Abstract:

Following a brief introduction to the Archean basement of the North China Craton (NCC), this paper summarizes the age distribution pattern, geochemistry and Nd-Hf-O isotopic compositions of late Neoarchean (mainly 2.55-2.5 Ga) magmatic rocks in the NCC. The late Neoarchean basement has the following features: 1) Late Neoarchean rocks are widespread while late Mesoarchean-early Neoarchean strata occur in many areas. 2) The magmatic zircon ages are mainly between 2.55-2.5 Ga and has a peak around 2.52 Ga. 3) Compared with pre-early Neoarchean (> 2.6 Ga) TTG (tonalite, trondhjemite, granodiorite), late Neoarchean tonalite and granodiorite are much more abundant, so are potassic granite, diorite-gabbro and sanukitoid with increased distribution range and scale. Voluminous potassic granites are mainly distributed in the east, forming a potassic granite belt-one of the double magmatic rock belts of the late Neoarchean (another is a TTG belt in the west). 4) Late Neoarchean supracrustal rocks are present in almost every area of basement outcrop, but occurring on a small scale when associated with TTG and potassic granite. The supracrustal rock types include metabasalt, meta-andesite, metadacite and clastic metasedimentary rocks, with meta-ultramafic rocks occurring in some areas. 5) In general, geological evolution of the late Neoarchean basement rocks began with supracrustal rock formation, then TTG intrusion, followed by metamorphism, deformation and emplacement of crust-derived potassic granite. The 2.6-2.55 Ga interval was a “quiet period” of magmatic and tectonothermal activity. 6) The late Neoarchean TTG rocks show large variations of Sr/Y and La/Yb ratios, consistent with medium-high formation pressures; their high abundances indicate significant continental crustal thickening in the late Neoarchean. Potassic granites were mostly derived from re-working of continental crust, with some, at least, involving sedimentary rocks. 7) All TTG rock types have similar whole-rock Nd and magmatic zircon Hf isotopic compositions, and the Nd-Hf isotope depleted mantle model ages are mainly between 3.0-2.5 Ga, similar to or slightly younger than that of late Mesoarchean-early Neoarchean rocks. The Nd-Hf isotopic composition of potassic granites is mostly constrained by the formation and evolutionary history of the source region on early Earth. Magmatic zircon has similar but more variable O isotopic composition compared to Archean magmatic zircon worldwide. Combined with results from other studies we arrive at the following conclusions: i) Similar to many other cratons, the late Mesoarchean-early Neoarchean was the most important period of rapid production of continental crust in the North China Craton, however, the NCC underwent strong magmatic and tectonothermal modification during the late Neoarchean. ii) “Modern-style” plate tectonic regimes began to form in the late Neoarchean in the NCC. iii) BIFs (banded iron formations) are mostly distributed along the double magmatic rock belts along the western margin of the Eastern Ancient Block of the NCC, thus the important exploration targets for BIF-hosted Fe resource should be between Anshan-Benxi and eastern Hebei and between eastern Hebei and western Shandong. iv) The initial cratonization of the NCC had completed by the end of the late Neoarchean.

Key words: North China Craton, late Neoarchean, magmatism, Nd-Hf-O isotopes, tectonic regime