Contribution Of Experimental Petrology On The Rock-forming And Mineralization Processes Of Jurassic Granites In South China

As a natural laboratory,the huge amounts of Mesozoic granite distributing in South China provided a unique opportunity to unravel the Mesozoic crust formation and evolution in southern China as well as for guiding mining exploration efforts in this area.The studies of mineralogy,petrology,geochemistry and geochronology had been carried out in South China for many decades,however,the conditions for the formation of these Mesozoic granites remain controversial.In this thesis,based on the systematically study of whole-rock geochemistry,geochronology,petrography,mineralogy,we use internally heated pressure vessel to explore the phase equilibrium of the granite from South China and successfully set up the phase diagrams.In terms of experimental petrology,our experimental results provided the first quantitative framework for the understanding of the petrogenesis of Mesozoic granites in South China.With 520 km2 out crop area,the Qitianling pluton is representative metaluminous(amphibole-bearing)Mesozoic granites in the central part of the Nanling Range in SE China,which is associated with world-class tin mineralization.It consisted of three main phases:phase-1,porphyritic,medium-to coarse-grained amphibole-biotite-rich monzonitic granite;phase-2,medium-grained biotite±amphibole-bearing granite;phase-3,fine-grained,biotite-bearing granite.The geochemical data show that Qitianling pluton is metaluminous,and belongs to aluminous A-type granites(A2 postorogenic subtype)with high Ga/Al ratio and high contents of HFSE(Zr,Nb,Ce,Y)and REE.Harker and REE spider diagrams show that the Qitianling pluton display well organised fractionation trends.Radiometric data show that the emplacement of Qitianling pluton occurred during Jurassic times.We have found crystallization ages(Zircon U-Pb age)for Qitianling pluton ranging between 149.5±4.8 Ma to 162±2 Ma being coincident with previous constraints(146±5 Ma to 163±3 Ma).The Ar-Ar dating of Qitianling granite shows that the amphibole and biotite record ages ranging from 151 Ma to 155 Ma which represent the cooling age of Qitianling.Located in the central part of the pluton,sample SC-52 acquired the ages from zircon(U-Pb system at 157.5± 1.4 Ma),amphibole(Ar-Ar system at 153.4±0.4 Ma)and biotite(Ar-Ar system at 152.6±0.8 Ma)which give a cooling rate of 80?/Ma for the cooling process of Qitianling pluton.We have experimentally established the phase relationships for the Qitianling granite.Three representative amphibole-bearing,metaluminous granitic samples were chosen for constraining crystallization conditions of the Qitianling pluton.Crystallization experiments were performed at 100-700 MPa,albeit mainly at 200 MPa or 300 MPa,at an JO2 of?NNO-1.3(1.3 log unit below the Ni-NiO buffer)or?NNO+2.4,at 660? to 900?,and at variable melt water contents(-3-8 wt%).Amphibole stability field and barometry both show that the pressure of magma emplacement was around 300-350 MPa.Amphibole and biotite Fe/Mg ratios further suggest that magmatic fO2 was around NNO-1±0.5 near solidus,while Fe-Ti oxides record an fO2 increase up to NNO+1 below solidus.Amphibole crystallization is restricted to near H2O-saturation conditions,requiring at least 5.5 wt%H2Omelt at 200 MPa,or 6-8 wt%at? 300 MPa.Amphibole occurrence in K2O-rich metaluminous silicic magmas thus indicates water contents significantly higher than the canonical value of 4 wt%.The experimental liquid line of descent obtained at 200-300 MPa mimic the geochemical trend expressed by the pluton suggesting that fractionation in the upper crustal reservoir could happen.Based on the phase equilibria of intermediate-acid igneous rocks,combining the composition of rock forming minerals(Amp and Bt)and bulk rock composition,we proposed an empirical oxygen barometer.The database of our oxygen fugacity modeling including Qitianling granite,Santa Maria dacite,Pinatubo dacite,Jamon granite,Lyngdal granodiorite which cover a SiO2 content of bulk rock ranging from 60 wt%to 71 wt%.All the required parameters are temperature,pressure,whole rock composition:ANNO=-0.0019×T(?)-12.5277×Fe#Amp+0.00142×P(MPa)+4.9068×Fe#Amp×Fe#w+0.4025×wTiO2-9.6175×w(Al2O3/SiO2)+0.459×w(Na2O/K2O)+6.5871(1)ANNO=-0.0052×T(?)-12.243×Fe#Bt+0.00177×P(MPa)+5.9615×Fe#Bt×Fe#w+0.4366×wTiO2-0.0526×w(Al2O3/SiO2)+1.2351×w(Na2O/K2O)+ 5.8933(2)These equations allow to calculate magmatic fO2 to within 1 log unit:the retrieved values most likely correspond to conditions during the late crystallization stage of the magma,owing to the easiness of re-equilibration of the used minerals.Application of our oxygen barometer to three Jurassic plutons(Jiuyishan,Huashan and Guposhan pluton)in South China demonstrates that the oxygen fugacity of those three plutons suggest a relative reduced redox environment,similar to that inferred for Qitianling granite.Besides,a highly evolved,composite ongonite-topazite with Nb-Ta-and Snbearing ore minerals dike was also involved into our study.The crystallization of NbTa-and Sn-bearing ore minerals was strongly controlled by the separation of the aluminosilicate and hydrosaline melt phases,which both are rich in F.Exsolution of the two melt phases and their efficient segregation into core and rim zones of the dike may have been driven by dike propagation/widening subsequent to the initial dike emplacement.The different content of F and other volatile components in two different melts may cause the early Nb-Ta mineralization in ongonite and later Sn mineralization in topazite.The experimental constraints on the emplacement conditions of Jurassic Qitainling granite gave several clues for better understanding the tin mineralization.The experiment of solubility of SnO2 suggested that low oxygen fugacity can be a significant constrain on the Sn incorporation in a granitic melt.Qitianling magma was emplaced at relative low oxygen fugacities(NNO-1±0.5),hence providing a beneficial environment for incorporating the Sn in the melt,at least during the early stages of its production/crystallisation.Besides,High liquidus temperature of magma plays an important role on extracting ore elements,especially in thoes anatectic melts,thus also allows high concentration of tin being reached in the silicate melt during the different geological process.Lastly,water rich magmas dissolve other volatiles such as chlorine,fluorine,boron and phosphorus which will be also helpful in capturing and transiting the ore elements such as Sn.Based on the low oxygen fugacity,high temperature and rich in melt water content,we deduced that Qitianling magma system is beneficial for the tin mineralization.What is more,high melt content in the Qitianling magma system may raise the question:Where is the water come from?Assume that there was no excess and extraneous water from the source or the country rocks,then the hydrous minerals such as amphibole and biotite bearing rocks could be a possibility of source rock of Qitianling pluton which can produce high content of water after the dehydration melting.