The Dissecting And Geochronology Of “Shenshan Formation” And “Kuli Formation” In Shenshan Area,Xinyu,Jiangxi Province:Implications For The Tectonic Evolution Of West Cathaysia

The South China Block is one of the major continental blocks in China and consists of the Yangtze Craton(Yangtze)in the northwest and the Cathaysia orogenic belt(Cathaysia)in the southeast.The Jiangshan-Shaoxing-Pingxiang(JSP)Fault is one of the most important fault zones in South China and is traditionally considered as the boundary between Yangtze and Cathaysia.The tectonic history of South China is complex and remains highly debatable.However,the initial assembly of South China(in other words,when and how the Yangtze and the Cathaysia were amalgamated)is critically important for understanding other controversial scientific issues in South China.To address this problem,we focus on the Shenshan area in the Xinyu City which is located at the northwest margin of Cathaysia and in the middle segment of the JSP Fault.In this study,following a detailed field-based investigation of the original Neoproterozoic “Shenshan” and “Kuli” formations in this area,we determined their compositions,structural characteristics and contact relationships.Then,we carried out a series of biochronology,U-Pb geochronology,sedimentology,petrology and geochemistry study of the various litho-tectonic units to constrain their ages,origins and original tectonic settings.The results of this study provide key evidence and new understanding for the tectonic evolution of Cathaysia.The main results and ideas are as follows.Detail field work in this study shows that the “Shenshan and Kuli formations” is not part of a continuous Neoproterozoic stratum as previously proposed but a tectonic mélange at the northwestern margin of Cathaysia,here termed the Shenshan tectonic mélange,and the ultramafic and mafic rocks exposed in it are not later intrusions but older tectonic blocks.The Shenshan tectonic mélange has a block-in-matrix structure at the outcrop scale and consists of various fault-bounded slivers characterized by strong deformation,which were divided into 9 slivers and numbered as A to I.Its sedimentary matrix includes three lithological units: tuffaceous turbidite,carbonaceous shale and red mudstone.The ultramafic to mafic blocks are identified as olivine pyroxene hornblendite(or olivine hornblende pyroxenite),pyroxene hornblendite(or hornblende pyroxenite)and gabbro.Detailed biochronology and U-Pb geochronology were conducted on the matrix of the Shenshan tectonic mélange.The acritarch assemblages found in the carbonaceous shale(Slivers A and C)are characterized by Skiagia-Asteridium,AsteridiumComasphaeridium and Celtiberium-Skiagia-Leiofusa assemblages and are correlated with the acritarch assemblage distributed in the lower to middle Cambrian.This result proves that the carbonaceous shale was originally deposited during the early to middle Cambrian.The Leiosphaeridia-Germinosphaera-Palaeopleurocapsa reniforma acritarch assemblage obtained from the tuffaceous turbidite(Slicver F)are typical middle to late Neoproterozoic taxa.In addition,zircon U-Pb ages of four tuffaceous turbidite samples(Sliver B,D,G and I)have youngest peak ages of 771 Ma,779 Ma,742 Ma and743 Ma respectively,which are consistent with the acritarch ages.These all indicate that the tuffaceous turbidite is middle to late Neoproterozoic in age.We also obtained a zircon SHRIMP U-Pb age 812.5 ± 7.5 Ma for a tuff sliver and a LA-ICP-MS zircon UPb age of 780 ± 6 Ma for a gabbro block.These new results indicates that the different lithological units are characterized by strong deformation,mixing,and markedly different ages.The lower to middle Cambrian age of the carbonaceous shale constrains the maximum formation age of the Shenshan tectonic mélange.Considering that there is no evidence for a major tectonic event in the region after the Early Devonian(marked by the sub-Middle Devonian regional unconformity),we conclude that the mélange most likely formed during the Ordovician to Silurian.The tuffaceous turbidite matrix is rich in volcanic debris and shows immature sedimentary features and single Neoproterozoic zircon U-Pb age peak patterns.These findings suggest that they are pelagic to hemipelagic sediments deposited in a moderately deep marine environment and the existence of syn-sedimentary igneous activity in a proximal source area.Mixture modeling results show that these tuffaceous turbidites record multiple volcanic activity ages(740 Ma,770 Ma,815 Ma and minor860 Ma).The whole rock geochemical data show that the tuffaceous turbidites have higher La/Sc(1.3–7.1),Th/Sc(0.40–2.79)and Cr/Th(0.5–12.5)ratios and lower Co/Th(0.15–0.92),Cr/Zr(0.05–0.47)and Sc/Th(0.36–2.48)ratios and indicate that their detritus were likely derived from felsic to intermediate igneous rocks.In the plots of LaTh-Sc,Th-Co-Zr/10 and Th-Sc-Zr/10 and multi-elemental diagrams,they all show trace element characters similar to those deposited in the continental island arc setting.A comparison of their zircon age peaks and the distribution of the εHf(t)with those of igneous rocks in the region indicates that the tuffaceous turbidites were more likely sourced from a Neoproterozoic arc in Cathaysia rather than the Jiangnan Orogenic belt and were deposited in an arc-back setting rather than a continental rift basin.We present whole rock major and trace elemental,and Sr-Nd isotopic data,in combination with petrographic data and mineral chemistry of the ultramafic to mafic cumulate blocks to constrain their parental magma and tectonic setting.The three groups of cumulates show similar trace element and Sr-Nd isotope characteristics(initial 87Sr/86 Sr ratios of 0.702679 to 0.709742 and εNd(t)values of +2.06 to +4.48),indicating that they were from the same or similar parental magmas.Their relative crystallization sequence is olivine + spinel,clinopyroxene,hornblende,plagioclase.The clinopyroxene in the gabbro cumulates is chemically similar to those formed in hydrous magmas,with the presence of magmatic hornblende and the Mg#(53-77)of the liquid in equilibrium with the clinopyroxene suggesting that their parental magmas are waterenriched basaltic magma.The geochemical distribution patterns,the depleted εNd(t)values and results of amphibole barometry suggest that these ultramafic to mafic cumulates were derived from a subduction-related hydrous basaltic magma in a convergent plate setting and accumulated in a magma chamber located at a mid to lower crustal level.The lithological components of different ages and provenances and the mélange structure imply that they were assembled at an active tectonic boundary.The Shenshan tectonic mélange is spatially closely associated with the JSP Fault.Here we propose a model to reconcile evidence for the previous different interpretations and provide new insight into the tectonic evolution of Cathaysia.(1)In our model,Yangtze and Cathaysia were two separate microcontinents,as indicated by their contrasting pre-Devonian geological history,and their boundary is JSP Fault not the Jiangnan Orogen.(2)The Jiangnan Orogen,as part of a folded basement of Yangtze,formed due the closure of a early Neoproterozoic arc-back arc system.After the collision,the area underwent extension.At this time,West Cathaysia was part of a magmatic arc system.(3)From Ediacaran to Ordovician,Yangtze was drifting in the Proto-Tethys ocean and was surrounded by a passive margin receiving detritus exclusively from its basement,whereas Cathaysia was likely colliding with Gondwana and receiving detritus from Gondwana continental regions.(4)Following the closure of the Proto-Tethys ocean,accretion and collision occurred between various Asian blocks and eastern Gondwana.Against this background,the southern margin of Yangtze collided with an unknown block or obliquely collided with Cathaysia with the formation of an O-S foreland basin.(5)Late during or following the collision,the JSP Fault was initiated subparallel to the continental margin(possibly due to oblique collision or transform faulting),and sinistral strike-slip motion along the fault juxtaposed Cathaysia with Yangtze.(6)The Shenshan tectonic mélange either formed during the subduction in the northwest margin of Cathaysia then emplaced by the strike-slip motion of the fault or straightforwardly formed and emplaced by this strike-slip motion.