Archean Continental Crust in the Southern North China Craton

Abstract

The Archean-Paleoproterozoic metamorphic basement is well exposed in the southern North China Craton (NCC), and can be divided into three distinct tectonic complexes, naming, the Sushui, Dengfeng and Taihua complexes. Their equivalents probably extend into the Huoqiu and Bengbu areas of Anhui Province in the southeastern margin of the NCC, and the Longshan area of Gansu Province in the southwestern margin. In the last decade, a large number of the Mesoarchean–Neoarchean (ca. 2.85–2.50 Ga) rocks have been widely recognized in those complexes, which provides important insights into the formation and evolution of NCC during this period. Based on the available isotopic data, the Archean continental crust of the southern NCC can be divided into the 2.85–2.70 Ga and ca. 2.50 Ga rock associations. Most of zircons in the 2.85–2.70 Ga rocks that consist predominantly of TTG gneisses have variable but positive e Hf(t) values, and their magmatic zircon domains that are characterized by oscillatory zoning in CL images present mantle-like O isotopes, reflecting a somewhat geochemically heterogeneous but depleted mantle Hf isotope reservoir dominates the source of these rocks. The whole-rock Nd isotopic data show similar geochemical characteristics, confirming juvenile sources for their provenance. Although majority of Chinese Precambrian geologists suggested that the ca. 2.50 Ga tectonothermal events were mainly involved in crustal reworking or partial melting of the early Neoarchean mafic crust formed at 2.85–2.70 Ga, we emphasize that ca. 2.50 Ga is another major period of continental crust growth in the southern NCC as well as the whole NCC based on the following lines of evidence: (1) a variable proportion of metabasaltic rocks with ca. 2.50 Ga zircon U–Pb ages exposed in both the high-grade gneissic complexes and the low-grade granite-greenstone terrenes. (2) the metabasaltic rocks present mixed MORB- and/or arc-like geochemical affinities, suggesting that they were derived from mantle source with minor continental crust contamination. (3) most of the ca. 2.50 Ga magmatic zircons from TTG gneisses, amphibolites and related rocks have positive e Hf(t) values that are similar to those of the contemporaneous depleted mantle, and their Hf model ages of 2.85–2.49 Ga are close to corresponding U–Pb ages, suggesting that these rocks originated from the juvenile crust at ca. 2.50 Ga. (4) the whole-rock Nd isotopic data of the ca. 2.50 Ga rocks show similar geochemical characteristics, confirming juvenile sources for their provenance. In summary, we suggeste that the southern NCC underwent two marked episodes of continental crust growth at 2.85–2.70 Ga and ca. 2.50 Ga during the Archean, and all the complexes in the southern NCC were welded together to form a coherent ancient terrane at the end of the Neoarchean, we named it the “Southern Archean Block (SAB)”. The SAB show an east–west trending belt from Gansu across Shaanxi and Henan into Anhui Province, stretching over 1000 km long, it may has experienced a long geological history which can be traced back to the Hadean. The ca. 2.50 Ga K-rich granitoid rocks were recognized from the southern NCC, which are commonly considered as the proxy of the final stabilization of the block lithosphere.