北秦岭西段早古生代埃达克岩地球化学特征及岩石成因

唐藏石英闪长岩体出露于北秦岭造山带西段,侵入于丹凤群火山沉积岩系之中,主要矿物组成为斜长石、角闪石、石英、钾长石、黑云母等,形成时代为454Ma。该岩体SiO2= 62.02％～64.26％;Al2O3=16.55%～17.45%,MgO=1.71%～2.09%, Sr = (1069～1296)×10-6（>400 ×10-6）,Y = (11.6～13.81)×10-6 （<18×10-6）,Yb = (0.96～1.14)×10-6（<1.9 ×10-6）,Sr/Y=80.98～108.19（>40）,稀土总量（∑REE=217.88×10-6～355.07×10-6）较高,LREE强烈富集,重稀土亏损,Eu负异常不明显（LREE/HREE=17.72~25.82;LaN/ YbN=44.56～53.85;δEu＝0.82～1.0）,显示出典型埃达克岩的岩石地球化学特点。在以原始地幔标准化的微量元素分布图中,大离子亲石元素（LILE）K、Th、Rb、Ba、Sr等富集,高场强元素(HFSE)Ta、Nb、Y、Yb等较相邻元素相对亏损,微量元素曲线分布形式及稀土元素配分模式与典型地区埃达克岩分布形式一致。研究表明,该岩体Na2O＝4.99%～6.91%, Na2O-2＞K2O,Mg#＝0.59～0.63, A/CNK=0.82～1.01,σ=4.24～2.24,为富钠偏铝质高镁埃达克岩。唐藏埃达克岩体岩浆源于早古生代秦岭商丹洋向北俯冲消减洋壳板片的部分熔融,岩浆在上升过程中与地幔楔发生反应。北秦岭早古生代埃达克岩的确定为探讨秦岭早古生代构造演化提供了新的证据。     

西昆仑布孜完达坂岩体成因：LA-ICP-MS锆石U-Pb年代学与地球化学制约

布孜完达坂岩体位于新疆西昆仑地区的中酸性岩浆岩带中,其主要岩性为不等粒石英闪长岩和二长花岗岩。通过LA-ICP-MS锆石U-Pb定年,结合锆石阴极发光图像（CL）和U、Th元素特征,获得布孜完达坂岩体的年龄为225.5±2.3 Ma（n=18,MSWD=1.6）,时代属晚三叠世（T₃）。石英闪长岩和二长花岗岩均含有角闪石, 副矿物中出现少量榍石, SiO₂含量为61.41％～73.01％,全碱变化于6.64％～8.25％;铝饱和指数A/CNK为0.98～1.04,属准铝质—弱过铝质;岩石轻重稀土元素分馏明显,（La/Yb）_N为6.38～43.73,负Eu异常,δEu为0.53～0.85;富集Rb、Th、K和LREE等大离子亲石元素,而贫Ba、Sr、Ti、Nb、Zr等元素,属高钾钙碱性I型花岗岩系列。根据岩体的成因类型并结合区域构造环境演化,分析认为随着古特提斯洋在中三叠世晚期向北消减直至最终闭合过程中,岩体温度在707℃～718℃的条件下,地壳较深处且受一定程度幔源物质混染发生部分熔融作用,于晚三叠世后碰撞演化形成了布孜完达坂岩体。     

Geochronology and magma oxygen fugacity of Ehu S-type granitic pluton in Zhe-Gan-Wan region,SE China

In this paper, we determined the U-Pb isotopic and trace element compositions of zircons from the Ehu S-type granite in the Zhe-Gan-Wan region, SE China, using in-situ laser ablation (LA) ICP-MS. The weighted mean ²⁰⁶Pb/²³⁸U age of 132.0 ± 0.6 Ma for the Ehu granite indicates that the pluton was formed in the Early Cretaceous and during the Late Mesozoic Cu-Mo mineralization quiescence in Zhe-Gan-Wan region. The calculated logarithmic magma oxygen fugacities for Ehu granite range from −19.19 to −11.43 with an average magma oxidation state of FMQ-0.29, which is much lower than those of Cu-Mo bearing granites in the Zhe-Gan-Wan region. Since Ehu granite was derived from partial melting of metasedimentary basement without fractional crystallization and mantle-derived magma contamination, the low oxidation state of this granite suggests that the assimilation of metasedimentary basement component may not significantly increase the oxidation state of reduced melts from asthenospheric mantle and could not generate oxidized magmas that are favorable for Cu-Mo mineralization.     

冈底斯中段达居复式花岗岩体成因:锆石U-Pb年代学、元素地球化学与Hf同位素制约

本文报道了冈底斯中段达居复式岩体的锆石U-Pb年龄与Hf同位素组成,以及全岩元素地球化学组成,据此探讨了岩浆的形成机制与演化过程,进而全面揭示了岩体的成因。根据岩性和岩相特征,达居复式岩体可区分为昌那和桑嘎两个单元,前者岩性主要为花岗闪长岩,岩体内含丰富的闪长质包体;后者岩性主要为中细粒二长花岗岩,岩性总体较均匀。锆石LA-ICP-MS U-Pb定年结果表明,昌那单元花岗闪长岩和其中的闪长质包体具有一致的成岩年龄,分别为52.7±0.7Ma和52.3±0.6Ma,桑嘎单元二长花岗岩的成岩年龄为45.3±0.7Ma。地球化学组成上,二单元岩石均具有亚碱、准铝的特征,碱铝指数(AKI值)多低于0.9,铝饱和指数(A/NKC值)均小于1.0,属于准铝质钙碱性I型花岗岩。它们均富集轻稀土和大离子亲石元素(如Rb,Pb),亏损高场强元素(如Nb,Ta,Ti),具有弧型岩浆岩的地球化学组成特征。与昌那单元岩石相比,桑嘎单元岩石富硅、富碱,具有较高的Rb/Sr、Rb/Ba比值,并表现出较显著的Eu负异常,指示其经历了较高程度的分异演化。昌那单元花岗闪长岩和其中的闪长质包体具有一致亏损的Hf同位素组成,二者的ε_(Hf)(t)值分别为+4.6~+11.5和+5.0~+11.1,桑嘎单元总体也具有亏损的Hf同位素组成,但其ε_(Hf)(t)值具有较大的变化范围(-0.5~+14.7)。综合分析表明,昌那单元岩石应为底侵的亏损地幔岩浆与其诱发初生地壳熔融产生的岩浆经混合作用形成,桑嘎单元岩石为这一混源岩浆经进一步分异演化后侵位结晶的产物,且成岩过程中极可能有古老的印度地壳物质参与。     

滇西始新世巍山岩体的岩石学成因及构造意义

巍山岩体位于云南西部哀牢山—红河断裂带附近,其主体岩性为二长斑岩和石英二长斑岩。LA-ICP-MS锆石U-Pb定年表明该岩体侵位于始新世,年龄为(38.5±0.6)Ma。巍山岩体为过铝质岩石,其铝饱和指数ASI(=Al2O3/(Ca O+Na2O+K2O),分子数)为1.09~1.44。所有样品具有高的K2O含量(4.50%~4.98%)和K2O/Na2O比值(1.10~1.41)。这些花岗闪长岩类呈现出埃达克质岩石的属性,例如高的Al2O3、Sr和低的重稀土(HREE)以及Y含量、Eu负异常缺失等。它们比云南西部同期的浅色过铝质侵入岩有着低的ISr和高的εNd(t)组成,暗示基性岩浆对该岩体的形成做出了贡献。岩石地球化学证据表明,该岩体最可能形成于幔源玄武质岩浆底侵诱发增厚地壳中沉积岩或者变质沉积岩发生了部分熔融。巍山岩体侵位年龄早于哀牢山—红河断裂带走滑过程(32~22 Ma),因此其形成可能与该断裂带的走滑无关。     

柴北缘果可山岩体的岩浆混合作用:来自岩相学、矿物学和地球化学证据

青海南山岩浆岩带沿柴达木盆地和西秦岭造山带北缘分布,主要由与古特提斯洋俯冲-碰撞相关的晚二叠世-三叠纪花岗岩组成。本文对柴北缘东段果可山石英闪长岩及其中镁铁质微粒包体开展岩相学、矿物学、地球化学和Sr-Nd同位素综合研究及成因约束,为探究壳-幔相互作用提供新信息。LA-ICP-MS锆石U-Pb定年结果显示,果可山石英闪长岩与镁铁质包体形成于～247Ma。石英闪长岩属中钾钙碱性I型花岗岩,具有中等K_2O含量(1.43%～2.18%)和高Mg~#值(48.9～52.4),还表现出富集轻稀土元素、大离子亲石元素(如U、K、Pb),亏损重稀土元素和高场强元素(如Nb、Ta、Ti)和弱Eu负异常特征(Eu/Eu~*=0.71～0.85)。Sr-Nd同位素组成和矿物不平衡结构(如斜长石韵律环带和突变环带及角闪石包裹黑云母)表明,石英闪长岩主要源自镁铁质下地壳部分熔融并伴有富集地幔来源镁铁质组分的参与。镁铁质微粒包体呈椭球状,发育冷凝边和特殊的显微结构(如针状磷灰石、斜长石反环带和斜长石含刀刃状角闪石),具有更低的SiO 2含量(56.68%～59.28%)、更高的ε#Nd(t)和Mg值(58.5～62.2)以及更平坦的稀土配分曲线,暗示果可山镁铁质包体和寄主花岗岩是由源自遭受俯冲交代地幔的镁铁质岩浆与古老下地壳(古元古代晚期)来源的长英质岩浆不同比例混合形成的。结合前人对区域沉积学和构造解析研究结果,本文认为青海南山早-中三叠世岩浆活动与古特提斯洋向北俯冲诱发的幔源岩浆底侵和岩浆混合作用有关。     

赣南兴国东固高分异花岗岩成因及地质意义

本文对江西省兴国县东固花岗岩体进行了岩石学、全岩元素地球化学、锆石U-Pb及Lu-Hf同位素研究。研究结果表明,东固岩体中东固、雪竹坑和佛子山单元均形成于152±2Ma,为近同期岩浆作用的产物,而非前人认为属于三个不同时期,这一发现对于认识东固岩体的成因有着重要作用。东固岩体具有较高的全岩Si O2含量、Rb含量(350μg/g),较低的Zr/Hf、Nb/Ta值,地球化学上兼有部分铝质A型花岗岩和高分异S型花岗岩的特征,其成因类型较难界定。其中白云母花岗岩单元分异程度最高,与成矿作用密切相关,已经发现的矿床多产于其中或其与围岩接触带。东固岩体中不同单元的锆石εHf(t)值均集中,佐证前人划分的不同单元可能来自于同一源区。其锆石εHf(t)值介于-9~-11之间,对应两阶段亏损地幔模式年龄为1.7~1.9 Ga,同位素特征显示其可能起源于古元古代地壳变质基底的部分熔融。结合附近存在的"裂谷型"正长岩类,我们认为东固岩体可能产生于古太平洋板块向华南板块俯冲所产生的拉张构造背景之下,高分异岩体的界定有利于后期进一步找矿工作的开展。     

桂东北苗儿山张家花岗岩LA-ICP-MS锆石U-Pb年龄与地球化学特征

对张家岩体乍古田矿床及张家矿床的2件赋矿围岩样品,开展LA-ICP-MS锆石U-Pb同位素定年,获得乍古田矿床赋矿围岩形成年龄为218.0±1.4Ma,张家矿床赋矿围岩形成年龄为226.8±5.7Ma。另外,对张家岩体8件样品的主量、微量元素及Sr-Nd同位素分析测试显示,张家花岗岩富硅(Si O2=74.35%~76.29%)、高铝(Al2O3=12.54%~14.03%)、高碱(Na2O+K2O=6.95%~9.17%)、低镁(Mg O=0.34%~0.45%)、低钙(Ca O=0.68%~1.03%)、低钛(Ti O2=0.106%~0.18%),属高钾钙碱性花岗岩;富集Rb,亏损Ba、Sr,Rb/Sr值高,属于低Ba-Sr花岗岩;轻稀土元素富集,负Eu异常;富U(平均21.5×10-6),Th/U值低(平均1.63),有利于铀矿化;(87Sr/86Sr)i高(约0.7247),低εNd(t)值(平均-13.95),二阶段Nd模式年龄为2.07~2.19Ga。张家花岗岩属于强过铝质S型花岗岩,可能是华夏板块古元古代基底变泥质岩经部分熔融形成的。     

Metamorphism and deformation of mafic and felsic rocks in a magma transfer zone,Stewart Island,New Zealand

In the mingled mafic/felsic Halfmoon Pluton at The Neck, Stewart Island (part of the Median Batholith of New Zealand) some hornblende gabbros and diorites retain magmatic structures, whereas others show evidence of major changes in grain and inclusion shapes, and still others are amphibolite‐facies granofelses with few or no igneous relicts. These mafic to intermediate magmas crystallized in felsic magma relatively quickly, with the result that most deformation occurred at subsolidus conditions. It is suggested that mafic‐intermediate rocks with predominantly igneous microstructures spent less time in the magmatic system. The metamorphism of the mafic rocks appears to be ‘autometamorphic’, in the sense that elevated temperatures were maintained by magmatic heat during subsolidus cooling. Elevated temperatures were maintained because of repeated sheet injection and subconcordant dyke injection of hot basaltic and composite mafic‐felsic magmas, into a dominantly transtensional, km‐scale, outboard‐migrating, magmatic shear zone that operated semi‐continuously for between c. 140 and c. 130 Ma. Complete cooling occurred only when the system evolved to transpressional and the locus of magmatism migrated inboard (southward) between c. 130 and c. 120 Ma, associated with solid‐state mylonitic deformation. Intermingled granitic rocks escaped metamorphism, because they remained magmatic to lower temperatures, and experienced shorter and lower‐temperature subsolidus cooling intervals. However, the felsic rocks underwent relatively high‐temperature solid‐state deformation, as indicated by myrmekite replacing K‐feldspar and chess‐board subgrain patterns in quartz; locally they developed felsic mylonites. The felsic rocks were deformed in the solid state because of their high proportion of relatively weak minerals (quartz and biotite), whereas the mafic rocks mostly escaped subsolidus deformation, except in local high‐strain zones of hornblende‐plagioclase schist, because of their high proportion of relatively strong minerals (hornblende and plagioclase). We suggest that such contrasting microstructural features are diagnostic of long‐lived syntectonic magma transfer zones, and contrast with the more typical complex, batholith‐scale magma chambers of magmatic arcs.     

丹凤地区光石沟铀矿床大毛沟岩株锆石U-Pb同位素定年及其地质意义

丹凤地区是我国伟晶岩型铀矿床的重要产区,光石沟铀矿床是该区最大的铀矿床之一,大毛沟岩株与光石沟铀矿床有密切的空间关系与成因联系。本文采用锆石LA-ICP-MS定年方法测定了大毛沟岩株的U-Pb同位素年龄,获得了代表大毛沟岩株形成年龄的锆石谐和曲线加权平均值年龄为418.3±8.8 Ma,与光石沟铀矿床的成矿年龄相当,说明光石沟铀矿床的成矿作用在时间上与该岩株的形成时间相当。同时,残留锆石中1980.5±19.47 Ma年龄的发现,说明大毛沟岩株白岗质花岗岩的物源可能与中下地壳秦岭群有关;E颗粒内核465Ma的年龄则可能代表灰池子岩体片麻状二长花岗岩的形成年龄,表明大毛沟岩株与灰池子岩体的形成年龄之间存在约30 Ma的时差。