Ultrahigh Pressure Metamorphic Rocks In The North Qinling Orogen

The Qinling orogenic belt is a composite orogenic system,which is located in the western segment of the Central Orogenic Belt of China.It records continental subduction and arc-continental accretion in the Paleozoic and continental collision between the North and South China Blocks in the Mesozoic.Therefore,it shows widespread occurrences of Ordovician and Triassic high pressure(HP)and ultrahigh pressure(UHP)metamorphic rocks,which are not only an important target to study the tectonic evolution of convergent plate boundaries,but also an ideal laboratory to study the recrycling and crusta-mantle interaction in continental subduction channels.The North Qinling orogenic belt is a collision orogen with a complex tectonic evolution history in the early Paleozoic,which has undergone multi-stages metamorphic evolution during the continental subduction and exhumation.The metamorphic evolution and tectonic framework of the North Qinling orogen in the Early Paleozoic has long been controversial and is one of the widely concerned geological problems.Although much isotopic work has been done,the timing of peak eclogite-facies metamorphism is still debatable.Therefore,an intergrated study of petrographic observations,mineralogy and geochemistry research of garnet amphibolites and plagioclase amphibolites as well as their country-rocks from four areas(at Qingyouhe,Zhaigen,Guanpo and Songshugou)in the North Qinling orogen.The results provide new constraints on the tectonic evolution of HP-UHP metamorphic rocks in the North Qinling orogenic belt in the early Paleozoic.In this dissertation presents a combined study of the whole rock major element and trace element analysis of the metabasite rock from four areas in the North Qinling orogen.The results show that the garnet amphibolites contains high content of Mg,Fe,Mn and Ti(MgO=5.48-6.16 wt.%,Fe2O3=15.10-24.22wt.%,MnO=0.19-0.44 wt.%,TiO2=2.24-4.25 wt.%).The country rocks contain high content of Si(SiO2=66.11-73.99 wt.%),Na(Na2O=1.30-4.09 wt.%)and K(K2O=0.84-4.05 wt.%).The country rocks of plagioclase amphibolite are relatively enriched in LILE and LREE,and depleted in HFSE.Most of the country rocks have a characteristic with obvious negative Eu anomaly(Eu*=0.25-0.42),positive Th and U anomaly,and high Th content(11.6-20.8 ppm).Instead,the metabasite rock has lower Th content(0.86-3.77 ppm)and Th/U ratio(0.70-6.15).This dissertation presents the intergrated study of garnet geochemistry and zircon geochronology for the garnet amphibolite from the Songshugou area.The garnet's major element results indicate that the Songshugou garnet is almandine and grossular,which have obvious core-rim composition zones characteristics.From core to rim,the pyrope(Pyr)contents increase gradually(Grain ?:7.9-18.0 mol%;Grain ?:7.1-14.0 mol%)and the spessartine(Sps)contents constantly decrease(Grain I:9.4-0.50 mol%;Grain ?:8.7-2.1 mol%)in the garnet,which stands for the typical prograde growth zones.The Mg#(0.11-0.22)increased continuously from the core to rim,and the core is enriched in HREE and depletes MREE,indicating that the rock reached the metamorphic peak at the garnet rim.Based on the characristics of the petrographic microstructures,mineral inclusions,and mineral compositions,three metamorphic stages are recognized for mineral assemblages in the garnet amphibolite:prograde stage(522-611?/1.4-3.0kbar),peak stage(742-749?/>3.0GPa)and retrograde stage(726-764?/7.3-8.3kbar).Geochronology study reveals that zircon grains in thin sections and mounts record three episodes of growth or reworking,respectively:(1)relict magmatic cores(G1),showing typical oscillatory zoning with middle Neoprotrozoic U-Pb ages of 746±16 Ma to 720±7 Ma and steep HREE patterns with obvious negative Eu anomalies,indicating the protolith of garnet amphibolite was produced in the Neoproterozoic;(2)metamorphically recrystallized domains(G2),exhibiting faintish oscillatory zoning or unzoning with discordant U-Pb ages ranging from 547±17 Ma to 492±15 Ma and steep HREE patterns with positive to negative Eu anomalies;(3)metamorphically grown domains(G3),which occur not only as overgrown rim in mounts but also as mineral inclusion within garnet rims in thin sections.They gave consistent 206Pb/238U age of 491 Ma to 480 Ma(average age is 488±10 Ma),flat HREE patterns with low Th/U ratios of 0.01 to 0.04,high MREE contents and no obvious Eu anomalies,indicating the timing of peak eclogite-facies metamorphism.In addition,the rod-like exsolution composed of rutile+apatite+quartz is identified in the garnet rim that formed at the peak metamorphic stage from the garnet amphibolite,indicating that it might have ever underwent ultra-high pressure metamorphism.Therefore,the garnet amphibolite has its protolith from the North Qinling microcontinent that was separated from Rodinia supercontinent between 746 Ma and 720 Ma.It was deeply subducted for metamorphism in the Cambrian to achieve the peak eclogite-facies metamorphism at 488 Ma,and then underwent exhumation for the retrograde amphibolite facies metamorphism.