Crust periodic evolution: Evidence from the Taihua complex, southern North China Craton

Abstract

The Taihua complex in the southern North China Craton (NCC), recording muti-episodes continental crust growth events, was divided into the lower and upper units. The lower unit were mainly composed of Neoarchean-Mesoarchean TTG gneisses and amphibolites. Here we report the whole-rock geochemistry, zircon U-Pb dating, trace element and Lu-Hf isotope compositions from the Wugang TTG within the lower unit of the Taihua complex LTC. They are characterized by high-SiO2 (62.2–70.73 wt%), high-Al2O3 (13.23–16.66 wt%), low-Mg# (31–53) and Cr contents (20–60 ppm), HREE and HFSE depletion with low to moderate Sr/Y (12.0–76.3) and (La/Yb)N (5.2–48.3, N denotes normalization by chondrite) in compositions. Magmatic zircons from representative samples, one granodioritic gneiss and one tonalitic gneiss (both are drilled samples), gave a consistent upper intercept ages at 2.85 Ga, reflecting the time of crystallization of the first time TTG pluton within LTC. They have positive zircon εHf(t) values (0.9–7.1) with the TDM1(Hf) model ages ranging from 2.85 Ga to 3.09 Ga with an average of 2.95 Ga, and TDM2(Hf) model ages ranging from 2.85 Ga to 3.23 Ga with an average of 3.01 Ga, their youngest Hf model ages (2.85 Ga) were close to the crystallized ages. In combination with data from Lushan Mesoarchean TTG rocks, they shared similar geochemical features, except for the depleted degree in HREE, indicating that they are generated from similar geologic dynamic setting but in different depths. These TTG rocks were principally derived from a depleted mantle sources, indicating a key juvenile crust accretion in the southern margin of the NCC at 2.85 Ga. They were most likely formed from occasional subduction and partial melting of oceanic plateaus, leaving a garnet-bearing amphibolitic residue at shallow depths (>25 km, Wugang TTG) and deep depths (>50 km, Lushan TTG) with different degree’s fractionation. In associated with previous study, we identified at least four stages of TTG formation, ∼2.85 Ga, ∼2.80 Ga, ∼2.76 Ga and ∼2.72 Ga, from the Southern NCC, which reconfirmed that the Archean continental evolution is episodic and periodic at least in the Taihua complex.