The Paleoproterozoic To Mesoproterozoic Magmatism In The SW Yangtze Block And Its Geological Implications

The Columbia supercontinent is likely the first truly large supercontinent in the Earth’s history,which was formed by aggregation of various cratonic blocks on a global scale within the timeframe of 2100-1800 Ma and then experienced incomplete breakup from 1790 to 1300 Ma.Intriguingly,its assembly has a close relationship with the evolution and even transition of plate tectonics,and its dispersal witnessed a significant change in the outer layer of Earth.In light of this,studying the geodynamic processes of the assembly and dispersal of the Columbia supercontinent would help us to better understand the evolution of plate tectonics,as well as the evolution of the lithosphere,biosphere,and atmosphere during the Paleoproterozoic to Mesoproterozoic time.The Yangtze Block was involved in the assembly and breakup processes of the Columbia supercontinent,which provide us an important window to disentangle the aforementioned issue.In the present study,we focus on the early Paleoproterozoic metasediment-derived granitoids in the Cuoke area and the late Paleoproterozoic to early Mesoproterozoic mafic intrusions in the Huili-Dongchuan region in the SW Yangtze Block.An integrated study of detailed field investigation,petrography,whole-rock major and trace elements,Sr-Nd isotopes,as well as zircon U-Pb-Hf-O isotopes and baddeleyite U-Pb dating was carried out on these rocks,combing with statistical analyses of geochemical compositions of global igneous rocks and zircon oxygen isotopes,the major achievements are summarized below:（1）LA-ICP-MS zircon U-Pb dating constrains that the crystallization ages of the Cuoke granitoids（including granitic gneiss,monzogranite,and granodiorite）are2362-2340 Ma.The peraluminous mineral-muscovite is widely distributed in the studied rocks and they have high A/CNK values（>1.1）but low zircon saturation temperature（725-840℃）,resembling strongly peraluminous granitoids and were produced at the collision-related tectonic setting.The granitic gneisses have whole-rockε_Nd（t）values of-3.6 to+0.7,magmatic zirconδ¹⁸O values of+5.6‰to+8.0‰andε_Hf（t）values of-10.8 to-3.9;The monzogranites have whole-rockε_Nd（t）values of-0.8 to+0.5,magmatic zirconδ¹⁸O values of+8.8‰to+9.8‰andε_Hf（t）values of-4.1 to-0.8;The granodiorites have whole-rockε_Nd（t）values of-2.9 to-2.4,magmatic zirconδ¹⁸O values of+7.7‰to+9.6‰andε_Hf（t）values of-5.1 to-2.1.These Nd-Hf-O isotopic characteristics indicate that the granitic gneisses originated from a mixing magma source involving meta-sedimentary and meta-igneous materials,while the monzogranite and granodiorites may be derived from the same magma source dominated by metasedimentary materials.The statistical analyses based on global zircon oxygen isotopes show that the zirconδ¹⁸O values rapidly increased at～2360 Ma,suggesting that the maximum timing of emergence of the continents above sea level on a global scale is～2360 Ma.（2）TIMS baddeleyite U-Pb dating for the Tongan gabbro yields the crystallization age of 1703 Ma.The Tongan gabbros are subdivided into tholeiitic rocks and have E-MORB like REE distribution patterns,slightly negative to positive Eu anomalies（Eu/Eu*=0.94-1.32）,enriched in large ion lithophile elements（such as Rb,Ba,Sr）,without pronounced negative anomalies of Nb（Nb/Nb*=0.84-1.27）,Hf（Hf/Hf*=0.84-1.59）and Ti（Ti/Ti*=0.85-1.37）.They have whole-rockε_Nd（t）values of-2.8 to+2.8 and theε_Hf（t）values of+0.1 to+9.2,suggesting that the primitive magma of the Tongan gabbros originated from the asthenospheric mantle characterized by element-enriched and heterogeneous Nd-Hf isotopes.LA-ICP-MS zircon U-Pb dating for the Dongchuan dolerite plutons shows that the first pulse of the magmatic event occurred at 1740 Ma whereas the second pulse at 1700 Ma.The 1740-1700 Ma Dongchuan dolerites have tholeiitic geochemical affinities and N-MORB to E-MORB REE distribution patterns.The 1740 Ma Dongchuan dolerites possess the whole-rockε_Nd（t）values ranging from+0.9 to+3.3,zirconε_Hf（t）values spanning from-2.1 to+9.6;The 1700 Ma Dongchuan dolerites have whole-rockε_Nd（t）values of+2.8 to+3.3,zirconε_Hf（t）values of+1.5 to+8.3.These Nd-Hf isotopes suggest that the 1740-1700 Ma Dongchuan dolerites share the same magma source in the asthenospheric mantle,which is characterized by moderately depleted Nd-Hf isotopes.Integrated with regional geology and the results of mantle potential temperature calculation,we speculate that these 1740-1700 Ma mafic rocks were produced at an intracontinental rift setting and were not directly products of the mantle plume.By comparison with global contemporaneous mafic magmatic events,we proposed that the Yangtze Block has a close spatial affinity with the northern Laurentia and southern Siberia during the late Paleoproterozoic,and a far‐field extensional setting beneath the Yangtze Block was activated by the 1750 Ma Timpton mantle plume beneath Siberia.（3）LA-ICP-MS zircon U-Pb dating for the Dongchuan dolerite dykes yields a crystallization age of～1500 Ma.They have alkaline geochemical affinities and typical OIB-like REE distribution patterns,obviously negative to slightly positive Eu anomalies（Eu/Eu*=0.60-1.10）,as well as significantly positive Nb-Ta anomalies（Nb/Nb*=1.1-1.4）,negligible Zr-Hf anomalies（Hf/Hf*=0.8-1.0）,slightly negative Ti anomalies（Ti/Ti*=0.6-1.0）.In addition,they have whole-rockε_Nd（t）values of-0.8 to+0.1 and zirconε_Hf（t）values of-3.3 to+8.4.Based on geochemical and isotopes characteristics,we propose that the Dongchuan dolerite dykes were produced in an intracontinental rift context and their primitive magma originated from the asthenospheric mantle characterized by element-enriched and heterogenous Hf isotopes.By comparison with the early Mesoproterozoic mafic magmatic events among other cratonic blocks,we further propose that the Yangtze Block was possibly the nearest neighbor to the northwestern part of Siberian Craton at～1500 Ma and finally drifted from the Columbia supercontinent,driven by the 1500 Ma Kuonamka mantle plumeIn combination with the comparison of global Paleoproterozoic to Mesoproterozoic mafic magmatic events and statistical analyses on geochemical compositions of global igneous rocks and zircon oxygen isotopes.We propose that the continents were emerged above sea level on a global scale during the early Paleoproterozoic,leading to the development of a global network of plate tectonics since～2100 Ma,and finally contributed to the assembly of the Columbia supercontinent.The incomplete breakup of the Columbia supercontinent was mainly governed by multiple mantle plumes,resulting in the abnormal hot mantle and crust（with high heat flow）,further promoting the highly compositional differentiation in the shallow crust,yielding high Si and low P felsic rocks in the upper continental crust,which finally stall the evolutionary pathway of life over the mid-Proterozoic.