江南造山带东段旌德岩体锆石LA-ICPMS年龄和Nd-Sr-Hf同位素地球化学

提要：旌德花岗质杂岩体位于安徽南部,由花岗闪长岩和二长花岗岩组成。本研究得到岩体中锆石U-Pb年龄为(141.0±1.0) Ma,认为该年龄代表岩体的侵位年龄。岩体全岩主量元素特征显示出中偏酸性 (SiO2＝66.01%~70.87%),富Al (Al2O3＝14.91%~16.24%),富碱 (alk = 6.64%~8.01%),K2O/Na2O变化范围在0.78~1.04,镁、铁含量较低,MgO：0.68%~1.06%,TFe2O3 (2.0%~3.34%),以及低磷 (P2O5 =0.10%~0.14%)的特点;微量元素主要富集Sr (189×10-6~452×10-6),贫Nb、Ta、P、Ti、Y、Yb,高Sr/Y比值 (23~66)和 (La/Yb)N (13~58),Eu有轻微的负异常到弱的正异常 (δEu＝0.81~1.18)。旌德岩体的地球化学特征与中国东部中生代埃达克质岩相似。ISr＝0.7096~0.7101, εNd(t)＝-6.28~-7.32,εHf(t)值变化于-6.5~-1.1,两阶段模式年龄 tDM2=1.4~1.5 Ga。较年轻的Nd同位素模式年龄、较高的εNd(t)值和εHf(t)值,以及岩体中发育有岩浆混合成因的暗色包体,指示源区可能有地幔物质的贡献。幔源岩浆底侵使下地壳发生部分熔融,并发生了岩浆混合作用,形成了旌德岩体。     

西藏冈底斯东部门巴地区晚白垩世埃达克质岩的岩石成因

本文首次报道了西藏冈底斯东部门巴地区晚白垩世花岗闪长岩的岩石学、锆石U-Pb定年和Hf同位素、全岩元素与Sr-Nd同位素地球化学数据。门巴花岗闪长岩锆石U-Pb定年结果为88.3Ma。花岗闪长岩的SiO2含量为66.4%～68.5%,K2O含量为3.1%～4.1%,A/CNK为0.90～0.99,属于高钾钙碱性准铝质岩石。岩石强烈富集轻稀土元素,(La/Yb)N为25.3～46.6,具弱的负Eu异常(δEu=0.83～0.91),高Sr(725×10-6～766×10-6)、高Sr/Y比值(Sr/Y=52.2～65.2),具有埃达克岩的特征,本文称之为门巴埃达克质岩石。门巴埃达克质岩石εNd(t)值为-4.1～-4.3,(87Sr/86Sr)t值为0.70671～0.70672。εHf(t)范围为-3.4～-1.2,Hf同位素地壳模式年龄为tCDM=1.2～1.4Ga。结合门巴埃达克质岩的主、微量元素、同位素以及含有闪长质包体等特征,判断门巴埃达克质岩石主要来源于古老下地壳物质的部分熔融,可能是来源于下地壳物质的熔体与幔源岩浆(如底侵的玄武岩浆)发生混合作用的结果。通过与在南冈底斯发现的埃达克岩的成因及地球动力学背景的对比,本文认为新特提斯洋洋脊俯冲作用不仅为镁铁质下地壳的熔融提供了能量,而且在岩浆形成演化过程中也起到了重要作用。     

燕山期中国东部高原下地壳组成初探：埃达克质岩Sr、Nd同位素制约

大陆地壳组成是地球科学家关注的课题，但困难的是如何确定深部地壳，尤其是下地壳的组成。埃达克质岩的厘定从一个方面有助于这个难题的解决。中国东部“C型”埃达克质岩（adakite-like)的地球化学性质表明其形成的深度大，熔融的温度高，为下地壳中基性麻粒岩部分熔融的产物。因此，可以尝试通过埃达克质岩的地球化学特征来反演下地壳的组成。本文的初步研究表明，中国东部埃达克质岩的Sr-Nd同位素组成有很大的变化，反映埃达克质岩熔融的源区下地壳组成有明显的不同。根据埃达克质岩石的Sr-Nd同位素组成，推测中国东部高原下地壳大体由胶东－大别、北京－辽西和下扬子（长江中下游）3个不同的块体组成。1，胶东－大别区：胶东区的下地壳以富Si,Na2O/K2O低（0．8－1．3）、Isr高（0．709－0．712）、εNd(t)低（－15－－21）和TDM高（1．9－2．1Ga)为特征；大别区的Isr值较低，可能代表了中生代时基底的特征，大体相当于该区下地壳的上限值。2，下扬子区位于扬子地块内，中生代埃达克质岩的Sr初始值，εNd和TDM变化大，说明元古宙的下地壳基底中可能有较多年轻的玄武质岩石底侵加入。相比而言，铜陵区的Isr较高（0．707－0．709）、εNd(t)较低（－11－－16）、TDM高（1．7－2．2Ga)，接近扬子下地壳的特征。3，辽西－北京地区的Sr-Nd高位素组成以低的Isr(0.705-0.707)、变化的εNd(t)(-4--20)和TDM（0．7－2．1Ga)为特征，反映辽西－北京区下地壳组成比较复杂。其中北京－彰武地区的Sr-Nd同位素组成与汉诺坝玄武岩中的麻粒岩包体类似，远高于华北下地壳的εNd(t)值（－32－－44），推测埃达克质岩石并非来自古老的华北太古宙下地壳，而可能是中生代时增生在华北下地壳底部的年轻基性麻粒岩部分熔融形成的。此外，文中还讨论了用埃达克岩反演下地壳组成存在的不足。     

南祁连拉脊山构造带早古生代岩浆混合作用:以马场岩体为例

拉脊山构造带位于南祁连构造带北缘,发育大量早古生代岩浆岩。其南缘马场岩体主要由黑云母花岗岩和花岗闪长岩组成,花岗闪长岩中发育大量的镁铁质微粒包体。黑云母花岗岩、花岗闪长岩和镁铁质包体LA-ICP-MS锆石U-Pb年龄分别为 467±7Ma、468±6Ma、466±6Ma。黑云母花岗岩具有低 K_2O/Na_2O(0.28～0.37)、高 Sr/Y(125～168)比值特征,为埃达克质岩;同时,黑云母花岗岩还具有相对低的MgO、Cr、Ni含量和Mg~#值,富集轻稀土、大离子亲石元素(Ba、K、Pb、Sr)以及Zr、Hf,亏损重稀土和高场强元素(Nb、Ta、Ti),具有Eu正异常(Eu/Eu~*=1.30～1.58),亏损Sr-Nd和锆石Hf同位素组成((~(87) Sr/~(86)Sr)_t=0.7044～0.7046 ,ε_(Nd)(t)=+2.05～+2.21,ε_(Hf)(t)=+8.2～+10.2),指示黑云母花岗岩是新生地壳物质重熔的产物。镁铁质微粒包体与寄主花岗闪长岩之间呈过渡-截然接触界线,发育反向脉角闪石嵌晶结构、斜长石不平衡生长结构、镁铁质凝块以及刀刃状黑云母、针状磷灰石等显微结构反映典型的岩浆混合特征。镁铁质微粒包体与寄主花岗闪长岩均富集大离子亲石元素(Cs、K、Pb、Sr),亏损高场强元素(Nb、Ta、Zr、Hf),具有与黑云母花岗岩一致的Sr-Nd同位素组成,但镁铁质微粒包体具有更高的 MgO 含量(6.15%～9.12%)、Mg~#值(57～60)和 Cr(271 × 10~(-6)～424 × 10~(-6))、Ni(54.7 × 10~(-6)～86.6× 10~(-6))含量,暗示镁铁质微粒包体与花岗闪长岩是由受俯冲流体交代的地幔熔体与新生地壳物质重熔形成的熔体经岩浆混合而成。结合区域背景分析,本文认为马场岩体的形成与南祁连洋俯冲过程中幔源岩浆底侵加热新生地壳以及岩浆混合作用相关。     

桂北圆石山早侏罗世A型花岗岩的岩石成因及意义

桂北圆石山花岗岩中发育大量镁铁质包体.LA-ICP-MS锆石U-Pb定年结果显示,花岗岩形成于早侏罗世(179±2Ma).花岗岩的地球化学特征表现为硅含量均一,富碱更富钾、相对富铁而贫镁,具有高的104×Ga/A1比值和Zr+Nb+Ce+Y含量,属于A型花岗岩.圆石山花岗岩具有比较均一的Sr、Nd同位素组成(ISr=0.701 7~0.710 8,εNd(t)=-7.77~-4.55).镁铁质包体则显示了稍低的ISr值(0.705 0~0.707 1)和稍高的εNd(t)值(-4.87~-2.63).花岗岩的锆石原位Hf同位素组成为:(176 Hf/177 Hf)i=0.282 62~0.282 70,εHf(t)=-1.68~1.17,相应的Hf同位素两阶段模式年龄TDM2变化于1.25~1.43Ga之间.圆石山花岗岩可能是在伸展环境下由低成熟度的下地壳物质部分熔融所形成.自早侏罗世(~200Ma)以来,伸展作用是华南内陆构造背景的主体,多期次的玄武质岩浆底侵作用可能是燕山期伸展作用的直接诱因.华南内陆早侏罗世时期可能仍处于板内"后碰撞"环境.     

北祁连东段冷龙岭地区毛藏寺岩体和黄羊河岩体的岩石成因及其构造意义

文中研究了北祁连东段冷龙岭地区毛藏寺岩体和黄羊河岩体的年代学、地球化学和Sr-Nd同位素组成。毛藏寺岩体主要岩石类型为花岗闪长岩。锆石U Pb定年获得花岗闪长岩岩浆结晶年龄为（424±4） Ma。花岗闪长岩具有高的Mg#（约55）,K2O/Na2O=0.77~0.91,A/CNK=0.92~0.94,表明岩石属准铝质。在微量元素组成上,花岗闪长岩富集LILE、亏损HFSE,轻重稀土分异明显［(La/Yb)N=16.9~19.5］,具有弱的Eu负异常（Eu/Eu*=0.75~0.83）;花岗闪长岩具有ISr=0.706 3~0.706 5,εNd(t) =-1.5~-1.1,TDM=1.10~1.16 Ga。这些地球化学特征和Sr Nd同位素组成表明,花岗闪长岩岩浆源区为基性下地壳变玄武质岩石,但在成岩过程中有少量幔源物质的加入。黄羊河岩体主要由钾长花岗岩组成,其岩浆结晶年龄为（402±4） Ma。岩石富碱（K2O+Na2O=6.91‰~7.66%）,K2O/Na2O>1,A/CNK=0.97~1.05。钾长花岗岩富集LILE及HFSE,轻重稀土元素分馏中等［(La/Yb)N =10.6~17.8］,并具有明显的负Eu异常（Eu/Eu*=0.43~0.68）,表明钾长花岗岩具有铝质A型花岗岩的地球化学特征。钾长花岗岩具有ISr=0.710 3~0.711 3,εNd(t)=-6.7~-6.0,TDM=1.46~1.55 Ga,反映岩浆主要来自地壳中长英质物质的部分熔融。冷龙岭地区花岗岩类的岩石成因及其岩浆演化揭示了北祁连山造山带从加里东早期的挤压构造体制向加里东晚期的伸展构造体制的演化。这些花岗岩类形成于碰撞后构造背景,岩浆的产生可能与俯冲的北祁连洋板片的断离作用有密切联系。     

周口店岩体矿物学、年代学、地球化学特征及其岩浆起源与演化

周口店岩体由三次侵入的中酸性岩石组成, 本次测得石英闪长岩锆石U-Pb年龄为131.6±2.1 Ma, 闪长玢岩锆石U-Pb年龄为128.1±1.4 Ma.周口店岩体各种类型岩石属高钾钙碱性系列、偏铝质, Mg#较高, 重稀土元素和Ta、Nb、P以及Ti明显亏损, 轻稀土元素和Ba、K以及Sr相对富集, Eu没有异常, Yb元素含量小于2×10-6, (La/Yb)_N和Sr/Y比值较高.斜长石复杂环带能谱线扫描表明, 花岗闪长岩中的斜长石核部牌号高, 完整的幔部由内向外由反环带和正环带组成, 微粒包体中的斜长石核部牌号低, 幔部以尘状环带开始, 然后演变为正环带, 这揭示存在多期基性岩浆的注入作用, 结合暗色微粒包体的形态、大小、数量、反向脉、矿物含量统计、矿物成分、地球化学和各类环带包体、岩墙状包体群等特征, 说明暗色微粒包体是在花岗闪长岩浆冷凝过程的不同阶段, 多期幔源基性岩浆注入并与酸性岩浆在围绕包体周缘的局部范围内发生不均一机械混合作用的结果.周口店中酸性岩石体现埃达克质岩的地球化学特征, 岩浆成分主要受源区控制, 形成于加厚下地壳环境.由石英闪长岩-花岗闪长岩至中酸性岩脉, 岩石(Er/Lu)_N和Nb/Ta比值升高, 说明源区残留相矿物组合由角闪石+石榴石向石榴石+金红石变化, 岩浆源区不断变深.      

长江中下游铜陵地区宝山岩体地球化学研究

长江中下游地区与铜金矿床有关的埃达克岩成因仍然存在分歧。对铜陵宝山地区的侵入岩和其中的包体进行了系统的矿物学、岩石学和元素地球化学研究。结果表明:(1)宝山岩体岩性变化较大,从辉石闪长岩到花岗闪长岩均有;(2)暗色基性岩常常被淡色酸性的花岗闪长岩所包裹,部分界限呈过渡状态;(3)LA-ICPMS锆石U-Pb定年结果表明,宝山花岗闪长岩和其中的辉石闪长岩包体分别形成于(143.8±3.2)Ma(2σ,MSWD=2.3)和(142.9±2.9)Ma(2σ,MSWD=2.2),属于早白垩世;(4)宝山岩体地球化学特征表明其主体具有典型的埃达克岩特征,如较大SiO_2含量变化范围(52.88%~63.66%),高Al_2O_3含量(平均值为16.03%)、高Sr/Y(33.4~102,平均为72.9)和Sr/La比值,低Y和Yb含量、低(La/Yb)N(14.6~26.6,平均值为21.4)和K_2O/Na_2O(平均值为0.39)比值。(5)野外岩石接触关系和岩石地球化学特征表明,宝山埃达克质岩可能为岩浆混合成因,混合的两个端元分别为高硅和低硅端元。高硅端元为俯冲洋壳熔融形成的熔体并发生不同程度磁铁矿的结晶分异,而低硅端元为高Ni、Cr、V、Co的岩石圈地幔熔体。铜陵宝山岩体的地质与地球化学研究结果表明,铜陵地区中生代埃达克质岩和大规模铜金成矿作用可能形成于俯冲背景之下。     

西藏尼木渐新世花岗岩中的岩浆混合作用:对岩石成因及陆壳增生的启示

尼木渐新世黑云母二长花岗岩中含有丰富的形态各异的暗色镁铁质微粒包体。本文报道了寄主岩和镁铁质包体的年代学、元素地球化学及Sr- Nd- Hf同位素组成,据此阐明了岩石的成因,并探讨了岩浆混合作用在成岩中的意义及其对陆壳增生的启示。锆石LA- ICP- MS U- Pb定年结果表明,寄主岩和镁铁质包体的成岩年龄在误差范围内基本一致,均为约30Ma,说明它们同时形成。元素地球化学组成上,寄主黑云母二长花岗岩为高钾钙碱性、准铝质,富集Rb、Th、U等大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素,且具有高Sr、低Y和Yb含量,Sr/Y比值高,缺乏明显铕负异常,表现出埃达克质岩石的特征。镁铁质包体贫硅,富铁、镁,具有与寄主岩相似的稀土与微量元素分布模式。二者的全岩Sr、Nd同位素和锆石Hf同位素组成也相近\[寄主岩:(87Sr/86Sr)i=0.7057~0.7064,εNd(t)=-1.45~0.35,εHf(t)=1.21~7.34;镁铁质包体:(87Sr/86Sr)i=0.7058~0.7064,εNd(t)=-2.23~-1.57,εHf(t)=2.40~7.04\]。综合分析表明,尼木渐新世黑云母二长花岗岩应主要起源于碰撞加厚的新生镁铁质下地壳的部分熔融,镁铁质包体应为幔源玄武质岩浆与其诱发加厚地壳熔融形成的埃达克质岩浆经不均匀混合作用的产物。结合对冈底斯带岩浆岩已有资料的全面分析,表明幔源玄武质岩浆的底侵及其诱发的岩浆混合作用既是冈底斯岩基形成的主要方式,也是导致青藏高原陆壳增生的重要途径。     

内蒙古中部达茂旗地区三叠纪花岗岩和钾玄岩的地球化学、年代学和Hf同位素特征

内蒙古索伦山缝合带南侧达尔罕茂明安联合旗北部分布有车根达来花岗闪长岩-闪长岩和西别河火山岩。车根达来花岗闪长岩为高钾或中钾钙碱性系列岩石,化学成分相当于埃达克岩。闪长岩为中钾钙碱性系列岩石,呈大小不等的包体产于花岗闪长岩中,化学成分相当于正常岛弧深成岩。但是,花岗闪长岩[εHf(t)=-4.56～-1.56]和闪长岩[εHf(t)=-5.01～-1.88]锆石Hf同位素组成指示其源区为古老的(镁铁质)陆壳或富集地幔。锆石SHRIMPU-Pb定年给出车根达来花岗闪长岩的年龄为245Ma±1Ma,闪长岩包体的形成年龄为239Ma±2Ma。西别河火山岩为钾玄岩,喷发年龄为224Ma±2Ma。火山岩以大量的、年龄分散的捕虏晶(260～1556Ma)为特征,指示了大陆背景。这些三叠纪侵入岩和火山岩形成于古亚洲洋闭合(中晚二叠世)之后,没有明显的构造变形,应为中亚造山带的造山后演化的岩浆岩记录。     

挥发份对高硅岩浆演化趋势的制约:以东南沿海白垩纪晚期花岗岩类岩石为例SCIEI北大核心CSCD

东南沿海分布大面积的白垩纪晚期侵入岩。这些岩石可分为两期:其中115~100Ma以钙碱性系列岩石为主,岩石组合为辉长岩-闪长岩-花岗闪长岩-二长花岗岩-碱性长石花岗岩;而100~86Ma的岩石为碱性系列,岩石组合为石英二长斑岩-正长斑岩-碱性长石花岗岩。115~100Ma的辉长岩以角闪辉长岩为主,具有极高的CaO、MgO和Al_(2)O_(3)含量,具有极低的SiO_(2)(42.9%~53.8%)、全碱(K_(2)O+Na_(2)O:0.86%~5.28%)、Ba、Nb、Th、Rb和Zr含量,也具有极低的FeO^(T)/MgO、La/Yb和Zr/Hf比值,较高的Eu/Eu^(*)、Sr/Y比值和Sr含量,为基性-超基性堆晶岩。与辉长岩同期的闪长岩和细粒暗色包体具有较高的SiO_(2)(50.34%~63.68%),较低的CaO、P_(2)O_(5)、MgO、Al_(2)O_(3)含量,相对低的Eu/Eu^(*)和Sr/Y比值,变化较大的La/Yb和Zr/Hf比值,代表了从基性岩浆储库中抽取的富硅熔体。115~100Ma的花岗闪长岩和二长花岗岩类岩石为准铝质岩石,SiO_(2)含量变化较大(61.7%~75.3%),具有较低的FeO^(T)/MgO、Ga/Al比值和Nb、Zr及Nb+Zr+Ce+Y元素含量,显示出典型I型花岗岩的特征。这些花岗岩具有相对高的La/Yb、Eu/Eu^(*)和Zr/Hf比值和高的Sr、Ba和Zr含量。结合岩相学特征,这些花岗岩为堆晶花岗岩。而115~100Ma的碱性长石花岗岩具有极高的SiO_(2)含量(大于75%),低的Eu/Eu^(*)、La/Yb、Zr/Hf和Sr/Y比值,具有低的Ba、Sr和Zr含量和高的Rb、Nb、Y和Th含量和Rb/Sr比值,表明这些花岗岩是由富硅岩浆储库中抽离的高硅熔体侵入地壳形成。100~86Ma期间形成的二长斑岩和正长斑岩具有极高的全碱含量,可以达到8%~12%,其SiO_(2)主要集中在60%~70%,具有极高的Zr、Sr和Ba含量和Eu/Eu^(*)、La/Yb和Sr/Y比值,显示出堆晶花岗岩的特征。而100~86Ma期间形成的大部分碱性长石花岗岩具有极高的SiO_(2)含量(大于75%),并显示出A型花岗岩的特征,具有高的Rb/Sr比值和高的Rb、Y和Th和低的Ba、Sr含量和低的Zr/Hf、La/Yb、Eu/Eu^(*)和Sr/Y比值,表明它们是由富硅岩浆储库抽离的高硅熔体侵入浅部地壳形成。东南沿海高硅花岗岩的形成和穿地壳岩浆系统密切相关,高硅花岗岩是由浅部地壳内晶体-熔体分异产生的熔体侵入地壳所形成,而高硅花岗岩的地球化学特征与岩浆储库的水及挥发份含量密切相关。115~100Ma期间,从富水的岩浆储库抽离的熔体形成具有低高场强元素含量和低Rb/Sr比值的高硅花岗岩,这一过程与古太平洋板块俯冲有关;100~86Ma期间,从富挥发份的岩浆储库抽离的熔体形成碱性特征、富含高场强元素和具有高的Rb/Sr比值的高硅花岗岩,这一过程和古太平洋板块回撤软流圈上涌有关。     

Geochronological and geochemical studies of adakites from Tethyan Belt,Western Pakistan: A clue to geodynamics and Cu-Au mineralization

ABSTRACT

Large porphyry Cu-Au deposits are associated with Early Miocene intrusive rocks in Tethyan belt, discovered along Chagai magmatic arc in Western Pakistan, adjacent to Southeast Iran. Two types of rocks were discriminated as granodiorite and monzodiorite from Saindak area. The granodiorites are associated with regional large Cu-Au mineralization, while the monzodiorites are mostly ore-barren. LA-ICP-MS U-Pb dating yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 22.16–24.2 Ma for granodiorites and ca. 22.6 Ma for a monzodiorite. These intrusive rocks in the Saindak area are all calc-alkaline rocks, with enriched LILEs and depleted HFSEs, and without Eu negative anomalies. The felsic granodiorites are characterized by high Sr contents and Sr/Y ratios, with intermediate to high (La/Yb)_N ratios that identify them as typical high-silica adakites. In contrast, the basaltic-andesitic monzodiorites are just normal arc-related rocks, showing less fractionated REE patterns than the granodiorites, with systematically lower LREE and higher MREE and HREE. Low K₂O/Na₂O ratios and decoupled Sr/Y-(La/Yb)_N ratios indicate the characteristics of slab-derived adakites for the granodiorites, most likely originated through partial melting of the subducted Neotethys oceanic crust beneath Eurasian continent followed by subsequent mantle interaction. The almost simultaneously melting of subcontinental lithospheric mantle induced by dehydration of Neotethys plate gave rise to the formation of the barren monzodiorites. These intrusive rocks in the Saindak area are characterized by similar Sr-Nd-Pb isotopes with high ⁸⁷Sr/⁸⁶Sr_i ratios, slightly negative εNd(t) values and radiogenic Pb isotopes, plotting in the field between the MORB and EM-II mantle endmembers or the Average Cadomian Lower Crust, suggesting subducting sediments or old continental crustal materials have contributed into the compositions of these rocks by source enrichment or crustal contamination. The slab-melting derived adakite is favourable for regional massive Cu-Au mineralization in the Saindak area.     

Discovery of two stages of the Early Paleozoic adakitic intrusive rocks in the Kunlun River area,East Kunlun: Implications for collisional orogenic processesSCIEI北大核心CSCD

早古生代志留纪-泥盆纪是东昆仑原特提斯洋闭合和碰撞造山过程发生的重要时期。在该阶段出现有榴辉岩和大量A型花岗岩、镁铁-超镁铁质岩,并产出有夏日哈木镍矿、白干湖钨锡矿等一批重要的矿产资源。然而,东昆仑早古生代详细碰撞造山的深部过程、榴辉岩的折返机制等问题还没有得到合理的解释。在东昆仑造山带昆仑河地区新发现的早古生代埃达克质侵入岩可能为上述问题的解决提供了重要依据。LA-ICP-MS锆石U-Pb测年结果表明昆仑河地区存在晚奥陶世(446Ma)和晚志留世(427～425Ma)两期埃达克质侵入岩。晚奥陶世埃达克质侵入岩为花岗闪长岩,SiO₂(67.55%～68.21%)和Al₂O₃(14.59%～15.89%)含量较高,富Na₂O(4.91%～5.15%)、贫K₂O(1.54%～1.64%),亏损重稀土,Y含量是7.76×10^-6～8.61×10^-6,Yb含量是0.67×10^-6～0.93×10^-6...     

黑龙江黑河地区晚侏罗世岩浆弧的火成岩记录:与蒙古-鄂霍茨克洋关系探讨

黑龙江黑河北部的黑花山地区广泛发育中生代的侵入岩，但该侵入岩的形成时代、岩石组合、成因仍然不清.对黑花山地区侵入岩进行了野外、岩相学、岩石学、年代学、岩石地球化学等研究，以探讨该套岩石形成的时代、原因及大地构造背景.对两件二长花岗岩开展的LA-ICP-MS锆石U-Pb测年获得加权平均年龄分别为148.7±0.6 Ma、153.50±0.56 Ma，表明其形成于晚侏罗世.岩石组合主要为含有少量英云闪长岩（T₁）、奥长花岗岩（T₂）的T₁T₂G（狭义花岗岩）组合，即弧环境的侵入岩组合.上述岩石组合富硅、碱、高钾、过铝（A/CNK大于1）、低Fe₂O₃T.其在SiO₂-K₂O图上为高钾钙碱系列，硅镁图中为低铁钙碱（LF-LA）系列，Peacock指数为钙碱性或碱钙性，痕量元素蛛网图显示富集大离子亲石元素，亏损Nb、Ta、P、Ti、Y、Yb等元素.稀土元素配分模式为右倾型，轻稀土相对富集，（La/Yb）_n=13.78~29.85，无负Eu异常或弱正Eu异常或弱的Eu负异常.花岗岩类具有高Sr低Y的特征，Sr/Y比值介于29.53~103.80，具有“C型”埃达克岩的特征.上述岩石组合及其特征均指示了弧火成岩的性质，结合构造环境判别认为该区晚侏罗世花岗岩类形成于蒙古-鄂霍茨克洋的南向俯冲有关的环境，岩浆可能源于兴安弧加厚下地壳的部分熔融作用.      

珲春小西南岔白垩纪花岗岩的地球化学及岩浆源区特征

珲春小西南岔地区白垩纪花岗岩主要有英云闪长岩和花岗闪长岩两种类型。英云闪长岩属于中钾钙碱性系列(Na₂O/K₂O=1.99~2.76),具有高Al₂O₃(15.46%~17.13%)、Sr(559×10^-6~731×10^-6)、Sr/Y(40~78)、La/Yb(16~21),低Y(9×10^-6~14×10^-6)、Yb(0.8×10^-6~1.3×10^-6)的特征,与埃达克质岩石地球化学特征类似。花岗闪长岩为高钾钙碱性系列,Na₂O/K₂O=1.01~1.56,w(Sr)=312×10^-6~410×10^-6、w(Yb)=1.23×10^-6~2.13×10^-6、Sr/Y=13~32,属正常的高钾钙碱性花岗岩。两类花岗岩的源区均为玄武质下地壳物质,英云闪长质岩浆形成压力较高(> 1.0 GPa),深度大于33 km,花岗闪长质岩浆形成压力相对较低(0.8~1.0 GPa),岩浆来源深度为26~33 km。     

Early Cretaceous adakitic granites in the Northern Dabie Complex, central China: Implications for partial melting and delamination of thickened lower crust

To date, few adakitic rocks have been reported in direct association with contemporary intra-continental extensional structures, which has cast doubt on genetic models involving partial melting of the lower crust. This study presents Early Cretaceous (143-129 Ma, new Sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb ages) adakitic granites, which are directly associated with a contemporary metamorphic core complex (i.e., the Northern Dabie Complex in the Dabie area). These granites exhibit relatively high Sr contents, negligible to positive Eu and Sr anomalies, high La/Yb and Sr/Y ratios, but very low Yb and Y contents, similar to subducted oceanic crust-derived adakites. They are also characterized, however, by very low MgO or Mg^# and Ni values, and Nd-Sr isotope compositions (ε_Nd(t) = −14.6 to −19.4 and (⁸⁷Sr/⁸⁶Sr)_i = 0.7067-0.7087) similar to Triassic continent-derived eclogites subducted in the Dabie-Sulu Orogen. Additionally, late granitic dikes in the adakitic intrusions exhibit low Sr contents, clearly negative Eu and Sr anomalies, low La/Yb and Sr/Y ratios, but relatively high Yb and Y contents, similar to 118-105 Ma granites in the Northern Dabie Complex. Based on composition and geochronology data of Neoproterozoic amphibolites and orthogneisses, Triassic high- to ultra-high pressure metamorphic rocks, and Early Cretaceous mafic-ultramafic intrusive rocks, and the constraints provided by experimental melt data for tonalites, metabasaltic rocks and eclogites, we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile-bearing eclogitic lower crust as a consequence of Triassic-Middle Jurassic subduction and thrusting. The late dikes probably originated from plagioclase-bearing intermediate granulites. Moreover, we suggest that late Mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of Early Cretaceous mafic-ultramafic intrusive rocks in the Dabie area, and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite. Asthenospheric upwelling in response to post-collisional delamination of lithospheric mantle was likely to have provided the heat source for the Cretaceous magmatism.     

New geological and geochemical data on the granitoids of the Uspensky massif in Southern Primorye

New geological, petrochemical, mineralogical, and geochemical data are presented on the Uspensky granitoid massif in Southern Primorye. The massif consists of the rocks of two associations: (1) the early association (103.3 ± 2.4 Ma) consisting of garnet-bearing biotite and two-mica granite-leucogranites and (2) the late association (99 ± 2 Ma) represented by biotite (±amphibole) granodiorites, melanogranites, and granites. The granitoids of both associations have moderate potassic alkalinity and elevated alumina contents but differ in the proportions of alumina, calcium, and alkalis. The garnet-bearing granite-leucogranites are characterized by the highest Rb, Th, and U contents and the lowest Sr, Ba, Hf, and Zr contents. The REE distribution patterns have a quasi-symmetric shape and deep Eu minimums. The melanogranites show higher Sr and Ba contents and, as granites, are characterized by asymmetric REE spectrums with an insignificant negative Eu anomaly. The porphyraceous granodiorites and granites are peculiar in their lowered Sr and Ba contents, while the granodiorites have lowered contents of K, REE, Zr, Hf, Th, and U; elevated Nb contents; and a distinctive Eu minimum.     

The relation between Rb,Ba and Sr in granitic rocks

The ternary relation between the elements Rb, Ba and Sr in granitoid rocks is discussed. This relation proved to be useful in tracing differentiation trends in acidic suites. Moreover, it has a potentiality for being used as a criterion in genetic problems of granites, i.e. in distinguishing magmatic from metasomatic or granitized granites. The averages of these three elements in the various types of granites are calculated. The Turekian and Wedepohl (1961) two-fold subdivision of granites into high- and low-Ca rocks is elaborated so as to comprise four distinctive types: quartz diorites and granodiorites, normal granites, strongly differentiated granites beside the group of anomalous granites.     

内蒙古宝音图钼矿成矿岩体地球化学、年代学及成因研究

内蒙古宝音图钼矿是狼山北段大型斑岩石英脉型钼矿床,成矿岩体为斜长花岗岩、二长花岗岩、钾长花岗岩及酸性细晶斑岩株组成的复合岩体,LA-ICP-MS U-Pb上交点年龄2400 Ma,反映花岗岩源区岩石是新太古代到古元古代陆壳变质岩。各岩性锆石LA-ICP-MS U-Pb年龄测定显示成矿岩体形成于印支期(225.9±4.4)Ma~(237.5±5.9)Ma、(236.8±4.5)Ma~(244.3±4.2)Ma、(247.5±4.4)Ma和(252.1±3.4)Ma~(258.8±3.3)Ma 4个阶段,同位素年龄反映的每个岩浆侵入阶段持续时间长,各侵入阶段间隔时间短。岩浆侵入期不同阶段元素经历了一定分异,各阶段岩石化学组成均显示铝过饱和,富钾特征,以晚期细晶岩K2O/Na2O最高,K2O对Ca O呈反相关关系。与世界主要岩浆岩带微量元素Sr/Ba-Zr/Y的比较显示,其不同于洋壳熔融成因花岗岩,而与燕山带、秦岭带岩浆岩地球化学特征一致,与矿区元古界变质岩围岩地球化学特征比较接近,这些特征显示本区成矿花岗岩是陆壳岩石熔融成因S型花岗岩。     

Altered mafic lower continental crust as a source for low δ18O-granodiorites (Damara orogen, Namibia)?

Summary New oxygen isotope data for metaluminous granites from the basement-dominated part of the Damara orogen (Namibia) range from 9.1 to 11.9‰. These data, together with previously published Sr, Nd and Pb isotope data indicate that these granites and associated peraluminous granites originated from felsic meta-igneous basement sources. New and unusually low oxygen isotope data for metaluminous granodiorites extend now the range of δ¹⁸O values from ca. 12 to 6‰ for this rock type. These low oxygen isotope values approach the values observed in mafic quartz diorites for which a model of derivation from depleted mafic lower crust has been established. In view of the higher Pb isotope ratios but lower oxygen isotope values of the granodiorites relative to the mafic quartz diorites, it is concluded that the granodiorites represent partial melts of an undepleted but strongly altered mafic lower crust. Most of the peraluminous and metaluminous granites and the metaluminous granodiorites have identical U–Pb monazite, allanite and zircon ages of ca. 510–500 Ma implying partial melting of distinct basement rocks of Archaean to Proterozoic age at the peak of regional high-grade metamorphism.