Study Of Key Issues Related To Tectonic Evolution Of South China

It was generally accepted that the South China block was built by the Neoproterozoic aggregation between the Yangtze block in the northwest and the Cathaysia block in the southeast,by which the Jiangnan Orogen was created between them.However,there are still various of controversies involving the collision time,mechanism,evolution process and subsequent multistage evolution during phanerozoic eon.This study focus on the several significant issues which have been payed limited attention and conducted some comprehensive and profound studies,by which we obtains a serious of new ideas and data.The first issue involves the boundary of Jiangnan Orogen and orogeny-related geometric and kinematic characteristics.Basing on previous work,by systematic field geological survey,sedimentary geotectonic methods and the differences of sedimentary sequences from both sides of Jiangshan-Shaoxing belt,this study confirmed the boundary between Yangtze and Cathaysia blocks in Hunan province,i.e.,the Shaoxing-Jiangshan-Dongxiang-Pingxiang-Qidong fault.Furthermore,this study conducted some geometrical and ductile deformation kinematics research.By observing the metamorphic deformation profiles from some regions along the Jiangnan Orogen,we obtained a series of occurrence elements for shear foliation,stretching lineation and kinematic marks in XZ facet.Finally,we conclude that there at least exist two stages of ductile deformation in the Pre-Nanhuan strata,which are the SE sliding in the early stage and the NE-SW sinistral strike-slip in the late stage.The second issue is about the collision time between Yangtze and Cathaysia blocks in the central Jiangnan Orogen.By detrital zircon U-Pb geochronological and Lu-Hf isotopic studies on the meta-sedimentary rocks from the strata on and under the angular unconformity surface(i.e.,the top part of Lengjiaxi group and the bottom part of Banxi group),we yield the maximum depositional age for Lengjiaxi Group from the NE Hunan in central Jiangnan Orogen is?825 Ma;the maximum depositional age for Banxi Group in Wangcheng County of Hunan is 797±14 Ma.Combined with published data for Banxi group near the study area,we infer the initial depositional age for Banxi Group is 798±11 Ma.These data constrain the collision time between Yangtze and Cathaysia blocks in the central Jiangnan Orogen is?825?800 Ma.The age distribution diagram indicate that both the Lengjiaxi and Banxi groups developed a single and prominent ages peaks between 968?820 Ma.Provenance analysis reveal that the source region for these detrital materials are basically Jiangnan Orogen.While the slightly paleo-proterozoic zircons probably derived from Cathaysia and Yangtze blocks.The third problems is related to the formation time,mechanism and evolutionary process of Fuchuan ophiolite in the mid-east Jiangnan Orogen.Based on detailed filed investigation,we identified some granitic dikes developed within the serpentinized peridotite of Fuchuan ophiolite By zircon U-Pb dating,we yielded the ages of the two analyzed granitic dikes is 841.2±5.5 Ma and 839.7±4.6 Ma.Geochemically,they show peraluminous,high silicon,high potassium and high K2O/Na2O ratios,and low Mgo,Fe2O3 and CaO contents.In addition,they show enrichment of LREE(light rare earth element)and obvious negative Eu,P and Ti anomalies and positive Pb anomaly.Besides,they high differentiation index and relatively low zircon saturation temperatures suggest that they are probably belong to S-type granites.Furthermore,zircons from this pluton all exhibit positive ?Hf(t)values,combined with their single-stage model ages,we suggest this pluton sourced from the partial melting of late Mesoproterozoic to early Neoproterozoic juvenile continental crust.According to present studies,the Fuchuan felsic dikes are probably belong to obduction-type granites which were formed by partial melting of rocks under hanging wall during the migration of ophiolite.Combined with regional geological setting,we infer the SE tendency both the granitic dyke and they country rocks show is generated by collision,squeezing and folding during?830-?800 Ma.It can't stand for its original tendency and also can't suggest a southeast subduction from Yangtze to Cathaysia block during-841 Ma.The fourth issue is about the kinetic mechanism of tectono-magmatic events in SE China during early Paleozoic.The majority of scholars regard the tectonic movement during early Paleozoic as intraplate tectonics.In addition,this view has been proved by a variety of sedimentological,tectonic and igneous petrological evidences.Recently,however,some scholars attributed the petrogenesis of the Longyou garnet amphibolite to the early Paleozoic collisional orogeny between Yangtze and Cathaysia blocks,just basing on some petrology and zircon U-Pb geochronology.In review of this,after detailed filed investigation and sampling,we conducted a systematically study involving petrology,zircon U-Pb geochronology,bulk rock geochemistry and mineral geochemistry(EPMA),and by using the relatively reliable P-T pseudosection method,we also figure out three stages of metamorphisms which are prograde(0.67?0.75 GPa/685?719 ?),peak(0.9-1.1 GPa/760-778?)and retrograde metamorphism(?0.68 GPa/?767 ?).The geochronological study suggest that the core-rim structures are rather common in the zircons from Longyou garnet amphibolite.The core part exhibit weighted average U-Pb age of 879±11 Ma,corresponding Th/U ratios of larger than 0.1;and the rim part show weighted average U-Pb age of 446±5 Ma,with Th/U ratios of lower than 0.1;indicating that the age of the protolith of Longyou garnet amphibolite is?879 Ma,and its metamorphic age is?446 Ma.The bulk rock geochemistry reveal that the protolith of Longyou garnet amphibolite E-MORB type basic magmatic rock.EPMA study suggest that,the plagioclase inclusion in garnet and clinopyroxene phenocrysts are anorthite,while the plagioclase distributed in the periphery of garnent and clinopyroxene phenocryst show An values of 44?66;not any Na-rich plagioclase(albite or oligoclase)which is indicator of omphacite has been identified,suggesting that the Longyou garnet amphibolite is not retrograded from high-pressure eclogite.The three stages of metamorphism constitute a clockwise P-T path,indicating the protolith of Longyou underwent a process of buried by compressive orogeny,and subsequently retrace.Combined with regional geological setting,we rebuilt the formation process:the paleo-South China Ocean subduction under Yantze plate in Neoproterozoic triggered back-arc extension,mantle-derived magma upwelling and forming E-MORB type basic rocks in Neoproterozoic,and during Silurian,a serious of medium to high-grade metamorphism result to the formation of Longyou garnet amphibolite.The fifth question is about the age,type and formation mechanism of the late Mesozoic granites from inland South China.This study focus on the Wugongshan and Lianyunshan granitic domes,the large granitic plutons from the boundary region of Hunan,Jiangxi and Hubei provinces,and the Lianshan pluton from NW Guangdong.Structure geology,bulk rock geochemical,zircon U-Pb geochronological and Lu-Hf isotopic studies indicate that the emplacement timing of Wugongshan late Mesozoic pluton is during late Jurassic(158?154 Ma),and it belong to peraluminous S-type granite,sourced from the reworking of Paleo-to Mesoproterozoic continental crust materials.As for the plutons from Lianshan and the boundary region of Hunan,Jiangxi and Hubei provinces,all of them show zircon U-Pb ages of late Jurassic(153?145 Ma).Geochemically,apart from the Lianshan pluton showing adakitic features,all the rest plutons belong to S-type or highly fractionated S-type granites.Zircon Lu-Hf isotopic analyses for these plutons suggest that these plutons are not sourced from the partial melting of their wall rocks(the Neoproterozoic-early Paleozoic sedimentary rocks),but probably from the partial melting of much deeper continental crust materials.By geochemical and isotopic analyses,we infer the Lianshan adakitic rocks are probably derived from a source region with high Sr/Y ratio at normal continent depth(30-40km).Combined with regional geological setting,we consider that the leading end(continental arc)of Paleo-Pacific plate subduction is probably located at Chenzhou-Ningyuan belt,and the formation mechanism for the large volumes of late Mesozoic S-type batholiths and stocks in the boundary region of Hunan,Jiangxi and Hubei is basaltic underplating or intraplate magmatism in the mutiplate aggregation background.