The early Paleozoic intracontinental orogeny in South China: A far-field response to the closure of the Proto-Tethys Ocean in the Indochina Block

The nature and geodynamics of the early Paleozoic orogen in South China are modeled as either products of direct subduction-collision or intracontinental orogeny. The synchronous basaltic-andesitic rocks within the orogen are at the heart of this scientific debate, but they have been less systematically investigated. The petrogenesis and tectonic setting of these mafic rocks are highly controversial, given their arc-like geochemical characteristics. We systematically investigated the patterns of field relations, ages, chemistry, and Hf-Nd isotopes of the early Paleozoic basaltic-andesitic rocks in the Yunkai massif and those reported elsewhere in the orogen. Based on chemical and isotopic signatures, these basaltic-andesitic rocks (ca. 460−420 Ma) can be divided into calc-alkaline light rare earth element (LREE)-enriched basalts, LREE-enriched andesitic rocks, and tholeiitic LREE-depleted basalts. The calc-alkaline suites mostly display arc-type geochemical features, with enrichment in large-ion lithophile elements, LREEs, and Pb, but depletion in high field strength elements relative to heavy rare earth elements. Their high La/Nb and Ba/Nb ratios, and low Nb/Th ratios, along with negative εHf(t) and εNd(t) isotope values, indicate the distinctive fractionation of incompatible trace elements that may have been derived from the partial melting of ancient metasomatized lithospheric mantle sources. On the other hand, the tholeiitic basalts show normal mid-oceanic-ridge basalt-like rare earth element patterns and positive zircon Hf isotope compositions, which reflect the partial melting of a depleted asthenosphere. Given the “intraplate-type” sedimentary, tectonic deformation, and metamorphic signatures across South China, we conclude that these planar-distributed basaltic-andesitic rocks were associated with the activation of the lithospheric mantle and asthenosphere, which occurred in a postorogenic tectonic environment rather than a subduction-related arc regime. Therefore, all geological facts and data patterns favor the Wuyi-Yunkai orogen as a typical intracontinental orogen. Moreover, the Indochina and South China blocks display comparable early Paleozoic faunas and sedimentary records, while the early Paleozoic Tam Ky−Phuoc Son Suture has been identified in Central Indochina. The timing of orogenesis in South China (ca. 460−440 Ma) overlaps with the pre-450 Ma accretionary orogeny in the Central Indochina Block. Therefore, we propose a geodynamic model of linked South China and Indochina blocks in the northern margin of Gondwana, with the geodynamic stress of the Wuyi-Yunkai orogeny derived from a subduction-collision event in Central Indochina. The Wuyi-Yunkai orogen is thus an intracontinental orogen resulting from the far-field effects of plate tectonics.