Neoarchean basement, mantle enrichment and crustal extraction in central Asia: petrogenesis of 2.5 Ga amphibolite and metadiorite in NE China

Abstract

Archean basement in the Central Asian Orogenic Belt (CAOB) is relatively rare, but it has the potential to provide additional information on the processes of lithospheric mantle enrichment and crust extraction processes during the early history of the Earth. We identified Neoarchean amphibolite (2537−2565 Ma) and metadiorite (2481−2539 Ma) in the Biliya area of the Erguna Terrane in the southeast CAOB. The amphibolite is geochemically MORB-like and has a weakly left-leaning REE pattern, and low zircon εHf(t) (−0.7−+6.2), and whole-rock εNd(t) (−1.7–+4.5) and εHf(t) (−1.9) values. Our petrogenetic modeling reveals that the amphibolite is derived from ∼20 % partial melting of the lithospheric mantle in the spinel stability field (∼65 km depth). The metadiorite shows near-zero εNd(t) (−0.5–+3.6) and εHf(t) (+0.5–+1.4) values and is likely derived from partial melting of mafic lower crust. The metadiorite and amphibolite likely formed in an extensional continental arc/back-arc setting and represent the Archean crystalline basement of the microcontinents within the CAOB. Three-staged mantle segregation and crust extraction processes have been proposed: (a) 20 % melt extraction from primitive mantle-like lithospheric mantle, leaving behind a depleted mantle; (b) subduction-related fluid/melt metasomatism of the lithospheric mantle and its partial melting, generating the arc-type enriched mantle and mafic lower crust; and (c) partial remelting of the mafic lower crust produced the Tonalite-trondhjemite-granodiorite (TTG) crust.