Precambrian Geochronology And Geochemistry Of The Tectonic Conjunction Of The Qinling-qilian Orogen:Evidence From The Longshan Complex

The conjunction area of the Qinling-Qilian Orogen is located in the "cross" region of the Central Orogenic System and the North-South tectonic belt.This area is influenced by the integrated effect of the paleo-Asian oceanic,the Tethys and the Pacific tectonic domains.Recent studies show that both Qinling and Qilian orogenic belt have early Paleozoic ophiolite,island arc magmatism and high-pressure/ultrahigh pressure metamorphism,which may indicate the existence of a unified ocean i.e.the primitive Tethys ocean.The conjunction area is divided into two subunits i.e.the North Qilian orogenic belt on the north and the West Qinling orogenic belt on the south by the Xinyang-YuanLong shear zone.The contact relationship of the two subunits is at an oblique angle with each other.Thus,the tectonic relationship between the Qinling and Qilian Orogenic Belts remained disputed for a long time.In this paper,we select the Longshan complex from Baoji area as the research object and carry out detailed field investigation,petrography,geochemistry,isotope chronology and whole rock Sr-Nd isotopic analysis on it,to discuss the of Precambrian history of the conjunction area of the Qinling-Qilian Orogen.The main achievements are as follows:The Tiefosi-Xiwali and Badu-Shanghetai profiles have been carried out to investigate the Precambrian Longshan complex.In Tiefosi-Xiwali profile,the Longshan complex has high-degree of metamorphism and deformation.The Main lithology include amphibolite,biotite plagioclase gneiss and biotite quartz schist etc.Based on the zircon U-Pb age dating on LA-ICP-MS,the biotite plagioclase gneiss was formed at ca.2.5 Ga,with metamorphic age at ca.1.85 Ga.Its zircon Lu-Hf isotopic analyses show that the ¹⁷⁶Hf/¹⁷⁷Hf ratios range from 0.280818 to 0.282425 with?Hf?t?values in the range of-28.9²2.In addition,most of the?Hf?t?values are positive except very few negative.The peak age of the two stage model age(T_DM2)is at ca.2.65 Ga,indicating that the biotite plagioclase gneiss originate from the juvenile crust.This is consistent with the study on rocks of the Shushui Complex in the southern margin of the North China Craton.The felsic gneiss showed high SiO₂?61 wt.%⁶9 wt.%?,Al₂O₃?14.71 wt.%¹5.82 wt.%?Sr(355×10^-6538×10^-6)contents,Na₂O/K₂O ratios?1.82².13?,and low Y(3.37×10^-612.2×10^-6),Yb(0.34×10^-61.49×10^-6)contents,?La/Yb?_N?12⁵7?,Sr/Y?31¹05?ratios,which is similar to Neoarchean TTG.While,the amphibolite displays high Mg^#value?49.2⁶7.6?.In addition,the the right-leaning type REE model,with La/Yb_CNN ratios?3.67⁷.44?,The?Eu values range from 0.71 to0.78 indicative of weak negative Eu anomaly.The primitive mantle normalized trace element concentrations are characterized by negative Nb,Ta,and positive Rb,Ba,Th anomalies,showing the characteristics of island arc magma.In the Badu-Shanghetai profile,banded gneisses expose widely,indicating that the rocks have once experienced regional migmatization.In addition,some plagioclase amphibolite and biotite monzon gneiss with weak deformation expose limitedly.Based on zircon U-Pb-Hf analyses on the LA-ICP-MS,the granitic gneiss was formed at ca.1.75 Ga.The?Hf?t?values range from-7.79 to-17.51.The peak age of the Hf two stage model ages is at ca.2.9 Ga,older than the TTG rocks in the Tiefosi area.The granitic gneiss show high K₂O?4.23 wt.%⁵.36 wt%?content,and low Na₂O?2.72 wt.%².83 wt.%?content,Na₂O/K₂O ratio?0.53⁰.64?,and Mg^#value?16²6?.The granite is plotted in the field of within plate granite.All these above indicates that this granitic gneiss resulted from the reworking of mature continental materials in the late Mesoarchean,Moreover,the granitic gneiss shows low?Nd?t??-7.5?value and its Nd model age is at 2875 Ma,close to its Hf two stage model age?3.4³.0 Ga?.The amphibolite shows high TiO₂?2.27 wt.%².29 wt.%?content,Mg^#?44⁴5?values,and a flat REE type pattern,indicative of low-degree of REE fractionation simiar with E-MORB.The ¹⁴³Nd/¹⁴⁴Nd ratio is 0.5126 with?Nd?t?value at 3.4,reflecting of mantle derived magma.All these above indicate that the grantic gneiss and amphibolite with E-MORB geochemical characteristics are formed in the tectonic setting of intraplate extensional environment.Zircon U-Pb dating of paracite quartz schist in paraceous metamorphic rock shows that its age is in the range of 471²713Ma.Limited by the youngest peak age and eldest intrusion age of the Longshan Complex,should deposited between the early Cambrian and the early Ordovician Preliminary.Thus,the formation time of this meta-sediments are obviously later than the TTG rocks in Longshan Complex formed in the late Archean-Paleoproterozoic suggesting that this meta-sediments are not the meta sedimentary rocks belonging to the basement of the NCB?North China Block?.It should be disintegrated from Longshan Complex.The meta-sediments yielding 4 age ranges including two major age populations of795¹097 Ma?49%,age peak at⁹29 Ma?and 2265²713Ma?22%,age peak at²435 Ma?,and two minor age populations of 471⁵75 Ma?12%,age peak at⁵41Ma?and 1539¹864 Ma?14%,age peak at¹717 Ma?,reflecting its complex source.Compared with the characteristic age peaks of the surrounding tectonic terrains,detrital zircons ranging of Archean to Paleoproterozoic?2713²265 Ma?and Mesoproterozoic?1864¹539 Ma?are thought to be derived from the southern margin of the NCB,the detrital zircons with Neoproterozoic ages?1097⁷95 Ma?are mainly derived from the North Qinling Orogenic Belt and Central Qilian Block,and the early Paleozoic zircons?575⁴71 Ma?are derived from the Wushan-Tianshui tectonic belt.Therefore,the biotite quartz schist of the Longshan complex is sourced from the southern margin of the NCB in the north as well as the Qilian-Qinling Orogenic Belt in the South,indicating that the meta-sediments deposited in a rifting or extending Hongtubu back-arc basin during the early Paleozoic period,which is similar to the tectonic environment of those parametamorphic rocks from the Kuanping complex and Erlangping complex in the eastern North Qinling Orogenic Belt.In summary,the Longshan Complex has rock assemblages formed in different times,thus it can be further disintegrated into two parts.The first part is the rocks with middle-deep degree metamorphism as the Precambrian crystalline basement there and the second part is the late Cambrian to early Paleozoic sedimentary rocks.The middle-deep degree metamorphic rocks of Longshan complex were formed at².5Ga and 1.75 Ga,and recorded the 1.85 Ga metamorphic event,which is similar with that of the southern margin of the North China plate.The Hf isotopic analyses show that the conjunction area of the Qinling-Qilian Orogen has experienced similar new crustal events at².5Ga with the North China plate.The granitic gneiss?¹.75Ga?was formed simultaneously with the Xiong'er Group volcanic rocks in the southern margin of the North China Plate,and both of them show the characteristics of a rift environment.All these evidence indicate the Precambrian Longshan complex has a strong affinity with the North China plate,and was probably a part of the southern margin of the North China plate basement.