Hu and 7 co-authors reported in 2019 that 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 ~2.62Ga tonalitic gneisses, ~2.59Ga granodioritic gneisses and ~2.61–2.58Ga monzogranitic gneisses, which generally intruded into ~2.70Ga tonalite trondhjemite–granodiorite (TTG) gneisses. The ~2.70Ga TTG gneisses show high SiO2 contents (70.17–72.89 wt%), but low MgO contents (0.58–0.69wt%) and high (La/Yb)N ratios (30–48) and originated from partial melting of low-K mafic rocks at lower crustal levels. The ~2.62Ga 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 ~2.61–2.58Ga monzogranitic gneisses show high SiO2 (70.35–75.12wt%) and K2O (4.35–5.81wt%) contents but low MgO (0.20–0.77wt%) contents with positive εHf(t2) values (+2.0−+6.6), suggesting they were derived from partial melting of juvenile greywackes. The ~2.59Ga granodioritic gneisses exhibit high SiO2 (66.34–73.04 wt%), K2O (2.23–4.32wt%) and MgO (1.39–3.51wt%) contents with dispersed εHf(t2) values (−0.2−+5.3), indicating a mixing process between crust- and mantle-sourced magmas. The ~2.59Ga 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 ~2.70Ga to 2.55 Ga, a subduction–delamination model is proposed in which a long-term lateral accretion beginning at ~2.7Ga and persisting through ~2.68–2.62Ga 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.
The authors concluded that:
(1) The middle Neoarchean intrusive granitoids in the WST are mainly composed of ~2620 Ma tonalitic gneisses, ~2591Ma high-Mg granodioritic gneisses and ~2613–2581 Ma monzogranitic gneisses. They generally intruded into ~2681–2692 Ga TTG gneisses.
(2) The early Neoarchean TTG gneisses originated from the partial melting of low-K mafic rocks in the lower crust. The middle Neoarchean tonalitic gneisses were derived from the dehydration melting of descending slabs. The magmatic precursor of themiddle Neoarchean monzogranitic gneisses originated from juvenile greywackes in the stability region of plagioclase. The pristine magma of the middle Neoarchean high-Mg granodioritic gneisses was derived from delaminated lower crustal materials that melted in the subcrustal mantle.
(3) Three episodes of geological events have been identified at ~2.71–2.68 Ga, ~2.68–2.62 Ga and ~2.61–2.58 Ga. The first episode was characterized by extensive crust-derived TTG gneisses. The second episode produced arc-related calc-alkaline volcanic or intrusive rocks. The rock assemblages in the third episode exhibit diversity, consisting of abundant delaminated granodioritic gneisses, crust-derived monzogranitic gneisses and trondhjemitic gneisses and contemporaneous mantle-derived
tholeiitic basalts and diorites.
(4) A middle Neoarchean subduction–delamination model is proposed in the WST, in which a long-term lateral accretion from ~2.7 Ga may have caused the lithosphere to thicken, lose stability and finally break off during ~2.61–2.58 Ga, leading to mantle uplift and intense crust–mantle interactions, forming the diverse granitoids.
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