Provenance Analysis On The Late Triassic-Middle Jurassic Sediments In The South China Continental Margin And Response To The Subduction Of The Paleo-Pacific Block

South China continental margin is situated at the intersection of the Paleo-Tethys and PaleoPacific tectonic domains.There exist well preserved Upper Triassic-Middle Jurassic sedimentary sequence in the area.Therefore,this area is the favorable research area to discuss the Paleo-Pacific Block subduction history by studying the Upper Triassic-Middle Jurassic sediments.Firstly,the zircon fertility factor of granitoids in South China Block was determined by using K-means clustering algorithm and difference significance test.Then,taking clastic rocks as the research object,this study adopted sandstone petrography,sedimentary rock geochemistry,and detrital zircon U-Pb chronology to determine the Late Triassic-Middle Jurassic main source regions in the southeastern margin of South China Block,and constructed sediment transport paths.It also provides an important reference to the subduction history of the Paleo-Pacific Block.At this stage,the following understandings have been obtained:(1)Based on Dickinson’s(2008)semi-quantitative approach that characterizes the ability of granites to produce detrital zircon grains(i.e.,"zircon fertility")under erosion,this study applied Kmeans clustering algorithm to analyze the dating data of 1784 granite rocks in the South China Plate in the past 20 years collected,and the age of the granite rocks in the South China Block could be divided into 6 categories:Group 1 aged 190-134 Ma,Group 2 aged 265-198 Ma,Group 3 aged 464381 Ma,Group 4 aged 852-785 Ma,Group 5 aged 2039-1816 Ma,and Group 6 aged 2875-2418 Ma.Satterthwaite-t’ test showed there were no significant differences in the mean zirconium contents between Neoproterozoic(900-700 Ma)and Jurassic(200-145 Ma)(t’=2.48,v=275,p>0.01),between Paleozoic(490-390 Ma)and Permian-Triassic(280-210 Ma)(t’=1.40,v=912,p>0.01),and between Neoarchean(2900-2500 Ma)and Paleoproterozoic(2000-1700 Ma)(t’=1.40,v=912,p>0.01)under the condition of α=0.01.Therefore,we combined these six groups into Group A,consisting of Group 1 and Group 4,Group B,containing Group 3 and Group 2,and Group C,comprising Group 5 and Group 6.The overall mean zirconium contents of Group A,Group B,and Group C are 139 ppm,182 ppm,and 331 ppm,respectively.The further Satterthwaite-t’ test revealed the differences between these three groups’ mean zirconium contents are statistically significant under the condition of α=0.01.Following the example of Dickinson(2008),we assigned Group A,Group B,and Group C zircon fertility factors of 1.0,1.4,and 2.5,respectively.(2)By using sandstone petrography,sedimentary rock geochemistry,major age groups and distribution patterns of detrital zircons,combined with crustal thickness and existing sedimentary facies distribution characteristics,this study believes that the major provenance areas of the southeastern margin of the South China Block are significantly different in both time and space.During the Late Triassic,the southeastern margin of South China Plate can be divided into two sedimentary sinks,the western and the eastern.The western sink includes Yangchun,Kaiping and Gaoming regions,showing three main age groups at 260-220 Ma,460-400 Ma and 1200-800 Ma,the main provenance area is Yunkai area,and the main source rocks include widely exposed Paleozoic and Mesozoic magmatic rocks,Neoproterozoic magmatic rocks,and widely exposed metamorphic complex.The eastern sink mainly includes Jiexi region of Guangdong Province,Nanjing and Zhangping regions of Fujian Province.Meanwhile,the provenance region is restricted by topographic factors,and there are some differences.The detrital zircons from the Late Triassic sedimentary rocks in Jiexi area of Guangdong Province are composed of four major age groups:290-250 Ma,460-400 Ma,900-700 Ma,and 2000-1800 Ma.The main source is Wuyi area,and the detrital strata also provide a large amount of detrital material in the area.The Upper Triassic samples from the Nanjing area of Fujian Province contain a large number of detrital zircons 260-220 Ma,900-700 Ma,and 2000-1700 Ma.The main source area is similar to Jiexi area of Guangdong Province,which is Wuyi area.However,Zhangping area of Fujian is closer to Wuyi area,the main age groups of 290-250 Ma and 370-320 Ma in the Upper Triassic samples are not consistent with the expected age groups.This may be due to the existence of uplift landforms between the Zhangping region and Wuyi area,thus preventing the Wuyi area from providing detral material to the Zhangping region,while the main provenance area of the Zhangping area is mainly confined to the uplift landforms around the area.In the Early Jurassic,due to the influence of large-scale transgression,the sedimentary sink area on the southeastern margin of the South China Block can be divided into Yangchun,Mingcheng and Huizhou in the west,Haifeng and Huilai in the center,and Zhangping in the east.The Lower Jurassic samples in the western Yangchun mainly include 280-210 Ma,460-400 Ma,1200-750 Ma,1600-1400 Ma and 2000-1800 Ma,and the main provenance areas are Yunkai area and Hainan Island.The Lower Jurassic samples in the western Mingcheng-Huizhou include 280-210 Ma,460-400 Ma,and 1200750 Ma,which can be traced to Yunkai area.The detrital zircon age distribution of Lower Jurassic samples in the central Haifeng-Huilai consists of 260-190 Ma,460-400 Ma,900-700 Ma,and 20001800 Ma,and the main provenance areas are Wuyi area and Nanling area.The 200-190 Ma detrital zircons may be derived from the magmatic arc formed along the southeast coast of the South China Block at the same time.The Lower Jurassic samples from the Zhangping region in the eastern sink have been recorded in two major age groups:2000-1800 Ma and 260-220 Ma,which correspond well with the major outcrops in Wuyi and Nanling.In the Middle Jurassic,the sea water completely withdrew from the southeastern margin of the South China Block,and a large number of restricted continental lacustrine basins developed.The rocks with a wide potential distribution of 2000-1800 Ma might be the main source rock,and the Dongguan area of Guangdong Province also received the detrital material from the igneous rock erosion formed in the Middle Jurassic.(3)In this study,detrital zircon age curve,MDS map,similarity coefficient and cross-correlation R2 coefficient were used to restore the Late Triassic-Middle Jurassic sediment transport system in the southeastern margin of the South China Block.The results show that Yangchun,Kaiping and Gaoping regions shared the same sediment transport system in the southeast margin of South China Plate during the Late Triassic.Kanshi,Nanshan and Jiexi areas share the same sediment transport system.Nankeng,Nanjing and Nanao regions share a sediment transport system.The Zhangping area is a separate sediment transport system.The paleogeographic restoration results show that the whole area of Fujian includes Wuyi Highlands,middle Fujian Highlands and northern Fujian Highlands during the Late Triassic.It is speculated that the age groups of detrital materials produced by denudation in different highlands have obvious differences:during the Late Triassic,a large number of Indochina rocks were uplifted and denudated in eastern Fujian,while the Proterozoic rocks were continuously eroded in western Fujian,and a large number of 370-320 Ma rocks were produced by Huainan Movement in central Fujian.In the Early Jurassic,Yangchun,Mingcheng and Huizhou area on the southeastern margin of South China Block shared the same sediment transport system.The detrital materials from Yunkai area and Hainan Island were transported eastward and pooled into the sea,and were deposited in Yangchun and Mingcheng area.Controlled by coastal current transport,the detrital materials entering Mingcheng area were transported to Huizhou area and deposited.Jiexi,Haifeng and Huilai regions share the same sediment transport system,which may be due to the rapid occurrence of largescale transgression in the Early Jurassic,resulting in the rapid inundating of the river valleys that may have formed before the Early Jurassic,such as the river valleys formed from Kanshi-Jiexi area in the Late Triassic.Therefore,detrite materials from Nanling and Wuyi areas can continue to be transported to Jiexi and other places through underwater river valleys.Since the main source areas of Zhangping area and Xinfeng area are Nanling area and western Fujian area,the age compositions of early Jurassic sediments in Zhangping area and Xinfeng area are highly similar.However,the distant geographical location indicates that Zhangping area and Xinfeng area should belong to different sediment transport systems.In the Middle Jurassic,the geomorphic pattern of the southeastern margin of the South China Block changed significantly,and the volcanic sedimentary basins were widely developed,and most of them were confined basins.Although there is some similarity in the age distribution patterns of detrital zircons from different regions,this is attributed to the fact that the source rocks composition of sediments from these regions are similar,and there is insufficient evidence to suggest that they share a sediment transport system.(4)This study also limited the subduction process of the Paleo-Pacific Block by combining with Upper Triassic-Middle Jurassic sandstone petrology,petrography,detrital zircon U-Pb cumulative probability density curve,detrital zircon special age group,characteristics of trace elements in detrital zircon and published geochemistry of Late Triassic to Middle Jurassic magmatic rocks.This study believes that the Paleo-Pacific Block subducted to the South China Plate in the Late Triassic,but only limiting in the southeast coast of the South China Block.At the end of the Late Triassic,the flat block of the Paleo-Pacific began to tear apart,possibly due to the eclogitization of the Paleo-Pacific Block or the subduction and collision of the Permian Akasaka-Kuzuu submarine plateau chain.In the Early Jurassic,the Paleo-Pacific Block began to subduct again,and magmatic rocks with magmatic arc background were formed in the area of Taiwan-South China Sea-East China Sea on the east side of the South China continental margin.Subsequently,the continuous subduction of the Paleo-Pacific Block led to the continuous development of this magmatic arc,which could already provide a large amount of detrital material to the South China continental margin in the Middle Jurassic.