中国东北部地区埃达克岩及其成矿意义

中国东北部地区埃达克岩分布较广，以吉林居多，内蒙古东部和黑龙江较少。本区埃达克岩分别形成于古亚洲洋、蒙古一鄂霍茨克洋和环太平洋三个构造带，时代为早古生代中期一中生代。区内以埃达克岩为成矿主岩的热液矿床矿种主要为Au和Cu，其次为Mo；成因类型主要为斑岩型、岩浆热液型和浅成低温热液型；成矿时代主要为早古生代中期、晚古生代晚期及中生代中晚期。依据本地区埃达克岩和与其相关的热液矿床特征及产出的构造部位，将本区以埃达克岩为含矿岩的热液矿床划分为大兴安岭北部、蒙吉黑和吉黑东部3个成矿区。     

Adakite-like signature of Late Miocene intrusions at the Los Pelambres giant porphyry copper deposit in the Andes of central Chile: metallogenic implications

Adakite-like features are recognized in the Late Miocene (~10 Ma) porphyritic intrusions of the Los Pelambres giant porphyry copper deposit, central Chile (32°S). Located within the southern portion of the flat-slab segment (28–33°S) of the Chilean Andes, the Al- and Na-rich porphyries of Los Pelambres display distinctly higher Sr/Y (~100–300) and La_N/Yb_N (~25–60) ratios than contemporaneous and barren magmatic units (e.g., La Gloria pluton, Cerro Aconcagua volcanic rocks) of the same Andean magmatic belt. Strong fractionation of heavy rare earth elements (HREE), absence of Eu anomalies, high Sr/Y and Zr/Sm and low Nb/Ta ratios suggest melt extraction from a garnet-amphibolite source. The Late-Miocene adakite-like porphyritic intrusions at Los Pelambres formed closely related in time and space to the subduction of the Juan Fernández Ridge (JFR) hotspot chain along the Chilean margin. Current tectonic reconstructions reveal that, at the time of formation of the Los Pelambres rocks, a W-E segment of the JFR started to subduct beneath them, producing a slow-down of a previously rapid southward migration of a NE-ridge—trench collision. These particular tectonic conditions are favorable for the origin of the Los Pelambres porphyry suite by melting of subducting young hotspot rocks under flat-slab conditions. The incorporation of crustal components into the oceanic lithopheric magma source by subduction erosion is evidenced by the Sr-Nd isotope composition of the Los Pelambres rocks different from the MORB signatures of true adakites. A close relationship apparently exists between the origin of this adakite-like magmatism and the source of the mineralization in the Los Pelambres porphyry copper deposit.Editorial handling: R.J. Goldfarb     

胶北地块与金成矿有关的郭家岭岩体和上庄岩体年代学及地球化学研究

胶东半岛是我国最主要的原生金矿矿集区,金矿的主要控矿围岩是郭家岭花岗岩,通过研究郭家岭花岗岩的地球化学特征对研究金矿的成因和物质来源具有指示性意义。本文研究的两个花岗岩岩体为上庄岩体和郭家岭岩体,两岩体同属郭家岭型花岗岩。通过对两岩体的花岗岩样进行岩相学矿相学观察、全岩主、微量元素和U-Pb同位素分析,获得胶东半岛中生代岩浆岩的成因机制与源区性质及自然金的产出形式等科学信息。LAICP MS锆石U-Pb年龄得出郭家岭岩体年龄125.4±2.2 Ma,上庄岩体U-Pb年龄128.8±2.0 Ma,都为中生代早白垩世,两岩体年龄相差3Ma,在年龄误差范围来看可以把两岩体作为同一期岩体,也在年龄角度证实两岩体都同属郭家岭花岗岩。两岩体的锆石组成都含有太古宙和晚侏罗纪的继承锆石,指示两岩体的成岩物质来源具相似性,都包含太古宙岩石成分和晚侏罗世花岗岩成分。两岩体具有相似的稀土元素和微量元素分配模式,表现出明显的LREE富集和HREE极度亏损,没有明显的铕负异常。郭家岭岩体和上庄岩体花岗岩都具有类似埃达克岩的特征,都具有高的Sr含量(913×10-6~1325×10-6),低的Y含量(2.2×10-6~8.4×10-6)和Yb含量(0.21×10-6~0.68×10-6),较高的(Dy/Yb)N比值1.62~2.28,暗示花岗岩岩浆形成时石榴石是一个重要的残留相,而没有斜长石作为残留相。两岩体具有较低的MgO、Cr、Ni含量和Mg#,反映郭家岭型花岗岩岩浆的形成可能是岛弧环境榴辉岩相压力条件下洋壳玄武质岩石的部分熔融。     

吉林东部大蒲柴河adakites锆石U-Pb年龄、Hf同位素特征及其意义

地球化学研究表明,大蒲柴河岩体具有典型的埃达克岩特征,来自加厚下地壳的部分熔融作用.本文采用激光等离子质谱对该岩体进行了U-Pb同位素定年,结果表明该岩体为晚侏罗世(165Ma)岩浆活动的产物.锆石的LA-MC-ICPMS Hf同位素研究结果显示,ε_(Hf)(165Ma)范围为-5.02～5.43,二阶段Hf模式年龄(t_(DM2))范围为965～1622Ma,暗示原始母岩浆为两种不同源区岩浆的混合.另外,Hf同位素研究表明,研究区在中-新元古代时(965～1304Ma)曾经历了一次重要的地壳增生事件.     

Evolution of the late Pleistocene Mojanda–Fuya Fuya volcanic complex (Ecuador), by progressive adakitic involvement in mantle magma sources

The Mojanda–Fuya Fuya Volcanic Complex consists of two nearby volcanoes, Mojanda and Fuya Fuya. The older one, Mojanda volcano (0.6 to 0.2 Ma), was first constructed by andesites and high-silica andesites forming a large stratovolcano (Lower Mojanda). This edifice was capped by a basaltic andesite and andesitic cone (Upper Mojanda), which collapsed later to form a 3-km-wide summit caldera, after large phreatomagmatic eruptions. The Lower Fuya Fuya edifice was constructed by the extrusion of viscous Si-rich andesitic lavas and dacitic domes, and the emission of a thick sequence of pyroclastic-flow and fallout deposits which include two voluminous rhyolitic layers. An intermediate construction phase at Fuya Fuya is represented by a mainly effusive cone, andesitic in composition (San Bartolo edifice), the construction of which was interrupted by a major sector collapse in the Late Pleistocene. Finally, a complex of thick siliceous lavas and domes was emplaced within the avalanche amphitheatre, forming the Upper Fuya Fuya volcanic centre. This paper shows that the general evolution from an effusive to an explosive eruptive style is related to a progressive adakitic contribution to the magma source. Although all the rocks of the complex are included in the medium-K field of continental arcs, the Fuya Fuya suite (61–75 wt.% SiO₂) shows depletion in Y and HREE and high Sr/Y and La/Yb values, compared to the less silicic Mojanda suite (55–66.5 wt.% SiO₂). The Mojanda calc-alkaline suite was generated by partial melting of an adakite-metasomatised mantle source that left a residue with 2% garnet, followed by fractional crystallization of dominant plagioclase + pyroxene + olivine at shallow, intra-crustal depths. For Fuya Fuya, geochemical and mineralogical data suggest either (1) partial melting of a similar metasomatised mantle with more garnet in the residue (4%), followed by fractional crystallization involving plagioclase, amphibole and pyroxene, or (2) mixing of mafic mantle-derived magma from the Mojanda suite and slab melts, followed by the same fractional crystallization process.     

Origin of high field strength element enrichment in volcanic arcs: Geochemical evidence from the Sulu Arc, southern Philippines

Lavas from the Sulu Arc, southern Philippines, exhibit an enrichment in high field strength elements (HFSE) that represents a departure from the typical volcanic arc geochemical signature. It has been postulated that this relative enrichment arises from metasomatism of mantle wedge peridotites by melts derived from the subducting oceanic lithosphere, through formation of amphibole which subsequently breaks down and enriches the mantle source of parental arc magmas in HFSE. Divergent chemical and isotopic characteristics between Sulu Arc HFSE-enriched lavas and the Sulu Sea crust being subducted—the presumed source of slab-derived melts—render it unlikely, however, that HFSE enrichment arises from the influence of such melts. New geochemical data suggest that the varying degrees of HFSE enrichment in Sulu Arc lavas are instead the result of variable amounts of mixing between enriched and depleted mantle end-components—the sources of South China Sea intraplate lavas and Sulu seafloor basalts, respectively—within a compositionally heterogeneous mantle wedge.     

冈底斯东段驱龙-甲玛矿集区新近纪花岗岩类区域岩石学及成矿学意义

对埃达克岩及其与成矿的关系的研究是近几年花岗岩地质研究的重要进展之一.本文通过对驱龙—甲玛矿集区新近纪花岗岩地质和元素、同位素地球化学综合分析,探讨了新近纪花岗岩类的成因及形成的动力机制、成矿意义.结果表明,新近纪花岗岩具有埃达克岩的地球化学特征,其岩浆起源于下地壳,在上升或演化过程中与幔源岩浆发生了混合,该混合作用对成矿具有重要意义.23～10 Ma岩石圈伸展导致软流圈上涌并诱发含石榴石镁铁质岩石拆沉,拆沉下地壳部分熔融形成埃达克岩浆.岩浆混合、岩浆流体出溶的地质、矿物学证据以及有利的大地构造背景、深部物质组成、蚀变特征表明驱龙—甲玛矿集区新近纪“似埃达克岩”具非常重要的成矿学意义.     

内蒙古那仁乌拉埃达克质花岗岩的发现、成因、锆石U-Pb年龄及其构造意义

在中亚造山带东段华北克拉通北缘,识别出一套晚古生代那仁乌拉黑云母花岗岩。其LA-ICP-MS锆石U-Pb年龄为267.2Ma±1.4 Ma(MSWD=1.3,n=21)和捕获锆石年龄为296.3Ma～296.1Ma、278.8Ma～277.4 Ma;岩石为低钾(拉斑)到高钾钙碱性,弱准铝质到过铝质I型;ΣREE较低,轻重稀土分馏较强(LaN/YbN:15.59～32.36)。在稀土元素配分模式图上,都表现为轻稀土富集,重稀土亏损的右倾散开式。弱到正的铕负异常(δEu:0.79～1.08);在微量元素蛛网图上,相对富集LREE(La,Ce),LILE(K,Rb)和HFSE(Zr,Hf),元素Nb,Ta,P,Ti,Y,Yb,Lu亏损程度较大,元素U,Th,Sr由相对亏损到富集。岩石整体具有埃达克质岩的亲合性,形成于弧向同碰撞过渡的构造环境;锆石εHf(t)值(0.95～4.05)显示具有弱亏损的幔源组分特性,与主量元素、微量元素及REE一致,变化范围都较大,共同暗示其源岩的壳幔混源性。较年轻的锆石Hf模式年龄(1 030.9Ma～1 226.8Ma)与兴蒙造山带范围一致;那仁乌拉花岗岩与西部乌拉特中旗克布岩体,东部吉林大玉山岩体共同组成一条华北克拉通北缘埃达克质岩浆带,其都为与古亚洲洋俯冲碰撞引起的具有弧岩浆性质的年轻的基性玄武质下地壳部分熔融有关的C型埃达克岩。这条埃达克质岩浆带与稍晚期的华北克拉通北缘碱性岩浆带相对应,都具有西部形成早于东部的特征,可能暗示古亚洲洋自西向东逐渐闭合的规律。     

俯冲带部分熔融

俯冲带是地幔对流环的下沉翼,是地球内部的重要物理与化学系统。俯冲带具有比周围地幔更低的温度,因此,一般认为俯冲板片并不会发生部分熔融,而是脱水导致上覆地幔楔发生部分熔融。但是,也有研究认为,在水化的洋壳俯冲过程中可以发生部分熔融。特别是在下列情况下,俯冲洋壳的部分熔融是俯冲带岩浆作用的重要方式。年轻的大洋岩石圈发生低角度缓慢俯冲时,洋壳物质可以发生饱和水或脱水熔融,基性岩部分熔融形成埃达克岩。太古代的俯冲带很可能具有与年轻大洋岩石圈俯冲带类似的热结构,俯冲的洋壳板片部分熔融可以形成英云闪长岩-奥长花岗岩-花岗闪长岩。平俯冲大洋高原中的基性岩可以发生部分熔融产生埃达克岩。扩张洋中脊俯冲可以导致板片窗边缘的洋壳部分熔融形成埃达克岩。与俯冲洋壳相比,俯冲的大陆地壳具有很低的水含量,较难发生部分熔融,但在超高压变质陆壳岩石的折返过程中可以经历广泛的脱水熔融。超高压变质岩在地幔深部熔融形成的熔体与地幔相互作用是碰撞造山带富钾岩浆岩的可能成因机制。碰撞造山带的加厚下地壳可经历长期的高温与高压变质和脱水熔融,形成S型花岗岩和埃达克质岩石。     

西藏改则县北亭贡南部地区埃达克岩的发现

在西藏改则县北亭贡南部地区发现一套岩石组合以辉绿玢岩-细粒闪长岩一细粒石英闪长岩-安山岩-凝灰岩组成的埃达克岩,根据岩石学、岩石化学、岩石地球化学及产出构造环境与典型埃达克岩对比,其完全符合埃达克岩类型,埃达克岩的发现为研究区火成岩成因问题和铜矿的找矿方向提供新思路。