Petrogenesis Of Igneous Rocks From Well NK-1 In The Nansha Block: Constraints On The Mesozoic Tectonic Evolution Of The Southern Continental Margin Of The South China Sea

Identifying whether and when the southern continental margin of the South China Sea（SCS）was part of the South China Block（SCB）is important for Mesozoic–Cenozoic paleogeographic reconstructions of Southeast Asia.Furthermore,it has been proposed that the proto-South China Sea（PSCS）existed in the southeastern South China Block（SCB）continental margin before the opening of the present SCS.However,the age of initial spreading and the geodynamics of the opening of the PSCS remain enigmatic.In this paper,we present geochronological and geochemical data for dacite,diorite and diabase samples from Well NK-1 in the Nansha Block（Dangerous Grounds）of the southern SCS margin to investigate their petrogenesis,tectonic affinity and their association with the formation of the PSCS.The main achievements of this dissertation are as the following.（1）There are three episodes of Mesozoic magmatism in the Nansha block.SIMS zircon U–Pb geochronology and plagioclase ⁴⁰Ar/³⁹Ar dating results show that the dacites,diorites and diabases were formed in the Late Triassic（218–217 Ma）,the Early Jurassic（ca.177 Ma）,and the Early Cretaceous（ca.124 Ma）,respectively.Both the diorites and diabases intruded the dacites as the dikes.（2）The Late Triassic dacites should be A-Type granite in their geochemical characteristics,and they were formed in the extensional environment which was probably related to the Paleo-Pacific plate subduction.The dacites have high total alkali（K₂O+Na₂O=7.39–8.42 wt%）and relatively low Ca O contents（1.87–3.38 wt%）,and show geochemical affinities with A-type granites,with high Zr saturation temperatures（806–855°C）,Fe O/Mg O ratios（5.22–7.48）,Zr+Nb+Ce+Y contents（530–680 ppm）and 10000×Ga/Al values（2.40–2.94）,and relatively low A/CNK values（0.86–1.04）,showing geochemical affinities with the A-type granite.In addition,the dacites show enriched Sr–Nd–Pb–Hf isotopes with old Nd-model ages(T_DM2–Nd=1.5-1.4 Ga),and their apatites exhibit high oxygen fugacity（higher than the NNO buffer）.The primitive magma for the dacites was likely generated by partial melting of ancient crustal materials in high temperature and oxidized conditions.Similar to the Triassic A-type granites distirbuted along the Zhenghe–Dapu Fault in the SCB,the Late Triassic dacites in the Nansha Block would be formed in an extensional setting at the continental margin of the SCB,which was probably related to the Paleo-Pacific plate subduction that restarted prior to the Early Triassic.（3）The Early Jurassic diorites were formed in the continental margin arc environment related to the subduction of the Paleo-Pacific plate,and the Early Cretaceous diabases were formed during the the spreading stage of the back arc basin formed by the rollback/retreat of the subducting Paleo-Pacific plate.The diorite samples are geochemically similar to continental arc basalt,with enrichment in Rb,Ba,Pb,and Sr,and pronounced negative Nb,Ta,and Ti anomalies.They have enriched Sr–Nd–Pb–Hf isotopes and high Th/Nb ratios（0.46–0.55）with decoupled Nd-Hf isotopes,indicating that their mantle source was metasomatized by subducted-sediment-derived melts in a continental arc setting related to subduction of the Paleo-Pacific plate beneath the SCB.In contrast,the diabases are compositionally similar to back-arc basin basalt,with enrichment of Rb,Ba,and Pb,and negligible Nb,Ta,Ti,Zr,and Hf anomalies.They have depleted Nd–Hf and enriched Pb isotopes,and are enriched in fluid-mobile elements with variable Sr isotopes,indicating that the diabases were derived from the partial melting of a mantle source metasomatized by aqueous fluids from the subducted slab（i.e.,mafic oceanic crust with the overlying sediments）in a back-arc basin associated with the Paleo-Pacific subduction.（4）We reconstructed the Mesozoic tectonic evolution model of the southern continental margin of the South China Sea.The formation age of dacites（218-217 Ma）in the Nansha Block is approximately consistent with the age peak（ca.228 Ma）of detrital zircons from the eastern Cathaysia Block.Late Triassic dacites in the Nansha Block and Triassic（246–215 Ma）A-type granites distirbuted along the Zhenghe–Dapu Fault in the SCB show similar geochemical characteristics with old Nd model ages,high zircon saturation temperatures,and enriched Sr–Nd isotopic compositions,etc.Furthermore,the two episodes of mafic magmatism in the Nansha Block,deonted by the Early Jurassic diorites and Early Cretaceous diabases,are coeval with the same episodes of magmatism in the SCB.On the other hand,the Upper Triassic deltaic sandstones from the Nansha Block contain a Dictyophyllum–Clathropteris flora,which migh have originated from the Cathaysia Block,and the Zhongsha,Xisha,and Nansha blocks show similar Bouguer gravity anomalies.Thus,we concluded that the southern continental margin of SCS was part of the SCB continental margin during the early Mesozoic.（5）We determined the age of initial breakup of the PSCS and illuminated the tectonic dynamics for the PSCS opening.The Early Cretaceous diabases are geochemically similar to mafic rocks and ophiolite complexes previously regarded as remnants of the PSCS oceanic crust.Therefore,the Early Cretaceous diabases in the Nansha Block are interpreted as representing coeval magmatism of the nascent ocean of the PSCS,indicating that the PSCS was a spreading back-arc basin associated with retreat of the subducting Paleo-Pacific plate during the Early Cretaceous.The compiled detrital zircon data from continental margin of SCB,Palawan and Mindoro Blocks,indicate that the Palawan Block was drifting southeastward in the Late Cretaceous due to the opening of the PSCS,while the Nansha,Reed Bank and Mindoro Blocks splited from the SCB and were drifting southeastward in the Cenozoic due to the spreading of the PSCS.