The Growth And Evolution Of The Paleo-Neoarchean Continental Crust In The Dharwar Craton,India

The Dharwar Craton in the southern India is one of the most important blocks which composed the “Indian shield”.Multi-stages tectono-thermal events were developed from Paleoarchean(~3.4 Ga)to Neoarchean(~2.5 Ga)in the Dharwar Craton.It is quite important to clarify the stages of these magmatic activities and to ascertain the growth and evolutionary history of the continental crust in the Dharwar Craton,which will provide significant information not only for understanding the cratonization process of ancient cratons,but also for the global comparative study.The Holenarsipur nucleus,Chitradurga-Sira-Kibbanahalli corridor and Kolar area are the mostly studied gneiss terranes in the Dharwar Craton,with Archean geological records well-preserved,and showing the typical rock assemblages of continental crust from the western Dharwar Craton to the eastern Craton.In this study,based on field observation,two types of granitoid gneisses including trondhjemitic gneisses and granitic gneisses from the above three regions were chosen for the petrological,petrographical,whole-rock geochemical,and zircon U-Pb geochronological and Lu-Hf isotopic analysis,combined with the previously published data,aiming to discuss the growth and evolutionary history of the continental crust in the Dharwar Craton.The following innovative understandings have been mainly obtained from this study.(1)The Archean magmatic activities in the Dharwar Craton,IndiaAccording to the different rock types,zircon U-Pb geochronology,and zircon Lu-Hf and whole-rock Nd isotopic features,the Archean magmatic activities in the Dharwar Craton can be broadly divided into three stages,including the Paleoarchean(3.4-3.2 Ga)stage,the Mesoarchean(~3.1-3.0 Ga)stage and the Neoarchean(2.7-2.5 Ga)stage,which is related to different stages of continental crust growth and reworking in the Dharwar Craton.(2)The Paleoarchean(3.4-3.2 Ga)is the major period of crustal growth in the Dharwar CratonThe Paleoarchean(3.4-3.2 Ga)granitoid magmatism in the Dharwar Craton generated numerous silicon-and sodium-rich,but potassium-poor TTG rocks of predominatly trondhjemitic gneisses,which is more extensively recorded in the western Dharwar Craton than in the eastern Dharwar.These TTG rocks have low Mg# values and Cr and Ni concentrations,as well as depleted zircon Hf isotopic compositions,with zircon ?Hf(t)values of-2.1~+5.7 and slightly older two stage zircon Hf model ages of 3677-3212 Ma,which isclose to the depleted mantle evolutionary line and approximate to the formation age of the Paleoarchean(3.4-3.2 Ga)TTG rocks,respectively.These features suggest that the Paleoarchean TTG rocks in the Dharwar Craton were produced by partial melting of juvenile mafic(basaltic)rocks,representing a major crustal growth event in the Dharwar Craton.(3)The Mesoarchean(~3.1-3.0 Ga)is an important period of crustal reworking and activation in the Dharwar CratonThe Mesoarchean(~3.1-3.0 Ga)granitic magmatism is predominantly developed in the western Dharwar Craton,represented by numerous granitic gneisses.Compared to the Paleoarchean TTG gneisses,these granitic gneisses are featured by higher contents of Si O2 and K2 O.In addition,there are minor ~3.0 Ga TTG gneisses reported in the eastern Dharwar Craton.These Mesoarchean(~3.1-3.0 Ga)granitoid rocks have low Mg# values and Cr and Ni concentrations,as well as relatively enriched zircon Hf isotopic compositions that with negative to positive zircon ?Hf(t)values of-5.7~+6.3 and older two stage zircon Hf model ages of 3569-2973 Ma,plotting on the 3.5-3.0 Ga crustal evolutionary line,indicating that these Mesoarchean(~3.1-3.0 Ga)granitoid rocks were derived from partial melting of Paleo-Mesoarchean continental crust,representing crustal reworking and activation event.Due to lacking of systematic petrographic observation,the geological significance of this event has not been paid enough attention.This study shows that the widely development of Mesoarchean granites probably marks the initial cratonization process of the Dharwar Craton,and also suggest that the continental crust of the Dharwar Craton has been developed and had a certain thickness at this period,with the crustal maturity gradually increased.By comparing the contemporaneous granitic magmatism occurred in the other cratons in the world,it is speculated that this event may have a global significance,which is worthy of attention in the future research.(4)The Neoarchean(2.7-2.5 Ga)is the period of final maturation and stabilization of continental crust in the Dharwar cratonAfter the crustal activation event at Mesoarchean(3.1-3.0 Ga),the Dharwar Craton experienced a relatively quiet period.Until the Neoarchean(2.7-2.5 Ga),the magmatic activities started again,generated numerous TTG rocks and high-K calc-alkaline granites in both the western and eastern Dharwar cratons.Except that the trondhjemitic gneisses in the western Dharwar Craton are featured by the relatively depleted zircon Hf isotopic compositions(?Hf(t)= +0.2~+7.5,TDM2=2942-2670 Ma),most of the trondhjemitic gneisses and granitic gneisses in this study have relatively enriched zircon Hf isotopic compositions,characterized by negative zircon ?Hf(t)values(-11.2~-0.2)and older two stage zircon Hf model ages(TDM2=3442-2916 Ma),indicating that they were derived from partial melting of Paleo-Mesoarchean continental crust,and representing a significant crustal reworking event.Combined with previously published data,the Neoarchean(2.7-2.5 Ga)is an important crustal reworking period in the Dharwar Craton,accompanied by small amount of crustal growth.The magmatism happened in this period leading to the final maturation and stabilization of continental crust in the Dharwar craton.The occurrence of large-scale of K-rich granites at the end of Neoarchean,together with the subsequent development of basic dyke swarms possibly indicate the final cratonization of the Dharwar Craton.(5)The Dharwar Craton in India underwent multi-stages crustal growth,reworking and cratonizationIn a summary,based on the spatial-temporal distribution of the Archean magmatic-thermal events and isotopic compositions of the granitic rocks in the Dharwar Craton,we speculate that the western Dharwar Craton and the eastern Dharwar Craton may have formed as part of a single terrane and underwent the same crustal growth and evolution history.However,the Paleoarchean to Mesoarchean crustal rocks in the eastern Dharwar Craton have been strongly reworked by the Neoarchean magmatic-thermal events compared to those in the western Dharwar Craton,and the mechanism needs to be further studied.The different rock types and zircon Hf isotopic compisions suggest that the Dharwar Craton is featured by multi-stages crustal growth(Paleoarchean 3.4-3.2 Ga and Neoarchean 2.7-2.5 Ga),reworking(Mesoarchean 3.1-3.0 Ga and Neoarchean 2.7-2.5 Ga)and cratonization(Mesoarchean ~3.0 Ga and Neoarchean ~2.5 Ga).