Response To Columbia Supercontinent In The Western Margin Of Yangtze Block

The western margin of the Yangtze block is surrounded by the Kangding region of Sichuan in the north,the Yuangjiang River in the north,the Puduhe fault in the east,and the Yuanmou-Lvzhijiang River fault in the west.The part of the Yangtze block contains the most extensively exposed Paleo-Mesoproterozoic rocks in southern China and presents an ideal region for Precambrian research.Such research can improve our current understanding of the formation and evolution of the Earth's crust,as well as the origin and development of the supercontinent.In this paper,the Paleo-Mesoproterozoic strata,magmatic rocks,and mineral deposits of the Tongan area on the western margin of the Yangtze block were studied.This study combined the whole rock chemical compositions,zircon U-Pb dating and Hf-isotope analysis with basic geological data to discuss the ancient geological records of the western margin of the Yangtze block as well as systematically and comprehensively summarize how this area responded to the assembly and breakup of Columbia supercontinent in terms of sedimentary rocks,magmatic rocks,and minerals.Through this study,the Precambrian basement of the Yangtze block and the Late Paleo-Mesoproterozoic tectonic evolution are clarified.Detrital zircon U-Pb age and Hf-isotope analysis are conducted on clastic rocks collected from Yinmin formation,on the bottom of the Dongchuan Group,on the western margin of the Yangtze block.Among the 230 effective points,the oldest zircon ?Hf(t)(t = 3654 Ma)value is—3.1,with TDMC ages 4081 Ma.One of the zircon's sHf(t)(t?2,406 Ma)value is-20.1,with TDMC ages 4,152 Ma.These results are similar to those of Zhang(2006),who reported detrital zircons with ages around 3.8 Ga.Among all the effective analyzed points,there are 10 with ages greater than 3.0 Ga and 82 between 2.5 and 3.0 Ga.These results indicate that the Yangtze block contains geological information from Archean even Hadean.At approximately 2.95 Ga,the age ratio increased significantly,reflecting the fact that more of the new crust was preserved at that time,which may suggest the occurrence of a structural system transformation related to the beginning of global plate tectonics at 3.0 Ga.In the age data obtained from Yinmin formation,the ages are primarily within the range of 1.7-3.0 Ga.For ages greater than 2.0 Ga,there are two age peaks of approximately 2.3 and 2.7 Ga respectively,where EHf(t)are mostly negative,with TDM2 ages from 2.5 to 4.0 Ga.This area is studied along with the scattered Archean to Early Paleoproterozoic continental crust rocks on the Yangtze block,metamorphic rocks,inherited zircon core existed in magmatic rocks and metamorphic rocks,the age of Archean-Early Paleoproterozoic samples and the ancient detrital zircons in the sedimentary rocks.All these information implies that the Archean-Early Proterozoic basement was widely developed on the Yangtze block,and the initial crustal materials may have crystallized from the mantle in Early Archean.Since 3 Ga,the continental crust preservation record has been characterized by the global cycle of peaks and valleys of U-Pb crystal ages,where the peak ages match the periods during the assemblage of supercontiental.Increasing evidence suggests that the peak age of zircon represents a better preservation of magma in the process of supercontinental assemblage and marks a period of accelerated crustal growth because more old crust remained in the new crust.The zircon ages obtained from Yinmin Formation indicates that at 2.0-1.8 Ga magmatism is also widely developed in the western margin of the Yangtze block,and there is a distinct peak with a primary peak age of about 1.84 Ga;this peak age is very close to the formation age of the Late Paleoproterozoic tuff of 1859.3±9.2 Ma and consistent with the ancient Proterozoic(?2.0-1.8 Ga)global collisional orogeny,as well as the Columbia supercontinent formation era.The samples are characterized by low SiO2 content of 50.84%on average,higher Al2O3 content of 16.71%on average,a higher K2O content of 8.9 1%on average,lightly positive ?Eu anomaly,ranging from 1.03 to 1.13 with an average of 1.06.On the whole,tuff samples are enrichment in large ion lithophile elements(Rb,Ba,U)and depletions in high field strength elements(HFSE;Nb,Ta,P,Zr,Ti),with the average values ranging from 207 to 283ppm.The right-slope REE pattern,similar with crustal derived melts,indicates that it is deposited in island arc environment.The sHf(t)values are between-9.4 and+4.8,with 5 positive values and 14 negative values.We analyzed 6 good points of Hf isotope and they all have negative sHf(t)values and the tDM?2385?2599 Ma,with an average of 2492 Ma,that are significantly older than their Zircon U-Pb age.This information indicates that the magma is formed from the partial melt of mantle wedge metasomatized by melt/fruid from ancient subducted continental crust.The age of the gabbro is 1694± 15Ma(n= 11,MSWD= 1.7),while the age of diabase is 1689:±34Ma(n?20,MSWD?2.1),and the weighted age of the diorite is 1715 ? 20Ma(n=20,MSWD = 0.19).These ages are very close within the error range,around 1.7 Ga,and consistent with the extensive development of magmatism event during this period.Most of sHf(t)values of Zircon are positive,with only two negative values,and the sHf(t)model ages are similar to the relevant zircon age,indicating that the material source of the 1.7-Ga magmatic rock in the western margin of the Yangtze block may be directly derived from the mantle or juvenile crust.In the spider diagram of trace elements and Chondrite-normalized REE distribution patterns,the characteristics are similar to those of E-MORB that are apparently different from OIB.The gabbro is characterized by slightly enrichments in Nb and Ta,which is different with island arc igneous rocks.In addition,there are extensively distributed A-type granite porphyry at 1.7 Ga,indicating that the magma is derived in the extensional tectonic setting on the west margin of the Yangtze block during Late Proterozoic.This period of magrmatism may represent the initial breakup of the western margin of the Yangtze block,which is the response of the breakup of Colombian supercontinent in this region.In this study,the age of gabbro associated with Zhuqing vanadium-titanium magnetite is 1647±39 Ma(MSWD?0.79).The zircon sHf(t)values are all positive,with an average of+10.7 and TDM 1661 Ma?which is almost consistent with the formation age of the gabbro,indicating that the source region may be directly from the mantle.The content of MgO,Fe2O3,and TiO2 has a significant negative correlation with SiO2,which reflects the magma crystallization and differentiation related to mineral genesis.The spider diagram of trace elements and Chondrite-normalized REE distribution patterns indicate that the gabbro has the same pattern with OIB.In the Nb-Zr-Y diagram,all the points fall into the intraplate region,which suggests that the vanadium-titanium magnetite is related to mantle plume or hot spot with OIB characteristics,and its material source may drive directly from the mantle.The metallogenic related magmatism is likely the response of breakup of the Columbia supercontinent,and the OIB-like mantle plume or hot spot in this period facilitate the occurance of the Columbia supercontinental breakup.In addition,the western margin of the Yangtze block was continuously stretch and breakup from the Paleo-Middle Proterozoic,and the Dongchuan Group,the Dahongshan Group,the Hekou Group and some relative stratums are filled with long-time breakup products.The weighted average ages of the two tuff samples in the upper Heishan Formation of the Dongchuan Group are 1414±25 Ma(MSWD=0.45,n?40)and 1394±26 Ma(MSWD?0.73,n=50),respectively.These results are consistent within the margin of error and represent the maximum sedimentary age of Dongchuan group.Meanwhile,in the Heishan Formation,the appearance of ocean island or seamount characterized by dual structure(basalt plus carbonate),the deep sea siliceous rock,the flysch-like structure,the pillow-shaped lava,and the deep carbon black carbon shale all demonstrate that Yangtze block broke up into oceans(the Tongan ocean)from the Columbia supercontinent at around 1.40 Ga.