Revisiting the Lushan-Taihua Complex: New perspectives on the Late Mesoarchean-Early Neoarchean crustal evolution of the southern North China Craton

Abstract

The Taihua Complex in the Lushan area was divided into gneisses series and supracrustal rocks. The former is mainly composed of TTG gneisses and amphibolites, and records the Late Mesoarchean-Early Neoarchean crustal evolution of the southern North China Craton (NCC). Late Mesoarchean amphibolites (>2.84 Ga) are enriched in light rare-earth elements (LREEs) and large-ion-lithophile elements (LILEs) with negative Nb-Ta and Ti anomalies, derived from partial melting of a metasomatic mantle source. Late Mesoarchean TTG gneisses (>2.81 Ga) have high La/Yb and Sr/Y ratios and low MgO, Cr and Ni contents, and are interpreted as resulting from partial melting of a thickened lower crust with residual garnet and amphibole in the source. Early Neoarchean amphibolites (2.80–2.73 Ga) show flat REE patterns but slightly negative Nb-Ta and Ti anomalies due to the fractionation of Ti-rich minerals and have depleted Hf-Nd isotopes, and were derived from large degree partial melting of a depleted mantle source. Most of the Early Neoarchean TTG gneisses show pronounced positive Eu anomalies and have relatively lower trace element concentrations and more depleted Nd-Hf isotopes than the Late Mesoarchean TTG gneisses, resulted from the partial melting of thickened juvenile lower crust with residual garnet and amphibole in the source and subsequent plagioclase accumulation. In addition, some of the Early Neoarchean TTG gneisses have high MgO, Cr and Ni contents, which were likely derived from partial melting of delaminated lower crust or a thickened lower crust interacting with mafic melt.Late Mesoarchean amphibolites and TTG gneisses are restricted in the center of Lushan area, denoting an ancient continental nucleus. In contrast, Early Neoarchean amphibolites and TTG gneisses outcrop in peripheral of the ancient nucleus, exhibiting as an elongated belt along the Dangzehe River, which is evidence for the subduction. Overall, the Lushan amphibolites and TTG gneisses show slightly negative to positive whole-rock εNd(t) and εHf(t) values and zircon εHf(t) values, indicating a dominant crustal accretion with subordinate crustal reworking. Therefore, the continental growth recorded in the Lushan-Taihua Complex was caused by the subduction-accretion process, suggesting a convergent margin in the southern NCC during the Late Mesoarchean and Early Neoarchean.