Petrogenesis of two types of gabbro from Neoproterozoic Fuchuan ophiolite complex, Jiangnan Orogen: Implication for a Japan Sea-like back-arc basin

Abstract

The Neoproterozoic Fuchuan ophiolite complex (FCO) represents the lithospheric remnant of the back-arc basin in the southeastern margin of the Yangtze Craton. However, the nature and development of this back-arc system are still confusing. This study focuses on the tholeiitic gabbros in the FCO, which can be divided into enriched and depleted types according to their geochemical characteristics. Enriched gabbros (843 ± 5 Ma) in the FCO are characterized by left-sloping rare earth element (REE) patterns and relatively low ε_Hf(t) (mostly in the range of 4.6–7.8). In contrast, depleted gabbros are slightly younger (838 ± 5 Ma) and isotopically more depleted (ε_Hf(t) mostly in 7.7–11.8) than enriched gabbros, exhibiting flat REE patterns. Based on geochemical variations and numerical modeling, the primary magmas of enriched gabbros were generated by a low degree (~14 %) partial melting of the melt-modified depleted mantle. In contrast, the more depleted isotopic composition and variable Ba/Th and V/Yb ratios of depleted gabbros suggest that their magmas have originated from a higher degree (~14–26 %) partial melting of the fluid-modified residual depleted mantle. Considering that the most ~860–830 Ma magmatism in the eastern Jiangnan Orogen is calc-alkaline, the Neoproterozoic back-arc basin where the FCO formed could be sialic and built on the thinning continental crust, resembling the Japan Sea. The occurrences of tholeiitic gabbros in the FCO thus provide crucial insights into the opening of the back-arc basin in the southeastern margin of the Yangtze Craton. The changes of mantle source nature and metasomatic agent reflect the upwelling of the depleted asthenosphere and oceanward retreading of the continental arc during the back-arc opening. Consequently, the early continental arc in the southeastern Yangtze Craton may have been transformed into a new island arc, resembling the formation of the northeast Honshu arc during the Miocene.