Diverse middle Neoarchean granitoids and the delamination of thickened crust in the Western Shandong Terrane, North China Craton

Abstract

Middle Neoarchean granitoids contain crucial clues for understanding crust–mantle interactions and further investigating the mechanism of the compositional transformation of the crust in the Neoarchean. In the Western Shandong Terrane (WST), diverse Neoarchean magmatism is recognized, including ∼2.62 Ga tonalitic gneisses, ∼2.59 Ga granodioritic gneisses and ∼2.61–2.58 Ga monzogranitic gneisses, which generally intruded into ∼2.70 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses. The ∼2.70 Ga TTG gneisses show high SiO2 contents (70.17–72.89 wt%), but low MgO contents (0.58–0.69 wt%) and high (La/Yb)N ratios (30–48) and originated from partial melting of low-K mafic rocks at lower crustal levels. The ∼2.62 Ga tonalitic gneisses exhibit low K2O/Na2O ratios (0.10–0.39), but high (La/Yb)N (7.60–12.7) and Sr/Y (18.8–48) ratios and Mg# (47–57). With high εHf(t2) values ranging from +2.7 to +7.4, these rocks were derived from dehydration melting of subducted oceanic slabs. The ∼2.61–2.58 Ga monzogranitic gneisses show high SiO2 (70.35–75.12 wt%) and K2O (4.35–5.81 wt%) contents but low MgO (0.20–0.77 wt%) contents with positive εHf(t2) values (+2.0 − +6.6), suggesting they were derived from partial melting of juvenile greywackes. The ∼2.59 Ga granodioritic gneisses exhibit high SiO2 (66.34–73.04 wt%), K2O (2.23–4.32 wt%) and MgO (1.39–3.51 wt%) contents with dispersed εHf(t2) values (−0.2 − +5.3), indicating a mixing process between crust- and mantle-sourced magmas. The ∼2.59 Ga granodioritic gneisses show consistent major and trace element compositions (i.e., high Mg#, lack of Eu anomalies and uniform chondrite-normalized REE patterns), and accordingly, their magmatic precursors were derived from partial melting of delaminated lower crustal materials.The lithological assemblages in the WST became increasingly diverse between ∼2.71–2.58 Ga, changing from TTG gneisses to arc-related rock series, and then to delamination-related granodioritic gneisses, crust-derived monzogranitic and trondhjemitic gneisses and mantle-derived tholeiitic basalts and diorites. By integrating these data with a reduction in the continental crustal thickness from ∼2.70 Ga to 2.55 Ga, a subduction–delamination model is proposed in which a long-term lateral accretion beginning at ∼2.7 Ga and persisting through ∼2.68–2.62 Ga led to the delamination of the lower crust at ∼2.61–2.58 Ga, which may have further triggered the transition of the continental crust composition from basaltic to andesitic.