【作者】 赵凯；

【导师】 戴立群；

【作者基本信息】 中国科学技术大学 ， 地质学， 2023， 博士

【摘要】 秦岭造山带是一个典型的复合造山带,记录了华南与华北陆块长期多阶段的大洋俯冲到大陆碰撞的构造过程。以洛南-栾川断裂,商丹缝合带和勉略缝合带为界,秦岭造山带被划分为北秦岭构造带与南秦岭构造带。早古生代期间,南秦岭北大巴山地区,普遍发育有NW-SE向延伸的基性岩墙群和碱性火山岩。前人对区内镁铁质岩浆岩及其幔源捕掳体的年代学、岩石学、矿物学和地球化学开展了大量的研究,并提出它们可能是拉张构造背景下的幔源岩浆活动产物。然而,对于镁铁质岩浆岩的同位素年代学、地幔源区性质、深部交代熔/流体性质以及岩浆岩形成机制等方面的认识还远远不够,一定程度上限制了我们对南秦岭早古生代构造演化的深入理解。为此,本博士学位论文对南秦岭早古生代碱性玄武岩和基性岩墙群进行了详细的岩石学和地球化学研究,研究结果将为深入认识该区域镁铁质岩浆岩的地幔源区性质、交代熔/流体性质以及岩浆作用产生的地球动力学机制和具体过程提供新的见解。为探究板片俯冲过程中洋壳衍生流体的化学性质与分异行为,我们对南秦岭岚皋地区早古生代富碳酸盐碱性玄武岩进行了系统的地球化学研究,包括全岩主微量元素,全岩Sr-Nd-Hf同位素和Mg-Ca同位素,碳酸盐矿物的主微量元素以及C-O同位素。研究结果显示,低硅碱性玄武岩富含碳酸盐矿物以及碳酸盐矿物包裹体,矿物微量元素特征指示这些碳酸盐矿物为岩浆成因。碱性玄武岩在化学组成上呈现洋岛型微量元素分布特征,同时具有较低的εNd(t)值(3.3～3.5)和高的(87Sr/86Sr)i比值(0.7040～0.7059),表明其地幔源区受到俯冲地壳组分的交代。此外,碱性玄武岩中碳酸盐具有高的δ18O值(16.2～18.0‰)以及δ13C值(-5.9～1.8‰),指示其地幔源区受到了沉积碳酸盐组分的交代。碱性玄武岩还具有低的SiO2含量(28.4～37.1 wt.%),高的CaO含量(11.6～24.3 wt.%)以及高的CaO/Al2O3比值(1.0～3.7),进一步指示其地幔源区为碳酸盐熔体交代形成的碳酸盐化橄榄岩。然而,富碳酸盐碱性玄武岩的Mg-Ca同位素组成却呈现出“解耦”特征,即岩石具有低于正常地幔的δ44Ca值(0.64～0.96‰)和高于正常地幔的δ26Mg值(-0.36～0.03‰)。低的Ca同位素特征进一步支持地幔源区的碳酸盐熔体交代作用,但其高的Mg同位素特征却与典型富镁沉积碳酸盐衍生熔体的交代作用不符。虽然幔源碳酸盐化岩浆在上升过程中,通过分离结晶或者液态不混溶作用会造成Mg-Ca同位素分馏(硅酸盐熔体相对于碳酸盐熔体会优先富集重的Mg和Ca同位素),但分馏会导致δ26Mg和δ44Ca值在样品之间呈现正相关性。然而,岚皋碱性玄武岩Mg-Ca同位素比值之间呈现出良好的负相关性,并不支持幔源岩浆在上升过程中发生的化学分异。相反,玄武岩中Mg-Ca同位素的这种“解耦”特征更可能继承自地幔源区以及交代介质本身。为此我们表示,俯冲板片在后弧深度部分熔融形成碳酸盐化硅酸盐熔体(富C熔体),在向上覆地幔楔迁移的过程中可能发生了不同程度的物理化学分异。首先,俯冲板片来源的富C熔体在上升过程中,碳酸盐组分不断从体系中抽离,致使初始富C熔体逐渐演变为含C熔体。其次,交代介质在分异过程中由于不一致的Mg-Ca同位素分馏(Mg同位素的分馏程度要远大于Ca同位素),造成残余含C熔体中δ26Mg值显著升高且高于地幔值,而δ44Ca值变化较小且依然低于地幔值。经过分异的交代介质交代地幔楔橄榄岩,最终产生Mg-Ca同位素解耦的碳酸盐化地幔源区。因此,碱性玄武岩“解耦”的Mg-Ca同位素组成,为示踪俯冲板片流体迁移过程中的物理化学分异行为提供了新的思路。这种流体分异过程将改变交代介质的物理化学组成,最终影响到俯冲带镁铁质岩浆岩的地球化学特征。为了探讨幔源岩浆上升过程中可能存在的熔体-地幔相互作用,我们在南秦岭岚皋富碳酸盐碱性玄武岩研究的基础上,进一步对同一火山岩中贫碳酸盐碱性玄武岩开展了系统性的地球化学研究工作。锆石U-Pb同位素定年结果显示,碱性玄武岩大约形成于454±4Ma。野外产出特征显示,富碳酸盐碱性玄武岩样品更加靠近火山岩中心,而贫碳酸盐碱性玄武岩样品更加趋近火山岩边缘。从富碳酸盐(SiO2=29～37 wt.%)到贫碳酸盐(SiO2=42～45 wt.%)碱性玄武岩,岩石的主微量元素和同位素(Sr-Nd-Hf、Mg-Ca)组成均呈现出系统的差别,表明这些特征并不能通过单一幔源的不同程度部分熔融以及岩浆分异等过程所形成。具体来看,从富碳酸盐到贫碳酸盐碱性玄武岩,全岩的SiO2和MgO含量逐渐升高,CaO和TiO2含量以及CaO/Al2O3比值逐渐降低,Sr-Nd-Hf同位素组成逐渐变亏损,Mg-Ca同位素组成逐渐接近MORB范围。C同位素组成上,贫碳酸盐样品要比富碳酸盐样品具有更低的C含量和更轻的C同位素组成,可能指示了幔源岩浆在上升过程中存在显著的去气作用。在单斜辉石矿物化学组成上,贫碳酸盐样品中的单斜辉石表现出更高的Mg#值,SiO2和Cr2O3含量,更低的A12O3和TiO2含量。因此,从富碳酸盐到贫碳酸盐碱性玄武岩的成分变化,很可能是硅不饱和的碳酸盐化硅酸盐岩浆在上升过程中与地幔围岩反应的结果。在熔体-地幔相互作用过程中,碳酸盐化硅酸盐岩浆会不断消耗围岩(橄榄岩)中的斜方辉石,而结晶沉淀橄榄石和单斜辉石,与此同时释放出CO2流体,造成碳酸盐化硅酸盐岩浆向着碱性玄武质岩浆演化。综合野外产出特征,我们进一步推断,靠近岩浆通道的岩浆受地幔围岩改造强烈,化学组成上表现为贫碳酸盐碱性玄武质熔体;而通道中心的岩浆受围岩改造相对较弱,化学组成上保持相对原始熔体成分,呈现为富碳酸盐碱性玄武质熔体。所以,我们认为硅不饱和的幔源岩浆在上升过程中可能受到熔体-地幔围岩相互作用的影响,使得其原始化学组成发生显著改变,而可能存在的宽的岩浆通道则造成熔体-地幔围岩反应不均一,在地表产生从碳酸盐化硅酸盐岩浆向碱性玄武质岩浆转变的成分变化范围。南秦岭早古生代地层中广泛出露NW-SE向延伸的基性岩墙群和碱性火山杂岩,为探讨南秦岭古生代构造演化、深部地幔性质和动力学过程提供了难得的地质载体。我们对南秦岭毛坝、高桥、孟石岭和镇坪地区的基性岩浆岩展开了详细的同位素年代学和地球化学研究,并尽可能完整地统计前人的研究数据,进一步制约基性岩墙群的岩石成因以及形成机制。样品中锆石U-Pb定年得到430±3 Ma至440±2 Ma的早古生代岩浆侵位年龄,落在前人基性岩墙群年代学研究结果(481～401 Ma)范围之内。这些基性岩脉具有洋岛型的微量元素特征,富集LILE和LREE,亏损Pb,不亏损HFSE,略富集的Sr同位素[(87Sr/86Sr)i=0.7043～0.7059]和弱亏损的 Nd-Hf 同位素[εNd(t)=2.5～3.4,εHf(t)=4.8～6.9]组成。综合已发表的数据资料,这些基性岩浆岩的地球化学特征表明它们来自于富集的地幔源区,可能是新元古代时期俯冲古洋壳来源的熔/流体与上覆地幔楔反应所形成的。为探究南秦岭早古生代时期被动大陆边缘构造背景下这些大规模镁铁质岩浆活动产生的动力学机制,我们系统检验了区域内已有的岩石地球化学数据随时间的变化情况。其中,Yb含量和La/Yb比值的系统变化,表明随着基性岩脉形成时代变年轻,其地幔源区由石榴石橄榄岩向着尖晶石橄榄岩演化,这指示地幔源区熔融深度有逐渐变浅的趋势。我们使用PRIMACALC2模型对基性岩浆岩地幔源区P-T条件进行了反演,结果同样显示年轻的岩脉具有更浅的熔融深度和更高的熔融温度。由此可见,早古生代期间,南秦岭岩石圈地幔热结构有着显著改变,厚而冷的岩石圈地幔逐渐被薄而热的岩石地幔所取代。此外,同一时期,处于主动大陆边缘的北秦岭微陆块和二郎坪单元的岛弧岩浆活动也十分发育,即原特提斯商丹洋两侧主动和被动大陆边缘的岩浆活动存在很好的时空耦合关系。为此我们表示,原特提斯商丹洋俯冲一方面造成了主动大陆边缘岛弧岩浆活动,另一方面导致了被动大陆边缘出现俯冲板块的拉拽,进而诱发被动陆缘产生裂谷型的碱性镁铁质岩浆作用。之后持续的板块俯冲和板块回卷,使得作用在被动陆缘的拉拽应力进一步增强,进而引发大规模的碱性镁铁质岩浆作用,最终导致古特提斯勉略洋的打开;与此同时,在大洋的另一侧,由于板块俯冲和回卷使得岩石圈底部地幔热流发生扰动,故而在主动大陆边缘(二郎坪群)发育有弧后盆地岩浆活动。因此,我们认为板块俯冲和板块拉拽控制着主动和被动大陆边缘的岩浆活动,进一步深化了我们对板块构造理论的理解与认识。本博士论文研究表明,造山带镁铁质岩浆岩为示踪造山带深部地幔源区性质、俯冲古洋壳衍生熔/流体的物理化学分异行为以及岩浆上升过程中与地幔围岩的相互作用提供了岩石学和地球化学记录,这些地质过程最终导致了造山带岩石圈地幔及其幔源岩浆在岩石学和地球化学组成上的不均一。与此同时,板块俯冲和板块拉拽共同控制着主动和被动大陆边缘的岩浆活动,对认识造山带的形成和演化具有重要意义。更多还原

【Abstract】 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.更多还原