湘南中生代火山岩中尖晶石二辉橄榄岩包体 Sm-Nd等时线年龄及地质意义

湖南宁远太阳山中生代玄武质火山岩内,产较丰富的尖晶石二辉橄榄岩包体。包体经人工碎样,镜下挑选了含铬尖晶石、镁橄榄石、含铬透辉石、斜方辉石、全岩5个样品,由国土资源部天津地质矿产研究所同位素室测试,得出 Sm-Nd全岩-单矿物等时线同位素年龄2702±19 Ma。进而根据测试结果和以往资料综合分析,认为华南陆块岩石圈是沿垂向演化增生的,由下往上依次为:上地幔岩垫托、结晶基底、褶皱基底、褶皱盖层、沉积盖层5个构造层。其中,上地幔岩起着底板垫托作用,形成于新太古代,为该地区最古老岩石。     

滇西地区富碱斑岩中地幔流体作用踪迹及其成矿作用意义

滇西地区大量产出的富碱斑岩及其包体岩石的形成和演化与该区新生代陆内变形、构造作用、幔源岩浆和深源流体活动,及其与此有关而广泛发育的内生多金属矿产存在必然的联系。本文通过岩相学、岩石化学、电子探针、扫描电镜和能谱分析,较为系统地分析论证了这一关联的内在统一制约和联系的纽带即深部地质过程和由此相伴的含矿地幔流体作用,初步揭示了这种深部过程和地幔流体作用的微观踪迹和方式可以直接表现为:(1)呈脉状和浸染状穿插于深源包体岩石中的富钠玻璃,透光镜下呈微晶和雏晶,颜色随成分差异而不均匀,化学成分以高硅、铝、钠、铁,低钾、钙、镁为特征,矿物成分以钠长石、角闪石、磁铁矿(镜铁矿)、钛铁矿组合为特征,是富碱岩浆携带包体岩石之前即已存在的上地幔流体;(2)呈脉状、团块状和浸染状穿插于主岩和各类包体岩石的富铁玻璃;(3)呈独立包体产出于霓辉正长斑岩中的富铁熔浆包体。后两者物质在透光镜下无光性,呈黑色不透明,反光镜下不反光,但在电子显微镜下呈显微晶质结构,化学成分以高硅、铝、铁,低钙、镁、钠、钾为特征,矿物组成上以硅酸盐和石英为主,含有碳硅石、含铬自然铁、钛铁矿、磷灰石等地幔标志矿物,其中微晶金属和非金属矿物之间呈熔离结构交生,反映了地幔流体的熔浆性质及其与富碱岩浆不混溶的特征;由地幔流体对岩石的交代浸染作用,引起主岩和包体岩石中普遍发育各种蚀变作用,如角闪石化、硅化和绿泥石化等,并导致矿物组合总体上表现为暗色矿物由辉石→角闪石→黑云母→绿泥石的退变序列。该地幔流体微观踪迹的三种表现形式与富碱岩浆共存,并共同运移,但两者由于组成和性质的差异而互不混溶;结合透岩浆流体成矿作用理论和本文论证的综合分析认为,当富碱岩浆和地幔流体系统封闭较好,地幔流体则伴随富碱岩浆的结晶过程对富碱斑岩进行同步自交代蚀变,在斑岩体或其深部形成矿床,构成正岩浆成矿体系,典型成矿实例如马厂箐斑岩钼矿床;若在此成岩成矿过程中发生构造作用扰动,则地幔流体进入岩体与围岩的接触带,或紧邻接触带的地层围岩中进行交代蚀变成矿,构成接触带成矿体系,典型成矿实例如马厂箐矿区中赋存于夕卡岩—大理岩带中的斑岩型铜矿和主要赋存于地层围岩中的斑岩型金矿,若金矿出现在斑岩体内,则一般受控于穿切斑岩体的成岩后断裂;若岩浆和流体运移通道的深大断裂体系发育,环境相对开放,则地幔流体伴随富碱岩浆的成岩过程而脱离岩浆沿分支断裂通道进入远离岩体的不同地层岩石中进行交代蚀变成矿,构成远程热液成矿系统,典型成矿实例如兰坪金顶超大型铅锌矿床;在这一成矿过程中,地幔流体可以随深度和环境变化引起的物理化学条件变化,其性质由熔浆→超临界流体→液相流体转化,并运载和沿途活化成矿物质至适宜容矿部位集中,促使幔壳物质叠加成矿;进而有利于深部成矿并形成大型和超大型矿床。     

Ultramafic xenoliths in mesozoic diorite in west shandong province

Deep-seated xenoliths are common in alkali ba-salts, kimberlites and lamprophyres[1]. These host rocks are all basic or ultrabasic volcanic and subvol-canic rocks originating from mantle. Reports of ul-tramafic xenoliths found in plutons are uncommon except those of Mesozoic diorites in North China[2—6]. This paper provides detailed mineralogy, petrology and geochemistry of ultramafic xenoliths in Tietong-gou pluton, Laiwu, Shandong Province, and discusses the possibility of their deep-sea…     

Helium isotope studies of the mantle xenoliths and megacrysts from the Cenozoic basalts in the eastern China

Helium isotope compositions of the mantle xenoliths and megacrysts in the Cenozoic basalts in the eastern China were measured. The samples were collected from Ludao of Heilongjiang, Huinan and Jiaohe of Jilin, Kuandian of Liaoning, Hannuoba of Hebei, Nüshan of Anhui, Dingan of Hainan. The³He/⁴He ratios of the mantle xenoliths and megacrysts from the most areas were about 1 × 10^-5, and were similar to those of the MORB, thus reflecting the characteristics of the MORB-typed depleted mantle. The³He/⁴He ratios of the mantle xenoliths from Jiaohe were 4.8×10^-6 and the³He/⁴He ratios of xenoliths from Hannuoba vary from 0.15× 10^-6 to 7.4 ×10^-6, obviously lower than those of the MORB, and even lower than the atmospheric helium isotope ratios, indicating that the continental mantle was strongly replaced in Jiaohe and Hannuoba areas. The helium isotope compositions of the mantle xenoliths and megacrysts in the same region vary in a very wide range. It is inferred that the mantle xenoliths and megacrysts were from different parts of the continental mantle. There were not necessary origin relations between the mantle xenoliths, megacrysts and their host basalts. An extremely high³He/⁴He ratio of garnet megacryst from Hannuoba, Hebei was found.     

Petrogenesis of ultramafic and mafic xenoliths from Mesozoic basanites in southern Sweden: constraints from mineral chemistry

Jurassic basanite necks occurring at the junction of two major fault zones in Scania contain ultramafic (peridotites, pyroxenites) and mafic xenoliths, which together indicate a diversity of upper mantle and lower crustal assemblages beneath this region. The peridotites can be subdivided into lherzolites, dunites and harzburgites. Most lherzolites are porphyroclastic, containing orthopyroxene and olivine porphyroclasts. They consist of Mg-rich silicates (Mg# = Mg/(Mg + Fe_tot) × 100; 88–94) and vermicular spinel. Calculated equilibration temperatures are lower in porphyroclastic lherzolites (975–1,007°C) than in equigranular lherzolite (1,079°C), indicating an origin from different parts of the upper mantle. According to the spinel composition the lherzolites represent residues of 8–13% fractional melting. They are similar in texture, mineralogy and major element composition to mantle xenoliths from Cenozoic Central European volcanic fields. Dunitic and harzburgitic peridotites are equigranular and only slightly deformed. Silicate minerals have lower to similar Mg# (83–92) as lherzolites and lack primary spinel. Resorbed patches in dunite and harzburgite xenoliths might be the remnants of metasomatic processes that changed the upper mantle composition. Pyroxenites are coarse, undeformed and have silicate minerals with partly lower Mg# than peridotites (70–91). Pyroxenitic oxides are pleonaste spinels. According to two-pyroxene thermometry pyroxenites show a large range of equilibration temperatures (919–1,280°C). In contrast, mafic xenoliths, which are mostly layered gabbronorites with pyroxene- and plagioclase-rich layers, have a narrow range of equilibration temperatures (828–890°C). These temperature ranges, together with geochemical evidence, indicate that pyroxenites and gabbroic xenoliths represent mafic intrusions within the Fennoscandian crust.     

中国吉林长白山地区地幔捕虏体中硫化物熔体包裹体

Mantle xenoliths are common in the Cenozoic basalts of the Changbaishan District,Jilin Province,China.Sulfide assemblages in mantle minerals can be divided into three types:isolated sulfide grains,sulfide-meh inclusions and filling sulfides in fractures.Sulfide-meh inclusions occur as single-phase sulfides,sulfide-silicate melt,and CO_2-sulfide-silicate melt inclusions. Isolated sulfide grains are mainly composed of pyrrhotite,but cubanite was found occasionally.Sulfide-meh inclusions are mainly composed of pontlandite and MSS,with small amounts of chalcopyrite and talnakhite.The calculated distribution coefficient K_(D3)for lherzolite are similar to that of mean experimental value.The bulk sulfides in lherzolite were in equilibrium with the enclosing minerals, indicating immiscible sulfide melts captured in partial melting of upper mantle.Sulfide in fractures has higher Ni/Fe and(Fe Ni)/S than those of sulfide melt inclusions.They might represent later metasomatizing fluids in the mantle.Ni/Fe and(Fe Ni)/S increase from isolated grains,sulfide inclusions to sulfides in fractures.These changes were not only affected by temperature and pressure,hut by geochemistry of Ni,Fe and Cu,and sulfur fugacity as well.     

南海北部及其沿岸中、新生代壳幔相互作用与构造演化——纪念“陆缘扩张带”概念的倡导者陈国达教授

新生代火山岩中的深源捕虏体资料反映,南海北部及其沿岸地区岩石圈地幔的主体由主量元素易熔组分相对饱满的、同位素组成类似MORB-OIB型的、高温型的二辉橄榄岩所组成;但在其顶部残留有古老的岩石圈地幔,它由主量元素易熔组分相对贫瘠的、同位素组成类似EM型的、较低温的方辉橄榄岩组成。在下地壳底部,分布着由晚中生代幔源岩浆分离结晶和堆晶的基性麻粒岩。由此提出了该区中、新生代壳 -幔或岩石圈 -软流圈相互作用与构造演化的简略模式: (1)印支期 -燕山早期为地壳岩石圈厚度增大的华夏型后地台活化造山带环境;(2)燕山晚期岩石圈快速减薄(如拆沉作用),造山带拉伸塌陷,地壳深处并发生广泛的底侵作用; (3)始新世 -渐新世软流圈再次上涌(如地幔柱的影响),岩石圈地幔发生底蚀减薄,地壳也因为下部层的塑性流展和上部层的张裂拉伸而减薄; (4)中新世以来,由于地幔热源在拉伸环境中被释放,壳幔发生冷却,部分软流圈地幔转化为“新生的”岩石圈地幔。研究进一步说明,南海北部陆缘扩张是该区大陆构造演化到大陆活化造山带后期,在深部壳 -幔的相互作用下,岩石圈所发生的垂向减薄和侧向伸展,既不同于弧后扩张,也不是受控于大西洋式的海底扩张。     

河北汉诺坝新生代玄武岩中幔源包体和高压巨晶的氦同位素地球化学研究

对该地区新生代玄武岩中幔源包体和高压巨晶的氦同位素进行了初步研究。幔源包体中橄榄石的3 He 4 He值为 (0 .15～ 7.4)× 10 - 6,较MORB值明显偏低 ,甚至低于大气的值 ,说明该地区曾发生过强烈的地幔交代作用。高压巨晶辉石和石榴子石的氦同位素组成与此明显不同 ,其3 He 4 He值为 (5 .7～ 2 4.3)× 10 - 6。提出幔源包体和高压巨晶不是同源的 ,二者可能与寄主玄武岩均无必然成因联系。在汉诺坝地区一件石榴子石巨晶中还发现了异常高的3 He 4 He值。     

Composition and evolution of lithosphere beneath the Carpathian–Pannonian Region: a review

Our knowledge of the lithosphere beneath the Carpathian–Pannonian Region (CPR) has been greatly improved through petrologic, geochemical and isotopic studies of upper mantle xenoliths hosted by Neogene–Quaternary alkali basalts. These basalts occur at the edge of the Intra-Carpathian Basin System (Styrian Basin, Nógrád-Gömör and Eastern Transylvanian Basin) and its central portion (Little Hungarian Plain, Bakony-Balaton Highland).The xenoliths are mostly spinel lherzolites, accompanied by subordinate pyroxenites, websterites, wehrlites, harzburgites and dunites. The peridotites represent residual mantle material showing textural and geochemical evidence for a complex history of melting and recrystallization, irrespective of location within the region. The lithospheric mantle is more deformed in the center of the studied area than towards the edges. The deformation may be attributed to a combination of extension and asthenospheric upwelling in the late Tertiary, which strongly affected the central part of CPR subcontinental lithosphere.The peridotite xenoliths studied show bulk compositions in the following range: 35–48 wt.% MgO, 0.5–4.0 wt.% CaO and 0.2–4.5 wt.% Al₂O₃ with no significant differences in regard to their geographical location. On the other hand, mineral compositions, particularly of clinopyroxene, vary according to xenolith texture. Clinopyroxenes from less deformed xenoliths show higher contents of ‘basaltic’ major elements compared to the more deformed xenoliths. However, clinopyroxenes in more deformed xenoliths are relatively enriched in strongly incompatible trace elements such as light rare earth elements (LREE).Modal metasomatic products occur as both hydrous phases, including pargasitic and kearsutitic amphiboles and minor phlogopitic micas, and anhydrous phases — mostly clinopyroxene and orthopyroxene. Vein material is dominated by the two latter phases but may also include amphibole. Amphibole mostly occurs as interstitial phases, however, and is more common than phlogopite. Most metasomatized peridotites show chemical and (sometimes) textural evidence for re-equilibration between metasomatic and non-metasomatic phases. However, amphiboles in pyroxenites are sometimes enriched in K, Fe and LREE. The presence of partially crystallized melt pockets (related to amphiboles and clinopyroxenes) in both peridotites and pyroxenites is an indication of decompression melting and, rarely, incipient partial melting triggered by migrating hydrous melts or fluids. Metasomatic contaminants may be ascribed to contemporaneous subduction beneath the Carpathian–Pannonian Region between the Eocene and Miocene.Sulfide inclusions are more abundant in protogranular and porphyroclastic xenoliths relative to equigranular types. In mantle lithologies, sulfide bleb compositions vary between pentlandite and pyrrhotite correlating with the chemistry and texture of the host xenoliths. While sulfides in peridotites are relatively rich in Ni, those in clinopyroxene-rich xenoliths are notably Fe-rich.     

Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al₂O₃contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al₂O₃ and TiO₂) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have ¹⁴³Nd/¹⁴⁴Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low ¹⁴³Nd/¹⁴⁴Nd values (0·5113)and extremely high ⁸⁷Sr/⁸⁶Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette