Early Paleozoic And Early Mesozoic Intracontinental Tectonic Evolution In South China

The South China Block is composed of the Yangtze and Cathaysia blocks which were amalgamated along Jiangshan-Shouxing-Pingxiang-Qidong-Chalin Fault during Neoproterozoic(900-800 Ma).During the Phanerozoic,South China was reworked by multi-stage intense tectonothermal events.There are many researches for issues about Precambrian evolution and Neoproterozoic geodynamic model.However,issues about not only the tectonic setting of granites,but also the tectonic nature and geodynamic evolution model of South China during Early Paleozoic(Caledonian)and Early Mesozoic(Indosinian)still remain controversial between plate subduction and intracontinental orogeny.To understand Early Paleozoic and Early Mesozoic tectonic evolution of South China,we take Yuechengling,Haiyangshan,Zhuguangshan of Nanling and Yunkai as our main research zone.Combine with the feature of petrography and deformation in field investigation,we analyzed the zircon U-Pb geochronology,whole-rock geochemistry and Hf isotopes of both stage granitic plutons.Then,we discussed the age,origin and tectonic background of the granites to provide evidences for the evolution process of South China during the Early Paleozoic and the Early Mesozoic.Field investigation shows that phanerozoic strata in South China are general complete.During Sinian-Late Ordovician,rock assemblage of muddy-sandy slate and neritic-face sedimentary structure are developed.Triassic strata are absent.From middle-late Devonian to middle Triassic,coarse clasolite and carbonatite are deposited,bearing neritic-face fossil and sedimentary structure.Moreover,there are no coeval oceanic records,such as turbidite with Bouma sequence,volcanic lava,and coarse volcaniclastic rock.Therefore,we infer that stable deposition setting other than active continental margin setting dominated in South China during Paleozoic.This work made detailed observation,sections and sampling for important phenomenon in Nanling and Yunkai domains,such as Yuechengling-Miaoershan,Nanyue-Tashan and Zhuguangshan-Luxi.Except nearly E-W striking,most folds and thrust sheets developed in South China are NW-striking and kinematics markers,such as pressure shadow,mica fish,rotated porphyroblast indicating top-to-NW thrusting.On one hand,the angular unconformity between the upper-middle Devonian and pre-Devonian strata widely developed in South China.On the other hand,late Triassic or early Jurassic strata unconformably overlay different-age of underlying strata.Therefore,we infer that two stages of orogeny happened during Cambrian-Jurassic:the Early Paleozoic tectonic event(Silurian)and the Early Mesozoic tectonic event(Middle-Late Triassic).The Early Paleozoic plutons are exposed extensively in South China(about 20,000 km2 for 200 plutons)and occur mainly as batholiths or sills.The plutons,named Yuechengling,Haiyangshan,Jinxi and Qingzhou,intruded into pre-Devonian strata and were overlain unconformably by Middle-Devonian strata.They are composed of biotite granites,muscovite granites and two-mica granites with massive and gneissic structure.From the core of the plutons to their margin,an obvious zoning is seen with coarse-grained massive granite,medium to coarse-grained massive granite,medium to fine-grained massive granite,weakly-oriented granite and foliated granite.Irregularly shaped relicts of country rocks can be observed in the margin.Moreover,euhedral feldspar are elongated and aligned,forming the magmatic foliation whose orientation is similar to country rocks,suggesting syntectonic emplacement.The cathodo-luminescence(CL)images of the plutons display oscillatory zoning typical of magmatic origin.Their geochronology results suggest that South China experienced intense magmatic activity during Silurian,their weighted age are 427.0 + 3.2 Ma?435.5 + 3.7 Ma?442.3 ± 3.5 Ma and 436.3 ± 3.4Ma,respectively,indicating Triassic plutons.Their sHf(t)values are-1.44?36.84 with two stage model age(TDM2)of 1.3-3.7 Ga,indicating magmas derived from partial melting of early-middle Paleoproterozoic and Archean basement with poor input of mantle material.Most of the samples contain Al-rich minerals,such as muscovite,with A/CNK values bigger than 1.1,suggesting they are peraluminous S-type granites.They show obvious LREE enrichment and Eu negative anomaly,suggesting fraction of plagioclase.Meanwhile,the negative anomaly of Ba,Sr,Nb and Ti and the enrichment of Rb,Th,U and Pb indicating they are derived from crustal material.In the tectonic setting diagram,they all fall in the post-cold area.Their Hf isotope compositions show that the magmas are closely related to the partial melting of early-middle Paleoproterozoic and Archean basement rocks.Finally,we summarized results of former research and found that the age of Early Paleozoic plutons is in the range of 379-468 Ma(peak cluster of 423-443 Ma).The palar-shaped granitic plutons didn't show any younging propagation trend along ocean-continent interior or Yunkai-Wuyi direction,indicating Early Paleozoic tectono-magmatic events have no direct relationship with plate subduction.The Early Mesozoic magmatic activity is intense,the area of the plutons is up to 20,900 km2.The plutons are palar-shaped distributed in Xuefeng,Wuyi,Nanling and Yunkai domains.They intruded into pre-Triassic strata and were overlain uncomformably by late Mesozoic strata.We take Jiangjunmiao,Tashan,Wangxianling,Nanyue,Luxi,Langnan and Zhuguangshan complex as important research object.They composed mainly of muscovite granites and two-mica granites with massive and gneissic structure.Their changes from the core of the pluton to their margin are similar to Early Paleozoic plutons,showing smaller grain-size and weaker directional alignment.In the margin,irregularly shaped relicts of country rocks can be found with similar orientation to country rocks.The Early Mesozoic granites are mainly composed of S-type granites with Al-rich minerals like muscovite(A/CNK values are larger than 1.1),suggesting strong peraluminous feature.Geochemical data show LREE enrichment and Eu negative anomaly and negative relation between Al2O3,TiO2,MgO,CaO,P2O5,(MgO +FeOT+TiO2),Ba,Sr,Yb,Zr and SiO2,suggesting fraction of rock-forming mineral like plagioclase.Meanwhile,the negative anomaly of Ba,Sr,Nb and Zr and the enrichment of Rb,Th,U and Cs,indicating they are derived from the partial melting of crustal material.Their zircon CL image show whole magmatic volatility band,suggesting magmatic origin.Their zircon U-Pb dating indicate that they belong to Triassic and their weighted ages are 217.5 ± 1.8 Ma(Jiangjunmiao);222.4 ± 3.6 Ma(Tashan);220.6 ± 2.0 Ma(Wangxianling);228.4 ± 1.6 Ma(Nanyue);224.4 ± 3.1 Ma(Luxi);231.3 ± 3.0 Ma(Langnan);222-234 Ma(Zhuguangshan Indosinian pluton)and 147-152 Ma(Zhuguangshan Yanshanian pluton).Their Hf isotope composition(?Hf(t)values of-3.32?-18.63;TDM2 of 1.4-2.5 Ga)indicate that magmas are derived from the partial melting of early-middle Paleoproterozoic basement rocks,without addition from juvenile mantle.We categorized the feature of geochemical of Xuefeng,Nanling and Yunkai,their temperature are 650-750° C,750-800° C and 780-830° C,respectively and their depth are 6.5-13 km.Finally,the summarizations of former results indicate that the crystallization age of the Early Mesozoic plutons are 195-260 Ma(two peak cluster of 228-244 Ma and 210-218 Ma).Similar to the Early Paleozoic plutons,the Early Mesozoic plutons do not show any define trend in age in any direction.They have no relationship with plate subduction.Both Early Paleozoic and Early Mesozoic tectonic events result in large-scale deformation,regional unconformity and granitic magmatic activity.According to the palar-shaped distribution of the strong peraluminous granites,combine with the geological evidence of absent of ocean record,such as ophiolite,arc magmatic rocks,we infer that the Early Paleozoic and Early Mesozoic granitic plutons are magmatism under intracontinental setting.They should be the result of far-field stress propagation from plate margins through lithosphere during the formation of the surrounding orogens.The compressive stress results in the reaction of the blocks in the continental,forming large-scale folds,thrusts,regional angular unconformity,low-level metamorphism and magmatism.