Coupling Mechanism Of Crust-and Mantle-derived Magmatism Under Different Tectonic Settings

The appearance of crust-derived magmatism is one of the Earth's outstanding features that distinguishe it from other solid planets.Nevertheless,the connection between crust-and mantle-derived magmatism is still not clear,and there are seveval problems to be solved.What role does the mantle-derived magma play in the process of producing crust-derived magma?What is the coupling mechamisn between crust-and mantle-derived magmatism?How does magma derived from different sources sustain the exchange of material and energy between mantle and crust?To solve the problems above,we choose the Neoproterozoic igneous rocks from northern Guangxi,and Early Mesozoic volcanic rocks from northern Vietnam along the southern margin of Yangtze Block to explore the process and coupling mechanism of crust-and mantle-derived magmatism behind the outcrop of natural rock through systemaic research on the petrography,geochemistry,isotopic geology and geochronology.On the one hand,the coupling and decoupling relationship between mantle-and crust-derived magmatism at different stages of sucduction-suture zone is explored through the petrogenesis research on Neoproterozoic high-Mg tonalite and the discussion on the magma source melting progress of mafic-ultramafic rocks from northern Guangxi.On the other hand,the magmatic evolution of Permian mafic-felsic volvanic rocks from Tu Le Basin of northern Vietnam is studied to explore the episodic evolution and eruption of volcanic rocks under the extension background related to mantle plume as well as the coupling relationship between mantle-and crust-derived magmatism.Furthermore,through comparing the geochemical covariant relationship between crust-and mantle-derived magma under different tectonic settings,the water content in granitic magma system is simulated and quantified in this thesis.The concept of granite hygrometer was firstly proposed based on numerical simulation.High-Mg tonalites were discovered in the southern part of the ca.830 Ma Dongma Pluton,northern Guangxi Province of South China.The tonalites(SiO2=59?65 wt%)are characterized by high MgO(6.7?8.9 wt%)contents and Mg-number[Mg#=100×Mg/(Mg+Fe)](69?73),in contrary to the associated medium-Mg(MgO=3.4?3.8 wt%,Mg#=59?63)granodiorites in the Dongma main body and the low-Mg(MgO=1.4?1.9 wt%,Mg#=46?51)granodiorites in the Bendong Pluton to the north,making these high-Mg tonalites to be ideal research object on the coupling mechanism of crust-and mantle-derived magmatism.Moreover,the high-Mg tonalites show surprisingly high Cr(595?640 ppm)and Ni(171?194 ppm)concentrations,which are beyond the ranges of most coeval mafic rocks in the study area.Correspondingly,chromite crystals were separated from the high-Mg tonalites and some of the medium-Mg granodiorites,and they show high Cr#[100×Cr/(Cr+Al)](average of 75),but low Mg#[100×Mg/(Mg+Fe)](0.34?2.51)and low Fe3+.The decoupling of Cr#and Mg#and the existence of quartz+apatite mineral inclusion in chromites suggest Mg-Fe exchange that may be facilitated by the disequilibrium resulted from magma mixing.The high-Mg tonalites show low La/Yb(6.8?8.5)and Sr/Y(2.1?3.1)ratios,significant negative anomalies of Nb and Ti and positive anomaly of Pb,resembling the Setouchi high-Mg andesites.All of the studied tonalites and granodiorites show enriched Nd isotope compositions,with ?Nd(t)values(-3.2 to-5.9)a bit higher than some of the associated mafic rocks.Some of the high-Mg tonalites show whole-rock ?Hf(t)(-6.0 to-6.2)coupled with Nd isotopes,similar to the associated mafic-ultramafic rocks in northern Guangxi,suggesting the metasomatism by melts of subducting sediments in the mantle source.Whereas,others show decoupled Nd-Hf isotopes that are similar to the medium-and low-Mg granodiorites[?Hf(t)=-1.8 to+0.05].Based on these petrological and geochemical observations,a two-stage model for the petrogenesis of the high-Mg tonalites is proposed:1)the mantle source was firstly metasomatized by melts from partial melting of subducting terrigenous sediments to form the enriched Nd-Hf isotopic characteristics;and then 2)the mantle-derived high-Mg mafic melts mixed with the crust-derived low-Mg granitic melts to form the high-Mg tonalites and medium-Mg granodiorites.The occurrence of high-Mg tonalites implies the existence of Neoproterozoic subduction-related metasomatism in the western part of the Jiangnan Orogen.Furthermore,the evolution of lithospheric mantle is explored through evaluating the relationship between the petrogenesis and crust-mantle mamgmatism from two episodes of Neoproterozoic mafic magmatism in the western segment of the Jiangnan Orogen.Field observation indicates the first episode of mafic magmatism took place at ca.830 Ma in the Baotan area and was associated with dioritic-granitic rocks.While the second formed at ca.770 Ma in the Longsheng area without significant granitic magmatism.T-P-XH2O estimation is conducted based on bulk-rock geochemistry to distinguish magma evolution and geochemical characteristics between the two episodes of mafic rocks.The ca.830 Ma Baotan mafic-ultramafic rocks with high SiO2 content(53?57 wt%)and low Mg#(28?42)indicate the mamga have undergone a certain degree of evolution.Their relatively enriched Nd-Hf isotopic characters[?Nd(t)=-8.3 to-0.9,?Hf(t)=-6.3 to-2.0]indicate the magma source resulted from hydrous melting of metasomatized mantle wedge with depth less than 60 km.Heat and volatiles can be released from the underplating mafic magmas to facilitate further crustal melting which generates the associated granitoid rocks.Whereas the ca.770 Ma Longsheng mafic-ultramafic rocks show relative lower SiO2 content(37?50 wt%),higher Mg#(37?65),with steep rare earth element patterns similar to oceanic island basalt(OIB),except for Nb(2.9?26.7)value obviously lower than OIB(Nb=67).Relatively radiogenic and coupled Nd[?Nd(t)values of-1.3 to+3.1)]and Hf isotopes[?Hf(t)values of+2.6 to+6.7]suggests that the Longsheng mafic-ultramafic rocks formed in a post-orogenic extensional setting as the product of partial melting during the upwelling of depleted asthenospheric mantle,which could be as deep as 140 km.The diverse petrogenesis between the two episodes of mantle-derived magmas shed light on the magma source trasition from lithospheric mantle to depleted asthenospheric mantle,which serves as an indicator of Neoproterozoic subduction related orogenesis and submergence in the western segment of the Jiangnan Orogen.Water could be an important factor determing whether crust-and mantle-derived magmatism are coupled or decouped for the two episodes of magma.Subduction zone is more favorable for water circulation,and partial melting of the crust to facilitate felsic magma evolution.In contrast,after subduction,the lack of water reduces the production of felsic magma.In order to further explore the coupling relationship between crust-and mantle-derived magma,formation and evolution of high-Si felsic volvanic rocks in mantle plume settings is investigated.The Permian basalts exposed in Tu Le Basin of northern Vietnam show high TiO2 content,with geochemical and isotopic characteristics similar to typical OIB,are almost identical geochemically to those of the Emeishan Large Igneous Province(ELIP)in southwestern China,confirming they are part of that province.While the accompanying felsic rocks of the Tu Le Basin formed within 260?252 Ma resemble typical A-type granites,with enriched high field strength element and obvious fractional crystallization.They can be divided into two groups according to SiO2 content:high-SiO2(>74 wt%)felsic rocks that formed earlier(260?255 Ma)according to zircon U-Pb dating,and low-SiO2(63?74 wt%)felsic rocks that were erupted later(254?252 Ma).According to calculation results of zirconium saturation thermometer,the high-SiO2 rocks tend to have higher melt temperatures than the low-SiO2 rocks(950?1100? vs.890?1010?).Geochemical modeling indicates that the felsic rocks were not generated by one continuous process of differentiation from mafic magma.Such multi-stage differentiation is consistent with episodic variations in Na2O,K2O,Rb/Sr,Rb,and Zr/Nb that are shown by multi-stage crystallization of plagioclase,K-felspar,apatite.Rhyolite-MELTS modeling results indicate that volvanic rocks in the Tu Le Basin was fromed from water-poor(H2O<0.3 wt%)magma under oxygen fugacity of ?FMQ?+1.During the evolution of a silicic magma chamber,ca.260?253 Ma high-SiO2 felsic rocks erupted at high temperature,leading to the relative low-Si residual melt to erupt during the mamga chamber cooling(ca.254?252 Ma).Volcanic rocks in the Tu Le Basin indicate the the felsic volcanic rocks is not derived from the basaltic volcanic rocks,but is the product of multi-stage differentiation of felsic magma chambers facilitated by crust melting in mantle plume.The occorance of later coupling between crust-and mantle-derived magmatism is mainly caused by the high temperature effect of the mantle plume.Under this regime,mass of crust-derived magma can be produced without much water.Through the above studies on the coupling mechanism of crust-and mantle-derived magma under subduction-suture zone and mantle plume settings,the effect of water and heat cannot be neglected.Thereinto,how to quantify water content in the magmatic system is an important issue in the study of granite genesis and evolution.Water influences the physics and chemistry of magmatic differentiation because it reduces the melting point,decreases melt viscosity,modifies phase equilibria,and controls how latent heat is released.Here,we explore how compositional trends of fractionating magmas can be used to evaluate the amount of water in the crystallizing system.MELTS modeling results indicate that water plays a vital role during the magma evolution from basaltic magma to silicic magmas endmenbers.Specifically,water expands the crystallization window over which silicic melts are stable(0 ? F ? 1).Such that granites with SiO2 increase are limited to small residual melt fractions(F<0.2)under dry conditions,but stable over a wider range of residual melt fractions(F<0.6)at high water contents.Therefore,in the SiO2-F trend,hygrometer can be used to indicate the water content for granitoids.To verify the reliability of SiO2-F hygrometer,we collected magmatic data from different tectonic settings.Using the abundance(C)ratio of incompatible element(Th or K)at different stages of magma differentiation relative to the initial magma abundance(Co),the residual melt fraction F duing magmatic evolution can be calculated.That is F=Co/C.From the SiO2-F trend,we confirm that arc magmas differentiate at higher water contents,while intra-plate magmas as well as mid-ocean ridge magma indicate less water content,which is consist with existing knowledge,therefore verified reliability of SiO2-F hygrometer.Moreover,from the compiled data indicate that Archean TTGs(tonalite,trondjhemite,and granodiorite)show similar Mg#-SiO2 systematics as modern arcs,suggesting similar petrogenetic conditions.Therefore,water content plays an important role on crust-and mantle-derived magma evolution,through which we can quantify the water content during magmatism,to order to figure out the coupling mechanism between crust-and mantle-derived magmatism.