Continental crust recycling in ancient oceanic subduction zone: Geochemical insights from arc basaltic to andesitic rocks and paleo-trench sediments in southern Tibet

Abstract

The recycling of continental crust via trench sediment subduction and subduction erosion is well established in modern oceanic subduction zones, and this is fundamental to subduction zone magmatism and mantle heterogeneity. Although a similar process likely took place in ancient oceanic subduction zones, its evaluation is difficult because most geological records were erased by subsequent continental collision and post-collisional reworking processes. To address this issue, a comprehensive geochemical study was conducted for Mesozoic basaltic to andesitic plutonic rocks and surviving contemporaneous paleo-trench sediments from the Himalayan orogen in southern Tibet. These plutonic rocks show variably enriched Sr-Nd-Pb-Hf isotope compositions with εNd(t) values varying from −5.6 to +4.7, which are neither positively nor negatively correlated with Mg# values. This indicates that they were not affected by crustal contamination but rather inherited from heterogeneous magma sources. There are consistently continuous variations in the Sr-Nd-Pb-Hf isotope space among the paleo-trench sediments, the arc plutonic rocks, and basaltic rocks from the Yarlung Tsangpo ophiolite suites, indicating that the binary mixing between the paleo-trench sediment-derived melts and the ambient mantle dictated the radiogenic isotope compositions of the arc igneous rocks. These qualitative interpretations are further verified by quantitative geochemical examination of the compositional links in Nd and Hf isotopes between the paleo-trench sediments and arc rocks. As a consequence, the present study has for the first time identified and systematically demonstrated the key role of paleo-trench sediments in constraining the continental crust recycling as well as the origin and compositional heterogeneity of arc igneous rocks above ancient oceanic subduction zones.