Decoding the primary magma composition and petrogenesis of the Neoarchean TTG rocks in the eastern North China Craton

Abstract

The primary magma of Archean tonalite-trondhjemite-granodiorite (TTG) is considered to originate from the partial melting of mafic crust. This process is pivotal for deciphering the formation and evolution of the Archean continental crust. However, defining the TTG primary magma is challenging due to the superposition of magmatic evolution and subsequent geological events. Based on detailed field survey and sampling, and subsequent petrological, geochemical and isotopic analyses, as well as extensive data compilation, we recently identified a suite of TT (tonalite-trondhjemite) rocks in the eastern Hebei region. These rocks are indicative of a primary medium-pressure (MP) TT magma composition. LA-ICP-MS zircon U–Pb dating shows that the TT samples emplaced at 2560–2507 Ma and metamorphosed at 2494–2473 Ma. Geochemically, these TT samples exhibit typical characteristics of MP-type TTG, with moderate SiO₂ (56.1–64.7 wt%) and Al₂O₃ contents (15.5–17.5 wt%), elevated Na₂O contents (4.04–5.17 wt%), low MgO contents (1.03–3.36 wt%), insignificant Eu/Eu* anomalies (0.94–1.1), near-chondritic Nb/Ta ratios (14.8–19.4), homogenous Dy/Yb ratios (2.10–2.75), and low Sr/Y (30.1–105) and (La/Yb)_N (7.45–30.1) ratios. Whole-rock Nd and zircon Hf isotopes indicate that their precursors were extracted from the depleted mantle at 2850–2650 Ma. By combining experimental petrology findings, geochemical data with Ti-in-zircon thermometer calculations, we suggest that these samples most likely originated from the partial melting of the mafic lower crust (ca. 10–12 kbar) in a high-temperature and dry to damp environment, with amphibole, plagioclase, and minor garnet as residual minerals. Their compositions have not been significantly influenced by magmatic mixing, contamination, and fractional crystallization (e.g., plagioclase or amphibole). Amphibole barometer calculations indicate that these MP-type TT magmas may have been emplaced at a mid-crustal depth of 20–26 km. Compared with globally reported TTG primary compositions, we propose that these TT samples can serve as archetypes of MP-type TT primary magma formed by the partial melting of mafic lower crust in the late Neoarchean. They could provide valuable insights in further examining the characteristics and evolution of TTG primary magmas in other ancient cratons globally.