Lu-Hf Isotope System in Zircons as an Indicator of Genesis of High-Alumina Rocks from Metamorphic Complexes

摘要

High-alumina rocks occur in most metamorphic complexes and are characterized by the presence of sillimaite, kyanite, or staurolite. They are usually considered as resulting from metamorphism of sedimentary rocks containing a significant share of the clayey component, although petrological and geochemical data demonstrate that some of them are metasomatic in origin being formed on account of acidic leaching under widely variable P-T parameters. Nevertheless, the genesis of these rocks remains debatable in most cases and requires additional criteria for its interpretation. The isotopic-geochemical properties, such as, for example, the specific behavior of the Lu-Hf isotope system in sedimentary rocks, may serve as such a criterion. In endogenic rocks, this system behaves similarly to the Sm-Nd isotope system, while its behavior in the sedimentary process is principally different. Terrigenous rocks are characterized by a low ~176Lu/~177Hf value (＜0.015) owing to the presence of Hf concentrating minerals such as, for example, detrital zircons. On the contrary, clayey rocks exhibit an extremely high ~176Lu/~177Hf value (＞0.1) due to sorp-tion of heavy REE by clayey minerals. In clays from oceanic pelagic sediments entirely free of the terrigenous component, this ratio is maximal (＞1).