2.85 Ga and 2.73 Ga A-type granites and 2.75 Ga trondhjemite from the Zhongxiang Terrain: Implications for early crustal evolution of the Yangtze Craton, South China

Abstract

The discovery of the Neoarchean Yangpo Group and Jinshan A-type granites in the Zhongxiang Terrain opened a new research window on the early crustal evolution of Yangtze Craton. We present here new zircon dating and detailed geochemical evidences to further reveal the magmatic events and their implication to related tectonic regimes. The combined study of zircon U-Pb ages, Hf isotope compositions, and whole-rock geochemistry were carried out for gneissic granites from the Huachong and Huashanguan complexes. Two groups of granite samples from the Huachong Complex have high SiO2, alkali (K2O + Na2O) and TFeO contents and relatively low Al2O3, CaO, MgO, TiO2 and P2O5 contents; all samples are noticeably peraluminous (A/CNK = 1.21–1.48) with negative anomalies in Nb, Sr, P and Ti, and positive anomalies of HFSE (Zr, Hf, Y) and LILE (Rb, U, Th, La) contents. These samples also show very high zircon saturation temperatures (Tzr (°C) = 910–942 °C), high 104 × Ga/Al (2.62–3.51), and high Zr + Nb + Y + Ce contents (571–767 ppm). Collectively, these observations illustrate typical characteristics of A-type granites. The samples from the Qinchong gneissic granites have high sodic, Sr/Y, Nb/Ta and (La/Yb)N with steep REE patterns, identical to those of TTG (tonalite–trondhjemite–granodiorite). These observations suggest that these rocks were generated by partial melting of juvenile crust materials that left a rutile-bearing eclogite residue. Zircon U–Pb dating for two A-type gneissic granites from the Huachong Complex yields weighted mean 207Pb/206Pb ages of 2856 ± 12 Ma and 2732 ± 18 Ma, and the Qinchong trondhjemite yields a upper intercept age of 2749 ± 46 Ma. Thus, we recognize three types of Archean magmatic events in the Zhongxiang Terrain as follows: ① ca. 2.85 Ga A-type granite, which is the oldest A-type granite currently known in South China; ② ca. 2.75 Ga trondhjemite, which may be resulted from arc-continental collision; ③ ca. 2.73 Ga A-type granite, which may be associated with back-arc extension. Therefore, combined with recent studies conducted in the Zhongxiang Terrain, the ca. 2.65 Ga Jinshan A-type granite in Huji region, these magmatic events suggest that the Yangtze Craton (including major Zhongxiang and Kongling terrains, and other microblocks) might have experienced two tectonic cycles of continental assembly and breakup from the Mesoarchean to Neoarchean era (ca. 2.90–2.85 Ga and ca. 2.75–2.65 Ga).