Early Paleozoic Ploy-metamorphism And Tectonic Evolution Of The Northern Qinling-Tongbai Orogenic Belt

The formation and evolution of continental collision orogen is a prominent feature of convergent plate margins, which is also one of the key subjects of geodynamics. It has been recognized recently that it may be a complex process in the formation and evolution of a collision orogen, which may include multistage compression and extension. Understanding crustal evolution, and in particular orogenic processes, involves reading the encoded geological record represented by the metamorphic rocks that make up the basement. Metamorphic rocks collected from ancient and modern orogens have followed a more or less complicated pressure (P) and temperature (T) path, from the Earth's surface, through the bowels of the orogen and back to the Earth's surface again. The P-T-t paths are a critical parameterisation of orogeny, as P paths reflect the depth history of rocks and T paths the thermal state as depth changed. Establishing P-T paths is essential to understanding orogeny and consequently the history of the continental crust.The Qinling-Dabie-Sulu orogen marks the suture zone between the North China and Yangtze cratons in Central China. Recent studies have detected that the Qinling-Dabie-Sulu orogen is a typical multiple evolution orogen, including the Early Paleozoic continental subduction and collision, the Silurian extension and rifting in relation to the opening of the Paleotethyan Ocean floor, and the Triassic continental material subduction, HP-UHP metamorphism and subsequent exhumation. Moerover, The results of recent studies show that the amalgamation of Qinling and North China Block in the Early Paleozoic also was a multistage process. In north Qinling-Tongbai orogen, not only exist500～480Ma HP/UHP metamorphism and magmatism, but also exist440-400Ma high-temperature granulites, migmatites, and some rocks record multi-stage metamorphism. This provides a good material for research poly-metamorphism in north Qinling-Tongbai orogen, is also an ideal place to study continental margin crustal evolution and plate dynamics. The nature and age of the protolith provide insight into the formation and early evolution of Qinling Group. Precise metamorphic ages and metamorphic evolution of Qinling Group are critical to decipher the tectonic evolution of the Qinling-Tongbai orogenic belt in Early Paleozoic, but also shed light on understanding crustal evolution at convergent plate boundaries. Additionally, Qinling Group is a unique area that contains both granulites and associated migmatite, magmatic rocks. Therefore, it is an optimal place to unravel the relationships among granulite-facies metamorphism, migmatite and granite.In this Thesis, the metamorphic P-T path of metamorphic rocks from Qinling Group have been determined based on microstructural analysis, P-T pseudosections, and mineral isopleths. Additional, trace elements, Hf isotopes of zircons and REE partitioning between zircon and garnet are adopted to track the formation condition of zircon and constrain the time of protolith, poly-metamorphism and Migmatization. Combining with published data, the poly-metamorphism and tectonic evolution history of north Qinling-Tongbai orogenic belt during Early Paleozoic, were discussed. The main reseach results are follows:1. This is the first report for sapphirine-bearing granulite from the Qinling Group in Tongbai area. And some sapphirine occurs in direct contact with quartz, indicates that the granulites from Tongbai area suffered ultra-high temperature matamorphism. According the results of equilibrium thermodynamics model indicate the peak metamorphic condition with>1020℃,>10kbar. The P-T-t path established for the Tongbai granulites shows an anticlockwise evolution trend. It records a first low-P heating followed by a dramatic pressure increase within the sillimanite field. The prograde metamorphism occurred under<6kbar and ca.800℃at443.3±3.1Ma. The peak metamorphic (M2) conditions, according to the petrogenetic grid and geothermobarometry, are around850-920℃at a pressure around8.5-9.5kbar, and the peak metamorphism age is ca.431.8±4.3Ma. At around419Ma, the granulites suffered a strong retrograde metamorphism (M3), represented by replacemet of garnet by biotite and plagioclase, or clinopyroxene by amphibole, and the metamorphic conditions were approximaely700℃at a pressure of7kbar. The last retrograde metamorphism (M4) represented as greenschist-facies overprint with age of ca.400Ma, and the metamorphic P-T condition was ca.500℃at5.8kbar.2. The results of equilibrium thermodynamics model and geochronology indicate that the leucosomes are produced via melting by quartz feldspathic gneiss or schist part, while the metamafic rocks should be of mafic dyke in the gneiss but not the residual of partial melting. Garnet-bearing gneiss or schist recod the peak meatamorphic P-T condition of around760℃,7.5kbar, followed by a nearly isobaric cooling. However, most rocks preserved relatively low temperature of720～670℃, and the leucosomes vein only recorded the P-T condition of～640℃,5.5～6.0kbar, just above the solidus. This is the result of some degree of hydrous retrogression and reequilibrium on cooling as the melt crystallizes and releases its H2O.3. LA-ICPMS zircon dating shows migmatization ranges from418to424Ma. This result is consistent with the retrograded age (-420Ma) of granulite in Tongbai, but younger than the peak metamorphic age (-430Ma). According to the phase equilibrium model, the age of leucosome in migmatite or garnet-bearing granite represent the melt crystallization age during retrograded process. It suggests that the onset of extensional tectonism in the Tongbai orogen is no later than420Ma. 4. The protolith age of orthogneiss in Qinling Group is around940Ma. The protolith ages of metabasic rocks in Qinling Group, including the metabasic veins or lens, range from750-800Ma. These ages mentioned above may correspond to the magma events during assembly-breakup of Rodinia supercontinent in Neoproterozoic.5. We get the peak temperature of670℃and pressure up to26kbar for eclogite in northern Qinling Group, the age of which ranges from485to500Ma. After experiencing isothermal decompression process to temperature of～620℃and pressure of～7kbar, it retrogrades to greenschit face at the age of470Ma (T<～430,P<～5kbar),6. The high pressure basic granulite in Songshugou area suffer the peak metamorphic conditions of T>800℃and P>16.5kbar. After experiencing isothermal decompression process to temperature of750℃and pressure of7kbar, it retrogrades to greenschit face. Zircon U-Pb dating reveals the age of granulite face metamorphism ranges from480to495Ma. The age of sphene that is paragenetic with actinolite is at the age of473±32Ma, revealing at that time the area having exhumated to greenschist face. In addition, high pressure granulite as well as revelant garnet amphibolite in Songshugou area records the metamorphic event at the age of400～430Ma, which imposes on the480-495Ma high pressure event. According to the zonation of amphibole and phase equilibrium simulation, we infer a increase of pressure and temperature from greenshcist face to lower amphibolite face (～500℃,～5kbar) caused the second metamorphism. The age of second metamorphism is consistent with the age of granulite metamorphism in Tongbai area and migmatization in Qinling, implying they being caused by the same tectonic event. All the result prove the metamorphism at age of400-440Ma is not the retrograded or thermal relaxation continuation of480-500Ma high pressure event, but representing another isolated thermal event.7. Zircon U-Pb dating results show the rocks in Qinling Group widespread suffer the480-500Ma high grade metamorphism. Northern part of Qinling Group experience low temperature but high-ultra high pressure metamorphism, implying a low geothermal gradient; however, southern part of Qinling Group experienced both high temperature and pressure metamorphism, exhibiting a higher geothermal gradient. It shows the eclogite and high pressure granulite form in the different thermodynamics but at the same time. We infer the eclogite in the northern part is derived from the subduction slab, but the formation of high pressure granulite is caused by the thickening of lower continental crust of the hanging side.8. During400to400Ma, North Qinling-Tongbai orogen suffer from considerable metamorphism and migmatization. From west to east, the degree of metamorphism reinforce gradually. In Tongbai area, granulites suffered UHT metamophism with an anticlockwise P-T-t path. Combining the considerable magma events, the addition of magma material with large scale mineraliztion, we raise a most possible tectonic modal:the north-heading of a ridge subduction cause a slab window open which is filled by upwelling mantle material, also leading to low P/T metamorphic condition. The difference between Qinling Gourp in east and west may be caused by the different erosion extent during uplifting, namely, the west part is eroded lesser than the east part.