| Popis: |
Evolution of the late Ordovician–Silurian Caledonian orogeny (ca. 460–420 Ma) in the South China Block (SCB) remains controversial due to the overprinting Triassic (Indosinian) and Jurassic-Cretaceous (Yanshanian) orogenic tectonics and the absence of enough petrologic records. This work reported a systematic study of whole-rock geochemistry, zircon U–Pb geochronology and Hf isotopes of the first Caledonian A-type granitoids (Dishui) in Jinxiu county, Guangxi (SW China). The Dishui granitoids are tectonically located in the southwestern Qin-Hang suture belt, which is the largest ancient orogenic belt formed by the collision of the Yangtze and Cathaysia blocks in South China. The Dishui granitoids consist primarily of a syenogranite pluton with a small-scale granodiorite porphyry dike. The Dishui syenogranite and granodiorite porphyry have LA-ICP-MS zircon U–Pb ages of 437.9 ± 1.0 Ma (MSWD = 0.77) and 436.6 ± 0.8 Ma (MSWD = 1.7), respectively. The syenogranite has high contents of SiO2 (73.20–77.35 wt.%), total alkalis (Na2O + K2O = 7.42–8.99 wt.%), total REE (198–445 ppm), high field strength elements (HFSEs: Zr + Nb + Ce + Y = 253–520 ppm), and 10,000∗Ga/Al (2.93–3.11) and FeOt/(FeOt + MgO) (0.77–0.96) ratios and high F concentration, resembling highly-fractionated A-type granite. The granodiorite porphyry displays similar A-type granite affinities, including high (Zr + Nb + Ce + Y) (410–485 ppm), 10,000∗Ga/Al (2.47–4.59), and zircon saturation temperature (TZr = 908–927 °C), medium to high SiO2 (64.56–64.95 wt.%), Fe2O3 (7.70–8.11 wt.%), and MgO (3.69–4.09 wt.%), and low FeOt/(FeOt + MgO) (0.61–0.64), resembling magnesian unfractionated A-type granitoid. The Dishui syenogranite and granodiorite porphyry dike have zircon εHf(t) value and two-stage model age (TDM2) of −3.9 to +1.5 and 1.32–1.66 Ga, and −1.7 to +11.8 and 0.94–1.53 Ga, respectively, both much more depleted than many Caledonian gneissic/massive granites and the Devonian A-type granites (εHf(t) = −14.9 to −3.5) in South China. All these data suggest that the Dishui syenogranite was derived from partial melting of the Proterozoic metamorphic basement with mantle-derived melt input, and the magma subsequently underwent extensive fractionation. Meanwhile, the Dishui granodiorite porphyry was likely derived from the same crustal melt with more mantle-derived input. New ages of the Dishui A-type granitoids (ca. 436–437 Ma) in the Qin-Hang belt indicate a tectonic transition from syn-collisional crustal thickening to post-collisional extension at ∼437 Ma in South China. |
