Formation and prolonged preservation of dense arc root cumulates: insights from retrograded eclogite xenoliths in the western Yangtze craton

Abstract

Delamination of dense mafic/ultramafic materials in arc roots has long been considered as the fundamental step in the paradigm of making an andesitic continental crust. However, the complexity in identifying ancient arc roots inherently with repeated modifications, poses a challenge in accurately determining the preservation time of the dense crustal materials and thus the delamination-driven model. Here, we conducted comprehensive petrographic, whole-rock, and mineral geochemical studies on 10 variably retrograded eclogite xenoliths from the ~ 35 Ma crustal-derived felsic porphyry in the Liuhe area, western Yangtze craton. Eclogite facies metamorphism is indicated by the fresh relic consisting of coarse-grained garnet, omphacite and rutile; the retrograde metamorphism is manifested by an additional assemblage of fine-grained diopside, amphibole, plagioclase, and biotite. Whole-rock element contents of the xenoliths generally display correlations with immobile Nb concentrations, suggestive of a dominant control from magmatic processes with negligible effects from the post-magmatic alteration. The protoliths of studied xenoliths are most likely accumulated garnet pyroxenites, where the negative correlation between heavy rare earth elements and Mg# (Mg/(Mg + Fe), atomic ratio), and the absence of positive Eu and Sr anomalies suggest the accumulation of garnet under high-pressure conditions. The parental magmas are inferred to be evolved and hydrous with arc-type trace-element patterns. Combined with studies on regional xenoliths, outcrops and tectonic history, the parental magmas likely record the melting of asthenospheric mantle wedge fluxed by recycled subducted slab in the Neoproterozoic (~ 800 Ma). The prolonged preservation (from ~ 800 Ma to at least 35 Ma) of the accumulated garnet pyroxenites with high densities in the deep continental crust can be ascribed to the support from the underlying refractory lithospheric mantle strengthened by plume head accretions. Therefore, we propose that the density-driven delamination of the arc root materials is more sluggish than previously expected and the longevity of dense crustal materials highlights the caution in understanding the role of arc root delamination in making an andesitic continental crust.