A Geochemical Study Of Early Paleozoic Mafic Igneous Rocks From South Qinling

The Qinling Orogen is a typical composite orogenic belt,recording the tectonic process of long-term and multi-stage from oceanic subduction to continental collision between the South and North China Block.The Qinling orogenic belt is a typical composite orogenic belt,which records the tectonic process of long and multi-stage oceanic subduction into continental collision between the South and North China Block.The Qinling orogenic belt is divided into North Qinling tectonic belt and South Qinling tectonic beltby the boundary of Luonan-Luanchuan Fault,Shangdan suture belt and Mianluo suture belt.During Early Paleozoic,the mafic dyke swarm and alkali lavas extending NW-SE were widely developed in the Northern Daba area of South Qinling.A lot of studies have been carried out on the chronology,petrology,mineralogy and geochemistry of mafic magmatic rocks and their mantle-derived xenoliths,suggesting that they may be the products of mantle-derived magmatic activity under the extensional tectonic background.However,the research of isotopic chronology,mantle source properties,deep metasomatic melt/fluid properties and magmatic formation mechanism of mafic magmatic rocks is still far from enough,which to some extent limits our in-depth understanding of Early Paleozoic tectonic evolution of South Qinling.Therefore,the petrological and geochemical studies of Early Paleozoic alkali basalts and mafic dykes in South Qinling have been carried out in detail in this thesis.And results of these studies will provide a new understanding for the mantle source properties,metasomatic melting/fluid properties,and the geodynamic mechanism and specific process of mafic magmatic rocks in this area.In order to explore the properties and differentiation behavior of oceanic crustal-derived fluids during subduction,a systematic geochemical study of carbonate-rich alkali basalts from Langao area in South Qinling was conducted,including whole-rock major and trace elements,Sr-Nd-Hf isotopes,Mg-Ca isotopes,carbonate major-trace elements and C-O isotopes.The results showed that these carbonate-rich alkali basalts are rich in carbonate minerals and carbonate inclusions,and the trace element characteristics indicate that these carbonate minerals are magmatic in origin.First,these alkali basalts show OIB-like trace element distribution features,high(87Sr/86Sr)i ratios(0.7040～0.7059)and low εNd(t)values(3.3～3.5),indicating that the mantle source has been affected by the assimilation of subducted crustal materials.Secondly,the alkali basalts also have low SiO2 content(28.4～37.1 wt.%),high CaO content(11.6～24.3 wt.%)and high CaO/Al2O3 ratio(1.0～3.7),indicating that the mantle source is the carbonated peridotite formed through carbonate melts metasomatism.Thirdly,the alkali basalts have high δ18O values(16.2～18.0‰)and δ13C values(-5.9～-1.8‰),indicating that the mantle source is affected by assimilation of sedimentary carbonate.However,the Mg-Ca isotopes composition of carbonate-rich alkali basalts show decoupled features,that lower δ44Ca values(0.64～0.96‰)and higher δ26Mg values(-0.36～0.03‰)than normal mantle.The lower Ca isotopic composition of the basalts further supports the injection of carbonate melts into the mantle source,but the higher Mg isotopic composition does not match the typical metasomatism of carbonate-derived melts with high Mg content.During the ascent of the carbonated magma,although fractional crystallization or liquid immiscibility can cause Mg-Ca isotopic fractionation(silicate melts are more enriched in heavy Mg and Ca isotopes than carbonate melts),this will generate a positive correlation between δ26Mg and δ44Ca values among the erupted lavas.However,the Langao alkali basalts show a good negative correlation between Mg-Ca isotopes compositions,which does not support the magma differentiation.On the contrary,this decoupled Mg-Ca isotopic feature is more likely to be inherited from the mantle source and the metasomatized medium itself.We thus suggest that the subducted slab-derived carbonated silicate melts(C-rich melts)at the back-arc depth,which may undergo different degrees of physical and chemical differentiation during its migration to the mantle wedge.First,as the C-rich melts from the subducted slab ascend,the carbonate components are continuously removed from the initial system,resulting in the gradual evolution of the initial C-rich melts into C-bearing melts.Secondly,during the differentiation process,the incongruent Mg-Ca isotopic fractionation(the degree of Mg isotopes fractionation is much greater than that of Ca isotopes)causes the δ26Mg value of the residual C-bearing melt to increase significantly and higher than the mantle,while the δ44Ca value changes little and is still lower than the mantle.The differentiated fluids finally modified the mantle wedge peridotite,producing alkali basaltic melts with decoupled Mg-Ca isotopic compositions.Model calculations further show that when the initial carbonated silicate melts(C-rich melts)are differentiated to a degree of F>0.1,the evolved melts begin to show heavier Mg isotopes and lighter Ca isotopes relative to the mantle;when the degree of differentiation F>0.5,highly evolved melts almost do not change the Mg-Ca isotopic composition of the normal mantle.Because with the continuous removal of carbonate components,the δ44Ca value of the residual melt will tend to the mantle,while its MgO content is significantly lower than the mantle.Therefore,the decoupled Mg-Ca isotopic characteristics of alkali basalts provide a new understanding for revealing the physical and chemical differentiation behaviors of subducted slab-derived fluids during their migration.This differentiation process will change the geochemical composition of the subduction zone fluids and ultimately affect the geochemical characteristics of subduction zone mafic magma.In order to investigate the possible melt-mantle interaction during the ascent of mantle-derived magma,we have carried out systematic geochemical research on the carbonate-poor alkali basalts in the same rock mass,on the base of the study of carbonate-rich alkali basalts in Langao,South Qinling.Zircon U-Pb dating indicates that the alkali basalt was erupted at about 454±4 Ma.Field observation shows that the carbonate-rich alkali basalt samples are closer to the center of the volcanic rock,while the carbonate-poor alkali basalt samples are closer to the edge of the volcanic rock.From carbonate-rich(SiO2=29～37 wt.%)to carbonate-poor(SiO2=42～45 wt.%)alkali basalts,the compositions of major and trace elements and isotopes(Sr-Nd-Hf,Mg-Ca)of the rocks show systematic differences.These characteristics can not be generated by partial melting and magmatic differentiation in a single mantle source.Specifically,from carbonate-rich to carbonate-poor alkali basalts,the contents of SiO2 and MgO in the whole rock gradually increase,the contents of CaO and TiO2 and the ratio of CaO/Al2O3 gradually decrease,the isotopic composition of Sr-Nd-Hf gradually becomes depleted,and the isotopic composition of Mg-Ca gradually approaches the MORB range.In terms of C isotopic composition,the carbonate-poor samples have lower C content and lighter C isotopic composition than the carbonate-rich samples,which may indicate the significant magmatic degassing during the ascent process.In terms of the chemical composition of clinopyroxene,the clinopyroxene from carbonate-poor samples showed higher Mg#values,higher contents of SiO2 and Cr2O3,and lower contents of Al2O3 and TiO2.Thus,the changes in composition from carbonate-rich to carbonate-poor alkali basalts are likely indicative of the reaction of carbonated silicate magma with mantle wall rocks during ascent.In the process of magma-mantle peridotite interaction,the orthopyroxene in the surrounding rock(peridotite)will be continuously consumed,while the olivine and clinopyroxene are crystallized and precipitated in the reacted melt.At the same time,the CO2 fluid is released,causing the carbonated silicate magma to evolve into the alkali basaltic magma.Combined with the distribution characteristics in the field,we further speculate that the rising melts near the magma channel is strongly modified by the mantle wall rock,and their composition show natural alkali basaltic melt.However,the rising melts at the center of the channel is weakly modified by the wall rock and still remain their original melt composition,presenting as carbonate-rich alkali basaltic melt.Therefore,we suggest that the possible magma-mantle interaction during the ascent of silica-unsaturated mantle-derived magma caused the conversion of carbonated silicate magma into natural alkali basaltic magma.The NW-SE extending mafic dyke swarm and alkali volcanic complex are widely exposed in the Early Paleozoic strata of South Qinling,which provides a rare geological carrier for the discussion of Paleozoic tectonic evolution,deep mantle properties and dynamic processes of South Qinling.We have carried out detailed isotopic chronology and geochemical studies on the mafic dykes in Maoba,Gaoqiao,Mengshiling and Zhenping areas of South Qinling,and collected the previous research data as complete as possible to provide convincing data support for solving the above scientific problems.Zircon U-Pb dating shows that the Early Paleozoic magma crystallization age is 430±3 Ma to 440±2 Ma,which is slightly earlier than Mianlue Ocean opening(430～400 Ma),and is consistent with previous chronology results(481～401 Ma).These mafic dykes with OIB-type trace element characteristics,enriched in LILE and LREE,depleted in Pb,no depleted in HFSE,slightly enriched in Sr isotopic composition,weak depleted Nd-Hf isotopic composition,the(87Sr/86Sr)ratio is 0.7043～0.7059,δNd(t)value of 2.5～3.4,εHf(t)value of 4.8～6.9.Based on the published data from the region,the geochemical characteristics of these dykes indicate that they are derived from the rich mantle source enriched in LILE and LREE but weakly depleted in radioactive isotopes.They might be formed through the metasomatism between the melt/fluid from the subducted paleo-oceanic crust and the overlying mantle during the Neoproterozoic.In order to investigate the dynamic petrogenesis of these large-scale mafic magmatic activities in the passive continental margin tectonic setting of South Qinling during the Early Paleozoic,we systematically examined the changes of the existing petrogeochemical data over time in the region.The results show that the variation of Yb content and La/Yb ratio in the mafic rocks indicates that the mantle source evolved from the garnet peridotite domain to the spinel peridotite domain over time,suggesting that the melting depth of the mantle source has a tendency to gradually become shallower.The PRIMACALC2 model was used to invert the P-T conditions in the mantle source region of the mafic rocks.The results also showed that the older dyke had a deeper melting depth,and the younger dyke had a shallower melting depth,and the melting temperature gradually increased over time.This indicates that the thermal structure of South Qinling lithospheric mantle changed significantly during the Early Paleozoic,and a thick and cold lithospheric mantle was replaced by a thin and hot lithospheric mantle,indicating that there may have been a continuous tensile stress at the passive continental margin.Therefore,we declare that the subducted slab pull may be the underlying dynamic mechanism of the above magmatic activity.In addition,during the same period,North Qinling micro-continent and the Erlangping unit arc magmatism were also well developed.That is,there was a good spatio-temporal coupling for the magmatism between the active continental margin and the passive continental margin on both sides of the Shangdan Ocean(Proto-Tethys Ocean).It is believed that the subduction of the Shangdan Ocean(Proto-Tethys Ocean)caused the arc magmatism in the active continental margin on the one hand,and the subducted slab pull in the passive continental margin on the other hand,which induced the rift-type alkali-basic-magmatism in the passive continental margin.Then,continuous slab subduction and slab rollback further enhanced the tensile stress acting on the passive continental margin,which led to large-scale alkali-basic-magmatic activity,and finally led to the opening of the Mianlue Ocean(Paleo-Tethys Ocean).At the same time,on the other side of the ocean,back-arc magmatic activity developed at the active continental margin(Erlangping Group)due to the disturbance of mantle heat flow at the bottom of the lithosphere caused by plate subduction and retreat.Here we suggest that together slab subduction and slab pull control the magmatism on the active and passtive continental margin,which to a large extent complement and improve our understanding of the theory of plate tectonics.This doctoral thesis is shown that the mafic magmatic rocks in the orogenic belt provide petrological and geochemical records for tracing the physicochemical differentiation of the subducted paleo-oceanic crust derived melts/fluids in the deep mantle source and the interaction between ascending magma and mantle rock.These geological processes finally lead to the heterogeneous petrological and geochemical composition of the lithospheric mantle and mantle-derived magma in the orogenic belt.At the same time,together slab subduction and slab pull control the magmatism on the active and passtive continental margin,which have important meaning on the formation and evolution of the orogenic belt.