Adakitic火成岩对大陆地壳增厚过程的指示:以青藏北部火山岩为例

Adakitic火成岩可以通过几种不同的岩浆作用方式产生,其中下地壳镁铁质岩石的直接部分熔融和拆沉下地壳的部分熔融可能是两种重要的adakitic火成岩形成方式。在一个大陆厚地壳背景,adakitic火成岩的产生指示了它们的岩浆源区位于大于40 km的下地壳之中,因此,暗示该大陆地壳的最小厚度超过40 km。青藏高原腹地的羌塘地区分布有40 Ma左右的“低镁”和“高镁”adakitic安山岩-英安岩-流纹岩,它们应分别是青藏高原厚大陆地壳下部镁铁质岩石直接部分熔融和拆沉的下地壳脱水熔融的产物。这套adakitic火山岩的厘定指示出在40 Ma左右时,青藏羌塘地区或更大范围的大陆地壳已经加厚到超过40 km,其地表在当时或稍后可能已经开始了隆升。     

试论大火成岩省与成矿作用

根据组成大火成岩省的岩浆类型不同,大火成岩省可以分为两类,一是以基性火成岩为主的镁铁质大火成岩省（MLIPs）,二是以酸性火成岩为主的长英质大火成岩省（SLIPs）。它们都是由于在异常高的地幔热流参与下导致地幔或地壳大规模熔融形成的。大火成岩省独特的巨量岩浆活动是引起多层次物质和能量交换的重要场 所。成矿物质的聚集导致成矿作用和矿床的形成是必然的,因此大火成岩省本身就是一个大成矿系统。在这个成矿系统中,由于物源、成分、温度、压力、流体和氧逸度等条件的差异性,形成不同种类的矿化和矿床,并构成一定的成矿系列。镁铁质大火成岩省中形成的矿床类型有岩浆硫化物型Cr-Cu-Ni-PGE矿床和Ti-Fe 氧化物型V-Ti-Fe 矿床,热液型的Cu-Pb-Zn-Au-Ag矿床,以及远程低温热液矿床等。长英质大火成岩省形成的矿床类型为岩浆和交代型、热液型Cu-Pb-Zn-Au-Ag,W-Sn,U-Th-REE矿床,以及Sb-As矿床等。加强对大火成岩省及其成矿机理的研究,有望形成新的成矿理论和加速超大型矿床的发现。     

为什么要提出西藏东南部早白垩世措美大火成岩省

近年在西藏东南部特提斯喜马拉雅带东段大规模白垩纪火成岩受到了很多学者的关注。这里的火成岩岩石类型包括玄武岩、镁铁质岩墙/岩床、辉长岩侵入体以及少量层状超镁铁质岩和酸性火山岩。锆石U-Pb定年结果指示现今覆盖面积约50000km2的岩浆活动发生在130~136Ma(峰期约132Ma)之间。镁铁质岩显示OIB型(高Ti)、N-MORB型(低Ti)和过渡型(介于二者之间)三种地球化学类型,其中未受地壳混染的镁铁质岩的Sr-Nd同位素和锆石Hf同位素成分类似于Kerguelen地幔柱产物。在扣除堆晶橄榄石之后,通过橄榄石-熔体平衡计算,苦橄玢岩母岩浆的MgO含量约20%,对应的地幔潜温1560℃。西藏东南部白垩纪火成岩浆活动这种覆盖范围大、持续时间短和地幔潜温高等特征,非常类似于世界上其它地幔柱成因的大火成岩省或热点,因而将其描述和命名为措美(Comei)大火成岩省是合理的。年代学、地球化学和古地理重建资料显示藏南措美大火成岩省和南西澳大利亚同期的Bunbury玄武岩可能代表了同一个大火成岩省(即Comei-Bunbury大火成岩省)。Comei-Bunbury大火成岩省很可能记录了Kerguelen地幔柱在132Ma左右的早期岩浆作用,拉开了大印度从澳大利亚分离出来的序幕,影响了同期Weissert大洋缺氧事件的形成。     

大火成岩省研新进展

大火成岩省的含义是指连续的、体积庞大的火成岩(包括镁铁质和长英质火成岩)所构成的巨型岩浆岩建造。镁铁质大火成岩省可分为:大陆溢流玄武岩、火山被动陆缘、大洋高原玄武岩、大岩墙群和大层状侵入体。镁铁质大火成岩省是地幔柱岩浆活动的直接产物,一般与聚敛板块边界无关。长英质大火成岩省主要由酸性、中酸性熔结凝灰岩及与之有成因联系的花岗岩构成,与岩石圈伸展构造和玄武岩浆底侵作用有不可分割的联系。今后研究方向包括大火成岩省的形成与地幔动力学的联系以及它与大陆增生、大陆裂解和生物绝灭的关系。此外还包括大火成岩省与成矿作用研究     

峨眉山大火成岩省中攀枝花A型花岗岩的锆石U-Pb年龄、元素及Sr-Nd-Hf同位素地球化学

了解A型花岗质岩浆的形成过程是探讨大陆内部地壳演化和生长的基本前提.特别是其同位素组成可以反映形成过程中壳幔物质的贡献并用于确定其构造背景.研究证实,地球演化历史中重要的初生地壳形成阶段均与大型的地幔上涌事件密切相关[1].大陆地壳之下的软流圈地幔上涌带来的巨大热量可以导致大陆岩石圈地幔和镁铁质下地壳的广泛部分熔融,从而形成双峰式的镁铁质和长英质侵人体[2].     

花岗岩类与大陆地壳生长初探——以中国典型造山带花岗岩类岩石的形成为例

本文主要基于东昆仑造山带、秦岭造山带、兴蒙造山带、阿尔泰造山带、燕山造山带以及华南过铝花岗岩带等花岗岩类形成的研究成果,讨论中国大陆内几个造山带的花岗岩类形成与大陆地壳生长方式和过程,我们的初步认识是:软流圈(对流地幔)的热和物质向大陆的输入(input)是大陆地壳生长和再改造的根本.大陆地壳的形成演化和再改造(reworking)主要通过岩浆作用完成,岩浆的形成、运移和定位是大陆地壳生长的基本过程.幔源玄武质岩浆底侵(underplating)于大陆地壳底部和内侵(intraplating)于地壳内部,是软流圈注入大陆的基本形式.造山带镁铁质下地壳的拆沉作用是致使陆壳总组成为中性火成岩(安山岩和闪长岩,或粗面安山岩和二长岩质的)的主要原因.收缩挤压构造作用使陆壳加厚达≥50 km,是诱发镁铁质下地壳拆沉作用的必需条件.火成岩构造组合及其时间序列是识别大陆地壳从软流圈地幔中分出,直至最终形成的过程的关键记录.     

造山岩套中镁铁质和长英质岩浆的相互作用研究进展

结合2002年9月14—21日在意大利中部召开的“欧洲花岗岩网络”年会的会议讨论和野外考察内容,评述了镁铁质和长英质岩浆的相互作用关系以及镁铁质熔体在巨量花岗岩浆形成中的作用。在花岗岩岩基内,不仅出露有狭义的花岗岩,还可见到更富镁铁质的岩石,例如辉长岩和闪长岩。一般,这些镁铁质岩石,既可能是慢源母岩浆结晶的产物,或是岩浆混合和分异一同化作用中的端员组分,也可能代表了下地壳源区的物质,或者是导致大陆壳部分熔融的热源物质记录。对镁铁质和长英质岩浆相互作用关系的研究,有助于限定造山作用过程中地壳物质再循环和新的地慢物质加入的相对贡献。     

莫霍面,下地壳与岩浆作用

美国地质学会近期在意大利北部阿尔卑斯山区召开的关于地壳分异作用的Ｐｅｎｒｏｓｅ会议,以Ｉｖｒｅａ带大陆地壳剖面为样板,讨论了与大陆生长和演化有关的主要过程。文中结合会议情况,评述了岩浆作用与大陆生长、莫霍面演化和地壳动力学的关系。大陆壳特有的成分和大的花岗岩基的形成,要求下地壳有大体积的基性超镁铁质岩存在。这样的基性超镁铁质岩,既可能代表囤积在下地壳的幔源岩浆,也可能是幔源岩浆分异的堆积岩,或是在壳内分异作用过程中产生出花岗质熔体后的残余体。在伸展构造区,幔源岩浆的底侵作用强烈地影响了大陆壳的结构、组成和热状态,而在陆陆碰撞带,由于地壳加厚,下地壳的基性超镁铁质岩石会转变为“榴辉岩”,促进地壳沉没作用和下地壳拆沉作用。由于地壳岩石的榴辉岩相变质作用,地球物理莫霍面与壳幔边界可能并不对应。莫霍面和地壳对岩浆的密度过滤作用,又控制了大陆壳中岩浆的侵位和演化。     

藏南措美残余大火成岩省的西延及意义

在西藏南东部和澳大利亚南西部新识别出的早白垩世Comei-Bunbury大火成岩省的范围还未能得到很好约束。为探讨这一问题,本文报道了西藏南东部浪卡子-洛扎以西、江孜、康马地区广泛分布的近东西向基性岩墙和少量玄武岩的岩石学和地球化学数据。这些以辉绿岩、辉长岩为主的镁铁质岩石属于碱性-亚碱性玄武岩系列,在地球化学上主要包括高Ti的洋岛玄武岩型和低Ti的大洋中脊玄武岩型,两者的εNd(t)值分别为(+0.9～+2.0)和(+4.6～+5.8)。这两种类型岩石的地球化学特征均与措美残余大火成岩省内的对应类型可比,暗示它们与措美残余大火成岩省一样,均形成于由地幔柱作用于大陆岩石圈引起的伸展背景,代表该残余大火成岩省的西延。本文研究结果使得现今保留在特提斯喜马拉雅带上的Comei-Bunbury大火成岩省的残余面积接近50000km2。该大火成岩省的大规模岩浆活动可能是引发研究区甚至全球早白垩世大洋缺氧事件的一个重要因素。     

长江中下游中生代安山质火山岩记录的新元古代大洋板片-地幔相互作用

大陆弧安山岩的形成是大洋板片向大陆边缘之下俯冲的结果,但是在具体形成机制上存在很大争议.针对这个问题,对长江中下游地区中生代安山质火山岩及其伴生的玄武质和英安质火山岩进行了系统的岩石地球化学研究,结果对大陆弧安山质火成岩的成因提出了新的机制.分析表明,这些岩石形成于早白垩世,它们不仅表现出典型的岛弧型微量元素分布特征,而且具有高度富集的Sr-Nd-Hf同位素和高的放射成因Pb以及高的氧同位素组成.通过全岩和矿物地球化学成分变化检查发现,地壳混染和岩浆混合作用对其成分的富集特征贡献有限,而其岩浆源区含有丰富的俯冲地壳衍生物质才是其成分富集的根本原因.虽然这些火山岩的喷发年龄为中生代,但是其岩浆源区形成于新元古代早期的华夏洋壳俯冲对扬子克拉通边缘之下地幔楔的交代作用.大陆弧安山岩地幔源区中含有大量俯冲洋壳沉积物部分熔融产生的含水熔体,显著区别于大洋弧玄武岩的地幔源区,其中只含有少量俯冲洋壳来源的富水溶液和含水熔体.正是这些含水熔体交代上覆地幔楔橄榄岩,形成了不同程度富集的超镁铁质-镁铁质地幔源区.在早白垩纪时期,古太平洋俯冲过程的远弧后拉张导致中国东部岩石圈发生部分熔融,其中超镁铁质地幔源区熔融形成玄武质火山岩,镁铁质地幔源区则熔融形成安山质火山岩.因此,大陆弧安山岩成因与大洋弧玄武岩一样,可分为源区形成和源区熔融两个阶段,其中第一阶段对应于俯冲带壳幔相互作用.      

Largest Phanerozoic provinces of silicic volcanism: Tectonic position and genetic prerequisities

This paper presents a brief overview of largest Phanerozoic volcanic provinces of the Earth, which comprise the substantial volumes (over 10⁵ km³) of silicic extrusive rocks. The provinces of this kind may re- present a separate geodynamic class, having the volume comparable to that of intraplate Large Igneous Provinces (LIPs), and being formed in convergent margin settings. The thermal energy preserved in the crust after a previous major magmatic event is considered to be an important factor affecting the volume of crust-derived magmas and playing a significant role in the formation of large provinces of silicic volcanism.     

Origin of silicic volcanism in the Panamanian arc: evidence for a two-stage fractionation process at El Valle volcano

In the Central American Volcanic Arc, adakite-like volcanism has often been described as volumetrically insignificant. However, extensive silicic adakitic volcanism does occur in the Panamanian arc and provides an opportunity to evaluate the origin of this magma-type as well as to contrast its origin with other Central American silicic magmas. The Quaternary volcanic deposits of El Valle volcano are characterized by pronounced depletions in the heavy rare earth elements, low Y, high Sr, high Sr/Y, relatively high MgO, and low K₂O/Na₂O, when compared with other Quaternary Central American volcanics at similar SiO₂. These chemical features are also diagnostic of adakitic signatures. Our new ⁴⁰Ar/³⁹Ar ages of lava flows and ash flows that compose the volcanic edifice of El Valle volcano illustrate that the eruptive volume of adakitic-like volcanism is substantial during the Quaternary (~120 km³). Adakitic-like magmas dominate the stratigraphic record. Common to all models for the origin of an adakite geochemical signature is the involvement of garnet, as a residual or fractionating phase. The stability of garnet in hydrous magmas has been recently reevaluated with important consequences; garnet is a stable primary igneous phase at pressure and temperature conditions expected for magma differentiation at the roots of a mature island arc. Moreover, adakite-like volcanism erupted at El Valle volcano displays the middle rare earth element depletion observed in other Panamanian volcanic centers that has been attributed to significant amphibole fractionation. Extensive amphibole fractionation may have occurred in two stages. The first stage of fractionation, garnet + amphibole fractionation, occurs from hydrous basaltic–andesitic parental magmas that have ponded at the base of an overthickened crust. The second stage occurs at mid-lower crustal levels where abundant amphibole + plagioclase and minor sphene crystallized from water-rich magmas. These two stages combined may have resulted in an amphibole-rich cumulate layer. This amphibole layer is likely the source of the abundant amphibole-rich cumulate enclaves and blobs found in volcanic products across the Panamanian arc. Stalling of water-rich magmas during this two-stage fractionation process could drive the interstitial liquids to the evolved compositions typical of continental crust, while leaving behind amphibole-rich cumulate rocks that may eventually be returned to the asthenosphere. Differentiation of H₂O-rich magmas under the conditions appropriate for the roots of island arcs may therefore be a key process in developing a better understanding of the generation of continental crust in island arc environments.     

Magmatic processes that generate chemically distinct silicic magmas in NW Costa Rica and the evolution of juvenile continental crust in oceanic arcs

Northwestern Costa Rica is built upon an oceanic plateau that has developed chemical and geophysical characteristics of the upper continental crust. A major factor in converting the oceanic plateau to continental crust is the production, evolution, and emplacement of silicic magmas. In Costa Rica, the Caribbean Large Igneous Province (CLIP) forms the overriding plate in the subduction of the Cocos Plate—a process that has occurred for at least the last 25 my. Igneous rocks in Costa Rica older than about 8 Ma have chemical compositions typical of ocean island basalts and intra-oceanic arcs. In contrast, younger igneous deposits contain abundant silicic rocks, which are significantly enriched in SiO₂, alkalis, and light rare-earth elements and are geochemically similar to the average upper continental crust. Geophysical evidence (high Vp seismic velocities) also indicates a relatively thick (~40 km), addition of evolved igneous rocks to the CLIP. The silicic deposits of NW Costa Rica occur in two major compositional groups: a high-Ti and a low-Ti group with no overlap between the two. The major and trace element characteristics of these groups are consistent with these magmas being derived from liquids that were extracted from crystal mushes—either produced by crystallization or by partial melting of plutons near their solidi. In relative terms, the high-Ti silicic liquids were extracted from a hot, dry crystal mush with low oxygen fugacity, where plagioclase and pyroxene were the dominant phases crystallizing, along with lesser amounts of hornblende. In contrast, the low-Ti silicic liquids were extracted from a cool, wet crystal mush with high oxygen fugacity, where plagioclase and amphibole were the dominant phases crystallizing. The hot-dry-reducing magmas dominate the older sequence, but the youngest sequence contains only magmas from the cold-wet-oxidized group. Silicic volcanic deposits from other oceanic arcs (e.g., Izu-Bonin, Marianas) have chemical characteristics distinctly different from continental crust, whereas the NW Costa Rican silicic deposits have chemical characteristics nearly identical to the upper continental crust. The transition in NW Costa Rica from mafic oceanic arc and intra-oceanic magma to felsic, upper continental crust-type magma is governed by a combination of several important factors that may be absent in other arc settings: (1) thermal maturation of the thick Caribbean plateau, (2) regional or local crustal extension, and (3) establishment of an upper crustal reservoir.     

Naturaliste Plateau: constraints on the timing and evolution of the Kerguelen Large Igneous Province and its role in Gondwana breakup

Volcanism associated with the Kerguelen Large Igneous Province is found scattered in southwestern Australia (the ca 136 to ca 130 Ma Bunbury Basalts, and ca 124 Ma Wallaby Plateau), India (ca 118 Ma Rajmahal Traps and Cona Basalts), and Tibet (the ca 132 Ma Comei Basalts), but apart from the ～70 000 km² Wallaby Plateau, these examples are spatially and volumetrically minor. Here, we report dredge, geochronological and geochemical results from the ～90 000 km² Naturaliste Plateau, located ～170 to ～500 km southwest of Australia. Dredged lavas and intrusive rocks range from mafic to felsic compositions, and prior geophysical analyses indicate these units comprise much of the plateau substrate. ⁴⁰Ar/³⁹Ar plagioclase ages from mafic units and U–Pb zircon ages from silicic rocks indicate magmatic emplacement from 130.6 ± 1.2 to 129.4 ± 1.3 Ma for mafic rocks and 131.8 ± 3.9 to 128.2 ± 2.3 Ma for silicic rocks (2σ). These Cretaceous Naturaliste magmas incorporated a significant component of continental crust, with relatively high ⁸⁷Sr/⁸⁶Sr (up to 0.78), high ²⁰⁷Pb/²⁰⁴ Pb ratios (15.5–15.6), low ¹⁴³Nd/¹⁴⁴Nd (0.511–0.512) and primitive-mantle normalised Th/Nb of 11.3 and La/Nb of 3.97. These geochemical results are consistent with the plateau being underlain by continental basement, as indicated by prior interpretations of seismic and gravity data, corroborated by dredging of Mesoproterozoic granites and gneisses on the southern plateau flank. The Cretaceous Naturaliste Plateau igneous rocks have signatures indicative of extraction from a depleted mantle, with trace-element and isotopic values that overlap with Kerguelen Plateau lavas reflect crustal contamination. Our chemical and geochronological results therefore show the Naturaliste Plateau contains evidence of an extensive igneous event representing some of the earliest voluminous Kerguelen hotspot magmas. Prior work reports that contemporaneous correlative volcanic sequences underlie the nearby Mentelle Basin, and the Enderby Basin and Princess Elizabeth Trough in the Antarctic. When combined, the igneous rocks in the Naturaliste, Mentelle, Wallaby, Enderby, Princess Elizabeth, Bunbury and Comei-Cona areas form a 136–124 Ma Large Igneous Province covering >244 000 km².     

Petrogenesis of a basalt-comendite-pantellerite rock suite: the Boseti Volcanic Complex (Main Ethiopian Rift)

Petrological and geochemical data for basic (alkali basalts and hawaiites) and silicic peralkaline rocks, plus rare intermediate products (mugearites and benmoreites) from the Pleistocene Boseti volcanic complex (Main Ethiopian Rift, East Africa) are reported in this work. The basalts are slightly alkaline or transitional, have peaks at Ba and Nb in the mantle-normalized diagrams and relatively low ⁸⁷Sr/⁸⁶Sr (0.7039–0.7044). The silicic rocks (pantellerites and comendites) are rich in sanidine and anorthoclase, with mafic phases being represented by fayalite-rich olivine, opaque oxides, aenigmatite and slightly Na-rich ferroaugite (ferrohedenbergite). These rocks were generated after prolonged fractional crystallization process (up to 90–95 %) starting from basaltic parent magmas at shallow depths and fO₂ conditions near the QFM buffer. The apparent Daly Gap between mafic and evolved Boseti rocks is explained with a model involving the silicic products filling upper crustal magma chambers and erupted preferentially with respect to basic and intermediate products. Evolved liquids could have been the only magmas which filled the uppermost magma reservoirs in the crust, thus giving time to evolve towards Rb-, Zr- and Nb-rich peralkaline rhyolites in broadly closed systems.     

The petrogenetic characterization of intermediate and silicic charnockites in high-grade terrains: a case study from southern India

Large charnockite massifs occur in some of the Precambrian high-grade terrains like the southern Indian granulite terrain. The Cardamom Hill charnockite massif from the Madurai Block, southern India, consists of an intermediate type and silicic type, with the intermediate type showing similarities to high-Ba−Sr granitoids with low K₂O/Na₂O ratios and the silicic type showing similarities to high-Ba–Sr granitoids with high K₂O/Na₂O ratios. Within the constraints imposed by near basaltic composition of the most mafic samples and their relatively high concentrations of both compatible and incompatible elements, comparison with recent experimental studies on various source compositions, and trace- and rare-earth-element modeling, the distinctive features of the intermediate charnockites can be best explained in terms of assimilation–fractional crystallization (AFC) models involving interaction between a mantle-derived basaltic magma and lower crustal materials. Silicic charnockites on the other hand are high temperature melts of moderately hydrous basaltic magmas. A two-stage model which involves an initial partial melting of hydrous basaltic magma and later fractionation explains the geochemical features of the silicic charnockites, with the fractionation stage most probably an open system AFC. It is suggested that for massifs showing spatial association of intermediate and silicic charnockites, a model taking into account their compositional difference in terms of the effect of variations in the conditions (e.g., temperature, water fugacity) that prevailed, can account for plausible petrogenetic scenarios.     

Origin of metaluminous and alkaline volcanic rocks of the Latir volcanic field,northern Rio Grande rift,New Mexico

Volcanic rocks of the Latir volcanic field evolved in an open system by crystal fractionation, magma mixing, and crustal assimilation. Early high-SiO₂ rhyolites (28.5 Ma) fractionated from intermediate compositionmagmas that did not reach the surface. Most precaldera lavas have intermediate-compositions, from olivine basaltic-andesite (53% SiO₂) to quartz latite (67% SiO₂). The precaldera intermediate-composition lavas have anomalously high Ni and MgO contents and reversely zoned hornblende and augite phenocrysts, indicating mixing between primitive basalts and fractionated magmas. Isotopic data indicate that all of the intermediate-composition rocks studied contain large crustal components, although xenocrysts are found only in one unit. Inception of alkaline magmatism (alkalic dacite to high-SiO₂ peralkaline rhyolite) correlates with, initiation of regional extension approximately 26 Ma ago. The Questa caldera formed 26.5 Ma ago upon eruption of the >500 km³ high-SiO₂ peralkaline Amalia Tuff. Phenocryst compositions preserved in the cogenetic peralkaline granite suggest that the Amalia Tuff magma initially formed from a trace element-enriched, high-alkali metaluminous magma; isotopic data suggest that the parental magmas contain a large crustal component. Degassing of water- and halogen-rich alkali basalts may have provided sufficient volatile transport of alkalis and other elements into the overlying silicic magma chamber to drive the Amalia Tuff magma to peralkaline compositions. Trace element variations within the Amalia Tuff itself may be explained solely by 75% crystal fractionation of the observed phenocrysts. Crystal settling, however, is inconsistent with mineralogical variations in the tuff, and crystallization is thought to have occurred at a level below that tapped by the eruption. Spatially associated Miocene (15-11 Ma) lavas did not assimilate large amounts of crust or mix with primitive basaltic magmas. Both mixing and crustal assimilation processes appear to require development of relatively large magma chambers in the crust that are sustained by large basalt fluxes from the mantle. The lack of extensive crustal contamination and mixing in the Miocene lavas may be related to a decreased basalt flux or initiation of blockfaulting that prevented pooling of basaltic magma in the crust.     

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。     

Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil

High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser ⁴⁰Ar/³⁹Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (⁸⁷Sr/⁸⁶Sr?=?0.704 and ¹⁴³Nd/¹⁴⁴Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C.     

Fluid history of UHP metamorphism in Dabie Shan, China: a fluid inclusion and oxygen isotope study on the coesite-bearing eclogite from Bixiling

This paper characterizes late Holocene basalts and basaltic andesites at Medicine Lake volcano that contain high pre-eruptive H₂O contents inherited from a subduction related hydrous component in the mantle. The basaltic andesite of Paint Pot Crater and the compositionally zoned basaltic to andesitic lavas of the Callahan flow erupted approximately 1000 ¹⁴C years Before Present (¹⁴C years b.p.). Petrologic, geochemical and isotopic evidence indicates that this late Holocene mafic magmatism was characterized by H₂O contents of 3 to 6 wt% H₂O and elevated abundances of large ion lithophile elements (LILE). These hydrous mafic inputs contrast with the preceding episodes of mafic magmatism (from 10,600 to ∼3000 ¹⁴C years b.p.) that was characterized by the eruption of primitive high alumina olivine tholeiite (HAOT) with low H₂O (<0.2 wt%), lower LILE abundance and different isotopic characteristics. Thus, the mantle-derived inputs into the Medicine Lake system have not always been low H₂O, primitive HAOT, but have alternated between HAOT and hydrous subduction related, calc-alkaline basalt. This influx of hydrous mafic magma coincides temporally and spatially with rhyolite eruption at Glass Mountain and Little Glass Mountain. The rhyolites contain quenched magmatic inclusions similar in character to the mafic lavas at Callahan and Paint Pot Crater. The influence of H₂O on fractional crystallization of hydrous mafic magma and melting of pre-existing granite crust beneath the volcano combined to produce the rhyolite. Fractionation under hydrous conditions at upper crustal pressures leads to the early crystallization of Fe-Mg silicates and the suppression of plagioclase as an early crystallizing phase. In addition, H₂O lowers the saturation temperature of Fe and Mg silicates, and brings the temperature of oxide crystallization closer to the liquidus. These combined effects generate SiO₂-enrichment that leads to rhyodacitic differentiated lavas. In contrast, low H₂O HAOT magmas at Medicine Lake differentiate to iron-rich basaltic liquids. When these Fe-enriched basalts mix with melted granitic crust, the result is an andesitic magma. Since mid-Holocene time, mafic volcanism has been dominated primarily by hydrous basaltic andesite and andesite at Medicine Lake Volcano. However, during the late Holocene, H₂O-poor mafic magmas continued to be erupted along with hydrous mafic magmas, although in significantly smaller volumes. Received: 4 January 1999 / Accepted: 30 August 1999