Subduction-related subcontinental lithospheric mantle metasomatism and crustal thickening: origin for superchondritic Nb/Ta in mafic dykes

Abstract

Superchondritic Nb/Ta is rarely reported in terrestrial reservoirs and is usually attributed to carbonatite metasomatism or accessory rutile in the residue phase. Previously documented high Nb/Ta in rocks derived from subcontinental lithospheric mantle indicated a predominance of carbonatite metasomatism. This study evaluates Nb/Ta in conjunction with other trace elements of Neoproterozoic mafic dykes exposed in the eastern segment of the Jiangnan Orogen, where early subduction existed before the amalgamation of South China. These mafic dykes show mostly superchondritic Nb/Ta ratios from 19.6 to 24.5. Partial melting modelling suggested low-degree melting of rutile-bearing subcontinental lithospheric mantle for these mafic dykes. A literature review of Neoproterozoic mafic–intermediate rocks throughout the Jiangnan Orogen shows sporadically but coincidently superchondritic Nb/Ta near or beneath the Shuangxiwu arc, indicating rutile stability in the relict sub-arc mantle. Rutile in the lherzolite was formed sometime after Neoproterozoic subduction initiation in South China but contemporaneous with crustal thickening at c. 860 Ma. This study brings direct evidence to bear on the mechanism of rutile formation in the mantle wedge, as well as the link between crustal thickening and superchondritic Nb/Ta of mafic products derived from the metasomatized mantle.Supplementary material: Major and trace element compositions, photomicrographs of samples, and figures illustrating geochemistry, REE and incompatible trace element patterns and loss on ignition versus Nb/Ta and La/Yb are available at https://doi.org/10.6084/m9.figshare.c.5093535