Precambrian Tectono-magmatic Events Of Yuanmou Complex In Southwestern Margin Of Yangtze Block And Its Response To Supercontinent Evolution

Precambrian,especially the period from Archean to Paleoproterozoic,is a key period to understanding the formation,evolution,and geodynamic process of the early Earth.The tectono-magmatic-thermal events and geodynamic processes experienced during this period are the front topics of concern to geoscientists.The Yangtze Block is one of the ancient cratons in China.Compared with the widely reported Archean to Paleoproterozoic of early Precambrian crystalline basement exposed on the north margin of the Yangtze Block,there are fewer Paleoproterozoic to Archean rocks exposed on the southwest margin,and due to the limitation of the research,we have a limited understanding of the formation and evolution of the early continental crust of the Yangtze Block.Therefore,whether there is an ancient crystalline basement in the southwestern Yangtze Block has always been the goal of many geologists.Recently,this paper has carried out an in-depth study on Yuanmou complex located in the south-central Kangdian region of the southwestern Yangtze Block,and found that it has exposed granitic gneiss,migmatized gneiss,garnet mica schist and amphibolite containing Paleoproterozoic to Archean age information.These rocks are mainly exposed in the Pudeng Group and suffered from strong superimposed metamorphism-deformation.Based on the detailed field macroscopic structural observation,and combined with detailed microscopic-Cathodoluminescence observations,lattice optimization and orientation analysis of minerals,electron probe composition analysis,zircon U-Pb chronology,Lu-Hf isotope and whole-rock geochemical analysis,this study focus on the limitation of rock formation age,the petrogenesis of the rocks,the evolution process,and to elucidate the tectonic background during the Paleoproterozoic to Neo proterozoic.Meanwhile,the temperature and pressure conditions of different metamorphic and deformation stages are defined through the calculation from conventional geothermobarometry,which provides a direct geological basis for the study of Precambrian tectono-magmatic events and the evolution history of Yuanmou complex in Kangdian region,southwestern Yangtze Block.Several results have been achieved:（1）Through LA-ICPMs zircon U-Pb dating,four group ages of zircon cores in granitic gneiss,metasedimentary rock and amphibolite have been obtained,which are2589–2300 Ma,2200–1770 Ma,1580–1200 Ma and 1100–1000 Ma,respectively.These zircons are of magmatic origin,with magmatic oscillatory zones and high Th/U ratio（>0.4）,confirming that there is a 2589–2300 Ma magmatic event in the southwestern margin of the Yangtze Block.The Th/U ratio of the zircon rim was generally low（<0.1）,showing two groups of age peak,980–840 Ma and 780–750 Ma.（2）Zircon Lu-Hf isotope analysis shows that the zircon cores with Neoarchean to early Paleoproterozoic（2589–2300 Ma）have positiveεHf（t）value,the two-stage model(T_DM2)age is 2760–2688 Ma,indicating that the magma was derived from depleted mantle and there was a crustal accretion event in the Neoarchean-Paleoproterozoic.It further reveals that there was obvious magmatic activity at 2589–2300 Ma in the Neoarchean to Paleoproterozoic on the southwestern margin of the Yangtze Block,which may be the response to ancient continent or craton convergence orogenic event in the southwestern margin of the Yangtze Block.In the Middle to late Paleoproterozoic（2200–1770 Ma）,theεHf（t）is mainly negative,and the corresponding T_DM2 age is 2800–2400Ma,indicating that its magma is derived from ancient（Neoarchean）crustal material with a small amount of depleted mantle material involved.It corresponds to the convergence period of the Colombia supercontinent.Early Mesoproterozoic（1580–1200 Ma）zircons mainly have positiveεHf（t）values and the two-stage model age is 1739–2487 Ma,indicating that the magma originated from depleted mantle with ancient（Paleoproterozoic）crustal material involved.There were crustal accretion and crustal reworking events during the Mesoproterozoic.（3）The geological events recorded by the zircon metamorphic edge ages（two major age groups of about 980–840 and 780–750 Ma）are the responses of the Neoproterozoic Rodinia supercontinent on the southwestern margin of the Yangtze Block.These zircon rims obtained the negativeεHf（t）values from-10.0 to 2.1,indicating that the magma originated from ancient crustal materials and there was a crustal reworking event.The metamorphic age of the zircon rim is related to regional superimposed metamorphism and partial melting events.（4）The granitic gneiss samples show S-type granites with aluminous characteristics.The REE distribution pattern shows that it is characterized by significant enrichment of light REE compared with heavy REE,the significant negative anomaly of Eu,enrichment of Rb,K,Th,U and Pb,and depletion of Ba,Nb,Ta,Sr and Ti.The negative Eu anomaly is generally considered to be related to the existence of plagioclase as a refractory residual phase during the partial melting of plagioclase-rich rocks.The strong Ti depletion indicates that the rutile or apatite residual phase may exist in the magma source area,while the positive U and Th anomalies in the samples indicate that they may be affected by metamorphic fluid.（5）The metamorphic rocks of Yuanmou Complex have experienced three distinct stages of deformation-metamorphism and exhumation,that is,progressive metamorphism（M₁-D₁）,peak metamorphism with high-temperature crystalline plastic deformation（M₂-D₂）,and retrograde metamorphism-deformation and exhumation stage（M₃-D₃）.The high-temperature metamorphism corresponds to the ages（970–750 Ma）obtained from metamorphic zircon rims,which is a response to the tectono-thermal metamorphism in the southwestern margin of the Yangtze Block.Due to the influence of retrograde metamorphism,the high-temperature ductile deformation-metamorphism environment is at least up to high amphibolite facies.The calculated temperature ranges from 620℃to 690℃,the pressure is between 0.8 and 0.95 Gpa,and there is a clockwise P-T evolution paths.