| The southwestern Yangtze Block?i.e.traditional“Kanddian Axis”?preserves the best early Precambrian geological records and has been an ideal area for investigating the Precambrian geology and supercontinent reconstruction.On the basis of detailed field investigations,an integrated study of petrology,stratigraphy,sedimentology,whole-rock geochemistry,zircon U-Pb geochronology and Hf isotopes was conducted on meta-granitoids from the Cuoke complex and volcano-sedimentary sequence from the Tongan Formation and Huili Group in the southwestern Yangtze Block.Formation ages,petrogenesis and metamorphic history of the Cuoke complex were determined.Archean-Paleoproterozoic crustal evolution history of the southwestern Yangtze Block was reconstructed,and the initial cratonization of the Yangtze Block and its temporal-spatial linkage to the Nuna supercontinent assembly were also discussed.On the other hand,stratigraphic ages,sedimentary provenance and volcanic genesis of the Tongan Formation and Huili Group were revealed.The basin dynamic processes of the Yangtze Block in the Mesoproterozoic and their responses to the global supercontinent cycle were finally explored.The Cuoke complex is a recently identified early Precambrian basement complex in the southwestern Yangtze Block,which has key implications for the evolution of early continental crust.The analyzed zircon domains are of magmatic origin,as evidenced by their oscillatory zoning,high Th/U ratios and magmatic REE patterns.Regressions of U-Pb isotope data of three trondjemitic gneisses yield formation ages of 3061±23,3073±23 and 3110±6 Ma,documenting the oldest basement rocks in the southern Yangtze Block.Their?Hf?t?values range from-3.2 to2.6 and corresponding depleted mantle two-stage(TDM2)model ages range between3.52 and 3.43 Ga,suggestive of an origin from reworking of Paleoarchean crustal materials.Two granodiorites and one monzogranite crystallized at 2855±16,2853±14 and 2857±9 Ma,respectively,indicating a significant episode of magmatism at?2.85 Ga.They have positive?Hf?t?values from 0.2 to 5.0 and TDM2 model ages from 3.26 to 2.97 Ga,suggesting that they were generated by remelting of juvenile continental crust.Zircon U-Pb-Hf isotope data from the Cuoke complex reveal the earliest crustal growth at 3.5 Ga with significant crustal growth and reworking at 3.1Ga,followed by crustal reworking at?2.85 Ga.Through integrating our results with records from other Archean basement complexes in the Yangtze Block,it is confirmed that the formation and evolution of the basement complexes are different.Paleoproterozoic magmatic rocks are significant for investigating the early evolution of the Yangtze Block and its reconstruction in the Nuna supercontinent.SHRIMP zircon U-Pb dating yields magmatic crystallization ages of?2.22 and?2.35Ga for two meta-monzogranites,?2.36 Ga for a meta-granodiorite,and?2.34 Ga for granitic gneiss.The granitic gneiss also contains metamorphic zircons that give an age of 1.96 Ga.LA-ICP-MS zircon U-Pb dating results show that two meta-monzogranites formed at?1.94 and?1.89 Ga.The studied meta-granitoids show negative zircon?Hf?t?values from-14.0 to-1.9,with corresponding TDM2 model ages of ca.3.50?2.93 Ga,indicating substantial involvement of ancient crustal materials during multiple crustal reworking in the Paleoproterozoic.In combination with petrological and geochemical data,it is suggested that they were derived from partial melting of ancient crustal sources.These 2.36?2.34 Ga meta-granitoids represent the post-collisional magmatism immediately following a possible collisional event,while the?2.22 Ga A2-type monzogranites were likely formed in a back-arc extensional setting.These results,compiled with published geochronological data,suggest that the western part of the Yangtze Block features extensive magmatism with ages of 2.5?2.2Ga and multiple metamorphism with peaks?2.50,2.36,1.96 and 1.85 Ga.This is significantly different from the eastern part of the Yangtze Block,dominated by the2.15?1.95 Ga subduction-and collision-related metamorphism and magmatism with a peak?2.0 Ga.The widespread 2.0?1.9 Ga magmatism and metamorphism in the whole Yangtze Block likely responded to its initial cratonization,which may be related to the assembly of Nuna supercontinent.Volcano-sedimentary successions deposited in various basins are crucial records for exploring the geodynamics and surface response during the supercontinent cycle.The early Mesoproterozoic Tongan Formation is one of the most representative volcano-sedimentary successions in the southwestern Yangtze Block.Geochronology and geochemistry studies reveal that meta-basalts from the lowermost part of the 1st member of the Tongan Formation formed at 1.73?1.72 Ga and have geochemical signatures similar to OIB;by contrast,meta-tuffs and meta-basalts form the 3rd member crystallized at 1.55?1.45 Ga,and these meta-basalts show geochemical characters comparable with E-MORB.Sedimentological study confirms that meta-sedimentary rocks of the 1st member are diluvial fan-braided river-shoreland sediments in a continental rift basin,the 2nd member represents the stable platform environment during the post-rift thermal subsidence,the 3rd member has been evolved into deep-sea basin with turbidity sediments on the continental slope,and the4th member carbonate was deposited in a epicontinental basin with unrestricted marine circulation.Detrital zircons indicate that the Tongan provenance is dominated by the Paleoproterozoic magmatic rocks in the Yangtze Block,related to the multiple collisional events.In view of regional magmatic-metamorphic-sedimentary records,the early Mesoproterozoic volcano and sedimentary successions in the Yangtze Block,as represented by the Tongan Formation,recorded the basin dynamic processes from a continental rift to the passive continental margin in the period 1.75?1.40 Ga.They are the consequence of long-term lithosphere extension in the context of the Nuna supercontinent breakup.A systematic study of characteristic detrital zircon ages and Hf isotopes obtained from sedimentary sequences in basins can reveals shifts in sedimentary provenance and basin dynamics.The Huili Group,one of the representative late Mesoproterozoic strata in the southwestern Yangtze Block,has been considered as the sedimentary response to the assembly of the Rodinia supercontinent.The youngest detrital zircon peak ages of meta-sedimentary rocks,collected from the stratotype sections of the Huili Group,define the depositional time of this group as ca.1.1?1.0 Ga.A dramatic provenance shift within the Huili Group is expressed as the detrital zircon age spectra.The Limahe Formation of the lower Huili Group is characterized by these detrital zircons with ages of 2.34?1.72 Ga,likely derived from multiple provenances such as the sedimentary recycle of the early Mesoproterozoic strata.By contrast,the detrital zircon age spectra of the Tianbaoshan Formation,the topmost unit of the Huili Group,show bimodal characteristic.In other words,the syn-deposition magmatic rocks from the adjacent areas contributed abundant materials to the Tianbaoshan Formation.Through integrating our new data with regional geology observations,a new scenario has been proposed for the Meso-Neoproterozoic basin dynamic evolution.During1.18?1.06 Ga,the southwestern Yangtze Block was likely a rift basin in the passive continental margin,and then evolved into a back-arc extension basin induced by the slab-rollback during the period 1.05?0.94 Ga.Furthermore,we suggest that these late Mesoproterozoic strata represent sedimentary records prior to the Rodinia assembly. |
PDF Full Text Request