Geochemical and isotopic constraints on the age and origin of mafic dikes from eastern Shandong Province,eastern North China Craton

Post-orogenic mafic dikes are widespread across eastern Shandong Province, North China Craton, eastern China. We here report new U–Pb zircon ages and bulk-rock geochemical and Sr–Nd–Pb isotopic data for representative samples of these rocks. LA-ICP-MS U–Pb zircon analysis of two mafic dike samples yields consistent ages of 118.7 ± 0.25 million years and 122.4 ± 0.21 million years. These Mesozoic mafic dikes are characterized by high (⁸⁷Sr/⁸⁶Sr) _i ranging from 0.7082 to 0.7087, low ?_Nd(t) values from??17.0 to??17.5, ²⁰⁶Pb/²⁰⁴Pb from 17.14 to 17.18, ²⁰⁷Pb/²⁰⁴Pb from 15.44 to 15.55, and ²⁰⁸Pb/²⁰⁴Pb from 37.47 to 38.20. Our results suggest that the parental magmas of these dikes were derived from an ancient, enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials prior to magma generation. The mafic dikes underwent minor fractionation during ascent and negligible crustal contamination. Combined with previous studies, these findings provide additional evidence that intense lithospheric thinning beneath eastern Shandong occurred at ～120 Ma, and that this condition was caused by the removal/foundering of the lithospheric mantle and lower crust.     

Petrology of late proterozoic mafic dikes in the Nico Perez region,central Uruguay

Summary Tholeiitic basaltic and basaltic andesite dikes of Brasiliano (or PanAfrican) age ( 600 Ma) intrude the basement of the Nico Perez region, Uruguay. Major and trace element geochemistry of the basalts indicates that they suffered fractionation in shallow magma chambers. The variation in element ratios, which remain virtually unchanged during fractionation (K/Rb, Rb/Ba, Ba/Nb, La/Nb, Zr/Nb and Ti/Zr), indicate that the dikes are not strictly comagmatic. However, they have certain features in common: LILE and LREE enrichment with respect to HFSE and HREE; high Rb/Ba (> 0.9) and Rb/Sr (> 0.08); low K/Rb (< 214); negative Nb and Ti anomalies (La _n /Nb _n > 2; Ba/Nb > 22 Ti/Zr < 60).⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd at 665 Ma are in the range 0.7052 – 0.7119 and 0.51158 – 0.51177, respectively.The lack of correlations between isotope and trace element variations indicate that these characteristics are not controlled by crustal contamination of the melts. They are interpreted as being due to the melting of an enriched mantle (C1) under the influence of a fluid-rich component (C2) which stabilized a Nb-retaining titanate phase in the residuum. Although this process may be related to a subduction environment, it is also possible that it occurred in an ensialic region by the interaction of deep mantle fluids with the lithospheric continental mantle.The parent mantle underlying the early Proterozoic (1.8 Ga) Florida region had isotope and geochemical characteristics which could evolve to the values observed in the adjacent Nico Perez region. It is therefore proposed that mantle enrichment took place in the early Proterozoic and that this mantle melted under the influence of fluids in the late Proterozoic to derive the Nico Perez dikes.

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Petrologie von spätproterozoischen, mafischen Gängen in der Nico Perez-Region, Zentral Uruguay

Zusammenfassung In das Grundgebirge der Nico Perez-Region, Uruguay, intrudierten tholeiitische und basaltandesitische Gange panafrikanischen (Brasiliano) Alters ( 600 Ma). Die Hauptund Spurenelementgeochemie der Basalte zeigt eine Fraktionierung in seichten Magmakammern an. Die Streuung jener Elementverhältnisse, die während der Fraktionierung scheinbar unverändert bleiben (K/Rb, Rb/Ba, Ba/Nb, La/Nb, Zr/Nb und Ti/Zr), weist darauf hin, daß die Gänge nicht streng komagmatisch sind. Sie haben aber dennoch bestimmte Merkmale gemeinsam: Anreicherung von LILE und LREE in Bezug auf HFSE and HREE; hohes Rb/Ba (> 0, 9) and Rb/Sr (> 0, 08); niedriges K/Rb (< 214); negative Anornalien bei Nb and Ti (La _n /Nb _n > 2; Ba/Nb > 22, Ti/Zr < 60). Bei 665 Ma liegen⁸⁷Sr/⁸⁶Sr und¹⁴³Nd/¹⁴⁴Nd im Bereich von 0, 7052 bis 0, 7119 beziehungsweise von 0,51158 bis 0,51177.Fehlende Korrelationen zwischen Isotopen- und Spurenelement-Streuungen zeigen, daß these Eigenschaften nicht durch Kontamination der Schmelzen mit Krustenmaterial kontrolliert werden. Sie werden mit der Aufschmelzung von angereichertem Mantel (C1) unter dem Einfluß einer Fluid-reichen Komponente (C2) erklärt. Diese Komponente stabilisierte eine Titanat-Phase im Residuum, die Nb zurückhielt. Obwohl dieser Prozeß einem Subduktions-Milieu zugeordnet werden kann, ist es auch möglich, daß er in einem intrakontinentalen Bereich durch Wechselwirkung von Fluiden aus dem tiefen Mantel mit dem kontinentalen Lithosphärenmantel stattfand.Der Ausgangsmantel, der die frühproterbzoische (1, 8 Ga) Florida-Region unterlagert, weist isotopische and geochemische Merkmale auf, aus denen sich die in der benachbarten Nico Perez-Region beobachteten Werte entwickelt haben könnten. Daher wird angenommen, daß die Anreicherung im Mantel im frühen Proterozoikum stattfand und daß dieser Mantel unter tern Einfluß von Fluiden im späten Proterozoikum Schmelzen bildete, von denen die Gange von Nico Perez abgeleitet werden können.

     

东南极拉斯曼丘陵镁铁质麻粒岩的变质作用演化

拉斯曼丘陵（Larsemann Hills）位于东南极普里兹构造带的中部,研究该区麻粒岩的变质作用演化对于理解普里兹带的构造属性至关重要。通过对该区含石榴石镁铁质麻粒岩转石详细的岩相学观察表明,峰期前进变质阶段矿物组合（M1）由角闪石+斜方辉石+单斜辉石+斜长石+黑云母+钛铁矿±石英±磁铁矿组成,其峰期矿物组合（M2）为石榴石+斜方辉石+单斜辉石+角闪石+钛铁矿±磁铁矿±石英,而代表后期与降压有关的叠加变质组合（M3）为斜方辉石+斜长石+单斜辉石+黑云母+钛铁矿±磁铁矿。矿物化学分析,结果显示其中石榴子石和斜方辉石具有弱的成分环带特征。利用THERMOCALC软件在NCFMASHTO体系下对该麻粒岩进行了详细的热力学模拟,结合传统温压计和平均温压计算结果,得出不同阶段温压条件分别为650~750℃/5.5~6.5kb （M1）,850~950℃/8~8.5kb （M2）,800~900℃/5.5~7.5kb （M3）。其变质作用演化为典型的峰期后近等温减压的（ITD）顺时针P-T轨迹。通过区域上镁铁质麻粒岩的对比分析,我们认为该镁铁质麻粒岩可能来源拉斯曼丘陵基岩露头。结合已有的年代学资料,表明该镁铁质麻粒岩的峰期变质事件可能对应于晚元古代格林威尔期构造事件,而后期退变质作用与早古生代的泛非期构造事件有关,意味着泛非期普里兹带可能是陆内造山带。     

Petrogenesis of the Barcroft pluton, northern White-Inyo Mountains, east-central California

The White-Inyo Range lies within the regional transition from Paleozoic-Precambrian North American continental basement to outboard Mesozoic and younger accreted terranes and a superimposed Andean-type arc. In the central White Mountains, the metaluminous Barcroft granodiorite invaded a major NE-striking, SE-dipping high-angle reverse fault—the Barcroft break. Because it is a relatively isolated igneous body and is well exposed over an elevation range of 1,500–4,000 m, its thermal history and that of the surrounding superjacent section are clearer than those of nearly coeval, crowded plutons emplaced in the hotter Sierra Nevada belt. The Barcroft pluton was emplaced as a compositionally heterogeneous series of areally scattered melt pulses episodically injected over the approximate interval 167–161 Ma. The oldest dated rocks are relatively quartzofeldspathic, whereas the youngest is more ferromagnesian, suggesting progressive partial fusion of a relatively mafic protolith. Heavy rare earth-enriched zircons indicate that Barcroft melts were derived at mid-crustal depths from a previously emplaced metabasaltic protolith containing plagioclase but lacking garnet. Granodioritic magma genesis involved the possible mixing of mafic and felsic melts, as well as very minor assimilation of country rocks, but mainly by fractional fusion and crystallization. Bulk chemical, rare earth, and isotopic data suggest that analyzed Barcroft rocks are members of a single suite. Granodioritic rocks are slightly more magnetite-rich at higher elevations on the NE, nearer the roof of the pluton. Earlier thermobarometry chronicled cooling and re-equilibration of the Barcroft pluton from its margins inward, as well as from mid-crustal generation depths of ~25 km through ascent and stalling at ~10–12 km. Refractory phase assemblages crystallized along the pluton margins, whereas subsolidus minerals in the interior of the of body continued to exchange with upper crustal deuteric and surficial aqueous fluids during exhumation and cooling.     

Differentiation of the Cuthbert Lake ultramafic dikes and related mafic dikes

In the Cuthbert Lake region of north-central Manitoba, northeasterly trending ultramafic-mafic dikes, part of the Molson dike swarm, show a range of composition from gabbro to olivine-hornblende pyroxenite to hornblende peridotite. The major dike which is ultramafic in composition is 60 m thick. Olivine and chromian spinel were the earliest cumulus phases formed in a subcrustal magma chamber before the emplacement of the dikes. Orthopyroxene and clinopyroxene were formed following emplacement at about 1120° C. Plagioclase and hornblende were the latest phases to crystallize from the intercumulus melt. Mineralogical and chemical variations across the major dike are interpreted to have resulted from flow differentiation of multiple injections of magma carrying suspended olivine crystals. Olivine phenocrysts changed their compositions from about Fo₈₇ to values ranging from Fo₈₀to Fo₇₃ as a function of the amount of intercumulus melt. The composition of this melt is estimated to have been basaltic. A mafic dike, about 10 m thick and occurring about 20 m away from the main ultramafic dike, is believed to have been formed from magmas that were tapped from an upper layer overlying the olivine-rich zone in a subcrustal magma chamber. Separation must have occurred when clinopyroxene and plagioclase appeared on the liquidus.Geological Survey of Canada Contribution 36486     

Isotope geochemistry of the mafic dikes from the Vazante nonsulfide zinc deposit, Brazil

The Vazante Group, located in the northwestern part of Minas Gerais, hosts the most important zinc mine in Brazil, the Vazante Mine, which represents a major known example of a hypogene nonsulfide zinc deposit. The main zinc ore is represented by willemite and differs substantially from other deposits of the Vazante-Paracatu region, which are sulfide-dominated zinc-lead ore. The age of the Vazante Group and the hosted mineralization is disputable. Metamorphosed mafic dikes (metabasites) that cut the metasedimentary sequence and are affected by hydrothermal processes recently were found and may shed light on the geochronology of this important geological unit. Zircon crystals recovered from the metabasites are xenocrystic grains that yield U–Pb conventional ages ranging from 2.1 to 2.4 Ga, so the basement of the Vazante Group is Paleoproterozoic or has metasedimentary rocks whose source area was Paleoproterozoic. Pb isotopes determined for titanite separated from the metabasites have common, nonradiogenic Pb compositions, which prevents determination of their crystallization age. However, the Pb signatures observed for the titanite crystals are in agreement with those determined for galena from the carbonate-hosted Zn–Pb deposits hosted by the Vazante Group, including galena from minor sulfide ore bodies of the Vazante deposit. These similarities suggest that the metalliferous fluids that affected the metabasites may have been those responsible for galena formation, which could imply a similar lead source for both nonsulfide and sulfide zinc deposits in the Vazante–Paracatu district. This common source could be related to deep-seated, basin-derived, metalliferous fluids associated with a long-lived hydrothermal system related to diagenesis and deformation of the Vazante Group during the Neoproterozoic.     

山东中生代基性脉岩的元素地球化学及其成因

研究区脉岩 SiO2含量变化范围为 48.63%～ 56.02% ,岩性上以煌斑岩、辉绿岩和辉长岩为主 ,以富集轻稀土元素 (LREE)和大离子亲石元素 (LILE)、亏损重稀土元素 (HREE)和高场强元素 (HFSE)为特征.主元素和微量元素研究表明 ,基性脉岩源区存在明显的地壳混染作用和少量富碳酸岩交代作用,该交代作用可能与扬子下地壳物质的参与有关.     

Geochemistry of Precambrian mafic dikes,central Bighorn Mountains,Wyoming, U.S.A.

Precambrian quartz dolerites and metadolerites of the central Bighorn Mountains form dikes that intrude a Precambrian metamorphic and igneous terrane typical of the Laramide uplifts of the middle Rocky Mountains. They have a restricted range of major- and trace-element compositions and are typical of basalts in the middle stages of tholeiitic fractionation. Fractionation in the direction of iron enrichment occurred by removal of plagioclase. Average element concentrations of the two groups are nearly identical to one another, are comparable to those in Archean metabasalts from numerous shield areas, and are intermediate between those of modern oceanic tholeiites and continental tholeiites. These average concentrations suggest a depth of magma generation and thickness of crust intermediate between those for the oceanic and continental environments.     

高喜马拉雅中段基性捕虏体的变质作用及其构造意义

西藏南部聂拉木—樟木剖面出露的高喜马拉雅变质带主要由副变质片麻岩和花岗质片麻岩组成,其次为伟晶岩和淡色花岗侵入体,区域变质程度为角闪岩相。我们对其中的变质基性捕虏体进行详细的变质作用研究,内容包括变质矿物组合,矿物变质反应结构和变质作用的温度—压力条件分析。基性捕虏体中的石榴子石角闪片麻岩和斜长角闪片麻岩均保存了两期变质矿物组合。温度与压力计算结果表明,石榴子石角闪片麻岩早期变质阶段(M1)温度约为829 ℃,压力为7.3 kbar; 晚期(M2)变质温度为625 ℃,压力为4.3 kbar。斜长角闪片麻岩所经历的早期变质阶段(M1)温度约为776 ℃、压力约为10.6 kbar; 晚期(M2)变质温度超过692 ℃,压力为7.4 kbar。石榴子石角闪片麻岩和斜长角闪片麻岩捕虏体均记录了典型的顺时针P-T轨迹,表明高喜马拉雅变质带曾向北俯冲到下地壳深度,之后被抬升到地表剥蚀出露。变质基性捕虏体的研究说明高喜马拉雅结晶岩系经历过较高温度—压力的变质作用,支持了其沿着藏南拆离系和主中央逆冲断裂系向南挤出的大地构造模型。     

广东沿海地区基性岩脉地球化学及成因

广东沿海地区基性岩脉的成岩年龄是146~54 Ma,主要形成于白垩纪,次为侏罗纪和第三纪。可将工作区基性岩脉的形成时代分为5期：第一期146Ma,第二期138~132Ma,第三期112~105Ma,第四期99~82Ma,第五期75~54Ma。根据K₂O/Na₂O比值和K₂O含量,以及不相容元素分配模式的“Nb-Ta”异常,可以划分出两类基性岩脉。第一类K₂O/Na₂O比值<0.5和K₂O含量<1.3%,在微量元素MORB标准化图解中具有弱的“Nb-Ta”负异常。第二类K₂O/Na₂O比值>0.5和K₂O含量>1.3%,在微量元素MORB标准化图解中具有显著的“Nb-Ta”负异常。在K₂O-SiO₂图解中,第一类基性岩脉样品落入中钾岩石范围,第二类基性岩脉样品落入高钾岩石或钾玄岩范围。基性岩脉分布的重要特点是中钾基性岩脉分布于本区北部上地幔拗陷区,而高钾基性岩脉分布于本区南部地幔隆起区或斜坡区。两类基性岩脉有着不同的岩浆来源。在本区北部,软流圈物质入侵和熔蚀Ⅰ型富集地幔混合形成中钾基性岩脉,随着时代的由老至新和熔蚀作用的不断进行,岩浆中Ⅰ型富集地幔成分增加。晚期,岩浆活动向连平、新丰一带迁移。在本区南部,软流圈物质入侵并熔蚀Ⅱ型富集地幔,形成具有软流圈物质和Ⅱ型富集地幔混合特征的高钾基性岩脉。随着熔蚀作用的不断进行,岩浆中Ⅱ型富集地幔成分增加,晚期基性岩浆活动向三水裂谷、南澳裂谷迁移。     

东南极格罗夫山镁铁质麻粒岩的变质作用

东南极内陆－格罗夫山存在一套经历了麻粒岩相变质作用的镁铁质麻粒岩和斜长角闪岩。变质反应结构显示该区多为单一的区域性麻粒岩相变质作用。但是，对含石榴石的镁铁质麻粒岩的详细工作则显示了包括3个阶段的近等温降压（ITD）的顺时针PT演化轨迹，M1：0．93GPa＞800℃：M2:0.65GPa,733-850℃;M3:0.46-0.61GPa，并有着与拉斯曼丘陵相似的演化历史。根据矿物组合和成分、变质反应结构及温压计算结果，格罗夫山镁铁质麻粒岩可能为高压麻粒岩。     

基性岩脉与金成矿的关系

基性岩脉是大陆伸展背景下,主要来自陆下软流圈或地幔岩石圈的岩浆侵入岩体.大量研究成果表明基性岩脉发育与金矿脉在空间展布、构造格局和形成时间具有紧密依存关系,虽然对基性岩脉是否为金矿脉提供金源尚无统一定论,但是根据基性岩脉与金矿化紧密的时空关系,基性岩脉可以作为找金矿的良好标志.     

Metamorphism and PGE-Au content of chromitite from the Ipanema mafic/ultramafic Complex, Minas Gerais, Brazil

Summary The Santa Cruz massif, which forms part of the Ipanema mafic/ultramafic Complex, Minas Gerais, Brazil, has an exposed upward sequence of metadunite, metaharzburgite (including three separate chromitite layers), metapyroxenite, metagabbro, and meta-anorthosite. Primary igneous chromite grains in the main chromitite layer are poikiloblastic and tectonically fragmented, and have a narrow (10–20 μm) margin of chromian spinel. Cataclased chromite fragments are extensively replaced and mantled by chromian spinel; they have a composite margin comprised of an inner zone of more aluminous spinel and an euhedral outer zone of more Cr-rich spinel, representing granulite and amphibolite facies metamorphic events, respectively. The contents of platinum-group elements (PGE) and Au in chromite separates are relatively high (Os 45, Ir 23, Ru 136, Rh 19, Pt 98, Pd 63, and Au 83 ppb), and significantly enriched (∼ 4x) over whole rock values. Platinum-group minerals are not observed and micrometre-sized inclusions of sulfide minerals (chalcopyrite and pentlandite) in relict chromite are rare. However, comparison of mineral proportions in the separated chromite and whole rock shows that the precious metals are hosted predominantly in the relict igneous chromite grains, rather than the secondary chromian spinel and primary and secondary Mg-rich silicates. The major element composition and average chondrite-normalized PGE pattern of the separated chromite correspond to S-poor stratiform chromitite. We suggest that the precious metals accumulated with chromite during crystallization of a S-poor magma, and were not remobilized in the relict chromite during the subsequent high grade metamorphism.

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Zusammenfassung Metamorphose und PGE-Au-Gehalte von Chromitit aus dem mafischen/ultramafischen Ipanema Komplex, Minas Gerais, Brasilien Das Santa Cruz Massiv, das Teil des mafischen/ultramafischen Ipanema Komplexes, Minas Gerais, Brasilien, ist, beinhaltet eine (von unten nach oben) Abfolge von Metadunit, Metaharzburgit (mit drei Chromititlagen), Metapyroxenit, Metagabbro und Metaanorthosit. Prim?re magmatische Chromitk?rner in der Chromit-Hauptlage sind poikiloblastisch, tektonisch fragmentiert und werden von einem dünnen (10–20 m) Saum von Chromspinell umgeben. Kataklastische Chromitfragmente sind intensiv umgewandelt zu und werden ummantelt von Chromspinell. Dieser mehrphasige Saum beinhaltet eine innere Zone mit Al-reicherem Spinell und eine euhedrale Au?enzone mit Cr-richerem Spinell, die sich bei granulit- bzw. amphibolitfaziellen Metamorphoseereignissen gebildet haben. Die Gehalte an Platingruppen-Elementen (PGE) und Au in Chromit-Separaten sind relativ hoch (Os 45, Ir 23, Ru 136, Rh 19, Pt 98, Pd 63, Au 83 ppb) und signifikant angereichert (∼ 4-fach) im Vergleich zum Gesamtgestein. Platingruppen-Minerale sind nicht zu beobachten und Mikrometer gro?e Einschlüsse von Sulfiden (Chalcopyrit und Pentlandit) in den Chromitrelikten sind selten. Der Vergleich der Mineralverh?ltnisse in den separierten Chromiten und Gesamtgesteinen zeigt aber, dass die Edelmetalle haupts?chlich in den reliktischen Chromitk?rnern sitzen und nicht so sehr in den sekund?ren Chromspinellen und Mg-reichen Silikaten. Die Hauptelement-Zusammensetzung und die Chondrit-normierten PGE Muster der separierten Chromite stimmen mit denen S-armer stratiformer Chromite überein. Wir vermuten, dass die Edelmetalle sich mit Chromit bei der Kristallisation eines S-armen Magmas akkumulierten und w?hrend der anschlie?enden hochgradigen Metamorphose nicht remobilisiert wurden.

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Received April 3, 2000; revised version accepted October 12, 2000     

Geochemistry of Mesozoic dolerite dikes from eastern North America

Major and trace element analyses of over one hundred Mesozoic dolerite dikes from eastern North America have established three main chemical types: 1) olivine-normative; 2) high-TiO₂ quartz-normative; and 3) low-TiO₂ quartz-normative; and a less common high-Fe₂ O₃ ^* ( Fe as Fe₂O₃) quartz-normative type. Quartz-normative dikes predominate from Nova Scotia to Maryland whereas olivine-normative dikes predominate in North and South Carolina. In Virginia and Georgia these types occur in approximately equal abundance.The high-Fe₂O₃ ^* quartz-normative type may be a result of local differentiation. The other quartz-normative types are chemically distinct from each other and probably evolved from different parental magmas. The olivine-normative type may be representative of these parental magmas, and either the parental magmas overlap in composition or only one magma is represented by analyzed olivine-normative dikes.Simple crystal fractionation models coupled with constraints on liquidus phases imposed by recent experimental studies reveal that 1) all three quartz-normative types can be derived from the olivine-normative type by the removal of slightly different cumulate assemblages, but not by contamination with any common crustal composition, and 2) the two-main quartz-normative types are related to each other by neither crystal fractionation nor contamination processes. According to the models, any of the quartz-normative types can be derived from the olivine-normative type by 60–70% accumulation, with the cumulate consisting primarily of 50% plagioclase, 25–30% olivine, and 15% clinopyroxene.The concept of vertical inhomogeniety with respect to incompatible elements in the upper mantle source areas is invoked as a possible explanation for the chemically distinct parental magmas. The spatial distribution of the chemical types and the gross outcrop pattern of the dike swarm clearly indicate that the tectonic environment of the northern Appalachian region differed from that of the southern part during the early Mesozoic.     

Late Paleozoic serpentinites and mafic schists from the Coast Range accretionary complex,central Chile: their relation to aeromagnetic anomalies

Regional structural trends within the poorly exposed metamorphic basement complex of central Chile between 39°30' and 41°30'S latitudes are evident on total intensity aeromagnetic maps. Field and laboratory measurements of magnetic susceptibility indicate that anomalies are related to relic magnetite and rare ilmenite phase concentrations in serpentinites and lower greenschist facies mafic schists.Serpentinites and mafic schists representing fragmented ophiolites are common throughout a broad, highly magnetic central zone which separates a low-magnetic metaturbidites terrain in the Northeast from a southwestern, low susceptibility, metavolcanic rich area with isolated anomalies. One of the isolated anomalies represents a polyphase deformed serpentinite emplacement overlain by coarse grained eclogitic blueschist, highly magnetic undeformed serpentinites and amphibolite boulders thought to have been transported within the ultramafic body.The trend of the regional magnetic anomaly pattern is controlled by the main foliation, a crenulation cleavage (S2), rather than by primary factors, such as emplacement pattern of the ultramafics. This structural grain was established during the early Carboniferous event which culminated pre-Andean accretion in the area. The overall zonation, however, seems to reflect the composition of slices in a composite prism accreted during oblique subduction along the margin of Gondwanaland.

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Zusammenfassung Reliktische präkinematische (prä-S2) Magnetitanreicherungen kommen in Serpentiniten und Metabasiten der Grünschieferfazies des spätpaläozoischen Grundgebirges der Küstenkordillere Chiles häufig vor. Zwischen 39°30' und 41 °30' südlicher Breite wurden magnetische Messungen in situ in diesen Gesteinen durchgeführt, die deren enge räumliche Beziehung zu regionalen NW-SO streichenden Anomalien bestätigen.Die Serpentinite stellen Teile eines zerscherten Ophiolithkomplexes dar, der zwischen einer schwach-magnetischen Metaturbidit-Zone und einer schlecht aufgeschlossenen Metapyroklastit-Zone diapirartig intrudierte. Ein mehrfach deformierter Serpentinitkörper ist von Gerollen aus eklogitischem Blauschiefer überlagert, die höchstwahrscheinlich ursprünglichen tektonischen Einschlüssen entsprechen.Der regionale Trend der Anomalien ist eher vom Streichen und Einfallen der Hauptschieferung als von primären Strukturen kontrolliert. Es handelt sich um eine Krenulationsschieferung (S2) die durch eine frühkarbonische Phase entstanden ist. Sie stellt den Höhepunkt einer prä-Andinen Schrägakkretion dar. Die regionale lithologische Gliederung scheint jedoch die Zusammensetzung der einzelnen Akkretionskeile widerzuspiegeln.

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Résumén Serpentinitas y esquistos máficos metamorfizados en condiciones de facies esquistos verdes baja forman parte del basamento metamorfico de la Cordillera de la Costa de Chile central entre los 39°30' y los 41°30'l.s. Mediciones in situ y en laboratorio de suceptibilidad magnética muestran que sus concentraciones de magnetita pretectonica con respecto a la foliación principal son responsables de anomalías de rumbo NW-SE.Las serpentinitas constituyen parte de una asociación »ofiolítica« fragmentada que aflora dentro de una ancha banda entre metaturbiditas débilmente magnéticas al NE y metapiroclastitas con aisladas anomalías al SW. Una serpentinita del último sector presenta evidencias de deformación polifásica y esta recubierta por bolones de esquistos azules eclogíticos y anfibolitas, los cuales podrían representar inclusiones originalmente transportadas por el cuerpo ultramáfico.El diseño regional de las anomalías magnéticas está controlado por la actitud de la foliación principal, más que por el diseño del emplazamiento original. Esta, un clivaje de crenuación S2, se originó durante un evento carbonífero temprano que representa el climax de la acreción pre-andina en el área. La zonación litológica regional, sin embargo, parece reflejar más bien la composición de lonjas individuales dentro de un prisma compuesto acrecionado mediante subducción oblicua.

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- (prä-S2) . 39°30 41°30 in situ , , NW-SO. , , . , , , , , . , . (S2), . - . .

     

闽西晚中生代基性岩脉的地球化学研究

对闽西三个地区基性岩脉的地球化学研究发现,该区域的基性岩脉为亚碱性岩石系列,具有高 Al、低 Ti、Na2O > K2O的特征;相对富集大离子亲石元素,亏损高强场元素.半村辉绿岩脉和拔里角闪辉长岩脉以 Nb、Ta、Ti负异常为特征,林子坟辉长辉绿岩脉具有 Nb、Ta负异常和 Ti弱正异常.半村和拔里的岩脉稀土元素总量高,明显富集轻稀土元素,(La/Yb)N=6.8～8.4;林子坟的岩脉稀土元素总量相对较低,轻稀土元素弱富集,(La/Yb)N=2.0.结合基性岩脉的地球化学特征和区域构造演化分析,初步认为半村和拔里的基性岩脉来自与俯冲作用有关的富大离子亲石元素流体交代的富集地幔;林子坟基性岩脉是软流圈地幔部分熔融的产物,地壳混染作用是其相对富集大离子亲石元素的原因.结合玄武岩构造环境判别图解可知,半村和拔里岩脉具有的大陆边缘弧的特征并不指示基性岩脉形成的构造环境,而是说明其源区受到俯冲板片派生出来的流体交代作用.半村和拔里岩脉形成于大陆板内拉张带,林子坟岩脉形成于板内裂谷环境.与华南其他地区的基性岩脉对比表明,虽然晚中生代华南地区处于拉张的构造背景下,但是岩脉形成的构造环境与微量元素地球化学特征均有明显的差异,反映了各地区构造环境演变和地幔演化的复杂性.     

Geochemistry of mafic dikes in the Singhbhum Orissa craton: implications for subduction-related metasomatism of the mantle beneath the eastern Indian craton

Mafic dikes, which transect the Mesoarchaean Singhbhum Granitoid Complex, are the most abundant members of the Newer Dolerite dikes of the Singhbhum Orissa craton. These dikes are subalkaline and exhibit a tholeiitic differentiation trend. Studied dikes underwent fractional crystallization of clinopyroxene and plagioclase. They show enriched patterns for the light rare earth elements (LREE) and large ion lithophile elements (LILE). On primitive mantle-normalized multi-element patterns, they possess Ba, Nb, Sr, P, and Ti depletions similar to subduction-related basaltic rocks. The high (La/Yb) _n and (Gd/Yb) _n ratios suggest that the studied mafic dikes were derived by low degrees of partial melting of a garnet-bearing source. Judging by trace elemental ratios (e.g. Ba/Y, Nb/Y, Ba/Th and Th/Nb), the studied dikes were derived from a mantle source metasomatized by a subduction component (e.g. fluids derived by dehydration of the subducting slab). We conclude that interaction between these fluids and the overlying mantle was the main cause of (LREE and LILE) enrichment and Nb (high field strength elements) depletion in the mafic dikes.     

赣杭构造带金衢盆地燕山期基性脉岩地球化学特征及成因探讨

赣杭构造带金衢盆地内发育有燕山期基性脉岩,岩石类型主要为辉绿(玢)岩.K-Ar年代学研究显示基性脉岩产出时代为69.5～131.7Ma,为白垩纪岩浆活动的产物.SiO2含量范围为46.70%～50.23%,K2O+Na2O为4.01%～7.82%,可分为碱性和亚碱性岩两类.微量元素相对富集大离子亲石元素(LILE)(K...     

赣西北基性岩脉的地质地球化学特征及其意义

对赣西北三个地区基性岩脉的地质地球化学特征进行了研究，并初步讨论了基性脉岩的成岩方式和源区性质。基性岩脉包括闪斜煌斑岩，辉绿岩和橄榄录玢岩。微量元素相对富集大离子亲石元素，而亏损高场强元素，具有Nb,Ta和Ti异常。武山煌班岩稀土元素总量高，明显富集轻;益口辉绿岩稀土元素总量低，略微富集轻稀土元素，上高橄榄辉绿玢岩介于两者之间，三者都无明显的铕异常，结合基性脉岩的地球化学分析可知，三者的岩石化学变化受到部分熔融和分离结晶所控制，武山煌斑岩和上高橄榄辉绿玢岩明显发生分离结晶作用，而隘口辉绿岩主要受部分熔融作用影响。其源区可能为与俯冲流体有关的富集地幔，有待于进一步研究。