Late Paleozoic Tectonics Of Southeast South China:Evidenced From Igneous And Sedimentary Rocks

Late Paleozoic(Hercynian or Variscan)orogeny is a crucial and world-wide orogenic event,but its effect on South China was hardly reported.Recently,a few igneous and metamorphic rocks with late Paleozoic ages have been found in the southeast South China,and many late Paleozoic detrital zircons have been found in Permian-Jurassic sedimentary rocks in South China.Then,(1)are there wider Hercynian magmatism and/or metamorphism in South China?(2)If so,where are they?which kind of tectonic setting dominated the South China Block in the late Paleozoic?plate collision,intra-continental orogeny,rifing or subduction?(3)When did the Hercynian orogeny in South China start and end?(4)What is the relationship between the late Paleozoic(Hercynian)orogeny and early Paleozoic(Caledonian)orogeny and early Mesozoic(Indosinian)orogeny in South China?and how to transform from late Paleozoic orogeny to early Mesozoic orogeny?Petrologic,geochemical and isotopic chronological studies of Hercynian-Indosinian igneous rocks in the northeastern Fujian Province and central Hainan Island and Jurassic sedimentary rocks in the southeastern Fujian Province have been carried out,in order to understand the effect of Hercynian-Indosinian tectonothermal events on the South China Block,and construct a tectonic evolution model of Late Plaeozoioc South China.This study has further identified a late Carboniferous(317 Ma)Daixi-Wufenglou S-type granites and early Triassic(-247 Ma)Houzhang complex in the northeastern Fujian Province,which provides new insights into the bewildering tectonics of South China predating its final welding to the Eurasian continent.The Daixi-Wufenglou granites have high K20(4.01-6.54 wt%),A/CNK(1.02-1.34),bulk initial 87Sr/86Sr(0.7094-0.7160)and zircon ?18O(12.7-14.0‰),and low ?Nd(t)(-8.87 to-6.31)and zircon ?Hf(t)(-8.1 to-1.9).Comparison of petrologic features and isotopic compositions with the nearby basement metamorphic rocks in the study area indicates that these S-type granites probably derived from the partial melting of a Neoproterozoic basement.Higher formation temperature suggests the influx of exotic heat fluids,which probably resulted from the input of mantle-derived fluid in a backarc extensional setting,triggered by rollback subduction of the Paleo-Pacific plate beneath the outboard proto-Japan arc.The Houzhang complex is comprised of monzodiorites and monzogranites.The monzodiorites are shoshonitic,characterized by high K,Ni,Cr and Mg#.The monzogranites are adakitic,featured by high Sr/Y and(La/Yb),and low Y and Yb.They have similar 87Sr/86Sr(0.7085-0.7087),?Nd(t)(-5.92 to-5.34)and ?Hf(t)(-7.5 to-2.5)values.Geochemical characterisitcs suggest that the parental magma of monzodiorites originated from a metasomatically enriched nantle modified by subduction-related fluids,and the adakitic monzogranites resulted from the fractional crystallization of the garnet and clinopyroxene from the monzodioritic residual magma.Coupling geochronological,geochemical and isotopic data in this study with other geological information in South China and neighboring countries,such as Japan and Korea,we propose a novel model on the tectonic evolution of the late Paleozoic South China,which was controlled by the subduction of Paleo-Pacific oceanic slab.Firstly,the study area was affected by far-field effect of an intraocean subduction,bring up the proto-Japan arc.Secondly,as the approaching of the proto-Japan arc and subduction zone in Early Mesozoic,the eastern South China directly suffered the influence of the accretion of the proto-Japan are and the ocean subduction,and gradually transformed to an active continental margin setting.This late Plaeozoic to early Mesozoic subduction also strongly affect the Hainan Island in the southmost South China.Two stages of magmatism have been identified in the central Hainan Island.The early stage(253-262 Ma)of gneissic granitoids exhibit SiO2 of 64.0-69.8 wt%,MgO of 0.97-1.60 wt%,and Fe2O3T of 2.68-6.78 wt%with low A/CNK(most<1.1)and high zircon ?Hf(t)values(-2.92?0.57).These granitoids can be subdivided into two groups based on their REE contents.Relative to the low REE granites,the high REE granites have high K2O,Ba,K2O/Na2O and Th/Ta,and low U,Rb/Ba,Nb/La and U/Pb.The low REE granites show ?Nd(t)(-4.33 to-3.96)and ISr(ca 0.7074),while the high REE granites have slighty lower ?Nd(t)(-7.34 to-6.33)and higher ISr(0.7096 to 0.7109).The gneissic granitoids were thought to originate from the partial melting of the mixed different components in the Baoban Complex at different crustal level.Late stage(245-256 Ma)of massive gabbroic-dioritic rocks exhibit relatively low SiO2(48.9-59.7 wt%),high Fe2O3T(6.57-11.6 wt%)and ALK(3.91-7.35 wt%)with variable MgO(2.74-7.05 wt%),Ni(3.84-60.1 ppm)and Cr(7.22-299 ppm)concentrations.They also have low ?Nd(t)values(-7.92 to-5.11)and high ISr(0.7083-0.7118),and varied zircon ?Hf(t)values(-5.55 to-1.11),suggesting that their parental magmas were probably derived from an enriched and heterogeneous mantle source,and experienced strong AFC process.The rock assemblages and geochemistry indicate that both gneissic granitoids and subsequent gabbroic-dioritic rocks were formed in a back-arc extensional environment,which was related with the subduction of Paleo-Pacific oceanic plate.The northeastern-striking Pingtan-Dongshan metamorphic belt(PDMB)is located along the coastal region of Fujian Province,and consists mainly of voluminous Yanshanian granites and volcanic rocks with minor Aojiao group meta-sedimentary rocks.Detrital zircon U-Pb dating results of the metasedimentary rocks in the Aojiao Group demonstrate that the Aojiao Group was deposited between 183 Ma and 160 Ma,rather than pre-Devonian as previously proposed.These meta-sedimentary rocks in the PDMB contain abundant late Paleoproterozoic detrital zircons,suggesting more widespread Paleoproterozoic basement in the eastern South China.In addition of these old detritus,these rocks also have many Paleozoic and early Mesozoic ones.Comparised with the fragmental components of different period of sedimentary rocks in the eastern South China,it can be founded that late Permian to Jurassic sedimentary rocks contain abundant late Paleoproterozoic and early Permian(-280 Ma)detrital zircons,implying that late Paleozoic(early Permian)magmatism was probably very strong in the coastal region and nearby area.Abrupt addition of late Paleoproterozoic detritus in late Permian sediments inland suggest that the intense uplift and erasion of the Precambrian basement of eastern Cathaysia Block prior to late Permian.Evidently,this uplift was probably caused by the late Paleozoic orogeny along the eastern South China.