Formation Of Huge Composite Granitic Batholiths In Jiuling Area Of North Jiangxi Province

The formation of huge composite batholiths is one of the key issues in granitic petrogenesis and crustal evolution.This contains a lot of information on a series of complex magmatic processes,including granitic melt production by disequilibrium melting of crustal materials,melt extraction,segregation,and differentiation.It plays a vital role in revealing the formation of granitoids.This thesis chooses the Jiuling composite batholith from Jiangxi Province in the eastern Jiangnan orogeny between Yangtze and Cathaysia Blocks and provides systematic studies on the mineralogy,petrology,geochemistry,isotopic geology and geochronology of the Neoproterozoic and Mesozoic granitoids within the batholith.The petrogenesis and evolution of the Neoproterozoic and Late Jurassic plutons within the Jiuling composite batholith,as well as the genetic links among them,have been discussed.These plutons are also compared with the Late Jurassic granitoids in the Dahutang tungsten deposit area to provide implications for the potential ore(especially tungsten)deposits.A systematic LA-ICP-MS zircon U-Pb geochronology study has been carried on the huge Jiuling composite batholith.The Neoproterozoic granodioritic and tonalitic plutons consist of Meiling,Jiuling,Shihuajian and Jiuxiantang plutons,formed at ca.827.4-816.9 Ma,ca.827.4-810.1 Ma,ca.824.3-821.6 Ma and ca.818.0-814.2 Ma,respectively.In comparison,the Late Jurassic Shiqiao(granodiorites),Ganfang(muscovite and two-mica granites)and Guyangzhai(two-mica granites)plutons were crystallized at ca.148.3-147.2 Ma,ca.146.7-144.0 Ma and ca.147.0-146.0 Ma,respectively.The host rocks of the Dahutang tungsten deposit also include Neoproterozoic granodiorites and Yanshanian granites(named as Dahutang granites)with crystallization ages of ca.830 Ma and ca.148.3-145.5 Ma,consistent with the ages of different plutons within the Jiuling huge composite batholith.Combined with field relationship,the Neoproterozoic batholith and Late Jurassic granitic suite were both accumulated incrementally by multiple batches of granitic magmas.Despite of the differences in geochronology,the Neoproterozoic and Late Jurassic granitoids have many similarities in magma sources.LA-MC-ICP-MS zircon Hf isotopes and whole-rock Nd isotopes among the Neoproterozoic granitoids(εHf(t):-7.7～+13.6;εNd(t):-5.4～-1.9;t=820 Ma)and Late Jurassic granitoids(εHf(t):-19.5～-4.1;εNd(t):-9.5～-5.6;t=146 Ma)are similar to that of the wall rocks(i.e.the Shuangqiaoshan Group)(calculated at the same time),which indicates the sources are similar to the Shuangqiaoshan Group in geochemistry.Due to a large amount of Neoproterozoic zircon cores in the Late Jurassic plutons,we discuss the possible genetic links between the Neoproterozoic and Late Jurassic granitoids.Combined with the low-greenschist metamorphism in the field,the simulations using the Rhyolite-MELTS and Energy-Constrained Assimilation-Fractional Crystallization(EC-AFC)programs preclude the possibility that partial melting of the Neoproterozoic granitoids could generate the Late Jurassic granitoids.A systematic geochemical data indicate different batches of granitic magmas with different characters and mixing within each single bath in the formation of the Neoproterozoic granitoids.To be specific,different batches of magma are characterized by distinct geochemistry(e.g.major and trace elements and sources)and physical parameters(i.e.magmatic areal addition rates and temperatures).Moreover,magmatic zircon in different Neoproterozoic plutons exhibit large variation in Hf isotopes,with more than 7 εHf epsilon units in general.The δ18O values of the magmatic zircon rims of these plutons have variations of 2-7‰(Meiling pluton:7.3-10.2‰;Jiuling pluton:7.4-10.9‰;Shihuajian pluton:5.6-13.1‰;and Jiuxiantang pluton:7.6-9.3‰).In particular,the cores have higher δ18O than that of the rims in some Neoproterozoic granitoids,together with large Hf and O isotopic variations,which implies the extensive mixing in each single batch of granitic magmas during the generation of the Neoproterozoic Jiuling plutons.Therefore,a huge composite batholith can be accumulated by different batches granitic magmas and by mixing within each individual bath.A deep crustal hot zone(DCHZ)model is generally used to account for the genesis of intermediate and silicic magmas in continental arc areas.And this model could also be applied to intracontinental settings where granitoids formed with similar geochemical characters.Based on systematic and comparative studies in this area,we,for the first time,proposed that there could be widespread DCHZs beneath the South China Block in the Neoproterozoic and Late Jurassic periods.In the Neoproterozoic period,the range of DCHZs is relatively small but sustains a longer time.This results in huge production of the Meiling,Jiuling and Shihuajian plutons by partial melting of the Shuangqiaoshan Group,among which the Shihuajian pluton intruded the Jiuling pluton.With the migration of the melting boundary in the DCHZ,the magma slightly changed the composition,and crystallized as tonalite within the Jiuxiantang pluton,which also intruded the Jiuling pluton.In the Late Jurassic period,the granodioritic magmas generated in the deeper crustal levels of the DCHZ ascended before stalling in a shallower part of the upper crust,forming the Shiqiao intrusion.The other magmas formed by the migration of the melting boundary in the DCHZ and input of new melts,which changed the magmatic compositions.The magmas also underwent extensive fractional crystallization,which generated highly differentiated magmas that were fluid-enriched at the top of the magma chamber.Some of the fluid-enriched highly differentiated magmas migrated upward through some faults or weak boundaries,assimilated crustal material,and formed the Ganfang granites.The rest of the magma in the chamber continued evolving,producing a similar differentiation trend to that of the Ganfang magma.This evolved magma,which also experienced crustal assimilation,intruded the early-formed Ganfang intrusion to form the Guyangzhai intrusion.This model of magma growth could be applied to other composite plutons,account for these granitic magmatic progresses,and the geochemical and petrographic variations observed in granitic intrusions elsewhere,as well as providing new insights into the relationship between granites and possible metallogenesis.Generally,the highly differentiated magmatic compositions,enrichments of late-magmatic fluids,high metallic element concentrations of sources,and various geochemical indicators of granite-related mineralization all favor the potential formation of W-Sn mineralization in the Late Jurassic GF-GYZ intrusive suite,as evidenced by the presence of the world-class Dahutang W deposit within the northeastern part of the Jiuling huge composite batholith.This could shed lights on the metallogenic explosion in South China from Jurassic to Cretaceous,and provide perspectives in finding ore deposits.Furthermore,this thesis also provides dissolution experiments(in hotplate,bomb and sintering procedures)of zircon-bearing samples to demonstrate the heterogeneous Hf isotopes under disequilibrium melting.The results reveal partial dissolution yielded variable and more radiogenic Hf isotope compositions than fully dissolved samples.A case study from the Neoproterozoic Jiuling batholith in southern China shows that about half of the investigated samples show decoupled Hf isotopes between zircons and the bulk rocks.This decoupling could reflect complex and prolonged magmatic processes,such as crustal assimilation,magma mixing,and disequilibrium melting,which are consistent with the wide temperature spectrum from～630 0C to～900 0C by Ti-in-zircon thermometer.We suggest that magmatic zircons may only record the Hf isotopic composition of their surrounding melt during crystallization and it is uncertain whether their Hf isotopic compositions are primary and can represent the primary Hf isotopic compositions of the bulk magmas.In this regard,only using zircon Hf isotopic compositions to trace crustal evolution may be biased since most of which could be originally from disequilibrium partial melts.This is also verified that disequilibrium melting is very common in the crust.