溧水盆地晚中生代基性火山岩成因与深部动力学过程探讨

下扬子地区溧水盆地晚中生代龙王山组基性火山岩属高钾钙碱性-钾玄岩系列,具有低MgO(3.16%～4.97%)和相容元素含量(如Cr11～34μg/g、Ni 9～27 μg/g),强烈富集大离子亲石元素(Ba/Nb=94～144)、轻稀土元素(La/Nb=3.7～4.6)和亏损Nb-Ta,稍高的Sr同位素组成[(87Sr/86Sr)i=0.705 73～0.706 17]和具弱负εNd(t)值(-3.0～-1.9).龙王山组基性火山岩的元素-同位素地球化学特征反映其来源于富集轻稀土元素和大离子亲石元素的岩石圈地幔;它们在Sr-Nd同位素组成上与大别-苏鲁造山带同时代的基性火成岩具有明显的差别,暗示其熔融源区不大可能受到俯冲陆壳的改造富集作用,更可能是早期具高Rb/Sr、低Sm/Nd和亏损高场强元素的流体/熔体交代作用的结果,总体上具有俯冲大洋板片交代作用的特征.在岩浆熔融深部动力学方面,晚中生代高钾钙碱性-钾玄岩浆的起源主要归结于深部岩石圈地幔的热扰动作用和岩石圈伸展-减薄作用,在时空上匹配于同时代郯庐断裂带的巨大走滑活动.     

燕山地区中生代中基性火山岩地球化学对岩石圈性质的约束

燕山地区中牛代中基性火山岩包括钙碱性-高钾钙碱性-钾玄岩系列,岩石类型包括粗叫玄武岩、玄武质安山岩、玄武质粗面安山岩、粗安岩,具有LREE富集的右倾平滑稀土配分模式,富集LILE（如Ba,K）,亏损Nb-Ta和Th—U的微址元素特征,Sr-Nd同位素组成中等富集,与华北内部和兴蒙带中生代基性火山岩具有不同的特征,其Nd同位素组成和（Nb／La）s,（Hf／Sm）s比值都介于华北陆块内部和兴蒙造山带之间,反映了其熔融地幔源区继承了华北陆块内部的EMI型地幔特征外,还很可能受到俯冲板片交代作用的影响,暗示了古亚洲洋板块消减过程对华北陆缘岩石圈地幔的改造作用。类似于华北陆块南缘岩石圈减薄和置换机制,古亚洲洋俯冲板块对燕山地区古老岩石圈的交代作用在岩石圈减薄和置换过程中起了很重要的作用,而不同于华北板块内部以软流圈沿地幔薄弱带或剪切带上升熔蚀和置换岩石圈的机制     

扬子陆块北缘西大别地区新元古界定远组双峰式火山岩地球化学特征、成因及其地质意义

西大别地区定远组双峰式火山建造主要由流纹岩、流纹质凝灰岩和玄武岩组成,受区域变质作用影响较小,保留了良好的新元古代火山岩构造,成为理解扬子地块北缘大别山地区前寒武纪地质演化的极佳研究对象.为深化理解其成因和动力学背景,对该火山岩系开展了详细的岩相学、岩石地球化学和Sr-Nd-Hf同位素研究.流纹岩具较高的SiO2(70...     

内蒙古锡林浩特早石炭世构造环境：来自变质基性火山岩的年代学和地球化学证据

内蒙古锡林浩特地区变质基性火山岩位于锡林浩特市东南部,大地构造上隶属于兴蒙造山带东段,其野外产出状态为锡林郭勒杂岩中的斜长角闪岩。应用LA-MC-ICP-MS锆石U-Pb测年方法,测得两组年龄为334.5±3.5 Ma和323.4±2.4 Ma,表明岩石的形成时代为早石炭世。地球化学特征显示研究区变质基性火山岩具有高Fe(TFeO=6.07%~10.55%)、高Mg(MgO=3.90%~11.29%)、高Al(Al_2O_3=12.68%~17.22%)、贫Ti(TiO_2=0.56%~1.69%)、富Na贫K(K_2O+Na_2O=2.12%~7.37%,K_2O/Na_2O=0.02~0.59)的特征,属于低钾—中钾低钛高镁铝钙碱性玄武岩或者安山岩,部分变质基性火山岩表现出高镁安山岩的特征。变质基性火山岩的稀土元素总量较低(∑REE=15.97×10~(-6)~176.0×10~(-6)),轻、重稀土分异不太显著(LREE/HREE=0.77~6.26),无明显Eu异常(δEu=0.48~1.13)。多数样品微量元素表现出强烈富集大离子亲石元素K、Rb等,亏损高场强元素Nb、Ta、Zr、Hf。以上地球化学特征表明研究区变质基性火山岩的原岩可能为玄武岩或安山岩,形成于岛弧环境。岩石Sr-Nd同位素分析结果显示研究区变质基性岩的(~(87)Sr/~(86)Sr)_i较低,为0.701828~0.704096,ε_(Nd)(t)为+2.83~+6.61,表明其直接来源于亏损的岩石圈地幔。综上所述,基本可以判定锡林浩特地区变质基性火山岩的原始岩浆应起源于受俯冲板片脱水交代的亏损地幔楔,是岛弧环境岩浆活动的产物。结合区域上前人的研究成果,认为该基性火山岩应该是在古亚洲洋洋壳向北俯冲,俯冲板片脱水形成的富集大离子亲石元素的流体进入上覆地幔楔,使地幔楔橄榄岩发生部分熔融而形成的,表明在早石炭世,古亚洲洋在研究区并没有关闭,而是正处于俯冲消减阶段。     

华北克拉通岩石圈减薄机制: 来自山东白垩纪基性-中基性岩浆岩Re-Os同位素地球化学特征的制约

对华北山东地区不同构造单元中生代白垩纪基性-中基性岩浆岩进行了系统的Re-Os同位素研究.侵入岩样品采自鲁西济南岩体、鲁中沂南岩体和胶东的观水岩墙群, 火山岩样品采自鲁西蒙阴、费县、方城青山群和鲁中安丘青山群.所有样品均以低Re(0.076×10-9~0.711×10-9)和Os(0.006×10-9~1.030×10-9)含量以及高放射成因Os同位素比值为特征, 其相应的初始187Os/188Os比值和γ_Os(t)值分别为0.131 6~0.529 1和4.30~319.00(安丘粗面安山岩分别高达1.463和1 060.000), 并具有较大变化范围(0.125~2.090 Ga)的Os同位素模式年龄.在Os-Nd和Os-Sr同位素关系图上, 这些样品分布于典型的岩石圈地幔与地壳端元组成之间, 显示壳-幔源岩浆混合特征.然而, 除1件观水岩体(MGS0610)和费县玄武岩之外, 各地质单元样品之间或所有的中生代岩浆岩作为整体, 样品的γ_Os(t)与(La/Ta)_N和(Th/Nb)_N比值间缺乏明显的相关关系, 说明样品的混源Os同位素并非来自岩浆作用过程中的地壳围岩混染.区内基性岩浆岩Os同位素模式年龄T_MA明显小于古生代地幔橄榄岩包体的Re亏损模式年龄(T_RD), 可能为地幔条件下源岩与再循环地壳物质发生交代作用后Re/Os比值升高所致.蒙阴高镁安山岩和沂南闪长岩在Yb-Re/Yb图解中大致位于中生代辉石岩包体与地壳端元之间, 也指示了下地壳拆沉后产生的熔体与地幔岩石可能发生了交代作用.蒙阴安山岩MMY-12(SiO₂=56.4%)具有古老壳源特征的Os同位素模式年龄(2.090 Ga)、高Mg#(64.6)和高Os含量(0.089×10-9), 为这一解释提供了有力的佐证.      

琼中古-中元古代变基性火山岩地球化学特征及其地质意义

琼中古—中元古代抱板岩群斜长角闪(片麻)岩为高钾变基性-中基性火山岩。化学成分上具有富钾(K2O/Na2O值大于0.6)、高铝(Al2O3含量高于15%)和低钛(TiO2含量低于1.3%)等特点。岩石的K、Rb、Ba、Th等大离子亲石元素和轻稀土元素明显富集,LREE/HREE值较高,并具有显著的Ta、Nb、Ti负异常。微量元素和Sm、Nd同位素特征表明,琼中变基性火山岩岩浆源区偏离原始地幔,并且未受成熟地壳物质的明显污染,它们是俯冲消减带上部陆弧区地幔楔受俯冲洋壳析出的流体和硅酸盐融体交代作用后部分熔融的产物。古—中元古代时期,海南地块可能经历了由拉张到挤压的地球动力学转变,早期拉张阶段形成琼西洋脊型和过渡型拉斑玄武岩,中晚期则形成与洋壳俯冲作用相关的琼中岛弧型高钾钙碱性玄武岩,这一过程可能与华南大陆的裂解及随后洋壳向华夏古陆的深俯冲作用有关。     

大兴安岭北段牙克石地区玄武质火山岩的元素、同位素地球化学特征及其地质意义

牙克石地区出露一套早白垩世玄武质火山岩,其SiO2含量为52．81%-53．39％,K2O含量为1．86％-2．87％,岩性为玄武质粗面安山岩。富集大离子亲石元素Rb和Ba,高场强元素Nb和Ta亏损明显,Zr和H阮明显异常,8Eu为0．77-0．82。从同住素的特点看,（^87Sr／^86Sr）,变化于0．704762-0．704941之间,εNd（t）为2．00～2．54;在εNd（t）-（^87Sr／^86Sr）闺解上,样品投影点落入洋岛玄武岩（OIB）和美国盆岭省范围内。^206Pb／^204pb为18．3288-18．4225,^207b／^204Pb为15、4566-15．4893,^208Pb／^204Pb为37．9401-38．0523：在。^207Pb/^24Pb-^206Pb.^204pb和,^208b／^304pb-^306Pb／^204pb图解上．样品投影点都落在亏损洋中脊玄武岩地幔附近。综合考虑本区火山岩的地质、地球化学特点,认为其来源于被俯冲洋壳交代的岩石圈地幔。     

胶东青山群基性火山岩的Ar-Ar年代学和地球化学特征: 对华北克拉通破坏过程的启示

青山群火山岩是华北克拉通破坏期间最具代表性的地幔或地壳熔融产物,记录了华北深部地质演化的重要信息。本文对胶东青山群基性火山岩进行了40Ar/39Ar定年和岩石地球化学分析,结合前人报道的胶东青山群酸性火山岩资料,发现:(1)基性火山岩喷发年龄为122～113Ma,早于青山群酸性火山岩(110～98Ma);(2)基性和酸性火山岩显示了不同的元素和同位素地球化学特征。岩石成因分析表明,基性火山岩为交代富集地幔部分熔融作用的产物,而酸性火山岩为古老下地壳和中生代底侵岩浆的熔融产物(Ling et al.,2009)。因此,胶东地区青山群火山岩记录了岩浆熔融源区从地幔向下地壳的转变。这与长时间尺度的岩石圈减薄过程中热能由地幔向地壳传递过程相吻合,而不同于地壳拆沉作用所预测的岩浆演化趋势。     

祁连造山带中部拉脊山古地幔特征及其归属: 来自基性火山岩的地球化学证据

拉脊山火山岩带位于祁连造山带中部, 通过对该区早古生代基性火山岩系统的地球化学研究, 揭示该区早古生代地幔的性质及其地幔域的构造归属.研究表明, 拉脊山基性火山岩可以分为两类: Ⅰ类为大陆板内碱性玄武岩, 其稀土元素组成模式为轻稀土富集型, 并具有明显的Nb、Ta负异常, 而Zr、Hf无明显的负异常; Ⅱ类为与地幔柱活动有关的拉斑玄武岩, 具有洋岛玄武岩(OIB) 特征.稀土元素组成模式同样表现为轻稀土富集型, 但其富集程度比Ⅰ类基性火山岩的富集程度弱, 无Nb、Ta、Zr和Hf负异常.基性火山岩的Sr、Nd、Pb同位素组成特征显示, 基性火山岩的地幔源区具有亏损地幔(DM) 和第二类富集地幔(EMⅡ) 混合的特点, 而第二类富集地幔端元(EMⅡ) 占主导地位, 亏损地幔(DM) 物质混入的程度较低; 并具有Dupal异常的同位素特征.通过与华北南缘、北秦岭和扬子北缘西段地幔的Pb同位素组成相比, 表明拉脊山造山带古地幔与北秦岭、南秦岭西段和扬子北缘西段地幔的Pb同位素组成相似.进而表明拉脊山造山带古地幔属于扬子型富放射性成因铅地幔, 而非华北型贫放射性成因铅地幔      

华北陆块东部晚中生代基性岩浆作用的地球化学研究

华北陆块晚中生代(早白垩世)基性火成岩沿着郯庐断裂带存在显著的元素-同位素地球化学分区。其西侧(鲁西区)火成岩在主量元素上以高Mg、Fe和低Al、Ti为特征,在微量元素上富集LILE、LREE和强烈亏损HFSE(Nb、Zr、Hf、Ti和P),同时显示出EM1型Sr-Nd同位素组成;其东侧(鲁东区)火成岩则相对低Mg、Fe和高Al、Ti,在微量元素上也表现出LILE、LREE富集和Nb、P亏损的特征,Ti、Zr、Hf异常不明显,并表现出EM1+EM2的Sr-Nd同位素组成。鲁西区基性火成岩源于陆内岩石圈伸展-减薄作用下古老而富集的岩石圈地幔减压部分熔融作用的结果,鲁东区则为被俯冲陆壳物质改造的富集岩石圈地幔减压部分熔融作用产物。中生代华北陆下岩石圈地幔性质仍为古老富集型地幔,因此至少在早白垩世前不可能存在地幔柱作用和大规模的下地壳拆沉作用。     

大别-苏鲁地区晚中生代镁铁质岩地球化学对造山带岩石圈地幔性质的约束

来自大别-苏鲁地区晚中生代镁铁质岩石表现出类似于岛弧火山岩的富集大离子亲石元素（LILE）、轻稀土元素（LREE）和相对亏损高场强元素（HFSE）的微量元素地球化学特征,和高度富集的放射成因Sr（ISr：0．7065～0．7090）和低放射成因Nd（εNd（t）=-19～-10）的同位素组成;同时它们显示出一定程度的Nb／Ta和Zr／Hf内部分馏特征,反映其地慢源区曾受到了相对富金红石和CO2的熔体交代作用。我们倾向认为深俯冲陆壳在俯冲或折返过程中发生部分熔融作用形成的熔体与地幔反应是形成大别-苏鲁地区造山带富集岩石圈地幔的重要机制。     

Secular evolution of the lithospheric mantle beneath the eastern North China craton: evidence from peridotitic xenoliths from Late Cretaceous mafic rocks in the Jiaodong region,east-central China

The Mesozoic lithospheric mantle beneath the North China craton remains poorly constrained relative to its Palaeozoic and Cenozoic counterparts due to a lack of mantle xenoliths in volcanic rocks. Available data show that the Mesozoic lithospheric mantle was distinctive in terms of its major, trace element, and isotopic compositions. The recent discovery of mantle peridotitic xenoliths in Late Cretaceous mafic rocks in the Jiaodong region provides an opportunity to further quantify the nature and secular evolution of the Mesozoic lithospheric mantle beneath the region. These peridotitic xenoliths are all spinel-facies nodules and two groups, high-Mg# and low-Mg# types, can be distinguished based on textural and mineralogical features. High-Mg# peridotites have inequigranular textures, high Mg# (up to 92.2) in olivines, and high Cr# (up to 55) in spinels. Clinopyroxenes in the high-Mg# peridotites are generally LREE-enriched ((La/Yb)_N>1) with variable REE concentrations, and have enriched Sr–Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.7046–0.7087; ¹⁴³Nd/¹⁴⁴Nd = 0.5121–0.5126). We suggest that the high-Mg# peridotites are fragments of the Archaean and/or Proterozoic lithospheric mantle that underwent extensive interaction with both carbonatitic and silicate melts prior to or during Mesozoic time. The low-Mg# peridotites are equigranular, are typified by low Mg# ( < 90) in olivines, and by low Cr# ( < 12) in spinels. Clinopyroxenes from low-Mg# peridotites have low REE abundances (ΣREE = 12 ppm), LREE-depleted REE patterns ((La/Yb)_N < 1), and depleted Sr–Nd isotopic features, in contrast to the high-Mg# peridotites. These geochemical characteristics suggest that the low-Mg# peridotites represent samples from the newly accreted lithospheric mantle. Combined with the data of mantle xenoliths from the Junan and Daxizhuang areas, a highly heterogeneous, secular evolution of the lithosphere is inferred for the region in Late Cretaceous time.     

Geochemistry of Cretaceous mafic rocks from the Lower Yangtze region, eastern China: Characteristics and evolution of the lithospheric mantle

Geochemical and isotopic data for Cretaceous mafic rocks (basalt, gabbro, and diorite) from the Lower Yangtze region, northern Yangtze block, constrain the evolution of the lithospheric mantle. The mafic rocks, separated into the northeast and southwest groups, are alkaline and evolved, with low Mg# values (44–58) and variable SiO₂ contents (47.6–57.4 wt%). Enriched LREEs, LILEs, and Pb, together with depleted Nb, Zr, and Ti, suggest that the mantle sources were metasomatized by slab-derived fluid/melt. All samples show high radiogenic ²⁰⁷Pb/²⁰⁴Pb(t) (15.41–15.65) and ²⁰⁸Pb/²⁰⁴Pb(t) (37.66–38.51) ratios at given ²⁰⁶Pb/²⁰⁴Pb(t) (17.65–19.00) ratios, consistent with the mantle sources having been metasomatized by ancient slab-derived material. Mafic rocks of the southwest group show enriched Sr–Nd isotopic characteristics, with ⁸⁷Sr/⁸⁶Sr(t) ranging from 0.7056 to 0.7071 and ε_Nd(t) ranging from −5.3 to −8.3, indicating an origin from enriched lithospheric mantle. Mafic rocks of the northeast group, which record ⁸⁷Sr/⁸⁶Sr(t) ratios of between 0.7044 and 0.7050 and ε_Nd(t) of −2.8 to −0.7, possibly formed by the mixing of melts from isotopically enriched lithospheric mantle and isotopically depleted asthenospheric mantle. Taking into consideration the geochemical and isotopic characteristics of Cretaceous mafic rocks, Cenozoic basalts, and basalt-hosted peridotite xenoliths from the Lower Yangtze region, we propose that an isotopically enriched, subduction-modified lithospheric mantle was replaced by or transformed into an isotopically depleted “oceanic-type” mantle. Such a process appears to have occurred in the eastern North China Craton as well as the eastern Yangtze block, probably in response to subduction of the paleo-Pacific plate beneath East Asia.     

晚古生代古亚洲洋俯冲作用:来自珲春前山镁铁质侵入岩的年代学和地球化学记录

珲春地区前山镁铁质侵入岩主要由橄榄辉长岩、苏长岩和辉长闪长岩组成,形成年龄273±2Ma,为早二叠世侵入岩。该岩体显示出岛弧拉斑玄武岩的元素地球化学特征,弱富集LREE且正Eu(Eu/Eu*=1.05～1.44)异常的REE配分模式,在不相容元素中富集大离子亲石元素(LILE)如Ba和Sr,亏损高场强元素(HFSE)(La/Nb=2.8～4.8;Zr/Sm=8.0～22.5),与起源于流体交代地幔楔部分熔融的岛弧低钾拉斑玄武岩类似。在同位素特征上低放射成因Sr,高放射成因Nd和Hf(87Sr/86Sr(i)=0.70295～0.70375;εNd(t)=+4.5～+6.4;εHf(t)=+9.6～+14.6),反映其来源于同位素组成亏损的交代地幔。结合区域地质背景,我们认为前山镁铁质岩形成于晚古生代期间古亚洲洋向华北板块的俯冲作用背景,其亏损Nb-Ta、Zr-Hf及Hf-Nd同位素解耦特点说明该岩体最有可能来源于俯冲流体交代的地幔楔。     

华北中、新生代岩石圈地幔的交代作用：含金云母地幔岩提供的证据

本文对华北克拉通三个不同地区（河北汉诺坝、内蒙古集宁三义堂、河南鹤壁）新发现的含金云母尖晶石二辉橄榄岩和尖晶石橄榄单斜辉石岩捕虏体进行了详细的矿物组成、单斜辉石的微量元素和Sr-Nd同位素研究.通过与相同地区不含金云母尖晶石二辉橄榄岩捕虏体的系统对比发现通常含金云母的地幔橄榄岩比不含金云母的地幔橄榄岩岩富Al2O3、CaO、NaO、K2O、TiO2,但相对贫镁;其单斜辉石的LREE更为富集,但Sr、Nd同位素组成则相对亏损.这说明地幔交代作用不仅能够造成地幔橄榄岩的玄武质组分和稀土元素的富集,而且亦能够造成全岩和橄榄石Mg#的降低和同位素组成的相对亏损.捕虏体的Rb-Sr等时线年龄暗示地幔交代作用发生在中、新生代;其交代熔体来源于软流圈.同时说明华北新生代岩石圈地幔普遍存在的主、微量元素和同位素组成类似于“大洋型”岩石圈地幔的特征很可能是岩石圈地幔橄榄岩与软流圈来源的熔体的大规模反应的结果,而非真正意义上的新增生的岩石圈地幔.     

Early Cretaceous gabbroic rocks from the Taihang Mountains: Implications for a paleosubduction-related lithospheric mantle beneath the central North China Craton

SHRIMP zircon U–Pb ages and geochemical and Sr–Nd–Pb isotopic data are presented for the gabbroic intrusive from the southern Taihang Mountains to characterize the nature of the Mesozoic lithospheric mantle beneath the central North China Craton (NCC). The gabbroic rocks emplaced at 125 Ma and are composed of plagioclase (40–50%), amphibole (20–30%), clinopyroxene (10–15%), olivine (5–10%) and biotite (5–7%). Olivines have high MgO (Fo = 78–85) and NiO content. Clinopyroxenes are high in MgO and CaO with the dominant ones having the formula of En_42–46Wo_41–50Fs_8–13. Plagioclases are dominantly andesine–labradorite (An = 46–78%) and have normal zonation from bytownite in the core to andesine in the rim. Amphiboles are mainly magnesio and actinolitic hornblende, distinct from those in the Precambrian high-pressure granulites of the NCC. These gabbroic rocks are characterized by high MgO (9.0–11.04%) and SiO₂ (52.66–55.52%), and low Al₂O₃, FeOt and TiO₂, and could be classified as high-mg basaltic andesites. They are enriched in LILEs and LREEs, depleted in HFSEs and HREEs, and exhibit (⁸⁷Sr/⁸⁶Sr)_i = 0.70492–0.70539, ε_Nd(t) = − 12.47–15.07, (²⁰⁶Pb/²⁰⁴Pb)_i = 16.63–17.10, Δ8/4 = 70.1–107.2 and Δ7/4 = − 2.1 to − 9.4, i.e., an EMI-like isotopic signatures. Such geochemical features indicate that these early Cretaceous gabbroic rocks were originated from a refractory pyroxenitic veined-plus-peridotite source previously modified by an SiO₂-rich melt that may have been derived from Paleoproterozoic subducted crustal materials. Late Mesozoic lithospheric extension might have induced the melting of the metasomatised lithospheric mantle in response to the upwelling of the asthenosphere to generate these gabbroic rocks in the southern Taihang Mountains.     

北京西山东狼沟组钾质火山岩成因及其动力学机制

北京西山东狼沟组火山岩为钾质系列岩石(K2O=3.40%～4.70%,K20/Na20＞1),由玄武质粗安岩和粗安岩组成.它们表现出LREE富集[(La/Yb)N=31.4～44.6]和HREE弱分馏[(Dy/Yb)N=1.64～2.01]的REE分布模式,Eu表现出弱的负异常(Eu/Eu*=0.81～0.86),富集Ba、K、LREE和相对亏损Nb-Ta、Th-U.较高的Sr[(87Sr/86Sr)i=0.705 7～0.705 9]和低Nd[δNd(t)=-14.7～-12.4],类似于辽西义县组和华北陆内同时代的EM I型基性火成岩.其Th-U相对Ba、La的强烈亏损,暗示早期可能有华北古老的下地壳再循环进入源区.结合区域构造研究,东狼沟组火山岩可能为陆内伸展拉张环境下早期交代富集的岩石圈地幔部分熔融作用的产物,并在岩浆演化过程中经历了橄榄石 辉石 斜长石的分离结晶作用.     

华北东部中、新生代岩石圈地幔的不均一性：来自橄榄石的组成填图结果

华北中、新生代玄武质火山岩和基性脉岩携带的地慢橄榄岩捕虏体中橄榄石和／或橄榄石捕虏晶系统的组成填图显示华北东部中、新生代岩石圈地幔存在明显的时空分布规律和不均一性。这与通过岩石圈地幔源基性岩石的地球化学反演获得的华北中生代岩石圈地幔的时空不均一性及其块体特征完全一致。太行山和鲁皖地区新生代岩石圈地慢的差异演化主要反映古老地幔橄榄岩与熔体相互作用时熔体性质和来源的不同。同时,橄榄石Fo填图还揭示了郯庐断裂对华北东部中、新生代基性岩浆活动及其岩石圈地幔演化的重要制约作用。而且,华北东部中生代岩石圈减薄后尚存古老岩石圈地幔残留。因此,华北东部岩石圈减薄的整体拆沉模式很难成立。     

Erosion of lithospheric mantle beneath the East African Rift system: geochemical evidence from the Kivu volcanic province

This study presents new major and trace element and Sr–Nd isotopic results for a suite of Miocene–Recent mafic lavas from the Kivu volcanic province in the western branch of the East African Rift. These lavas exhibit a very wide range in chemical and isotopic characteristics, due to a lithospheric mantle source region that is heterogeneous on a small scale, probably <1 km. The chemical and isotopic variations are mostly geographically controlled: lavas from Tshibinda volcano, which lies on a rift border fault on the northwestern margin of the province, have higher values of ⁸⁷Sr/⁸⁶Sr, (La/Sm)_n, Ba/Nb, and Zr/Hf than the majority of Kivu (Bukavu) samples. The range of ⁸⁷Sr/⁸⁶Sr at Tshibinda (0.70511–0.70514) overlaps some compositions found in the neighboring Virunga province, while Bukavu group lavas include the lowest ⁸⁷Sr/⁸⁶Sr (0.70314) and highest _Nd (+7.6) yet measured in western rift lavas. The Tshibinda compositions trend towards a convergence for Sr–Nd–Pb isotopic values among western rift lavas. Among Kivu lavas, variations in ¹⁴³Nd/¹⁴⁴Nd correlate with those for certain incompatible trace element ratios (e.g., Th/Nb, Zr/Hf, La/Nb, Ba/Rb), with Tshibinda samples defining one compositional extreme. There are covariations of isotopic and trace element ratios in mafic lavas of the East African Rift system that vary systematically with geographic location. The lavas represent a magmatic sampling of variations in the underlying continental lithospheric mantle, and it appears that a common lithospheric mantle (CLM) source is present beneath much of the East African Rift system. This source contains minor amphibole and phlogopite, probably due to widespread metasomatic events between 500 and 1000 Ma. Lava suites which do not show a strong component of the CLM source, and for which the chemical constraints also suggest the shallowest magma formation depths, are the Bukavu group lavas from Kivu and basanites from Huri Hills, Kenya. The inferred extent of lithospheric erosion therefore appears to be significant only beneath these two areas, which is generally consistent with lithospheric thickness variations estimated from gravity and seismic studies.     

Multiple mantle metasomatism beneath the Leizhou Peninsula,South China: evidence from elemental and Sr-Nd-Pb-Hf isotope geochemistry of the late Cenozoic volcanic rocks

ABSTRACT

We analysed whole-rock major and trace elements and Sr-Nd-Pb-Hf isotopes of the late Cenozoic volcanic rocks in the Leizhou Peninsula, South China to investigate their mantle source characteristics. These volcanic rocks, collected from Jiujiang, Tianyang and Huoju areas of the Leizhou Peninsula, are characterized by incompatible element enrichment but variable isotopic depletion. The volcanic rocks from Jiujiang and Tianyang show prominent primitive-mantle-normalized positive Nb, Ta and Sr anomalies and depleted Sr-Nd-Pb-Hf isotope compositions, whereas those from Huoju show slight positive to negative Nb and Ta anomalies, a prominent positive Pb anomaly, and more enriched Sr-Nd-Pb-Hf isotope compositions. Two types of mantle metasomatism are required to explain the geochemical characteristics of these rocks. The Jiujiang and Tianyang samples were largely derived from a mantle source metasomatized recently by a low-F melt. Such low-F melt is generated within the asthenospheric mantle, which is enriched in volatiles and incompatible elements with positive Sr anomaly and depleted Sr-Nd-Pb-Hf isotope compositions. The Huoju samples were largely derived from a mantle source metasomatized by recycled upper continental crust material. These two types of mantle metasomatism beneath the Leizhou Peninsula are consistent with trace element characteristics of mantle mineralogy (e.g. clinopyroxene vs. amphibole), which reflects source evolution in space and time (e.g. tectonic setting change).