Neoproterozoic Tectonic Evolution Of The South Qinling Belt

The Neoproterozoic tectonic evolution of the Yangtze Block is of great significance for exploring the assemblage and break-up of the Rodinia supercontinent.Large numbers of works around the Yangtze Block provide well constraints for revealing the Neoproterozoic tectonic evolutionary history.Previous studies have shown that the tectonic regime of the northern margin of the Yangtze is different from that of the western and northwestern margins.The South Qinling Belt is traditionally realized to be the northern part of the Yangtze Block.Well-preserved basement rocks have witnessed the Neoproterozoic tectonic evolutionary history of the South Qinling Belt.Numerous studies have been done on these rocks and many significant insights have been achieved.Some key issues,however,remain unclear.What are the tectonic properties of the most widely distributed Neoproterozoic volcanic-sedimentary rock series in the South Qinling Belt?Was the South Qinling Belt in a long-term subduction-accretion regime or a rifting setting during the mid-Neoproterozoic?Was the tectonic setting of the South Qinling Belt in the late Neoproterozoic a continental rift or back-arc extension?These series of questions are of great significance for understanding the Neoproterozoic tectonic evolution of the Yangtze Block and even the reconstruction of the Rodinia supercontinent.The Suizhou area was little affected by high-grade metamorphism during orogenesis.The mid-Neoproterozoic Suixian Group,the late Neoproterozoic Yaolinghe Group,and a series of mafic dykes are exposed in the Suizhou area,which is an ideal place to study the Neoproterozoic tectonic evolution of the South Qinling Belt.Based on the above discussion,this paper has carried out a series of petrological,geochronological,whole-rock geochemical and isotopic studies on the volcanic rocks in the Suixian Group and the Yaolinghe Group and the late Neoproterozoic mafic dykes.The main conclusions are as follows:The Suixian Group was formed at 763–700 Ma.According to geochemical characteristics,the basalts of the Suixian Group can be divided into three groups.The first group of basalts have low concentrations of Si O₂（47.81%–49.63 wt%）,Ti O₂（1.11%–1.18wt%）and high Mg O（7.13%–8.27 wt%）.Their trace element composition is similar to that of the basalts in the Wudang Group previously reported,all of which are depleted in high field strength elements（such as Nb,Ta,Zr and Hf）and relatively enriched in LILEs and LREEs.These rocks have（Ta/La）_N ratios ranging from 0.35 to 0.53 and（Hf/Sm）_N ratios ranging from0.71 to 0.75,suggesting that they may have formed from partial melting of the mantle wedge metasomatized by subduction-related fluids.The second group of basalts exhibit low Si O₂contents（48.18–48.67 wt%）,Mg O contents（3.66–3.88 wt%）,and high Ti O₂ contents（3.30–3.38 wt%）.In addition,they have high Nb contents,（Nb/La）_PM ratios ranging from 0.60 to0.63,and Nb/U ratios ranging from 21.12 to 21.33,which are similar to Nb-enriched basalts and are related to the formation of slab-window.The third group of basalts are characterized by low Si O₂（45.50–50.05 wt%）,moderate Mg O（6.57–6.99 wt%）and Ti O₂（1.90–2.29 wt%）.In the primitive mantle-normalized spider diagram,these basalts show the characteristics of E-MORB,and also have some characteristics of back-arc basin basalts,indicating that they may have formed in the a back-arc extensional setting.The zircon U–Pb ages of 747±5 Ma,743±4 Ma and 742±5 Ma were obtained from the felsic volcanic rocks in the Suixian Group.These volcanic rocks are featured by high Si O₂contents（66.40–70.05 wt%）,moderate to high K₂O（2.77–3.71 wt%）contents and moderate Na₂O（3.04–4.29 wt%）,belonging to the calc-alkaline to high-potassium calc-alkaline series.They are enriched in LILEs and LREEs,and depleted in HFSEs such as Nb,Ta,P and Ti,showing the characteristics of arc-related magmatic rocks.The presence of inherited zircon with ages of 1873 Ma and 1852 Ma suggests that the source rock may have been formed in Paleoproterozoic.Furthermore,these samples exhibit heterogeneous zircon Hf isotopic composition(ε_Hf（t）values of-9.19 to-0.95),corresponding to two-stage model ages of 2233–1716 Ma,indicating that they were mainly derived from ancient middle to lower crust.Combined with previous works,we conclude that the Suixian Group is a set of volcanic-sedimentary rocks formed in subduction-accretion tectonic regime during the mid-Neoproterozoic.The Yaolinghe Group exposed in Suizhou area were formed around 685 to 620 Ma.The basalts from the Yaolinghe Group are characterized by low Si O₂ contents（45.07–49.65 wt%）,moderate Mg O contents（5.36–8.03 wt%）and Ti O₂ contents（1.59–1.96 wt%）.These samples have a flat trace element composition pattern with E-MORB-like geochemical signatures.The Th/Nb and Ti O₂/Yb ratios of these rocks are 0.137–0.161 and 0.652–0.772,respectively.Combined with high initial melt temperatures（>1510°C）,the asthenosphere mantle and the lithospheric mantle that had undergone the subduction-related metasomatism were involved in the formation of the Yaolinghe basalts.Three mafic dyke samples give zircon U–Pb ages of 635±2 Ma,631±3 Ma and 630±2 Ma,respectively,indicating that they were emplaced during the late Neoproterozoic.The mafic dykes are tholeiitic in composition and have low concentrations of Si O₂（44.26–49.08wt%）,varying Ti O₂（0.50–1.71 wt%）and Mg O（6.43–14.24 wt%）with Mg#values of 53 to74.They are enriched in LREEs with（La/Yb）_N ratios of 2.10–5.20 and display E-MORB-like trace element patterns.Isotopically,the mafic dykes show negativeε_Nd（t）values of-5.09 to-7.15 and low ratios of ²⁰⁶Pb/²⁰⁴Pb（16.77 to 16.92）,²⁰⁷Pb/²⁰⁴Pb（15.39 to 15.41）,and²⁰⁸Pb/²⁰⁴Pb（37.23 to 37.51）.These characteristics illustrate that the mafic dykes were mainly derived from the enriched lithospheric mantle,since the negligible crustal contamination has been assessed by the energy-constrained assimilation and fractional crystallization simulation of Sr-Nd isotopes.In addition,the mafic dykes haveΔNb values of-0.46 to+0.15 with zirconε_Hf（t）values of-1.5 to+10.0 previously reported,indicating that the depleted mantle source had participated in their formation.Furthermore,the non-modal batch melting calculation infer that,a mixing of melts from the asthenospheric mantle（18%extent of partial melting）at garnet-stable depth with peridotites from the lithospheric mantle could fit well with geochemical composition of the mafic dykes.All above characteristics indicate that the mafic dykes and the basalt from the Yaolinghe Group originated from the same mantle source.The correlation of incompatible elements reveals that these rocks were formed in a rifting continental margin during the late Neoproterozoic.Our works,together with previous studies,conclude that the South Qinling Belt has experienced a tectonic evolutionary history from early to middle Neoproterozoic subduction-accretion to late Neoproterozoic intra-continental rifting,mainly including the following 4 stages:（1）800–740 Ma slab subduction and formed the Suixian island arc basalts and rhyolites;（2）740–715 Ma slab subduction and opening of the slab window,formed the Nb-enriched basalts in the Suixian Group;（3）Back-arc extension at 715–685 Ma and formed the E-MORB-like basalts in the Suixian Group;（4）685–620 Ma continental rifting and formed the Yaolinghe basalt and a series of mafic dykes.