The Neoarchean To Late Neoproterozoic Magmatism In The Southwestern Yangtze And Its Response To Supercontinent Cycles

The Yangtze is one of the largest precambrian blocks in China.Its precambrian geological evolutional history is still ambiguous due to Archean to late Mesoproterozoic basement rocks sporadical outcrups in its peripheries.This further makes it hard to build the relationship with precambrian supercontinents.In this study,we took Neoarchaean-Mesoproterozoic rocks samples from the Zhongxiang complex in the northeastern part and Cuoke complex in the southwestern part of the Yangtze block,and displayed systematic petrology,whole rock geochemistry,zircon U-Pb geochronology and Hf isotope analysis,the following conclusions have been obtained:(1)The monzogranites from the Zhongxiang complex are formed at?2.73 Ga,and display high Si O₂(73.11-74.01 wt.%),K₂O(3.93-5.48 wt.%),Na₂O(3.93-4.86 wt.%)and low Ca O(0.30-0.69 wt.%),Mg O(0.17-0.30 wt.%),Ti O₂(0.14-0.17 wt.%),P₂O₅(0.01-0.06 wt.%),Al2O3(14.11-14.37 wt.%)content with weakly peraluminous affinity(A/CNK=1.04-1.11).Geochemically,they belong to I-type granites.The consistent whole-rock Nd and zircon Hf isotopic compositions indicate a homogeneous source.According to their?Hf(t)values(-2.0 to 0.8),two-stage Hf model ages(3.1-3.2 Ga)and positive?Nd(t)(1.4-2.1),we argue that they were probably generated by partial melting of a juvenile lower crust with little ancient materials.Their relatively high Nb/Ta(15.2-34.8)ratios suggest they formed under eclogite facies conditions.Combined with reported 2.73 Ga and 2.67-2.62 Ga A-type granites in the Zhongxiang Uplift,we infer that the transition from I-type to A-type magmatism may have resulted from a switch in the geodynamic regime from the late-orogenic or collisional compressive environment to an extensional environment caused by the subsequent lithospheric collapse and mantle upwelling,also suggesting an early Neoarchean orogenic event in the eastern Yangtze Craton.Based on the comparison of the precambrian magmatic records and Hf isotopic compositions of the Archean basements in the Yangtze block,we suggest that only the Kongling and Zhongxiang complexes were assemblied in the Neoarchean,while other Archean basements should be assemblied with the Kongling and Zhongxiang complexes after the Neoarchean.(2)Three episodic Paleoproterozoic magmatism were identified in the Cuoke complex and formed at?1.93 Ga,?1.88 Ga and?1.84 Ga respectively.Samples CKN09(?1.88 Ga)and DA060(?1.84 Ga)show the geochemical characteristics of high potassium I-type granite,while samples CKN08(?1.93 Ga)and DA854(?1.84 Ga)show the geochemical characteristics of albite granite.?1.93 Ga albite granites also show belong to I-type granite,while?1.84 Ga albite granites are classified as A type granite.Their whole rock geochemistry and Hf isotopic compositions(3.1-3.0 Ga)suggest that the high-k I-type granites were formed by partial melting of ancient felsic igneous rocks(e.g.,tonalite),while the albite granites were formed by partial melting of plagioclase-rich layers.Combined with previous research results,we suggest that the1.93-1.88 Ga granitic rocks formed in the transitional environment from late collisional to post-collisional tectonic setting,while the?1.84 Ga granitic melts formed in a post-collision environment.Based on the comparison of archean to Paleoproterozoic magmatism,metamorphic events and their geological evolutional histories of the Archean basements of the Yangtze block,we argue that the final assembly was completed at?1.93Ga,followed by extensive cratonization.Furthermore,the Archean to Paleoproterozoic magmatic records and Hf isotopic compositions in the southwestern margin of the Yangtze displayed the inimate relationship with the Rae craton in Laurentia continent;the Kongling and Zhongxiang complexes in the northeastern part also have a close correlation with the Aldan block in the Siberia continent;and the Yudongzi and Douling complexes in northwestern part are likely adjacent to the Anarbar block in the early Paleoproterozoic.Therefore,the Yangtze block was not only involved in the assembly of the Nuna supercontinent,but also the final assembly of the basements occurred at?1.93 Ga.(3)The newly identified monzogranites from the Cuoke complex in the southwestern Yangtze Block formed at 1.18-1.14 Ga.All the studied samples possess typical geochemical signatures of A₁-type granite.They have negative whole-rock?_Nd(t)values from-6.9 to-5.7 and zircon?Hf(t)values from-7.4 to-2.5,with consistent Nd and Hf model ages of 2.4-2.1 Ga.The parental magmas for these monzogranites were generated by partial melting of the early Paleoproterozoic crustal materials at low pressure and high temperature.Taking into account of the coeval within-plate mafic magmatism in the southwestern Yangtze Block,these ca.1.18-1.14 Ga A₁-type monzogranites were most likely emplaced in a continental rift setting.Available data indicate continental rifting may have occurred extensively in the Yangtze Block at1.2-1.1 Ga,followed by oceanic plate subduction starting at 1.1 Ga.We propose the Yangtze Block was an isolated plate during 1.2-1.1 Ga and started to drift toward the Rodinia supercontinent at 1.1 Ga.