Cretaceous Volcano-Sedimentary Sequences In Jianning-Taining Region Of Mts.Wuyi:Constraint On Cretaceous Tectonic Evolution Of Se China

South China consists of the Yangtze Block and the Cathysia Block,which were amalgamated in Neoproterozoic along the Jiang-Shao suture,and it experienced multiple tectonic-magmatic events in the Phaneozoic.Late Mesoroic magmatism and associated mineralization is the most remarkable one,along with the formation of widespread extensional basins.Numerous studies have been conducted in terms of petrology,petrogenesis,mineralization background,sedimentary tectonics and sedimentary basin analysis,and some classical models were put forward,including steepening of subduction slab angle,flat-subduction,mid-ocean ridge subduction.Nevertheless,the dynamic origin and deep process of late Masozoic tectonics of SE China remain highly debated.In this study,we examined the Cretaceous volcano-sedimentary sequences developed in the Jianning-Taining area of the Wuyi Mts.,Fujian Province by using geochronology,geochemistry and isotope geochemistry,with the purpose of establishing an episodic evolution of the Cretaceous tectonics of this continenal margin.We distinguished two volcano-sedimentary sequences,separated by an unconformity.Each one consists of volcanic rocks in the lower part and clastic sedimentary deposits in the upper part.Zircon U-Pb geochronology reveals that the lower volcanic rock series erupted in Early Cretaceous at 141 to 135 Ma,while the upper volcanic rock series erupted at 105 to97 Ma in the latest Early Cretaceous to Late Cretaceous.The lower volcanic rocks,geochemically,are characterized by enriched in LREEs,LILEs and Hf isotope(-20.2 to1.9),but depletd in HFSE(i.e.Nb,Ta)with the affinity of volcanic arc in a Andean-type continental margin.By contrast,the upper volcanic rock series are relatively depleted in Hf isotope(-16.1 to 10.2).This late volcanism was coeval with the intrusion of the A-type Baishiding granites,which imply a back-arc extensional setting.In addition,we made a stratigraphic correlation among the Cretaceous volcanic-sedimentary sequence established in the Jianing-Taining region of Mts.Wuyi with those of other regions of SE China,and also compared it with late Mesozoic magmatism in SE China.We found a distinct magmatism lull between the two volcanisms.It shows that Early Cretaceous magmatism displays a vanishing tendance from the SW area(135-110 Ma)to the NE region(at 120-110 Ma),and it ceased ca.120 Ma.During this migrated magmatic quiescence,SE China was deposited by a package of fluvial-lacustrine facies sediments,this was subsequently deformed by a compressional event.The resumed volcano-sedimentary activity in Late Cretaceous seems to be controlled by extensional faulted basins,in which accumulated a thick,gypsum-bearing red beds.The Early Cretaceous volcanism has geochemistry character of high-K calc-alkaline I-type granites with lower Zr contents and Na₂O/K₂O ratios while higher Ca O/Na₂O and Al₂O₃/Ti O₂ ratios,implying the condition of relatively low temperature and enrichments of water;however,the late volcanism was evolved toward alkali-rich,iron-rich,zirconium-rich and sodium-rich,which reflects relatively high-temperature and water-poor state of the parental magma.To sum up,these two Cretaceous volcano-sedimentary sequences recorded two tectonic stages in SE China with distinct features—During the early Early Cretaceous,the normal subduction of the Plaeo-Pacific Plate(PPP)produced a broad continental volcanic arc.At the turn of the late Early Cretaceous and early Late Cretaceous,the rollback of subducted PPP triggered the back-arc extension as well as the generation of faulted basins,in which was usually filled with a thick,gypsum-bearing red beds.It is remarkably,however,that tectonic inversion,induced by the subdudction plate shallowing,occurred between the two sequences and ended the early volcanic flare-up.