Magmatic-tectonic response of the South China Craton to the Paleo-Pacific subduction during the Triassic: a new viewpoint based on Well NK-1

The Nansha Block (NB) is one of the blocks separated from the southern margin of the South China Craton (SCC) by the western Pacific subduction, which contains rich information of geodynamic and tectonic transformation. To reveal the essence of western Paleo-Pacific subduction during the Triassic period, Well NK-1 in this block was selected for petrographic study, and published research data from other cooperative teams were compared. A double-cycle pattern of basic to neutral magmatic volcanism was established, and 36 lithological rhythmic layers and representative cryptoexplosive breccia facies and welded tuff bands were identified. Combined with a reanalysis of published geochronological data, geochemical elements, and isotope geochemistry, we found that the rock assemblages could be divided into an intermediate-acid dacite (DA) series (SiO 2 >65%) and basaltic (BA) series (Co<40 µg/g), which was formed during the early Late Triassic ((218.6±3.2)−(217.9±3.5) Ma). BA exhibits obvious calc-alkaline island-arc magmatic properties: ( 87 Sr/ 86 Sr) i ratio ranging 0.703 77–0.711 18 (average: 0.706 45), 147 Sm/ 144 Nd ratio ranging 0.119–0.193 (average: 0.168), and chondrite-normalized rare earth element (REE) curves being flat, while DA exhibits remarkable characteristics of subducted island-arc andesitic magma: ( 87 Sr/ 86 Sr) i ratio (0.709 39–0.711 29; average: 0.710 35), eNd(t) value (−6.2−−4.8; average: −5.6) and εHf(t) value (−2.9−−1.7, average: −2.2) show obvious crust-mantle mixing characteristics. BA and DA reveal typical characteristics of island-arc magma systems and type II enriched mantle (EM-II) magma. BA magma was likely resulted from the process whereby the continental crust frontal accretionary wedge was driven by the Paleo-Pacific slab subduction into the deep and began to melt, resulting in that a large amount of melt (fluid) joined the asthenosphere on the side of the continental margin. In contrast, DA magma was likely resulted from the process whereby the plate front was forced to bend with increasing subduction distance, which triggered the upwelling of the asthenosphere near the continent and subsequently led to the partial melting of the lithospheric mantle and lower crust due to continuous underplating. The lithospheric thinning environment in the study area at the end of Triassic created suitable conditions for the separation between the NB and SCC, which provided an opportunity for the formation of the early intracontinental rift during the later expansion of the South China Sea (SCS).