Depositional Age,Provenance And Tectonic Setting Of The Kuanping Group In The North Qinling Orogenic Belt,China

The North Qinling Orogenic belt,located between the southern margin of the North China Craton and the South Qinling Orogenic belt,has experienced strong metamorphism and tectonic compression.It is the most complex zone in the Qinling orogenic belt and is also important to understand the tectonic evolution of the whole Qinling orogenic belt and the original Tethys Ocean.However,due to the complexity of tectonic processes and the limitations of dating methods,the early tectonic evolution of the North Qinling Orogenic belt is still unclear.Located at the northernmost margin of the Qinling orogenic belt,the Kuanping Group has well preserved a large number of key Neoproterozoic-Early Paleozoic geological records and identified ophiolitic mélanges representing the Kuanping oceanic crust remnants.Therefore,it is very useful to select the Kuanping Group as the research object to solve this problem.In addition,because the Kuanping Ocean is closely related to the age,provenance and tectonic affinity of the North Qinling Orogenic belt,the study of the Kuanping Group can also well constrain the tectonic evolution of the North Qinling Orogenic Belt.This is of great significance to rebuild the tectonic framework in the Precambrian and reconstruct the global Rodinia supercontinent.Two widely distributed kinds of metasedimentary rocks,including two-mica schist and biotite quartz schist,were chosen from the Kuangping Group as the important objects of this study.We carried out detailed studies on field geological investigations and indoor lithofacies,zircon U-Pb chronology,whole-rock geochemistry and zircon Hf isotopes.Focusing on the key issueses,we aim to constrain the formation age and source characteristics of the Kuanping Group,discuss the crustal growth and tectonic background when the group deposited,and then to provide reliable evidence for the Precambrian evolution of the Qinling Orogenic Belt.Sedimentary samples from the Kuanping Group have wide range of major element contents with SiO₂ from 58 wt.% to 76 wt.%,Al₂O₃ from 10 wt.% to 16 wt.%,Fe₂O₃ T from 4.9 wt.% to 7.4 wt.%,MgO from 1.7 wt.% to 4.0 wt.%,CaO from 0.6 wt.% to 3.5 wt.%,NaO from 0.9 wt.% to 2.8 wt.%,and K₂O from 1.7 wt.% to 5.3 wt.%.Compared with the post-Archaean Australian shale,these samples have similar SiO₂,Al₂O₃ and K₂O contents,but lower Na2 O and CaO contents,higher Fe₂O₃ and MgO contents,approaching to the average composition of the upper crust.These low metamorphic sedimentary rocks show relatively similar REE distribution patterns.They show enriched light REE with relatively flat heavy REE??La/Yb?_N=6.3-12.5 and?Gd/Yb?_N=1.5-2.2?,and have relatively high total REE contents??REE=156-251 ppm?.These low metamorphic sedimentary rocks show significant negative Eu anomalies??Eu=0.58-0.68?,similar to post-Archaean Australian shales,and have significant negative Ba,Nb,Ti characteristics and clear negative Sr,Eu anomalies.The detrital zircons from the Kuangping Group are mostly semicircular to subcircular,with different grain sizes and poor psephicity,indicating that the detrital sediments are dominated by proximal provenances after a short distance of transportation.Most of the detrital zircons have Oscillating zone,showing an igneous origin.The zircons have well recorded the geological history from the early Archean to the early Palaeozoic?3440 to 488 Ma?,of which the oldest inherited zircon of 3.44 Ga is preserved.The detrital zircons show a wide range of initial 176Hf/177 Hf ratios and can be further divided into positive?from 0 to 8.3?and negative?from-32.0 to-0.4?groups.Based on the age of zircons,their calculated TDM2 ages are mainly distributed in the range of 1381-3138 Ma and 1567-4084 Ma,respectively.According to detailed studies of the Kuangping Group,we have obtained the following conclusions:1.The youngest age of metamorphic zircons from the Kuanping Group is 488 ± 3 Ma.In combination with the age of regional metamorphism?440 Ma?,the sedimentary age of the Kuanping sedimentary rocks is limited to the early Paleozoic?488-440 Ma?,rather than the Mesoproterozoic-Neoproterozoic or Paleoproterozoic as previously proposed,which is consistent with the Ordovician fossils found in the low metamorphosed sedimentary rocks of this group.2.The geochemical characteristics of the Kuangping sedimentary rocks indicate a material source mainly derived from the felsic upper crust and a depositional environment close to continental island arc in active continental margins,such as the back-arc basin.3.The detrital zircons have ages mainly distributed in the Neoproterozoic?1000-900 Ma?,with an age peak at 994 Ma,and other subdominated groups at 2500-2400 Ma,1300-1100 Ma and 850-700 Ma.By comparing zircon U-Pb ages and Hf isotopes with those of detrital zircons from adjacent areas,we suggest that the Kuanping Group has dual provenance,with a main source from the Qinling Group,and another source from the southern margin of the North China Craton.4.The source area of the Kuanping Group has experienced three important crustal evolution events,including a Neoproterozoic crustal reworking event and two significant Paleoproterozoic crustal reworking and growth events,which may be related to the convergence and breakup of supercontinents.