Composition,Forming Age And Metamorphism Of The Qinling Group In The Western Segment Of The North Qinling Orogenic Belt

The West Qinling Orogen(WQO)is located at the middle of the central orogenic system of China.It connects the East Qinling Orogenic Belt to the east and the Qilian and Kunlun orogenic belts to the west.Recently,with discovery of many kinds of HP-UHP metamorphic rocks in the Qinling Group of East Qinling Orogen(EQO),the westerly extension of this UHP belt has drawn wide concern.However,due to the limited outcrops and the influence of post tectonic overprint,research about the Qinling Group in WQO is relatively low.The questions about its composition,forming age and whether it has experienced HP-UHP metamorphism are still not clear,which restricts the understanding of tectonic evolutionary processes of the North Qinling Orogenic Belt.In this thesis,we selected three profiles(Taibai,Zhangjiazhuang,Lengshuihe)from the Baoji-Tianshui area based on extensive field study.We carried on systemic petrography,mineral chemistry,geochronology and geochemistry studies on gneiss,amphibolite and garnet amphibolite from the Qinling Group in WQO.Several conclusions of this thesis are as follows:1.The Qinling Group in the west segment of the North Qinling Orogenic Belt mainly consists of orthogneiss,paragneiss,schist,leptynite and marble with minor amphibolite and garnet amphibolite occur as lenses or layers in gneiss.Biotite-plagioclase paragneiss is the mainly rock type of the Qinling Group in WQO,and granitic gneiss,schist and leptynite are relatively minor.2.Zircons U-Pb dating shows that the inherited detrital zircons from the paragneisses yield four dominating age peaks at ~1.0Ga,~1.14 Ga,~1.45 Ga and ~1.57 Ga,and three secondary age peaks at ~1.33 Ga,~1.74 Ga and ~2.72 Ga,with the youngest detrital zircon age at ~1.0Ga.Considering the crystallization age of 954±6Ma obtained from the Neoproterozoic granite intruded into the paragneiss,we concluded that the forming age of the Qinling Group in WQO is 1.0-0.95 Ga.The metamorphic ages of paragneiss are in four age ranges of 491-501 Ma,442-459 Ma,398-425 Ma and 346-384 Ma,respectively.Among which,450-400 Ma is the most common range.Since some 500 Ma metamorphic zircons have flat HREE distribution patterns without Eu anomalies,we interpreted ~500Ma as the age of eclogite-facies metamorphism.3.The amphibolite and garnet amphibolite have protolith age of 797 Ma to 838 Ma,and they underwent metamorphism at ~495Ma,~424Ma,~400Ma and ~358Ma,which is similar with paragneiss.Geochemical datas show that the amphibolite and garnet amphibolite have within-plate basalts characteristics.Combined with its field occurrence and crystallization age,we proposed that the protolith of the amphibolite and garnet amphibolite are probably the basic dykes formed during the breakup of the Neoproterozoic Rodinia supercontinent.4.In combination with geothermobarometry and phase equilibrium calculation methods,we established a clockwise P-T path for the Taibai garnet amphibolite.The estimated peak P-T conditions is 10.5~10.6Kbar,650~660?by using mineral assemblage mantle composition of garnet and Pl+Amp in matrix.Whether it represents the peak mineral assemblage and condition remain to be further verified.5.A comprehensive comparison study shows that:(1)the clasitic rocks from Qinling Group in WQO and EQO have the same youngest detrital zircon age and analogical age spectrum,indicating that they were deposited in the same sedimentary environment.Combined with the maximum emplacement age of Neoproterozoic granite in the Qinling Group,we proposed that the forming age of the Qinling Group should be 1.0-0.96 Ga.(2)The most obvious Mesoproterozoic(1300-1600Ma)age peak in clasitic rocks from the Qinling Group is distinctly different from the chronological records of the North China Block,the South Qinling Belt and the Yangtze Block,indicating that the North Qinling Orogen may be an independent microcontinent in the Meso-Neoproterozoic.(3)The meta-basic rocks in WQO and EQO have similar geochemical characteristics and protolith ages.Moreover,both the meta-clastic and meta-basic rocks recorded multiple metamorphic events of ~500Ma and 450-400 Ma,which are basically consistent with the EQO.Therefore,we suggest that the WQO and EQO should be a uniform early Paleozoic collision orogenic belt.