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The early Palaeozoic orogen in South China is considered as one of the typical examples of intraplate orogens in the world. However, the nature of the orogen, for example, the onset time and the timing of tectonic transition from syn-orogenic compression to post-orogenic extension, are still controversial. Here, we present new zircon U–Pb age and Hf isotope data, and whole-rock geochemical and Sr–Nd isotopic data for migmatites and potassic mafic rocks in southern Guangdong, from the Cathaysia Block, South China. Migmatites consist of metatexite and diatexite, including various amounts of palaeosome, melanosome, and leucosome. Leucosomes from metatexite and diatexite/migmatite granite samples yield consistent U–Pb ages of 451.5 ± 2.0 Ma to 458 ± 16 Ma, respectively. Zircon U–Pb dating from two potassic plutons yield consistent crystallization ages of 451–446 Ma. Our new results of field investigation, mineralogy and microstructures, geochronology, and geochemical compositions demonstrate that the protoliths of migmatites are metasedimentary rocks, while metatexites and diatexites are the products of variable degrees of partial melting of the protolith. The potassic rocks have SiO2 contents ranging from 52.5 to 60.2 wt.%, high K2O/Na2O ratios (0.7–1.3), with MgO contents of 3.0–6.2 wt.% and Mg number of 42–57. They have enriched Sr–Nd–Hf isotopic compositions with (87Sr/86Sr)i ratios of 0.7085–0.7151, ɛNd(t) values of −9.9 to −6.2, and εHf (t) values of −10.5 to −5.3, indicating that they were generated from an enriched mantle source modified by ancient oceanic subducted sediments. Regional crustal anatexis occurred at about 451 Ma, and the timescale from crustal anatexis to melt homogenization and accumulation spans over an interval of ~15 Myr. The association of migmatites and the potassic rocks may form in post-orogenic extension setting. Thus, we suggest that the tectonic transition from syn-orogenic compression to post-orogenic extension occurred by the Late Ordovician (ca. 451–446 Ma) for the early Palaeozoic orogen in South China. [ABSTRACT FROM AUTHOR]

Abundant evidence points to the Cretaceous crust-mantle interaction and plate subduction in the Gan-Hang Tectonic Belt (GHTB), southeastern China, but the evolutionary process remains poorly constrained. Here we conduct a comprehensive study on Daqiaowu granitic porphyry and diabase dikes in the eastern GHTB, in conjunction with previous studies on simultaneous felsic and mafic rocks along the GHTB, to demonstrate their petrogenesis and geodynamic evolutionary process. The Daqiaowu granitic porphyry (125 Ma), as well as the coeval granitic rocks, exhibits high zircon saturation temperatures, alkalis, 104*Ga/Al ratios, and Zr + Nb + Ce + Y contents, concluding a distinctive belt of the Early Cretaceous (~137-125 Ma) A-type volcanic-intrusive rocks in the GHTB. Their εNd(t) and zircon εHf(t) values gradually increased through time from approximately −9.0 to −1.0 and −10.0 to +4.0, respectively, implying increasing contribution of mantle-derived components to their formation, and hence progressively intensified crust-mantle interaction in an intra-arc rift environment (a geodynamic transition stage from continental arc to back-arc) during the Early Cretaceous. This plausibility is further supported by the Early Cretaceous Daqiaowu diabase dikes and coeval mafic rocks which exhibit arc-like magmatic signatures and were derived from mantle wedge. In contrast, the Late Cretaceous mafic rocks show ocean island basalt-like geochemical characteristics, reflecting a depleted asthenosphere mantle source. This discrepancy of mantle sources concludes that the geodynamic setting in the GHTB may have basically transferred to back-arc regime in the Late Cretaceous. Thus, the Cretaceous geodynamic evolutionary process in the GHTB can be defined as the Early Cretaceous gradually intensified crust-mantle interaction in a geodynamic transition stage (from continental arc to back-arc extension) and the Late Cretaceous back-arc extensional setting. [ABSTRACT FROM AUTHOR]

The extensively exposed late Carboniferous volcanic and volcaniclastic successions along the northern margin of the North Tianshan are called the Arbasay Formation. We present field-based mapping, petrography, zircon cathodoluminescence (CL) images, and U–Pb dates, as well as whole-rock geochemical data for these rocks, in order to constrain their formation age and petrogenesis, and understand the geodynamic setting. Conspicuously, the Arbasay Formation shows typical basalt–andesite–dacite–rhyolite volcanic series, and is dominated by andesites with a small amount of basalts and rhyolites based on the geological profile. U–Pb isotopic dating using the LA-ICP-MS method on zircons reveals that the volcanic rocks in the Arbasay Formation formed at 308–305 Ma, that is, late Carboniferous, rather than early Permian as previously proposed. Geochemically, the volcanic rocks mainly belong to the calc-alkaline series and have arc-like geochemical compositions. They are enriched in LREEs ((La/Yb)N = 2.9–7.5) and LILEs (K, Rb, Ba) and depleted in HFSEs (Nb, Ta, Ti). In the tectonic discrimination diagrams, the basalts mainly fall into the area of continental arc. Given our U–Pb dating results, geochemical characteristics, and the regional geological framework, we propose that the late Carboniferous volcanic rocks originated from the arc-related setting, not the intracontinental rift-related setting. They are possibly the major constituents of a continental arc that is formed with the southward subduction of the North Tianshan Oceanic lithosphere. [ABSTRACT FROM AUTHOR]