Crustal Metasomatism at the Slab‐Mantle Interface in a Continental Subduction Channel: Geochemical Evidence From Orogenic Peridotite in the Sulu Orogen

Abstract

AbstractTo understand crust‐mantle interactions in a continental subduction channel, we carried out a combined study of whole‐rock elements and Sr‐Nd isotopes, mineral elements, and whole‐rock and mineral O isotopes in orogenic peridotite and its host gneiss from the Sulu orogen in China. The results indicate that the peridotite originated from highly depleted subcontinental lithospheric mantle beneath the North China Craton and underwent three episodes of metasomatism via crustally derived fluids. The peridotite was offscraped from the subcontinental lithospheric mantle wedge base and experienced petrographic transformation from a spinel peridotite to a garnet peridotite during continental subduction from forearc to subarc depths. The peridotite underwent the first episode of metasomatism via aqueous solutions at high‐pressure conditions. Afterward, the peridotite underwent a second episode of metasomatism by carbonate melts at ultrahigh‐pressure conditions and a third episode of metasomatism by aqueous solutions at high‐pressure conditions. While the aqueous solution was derived from the metamorphic dehydration of the subducting/exhuming continental crust at the forearc depths, the carbonate melts were produced by partial melting of the ultrahigh‐pressure metasedimentary rocks in the exhuming continental crust at subarc depths. These metasomatic agents did not originate from the gneiss directly hosting the peridotite but from another part of the deeply subducted continental crust that was not in direct contact with the peridotite. Therefore, the orogenic peridotite recorded the geochemical transfer from the subducting crust into the mantle in the continental subduction zone.