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<40μg/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 0.70377–0.71118(average:0.70645),^(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(0.70939–0.71129;average:0.71035),εNd(t)value(-6.2–-4.8;average:-5.6)andε_(Hf)(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 typeⅡenriched 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 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).

Petrogenesis and Geodynamic Implications of the Carboniferous Granitoids in the Dananhu Belt,Eastern Tianshan Orogenic Belt

This paper presents new LA-ICP-MS zircon U-Pb geochronology,whole-rock major and trace element geochemistry,and Sr-Nd isotopes systematically on porphyritic granitic and K-feldspar granitic intrusions from the Dananhu belt,eastern Tianshan orogenic belt(ETOB).Zircon U-Pb dating indicates that the porphyritic granitic and K-feldspar granitic plutons were formed at 357±3 and 311±3 Ma,respectively.The porphyritic granites show geochemical and isotopic characteristics(high Si O2,low Mg O and Mg^#,depleted Sr-Nd isotopic values(about 0.7034 and 6.13,respectively),with Nb/Ta(13.3–14.7)and Zr/Hf(31.0–33.9)ratios)similar to those of the crustal-derived magmas.The above characteristics suggest they were probably originated from juvenile lower crustal materials.The K-feldspar granites also have high Si O2,low Mg O and Mg^#,depleted Sr-Nd isotopic values(0.7033–0.7046 and 4.41–5.67,respectively).But some trace elements contents vary widely,with variable Nb/Ta(12.7–22.7),Zr/Hf(21.3–36.1)and Nb/La(0.38–1.07)ratios,indicating that the K-feldspar granites were formed by partial melting of juvenile lower crustal materials with old crustal materials.Combined with previous data on Carboniferous granitoids in the Dananhu belt,we infer that all the Carboniferous granitic plutons in the Dananhu belt were most likely emplaced in an island arc environment(Dananhu arc).Subsequently,a tectonic transition from oceanic subduction to post-collisional extension probably occurred in the ETOB.