Geochronology, petrogenesis and tectonic implications of Late Triassic and Late Jurassic granitoids in the South China Block

The Early Mesozoic tectonic transition from the Palaeo-Tethys to Palaeo-Pacific tectonic regimes in the South China Block has long been a matter of debate. Zircon U-Pb dating, and whole-rock elemental and Sr-Nd-Hf isotoic analyses were conducted for the Early Mesozoic Dengfuxian, Xitian, and Wufengxian plutons to investigate their petrogenesis and the implications for the tectonic transition of the South China Block. These plutons were emplaced in two stages: 1) Late Triassic (227-226 Ma) Dengfuxian and Wufengxian monzogranite and 2) Late Jurassic (152-151 Ma) Dengfuxian and Xitian alkaligranites. Monzogranites are characterized by decrease in P2O5 contents when SiO2 contents increasing, low A/CNK (molar Al2O3/(CaO + Na2O + K2O)) indexes, high K2O contents and Mg# (molar 100 x Mg/(Mg + Fe)) values, and they belong to high-K calc-alkaline to shoshonite I-type granites. They have negative eNd(t) (-10.2 to -9.0) and eHf(t) (-20.6 to -4.4) values with old, two-stage crustal Nd-Hf isotopic model ages (1.40-2.68 Ga) and high zircon saturation temperatures (896 degrees C-861 degrees C), indicating that the monzogranites were derived from the partial melting of Mesoproterozoic meta-basaltic rocks. Alkaligranites are characterised by high A/CNK values, Fe# (FeOT/(FeOT + MgO)) indexes and low Al2O3 and CaO contents, and Mg#, and they belong to A-type granites. They have negative eNd(t) (-7.1 and -5.5) and eHf(t) (-8.5 to -4.7) values with old, two-stage crustal Nd-Hf isotopic model ages (1.20-1.49 Ga), low zircon saturation temperatures (799 degrees C-830 degrees C) and geochemical features, indicating that they were produced by the partial melting of Neoproterozoic meta-sedimentary rocks. Combined with previous results, we suggest that the Late Triassic monzogranites formed in a syn-collision environment related to the collision of the continental plates in the Palaeo-Tethys oceans, whereas the Late Jurassic syenogranites formed in a post-orogenic environment in the South China Block resulting from the rollback of the Palaeo-Pacific Plate. The tectonic transition from the PalaeoTethys to Palaeo-Pacific tectonic regimes in the South China Block could have occurred between the Late Triassic and Early Jurassic.