occurrences of chalcophile elements (Hg, Sb, Au, Ag, Zn, Cu, and Pb), F, Ba, and Sr. Late Cenozoic celestite deposits deserve special attention in this context. They form the largest Sr province in the Mediterranean mobile belt and were presumably also related to mantle sources.