Recycling of granulitic lower crust into the mantle

Abstract

Foundering of eclogitic lower crust into the convecting mantle is a well-recognized process invoked to explain the relatively evolved chemical composition of Earth's continental crust. However, it remains unclear the extent to which the foundering could control the newly-made lowermost crust, and whether mafic granulite in a thickened eclogitic-granulitic lower continental section is effectively removable. Here we report a geochemical study of zoned clinopyroxene phenocrysts in Early Cretaceous high-Mg, Eu-depleted andesites (adakite-like intermediate lavas with high La/Yb and Sr/Y ratios and low heavy rare-earth element and Y contents) from the eastern North China craton (NCC), which suggests crust-to-mantle recycling of plagioclase-rich lithologies. Three types of zoned clinopyroxenes are recognized. The type I and II zoned clinopyroxenes show normal- and reverse-zonation, respectively, and both are featured by clear negative Eu anomalies in their cores. The type III zoned clinopyroxene is poikilitic with a high-Mg# core but no Eu anomaly, and enclosing low-Mg#, partially resorbed olivine grain. We therefore propose that these high-Mg andesites were derived from the partial melting of recycled plagioclase- and garnet-bearing granulitic lower crust, which have interacted with refractory peridotites at spinel-facies depth. Owning to mafic plagioclase- and garnet-bearing granulite with density higher and viscosity lower than refractory peridotite at the same temperature, the subcontinental granulitic lower crust could be preferentially recycled into the mantle through delamination. This correlates well with the continental Moho depth, as lower crustal xenolithes and geophysical studies in the eastern NCC revealed the existence of a granulite-facies rather than eclogite-facies lowermost crust residue (present-day ~30-km-thickness) with a pronounced Eu depletion (Eu/Eu* = ~0.8). Consequently, our preferred model has important implications for the compositional evolution of the Earth's continental crust, and offers new constraints on the scale and timing of lithosphere removal beneath the eastern NCC.