Delineation of the ca. 2.7 Ga TTG gneisses in the Zanhuang Complex, North China Craton and its geological implications

Through detailed studies we have delineated a suite of banded TTG gneisses from the Zanhuang Complex. The protolith of the gneisses, predominantly tonalite, has undergone intensive metamorphism, deformation and anatexis and in a banded structure is intimately associated with melanocratic dioritic gneiss and leucocratic trondhjemitic veins. SHRIMP Zircon U–Pb data show that the tonalite was formed ca. 2692 ± 12 Ma ago. The tonalitic gneiss has the features of high SiO2 (67.76–73.31%), high Al2O3 (14.38–15.83%), rich in Na2O (4.48–5.07%) and poor in K2O (0.77–1.93%). The gneiss is strongly fractioned in REE ((La/Yb)N = 12.02–24.65) and shows a weak positive Eu anomaly (Eu/Eu* = 1.05–1.64). It has high contents of Ba (199–588 ppm) and Sr (200–408 ppm), low contents of Yb (0.32–1.00 ppm) and Y (3.41–10.3 ppm) with high Sr/Y ratios (21.77–96.77) and depletion in HFSE Nb, Ta and Ti. These characteristics are similar to those of the high-Si adakitic rocks. The melanocratic dioritic gneiss has low SiO2 (59.81%), high MgO (6.34%), high Al2O3 (14.02%) contents, rich in Na2O (3.7%) and poor in K2O (1.79%), with high Mg index (Mg# = 67). REE and trace elements are on the whole similar to that of the tonalitic gneiss, but compatible element abundances V (116 ppm), Cr (249 ppm), Co (37 ppm) and Ni (179 ppm) are higher. The leucocratic felsic bands (approximating trondhjemite in composition) have major oxides similar to that of the TTG gneisses but the REE and compatible elements are extremely low, which are indicative of the products of anatexis. The tonalitic gneiss has positive εNd(t) (2.37–3.29) and low initial Sr (0.69719–0.70068) values with depleted mantle Nd model age of ca. 2.8 Ga, suggesting its generation from partial melting of mantle-derived juvenile crust. The dioritic gneiss was also derived from subduction environment, but has undergone significant metasomatism of mantle wedge. The delineation of the ca. 2.7 Ga TTG gneisses in the Zanhuang Complex further proves that the North China Craton experienced large-scale continental crustal accretion in early Neoarchean, and gives new constraints on the subdivision of the early blocks and greenstone belts of the craton.

► Occurrence of the ca. 2.7 Ga TTG gneiss of large area in the Zanhuang Complex. ► The TTG formed by partial melting of the juvenile crust from the depleted mantle. ► The Zanhuang Complex experienced a voluminous continental accretion at ca. 2.7 Ga. ► A granite–greenstone belt of ca. 2.7 Ga may be present in the central NCC.