Geochemistry, Petrogenesis and Metallogenesis of the Panzhihua Gabbroic Layered Intrusion and Associated Fe-Ti-V Oxide Deposits, Sichuan Province, SW China

The Panzhihua gabbroic layered intrusion is associated withthe 260 Ma Emeishan Large Igneous Province in SW China. Thissill-like body hosts a giant Fe–Ti–V oxide depositwith 1333 million ton ore reserves, which makes China a majorproducer of these metals. The intrusion has a Marginal zoneof fine-grained hornblende-bearing gabbro and olivine gabbro,followed upward by Lower, Middle, and Upper zones. The Lowerand Middle zones consist of layered melanogabbro and gabbrocomposed of cumulate clinopyroxene, plagioclase, and olivine.These zones also contain magnetite layers. The Upper zone consistschiefly of leucogabbro composed of plagioclase and clinopyroxenewith minor olivine. Most rocks in the body show variable-scalerhythmic modal layering in which dark minerals, primarily clinopyroxene,dominate in the lower parts of each layer, and lighter minerals,primarily plagioclase, dominate in the upper parts. The oxideores occur as layers and lenses within the gabbros and are concentratedin the lower parts of the intrusion. Ore textures and associatedmineral assemblages indicate that the ore bodies formed by verylate-stage crystallization of V-rich titanomagnetite from animmiscible oxide liquid in a fluid-rich environment. The rocksof the Panzhihua intrusion become more evolved in chemistryupward and follow a tholeiitic differentiation trend with enrichmentin Fe, Ti, and V. They are enriched in light rare earth elementsrelative to heavy rare earth elements, and exhibit positiveNb, Ta, and Ti anomalies and negative Zr and Hf anomalies. Thesilicate rocks and oxide ores of the Panzhihua intrusion formedfrom highly evolved Fe–Ti–V-rich ferrobasaltic orferropicritic magmas. The textures of the ores and the abundanceof minor hydrous phases indicate that addition of fluids fromupper crustal wall-rocks induced the separation of the immiscibleoxide melts from which the Fe–Ti–V oxide ore bodiesin the lower part of the intrusion crystallized. KEY WORDS: magnetite; Fe–Ti-rich gabbro; layered intrusion; Panzhihua; SW China     

Petrogenesis and Metallogenesis of the Niumaoquan Gabbroic Intrusion Associated with Fe-Ti Oxide Ores in the Eastern Tianshan, NW China

The Niumaoquan layered gabbroic intrusion is in the southern margin of the Central Asian Orogenic Belt in North Xinjiang, China, and hosts a Fe-Ti oxide deposit in its evolved gabbroic phases. In this paper, we report zircon U-Pb age, Sr-Nd-Hf isotopes, plagioclase chemistry, and whole-rock geochemistry of the Niumaoquan layered gabbroic intrusion. Zircon grains separated from an anorthosite sample analyzed by laser ablation inductively coupled plasma mass spectrometry yielded a concordia age of 314.7±0.74 Ma, indicating that the Niumaoquan ore-bearing gabbroic intrusion was emplaced during the Late Carboniferous. The olivine gabbro texture and plagioclase chemistry suggest that plagioclase was an early crystallized silicate phase that crystallized prior to olivine. Fractional crystallization and accumulation of plagioclase significantly controlled the evolution of the Niumaoquan gabbroic intrusion and contributed to the formation of anorthosite layers, causing metallogenic elements to become enriched in the residual melt. The Niumaoquan gabbroic intrusion is characterized by the enrichment of large ion lithophile elements and depletion of high field strength elements, positive zircon εHf(t) values (+2.1 to +12.2), positive εNd(t) values (+3.3 to +5.2), and low initial 87Sr/86Sr ratios (0.7039 to 0.7047), suggesting that the parental magma was produced by interactions between metasomatized lithospheric mantle and depleted asthenospheric melts at an early post-collision stage. The Fe-Ti oxide mineralization of the Niumaoquan intrusion benefited from interactions between depleted asthenospheric melts and lithospheric mantle, and fractional crystallization of abundant plagioclase and magnesian minerals.     

攀西地区新街层状岩体粒间不混熔作用:来自斜长石环带结构的记录

位于攀西地区的新街层状岩体赋含大量钒钛磁铁矿,是峨眉山大火成岩省的一部分。岩体下部带和中部带以单斜辉石岩为主,并伴生浸染状钒钛磁铁矿矿化;上部带以辉长岩为主,赋存厚层的钒钛磁铁矿矿体。之前研究认为厚层的钒钛磁铁矿矿体的形成与粒间不混熔的富Fe熔体有关,但对富Fe熔体的演化过程缺乏细致研究。本文通过对新街岩体上部带的富矿辉长岩层和上覆浅色辉长岩中斜长石环带结构和成分的研究,揭示了富Fe熔体的演化过程。在浅色辉长岩中保存的岩浆不混熔的直接证据表现为矿物粒间共轭的富Si交生体和富钛铁矿交生体代表的非反应结构。本次研究发现,与粒间富Si交生体接触的斜长石边部的FeO和TiO_2含量随斜长石牌号(An)值的降低而降低,而与粒间富钛铁矿交生体接触的斜长石边部的FeO和TiO_2含量随An值的降低而升高,说明斜长石的边部成分变化记录了粒间共轭的富Si和富Fe熔体的成分特征。在富矿辉长岩中,斜长石可分为初生和新生两种,初生斜长石的An值介于57~62,FeO含量为0.34%~0.50%,TiO_2含量为0.06%~0.13%,新生斜长石具有相对较高的An值(61~81)和FeO、TiO_2含量,二者的内部和边部还发育增生斜长石,其An值(~50)相对较低;在初生斜长石边部可见不连续的新生斜长石环带和增生斜长石边,造成其内部成分显著不均一,并发育复杂的环带结构。本文认为,初生斜长石是岩浆正常分离结晶作用的产物。在粒间熔体发生不混熔后,不混熔的富Fe熔体逐渐向岩浆房下方迁移并结晶出了一些相对高An值的新生斜长石,或沿一些初生斜长石边部生长形成不连续的高An环带。当富Fe熔体演化至晚期,由于矿物生长空间受限,仅在初生和新生斜长石局部形成了相对低An值的增生边、或沿颗粒裂隙进入斜长石内部形成增生斜长石核。     

Petrogenesis and Metallogenesis of the Mazaertag Layered Intrusion in the Tarim Large Igneous Province, NW China

The Mazaertag layered intrusion is located in the northwestern part of the Tarim large igneous province where several early Permian layered mafic-ultramafic intrusions host important Fe-Ti oxide deposits. The intrusion covers an area of ～0.13 km~2 and has a vertical stratigraphic thickness of at least300 m. It consists chiefly of olivine clinopyroxenite, and is cut through by the nearby mafic-ultramafic dykes. In this paper, we report new mineral chemistry data and whole-rock chemical and isotopic compositions for the Mazaertag intrusion along with whole-rock isotopic compositions for the nearby mafic dykes. The averaged compositions of cumulus olivine, clinopyroxene and intercumulus plagioclase within individual samples range from Fo_(71-73),Mg~# = 76 to 79 and An_(65-75) but they do not define sustained reversals. The observed mineral compositions are consistent with the differentiation of a single batch of magma in a closed system. Rocks of the Mazaertag intrusion are characterized by enrichment in light REE relative to heavy REE, positive Nb and Ta anomalies and a small range of age-corrected ε_(Nd)(t)(-0.1 to +0.9) and initial ~(87)Sr/~(86)Sr values(0.7044 to 0.7068). The slightly lower ε_(Nd)(t), initial ~(206)Pb/~(204)Pb and higher initial ~(87)Sr/~(86)Sr values of the intrusion compared to those of the least contaminated dykes[ε_(Nd)(t) =+2.8 to +3.4;(~(206)Pb/~(204)Pb)_i = 18.516-18.521;(~(87)Sr/~(86)Sr)_i = 0.7038-0.7041] imply that the Mazaertag magma was subjected to small to modest degrees of contamination by the upper crust. The Sr-Nd isotopic compositions of the least contaminated dykes are consistent with derivation from a FOZO-like mantle source. The parental magma of the Mazaertag intrusion, estimated from clinopyroxene compositions using mineral-melt partition coefficients, has trace element compositions similar to some of the most primitive mafic dykes in the same area. This suggests that the Mazaertag intrusion and mafic dykes shared a similar mantle source. Therefore, the parental magma of the Mazaertag intrusion was interpreted to have originated from a mantle plume. Based on the Cr_2 O_3 contents in titanomagnetite and less-evolved characteristics of the Mazaertag intrusion compared to the Wajilitag Fe-Ti oxide deposit in Bachu, it is speculated that there might not be a potential to find economic Fe-Ti oxide mineralization in the intrusion.     

The Bjerkreim-Sokndal Layered Intrusion, Rogaland, SW Norway: Evidence from marginal rocks for a jotunite parent magma

The Late-Proterozoic Bjerkreim-Sokndal Layered Intrusion (BKSK) consists of andesine anorthosite, leuconorite, troctolite, norite, gabbronorite, jotunite, mangerite, quartz mangerite and charnockite. The sequence of appearance of cumulus minerals and their compositions suggest a parent magma that was evolved, had plagioclase (±olivine) on the liquidus, was sufficiently TiO₂-rich for hemo-ilmenite to crystallise early, and low in CaO and CaO/Al₂O₃compared to basalts as reflected by the sodic plagioclases and the delayed appearance of cumulus augite. Fine- to medium-grained jotunites found along the northern contact of the BKSK consist of plagioclase (An_45–53), inverted pigeonite (Mg# = 55-50), sparse augite (Mg# = 69-59), Fe-Ti oxides, K-feldspar, quartz and apatite. They are basic to intermediate rocks with relatively high FeO^total, high TiO₂, low MgO/MgO + FeO, moderate Al₂O₃ and low CaO and normative diopside. The jotunites have compositions that are consistent with the parental magma for the lower part of the BKSK Layered Series, and are interpreted as being marginal chills. Similar, but slightly more differentiated, jotunite magmas were subsequently emplaced into the BKSK and the surrounding region as broad dykes and small plutons. Jotunite is a minor rock type in most massif-type anorthosite provinces but may have an important petrological significance.     

Textures in Partially Solidified Crystalline Nodules: a Window into the Pore Structure of Slowly Cooled Mafic Intrusions

Abundant glass is present along grain boundaries in coarse-grained,glass-bearing, crystalline gabbroic and peridotitic nodulesentrained and erupted in lavas from Iceland, Santorini and MaunaLoa (Hawaii), even when the total porosity is less than a fewvolume per cent. The glass films vary from a few microns toa few tens of microns thick, and are associated with stringsof small lensoid grain boundary pockets formed by impingementduring crystal growth. Additional porosity occurs as extensiveliquid-filled pockets adjacent to included grains within oikocrystsand as large triangular pockets formed by impingement of planar-sidedgrains. Interstitial material within glass films, and the irregularityof film thickness along a single grain boundary, suggest thatthe present pore structure is representative of the pore structurebefore entrainment and eruption. Pore geometry is consistentwith a dominant control by crystal growth during solidification,with little or no evidence for control by minimization of internalenergies driven by textural equilibration. Similarities betweenliquid distribution in the crystalline nodules and that of late-stage,interstitial phases in fully solidified mafic cumulates fromthe Rum and Skaergaard intrusions demonstrate that the crystallinenodules provide information about the latest stages of solidificationin slowly cooled mafic plutons. The highly permeable networkof intersecting liquid films, lenses and pockets may promotein situ crystallization in the solidifying mush, explainingthe common presence of adcumulates in such intrusions. KEY WORDS: textures; liquid distribution; mafic cumulates; crystalline nodules     

Textural Maturity of Cumulates: a Record of Chamber Filling, Liquidus Assemblage, Cooling Rate and Large-scale Convection in Mafic Layered Intrusions

Textural maturity describes the extent to which a rock has evolvedfrom the initial reaction-controlled texture towards texturalequilibrium controlled by the minimization of interfacial energy.Solidification in a magma chamber results in the formation ofan impingement texture by the random juxtaposition of planar-sidedgrains. Orthocumulates, in which the initial melt-filled poresare pseudomorphed by later-crystallizing phases, have an ophiticor intersertal texture immediately after complete solidification,which then evolves towards solid-state equilibrium by roundingof initially planar grain boundaries and an increase in themedian dihedral angle subtended at the junctions of two primocrysticgrains with the interstitial phase. The bulk of the increasein angle occurs just below the solidus temperature in kilometre-scalemafic plutons. Quantification of textural maturity via measurementof dihedral angle populations in troctolitic and gabbroic cumulatesfrom the Rum Eastern Layered Intrusion and the Skaergaard Intrusiondemonstrates that the rocks preserve a record of thermal eventsrelated to magma chamber replenishment and the onset of chamber-wideconvection. Textural maturity is also a function of the liquidusphase assemblage: for systems in which only olivine and plagioclaseare liquidus (i.e. cumulus) phases in the main magma body abovethe crystal mush, the texture is significantly less mature thanthat in systems in which clinopyroxene is an additional liquidusphase. The difference in textural maturity reflects differencesin the cooling and solidification rate, and demonstrates directlythat the liquidus phase assemblage plays a role in determiningthe thermal history of plutons. KEY WORDS: cumulates; dihedral angles; Rum; Skaergaard; textures     

Intrusion and Crystallization of a Spinifex-Textured Komatiite Sill in Dundonald Township, Ontario

Although komatiite has been defined as an ultramafic volcanicrock characterized by spinifex texture, there is a growing recognitionthat similar textures can also form in high-level dykes andsills. Here, we report the results of a petrological and geochemicalinvestigation of a 5 m thick komatiite sill in Dundonald Township,Ontario, Canada. This unit forms part of a series of komatiitesand komatiitic basalts, some of which clearly intruded unconsolidatedsediments. The komatiite sill is differentiated into a spinifex-texturedupper part and an olivine cumulate lower part. Features characteristicof the upper sections of lava flows, such as volcanic brecciaand a thick glassy chilled margin, are absent and, instead,the upper margin of the sill is marked by a layer of relativelylarge (1–5 mm) solid, polyhedral olivine grains that gradesdownwards over a distance of only 2 cm into unusually large,centimetre-sized, skeletal hopper olivine grains. This is underlainby a 1 m thick zone of platy spinifex-textured olivine and coarse,complex, dendritic, spinifex-textured olivine. The texture ofthe olivine cumulate zone in the overlying unit is uniform rightdown to the contact and a lower chilled margin, present at thebase of all lava flows, is absent. The textures in the silland the overlying unit are interpreted to indicate that thesill intruded the olivine cumulate zone of the overlying unit.Thermal modelling suggests that soon after intrusion, a narrowinterval of the overlying cumulate partially melted and thatthe liquid in the upper part of the sill became undercooled.The range of olivine morphologies in the spinifex-textured partof the sill was controlled by nucleation and crystallizationof olivine in these variably undercooled liquids. KEY WORDS: komatiite; intrusion; spinifex texture; olivine     

攀西地区层状辉长岩体及钒钛磁铁矿床的成因

峨嵋山大岩浆省由大量的溢流玄武岩及其伴生的镁铁和超镁铁侵入岩组成。攀西地区的一些层状辉长岩体形成于260Ma,与早期报道的峨嵋山大岩浆省的年代相同。这些岩体中含有巨大的钒钛磁铁矿床,矿体呈似层状及透镜状产在层状辉长岩体的下部层位,不同于典型的层状岩体(如布什维尔德岩体)的磁铁矿床。地球化学资料表明,攀西地区含磁铁矿的岩体是从高度演化的基性岩浆中结晶而成,因为富硅的岩浆分离使得母岩浆高度富集铁、钛和钒。相对围岩来说,磁铁矿石形成较晚,是从氧化物矿浆中结晶的产物。矿石中有丰富的含水矿物相,流体的参与对氧化物矿浆的形成有重要的作用。     

Cyclicity in the Main and Upper Zones of the Bushveld Complex, South Africa: Crystallization from a Zoned Magma Sheet

The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock ⁸⁷Sr/⁸⁶Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial ⁸⁷Sr/⁸⁶Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An₇₂ plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An₄₃; Mg# 5 clinopyroxene; Fo₁ olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V₂O₅ of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO₂ and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO₂ and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation     

Interaction of magma with sedimentary wall rock and magnetite ore genesis in the Panzhihua mafic intrusion,SW China

In southwestern China, several large magmatic Fe–Ti–V oxide ore deposits are hosted by gabbroic intrusions associated with the Emeishan flood basalts. The Panzhihua gabbroic intrusion, a little deformed sill that contains a large titanomagnetite deposit at its base, concordantly intrudes late-Proterozoic dolostones. Mineralogical and chemical studies of the contact aureole in the footwall dolostones demonstrate that the metamorphism was largely isochemical but released large quantities of CO₂ as the rocks were converted to marble and skarns during intrusion of the gabbroic magma. Petrological modelling of the crystallization of the intrusion, using H₂O-poor Emeishan basalt as parent magma, shows that under normal conditions, Fe–Ti oxides crystallize at a late stage, after the crystallization of abundant olivine, clinopyroxene and plagioclase. In order for titanomagnetite to separate efficiently to form the ore deposit, this mineral must have crystallized earlier and close to the liquidus. We propose that CO₂-rich fluids released during decarbonatization of sedimentary floor rocks passed up through the magma. Redox equilibria calculations show that when magma with the composition of Emeishan basalt is fluxed by a CO₂-rich gas phase, its equilibrium oxygen fugacity (fO₂) increases from the fayalite–magnetite–quartz buffer (FMQ) to FMQ + 1.5. From experimental constraints on magnetite saturation in basaltic magma under controlled fO₂, such an oxidizing event would allow magnetite to crystallize near to the liquidus, leading to the formation of the deposit.     

Using LA-ICPMS Mapping and Sector Zonation to Understand Growth and Trace-Element Partitioning in Sector-Zoned Clinopyroxene Oikocrysts from the Norra Ulvo Gabbro, Sweden

Petrographic analysis and geochemical mapping are used to characterizesector-zoned clinopyroxene oikocrysts from the Norra UlvöGabbro, Sweden. The sector zones are distinguished opticallyand by differences in TiO₂ , Al₂O₃ , and incompatible traceelements. The oikocrysts grew with a branching morphology asevidenced by numerous triple points in a single oikocryst. Oikocrystsgrew from the center of domains outward and the growth withinindividual domains did not completely fill the space until theoikocryst had reached its full size. In some domains sectorzones are overgrown by growth zones that lack sectors. The lastcrystallization infilled channels that allowed late-stage liquidsto move through the oikocryst at the contacts with chadacrysts.Calculation of partitioning behavior of trace elements betweenadjacent sectors shows it to be regular and consistent withexperimental results. The data show that the CaTiAl₂O₆ componentgreatly enhances the solubility of most incompatible elementsin clinopyroxene. KEY WORDS: cumulate; layered intrusion; oikocryst; sector-zoned; trace elements     

Petrology and Mineral Chemistry of Lower Crustal Intrusions: the Chilas Complex, Kohistan (NW Pakistan)

Mineral major and trace element data are presented for the mainrock units of the Chilas Complex, a series of lower crustalintrusions emplaced during initial rifting within the MesozoicKohistan (paleo)-island arc (NW Pakistan). Detailed field observationsand petrological analysis, together with geochemical data, indicatethat the two principal units, ultramafic rocks and gabbronoritesequences, originate from a common parental magma, but evolvedalong different mineral fractionation trends. Phase petrologyand mineral trace element data indicate that the fractionationsequence of the ultramafic rocks is dominated by the crystallizationof olivine and clinopyroxene prior to plagioclase, whereas plagioclaseprecedes clinopyroxene in the gabbronorites. Clinopyroxene inthe ultramafic rocks (with Mg-number [Mg/(Fe_tot + Mg] up to0·95) displays increasing Al₂O₃ with decreasing Mg-number.The light rare earth element depleted trace element pattern(Ce_N/Gd_N 0·5–0·3) of primitive clinopyroxenesdisplays no Eu anomaly. In contrast, clinopyroxenes from thegabbronorites contain plagioclase inclusions, and the traceelement pattern shows pronounced negative anomalies for Sr,Pb and Eu. Trace element modeling indicates that in situ crystallizationmay account for major and trace element variations in the gabbronoritesequence, whereas the olivine-dominated ultramafic rocks showcovariations between olivine Mg-number and Ni and Mn contents,pointing to the importance of crystal fractionation during theirformation. A modeled parental liquid for the Chilas Complexis explained in terms of mantle- and slab-derived components,where the latter component accounts for 99% of the highly incompatibleelements and between 30 and 80% of the middle rare earth elements.The geochemical characteristics of this component are similarto those of a low percentage melt or supercritical liquid derivedfrom subducted mafic crust. However, elevated Pb/Ce ratios arebest explained by additional involvement of hydrous fluids.In accordance with the crystallization sequence, the subsolidusmetamorphic reactions indicate pressures of 0·5–0·7GPa. Our data support a model of combined flux and decompressionmelting in the back-arc. KEY WORDS: Kohistan; Island arc; gabbro; trace element modelling; lower crustal intrusion     

Crystal Growth and Crystallization Time Scales of the Panzhihua Layered Intrusion: Constraint from Crystal Size Distribution

The Panzhihua layered intrusions is generated closely related to the Emeishan LIPs. This paper analyzes the spatial distribution of plagioclase and pyroxene. The quantitative texture analysis of 2209 plagioclase shows that the characteristic length of plagioclase is 0.54 to 0.96 mm, the intercept variation range is large, from ?0.67 to 0.96, and the slope is ?1.85 to ?1.04, the Aspect Ratio shows from 1.84 to 2.59 and fractal dimension D is 1.908–1.933. The quantitative texture analysis of 2342 pyroxene shows that the characteristic length of pyroxene is 0.38–0.64 mm, the intercept shows from 0.46 to 2.26, The slope ranges from ?2.6 to ?1.47, the Aspect Ratio value varies from 1.53 to 1.71, the fractal dimension D is 0.93 to 1.13. All the CSDs results of the Panzhihua intrusions indicate that plagioclase and pyroxene form in an open magma system and undergo four replenishment of magma injection. The plagioclase crystals do not grow as the lathlike shape, and the fractal growth leads to complex crystal surface. The plagioclase undergoes deformation compaction during the crystal process, and then is oriented. The pyroxene crystals grow along an approximately triaxial ratio and undergo texture adjustment and small crystal dissolution reabsorption. When all crystals in magma system grows up to 2 mm, the pyroxene undergoes cumulation in the Panzhihua layered intrusions. The plagioclase crystallization time scale is 171.23–304.41 years, representing that the crystallization is the more uniform in central part of the melt. The nucleation density continuously increases during the crystallization process of the magma system. The time scale to reach the final maximum crystal nucleation density is 15.28–58.98 years.     

辉长岩韵律层厚度的分形结构与模拟计算——以攀枝花层状侵入体为例

运用分形理论对四川攀枝花层状侵入体的韵律层厚度进行分维计算, 并结合计算机模拟手段, 对韵律层理的成因进行了探讨.通过分形计算表明, 攀枝花层状侵入体的韵律层在厚度上具有分形结构, 其分维数介于1.4~1.8之间, 不同级别韵律层的厚度分维数不同, 韵律演化的级别越高, 厚度分维数越大.通过模拟计算的韵律色度分维数与实测韵律层的厚度分维数比较接近, 变化规律也比较相似, 这不仅说明在韵律的演化过程中, 厚度和色率之间有一定的内在联系, 也说明了重力在韵律形成机制中是一个重要的影响因素.地球的重力作用与地球的其他物理化学作用在空间和时间上耦合, 形成了这种具有时空分形结构的韵律现象.      

Origin and Source Evolution of the Leucite Hills Lamproites: Evidence from Sr-Nd-Pb-O Isotopic Compositions

Whole-rock major and trace element and O, Sr, Nd and Pb isotopicdata are reported for 3·0–0·89 Ma lamproitesfrom the Leucite Hills, Wyoming, USA. The two main groups oflamproites, madupitic lamproites and phlogopite lamproites,are geochemically distinct and cannot be related to one anotherby either fractional crystallization or crustal contamination.It seems likely that the geochemical differences between thesetwo rock types are related to variations in source mineralogyand depth of partial melting. The high Mg-number and large ionlithophile element abundances and negative Nd values of thelamproites indicate a mantle source that has experienced stagesof both depletion and enrichment. The negative Nb, Ta and Tianomalies in mantle-normalized trace element diagrams and lowtime-integrated U/Pb, Rb/Sr and Sm/Nd ratios of both lamproitegroups and other Cenozoic igneous rocks from the Wyoming ArcheanProvince indicate an ancient metasomatic enrichment (>1·0Ga) of the mantle source associated with the subduction of carbonate-bearingsediments. Other chemical characteristics of the Leucite Hillslamproites, especially their high K₂O and volatile contents,are attributed to more recent metasomatism (<100 Ma) involvinginflux from upwelling mantle during back-arc extension or plumeactivity. KEY WORDS: isotopes; lamproites; metasomatism; Leucite Hills; Wyoming     

Chemistry of the Shiant Isles Main Sill, NW Scotland, and Wider Implications for the Petrogenesis of Mafic Sills

Major and trace element data for the Tertiary, Shiant IslesMain Sill, NW Scotland, are used to discuss its complex internaldifferentiation. Vertical sections through the sill exhibitsharp breaks in chemistry that coincide with changes in texture,grain size and mineralogy. These breaks are paired, top andbottom, and correspond to the boundaries of intrusive units,confirming a four-phase multiple-intrusion model based on fieldrelations, petrography, mineralogy and isotopes. Whole-rockchemistry is consistent with this model and necessitates onlyminor revisions to the intrusive and differentiation mechanismspreviously proposed. The rocks contain strongly zoned minerals(e.g. olivine Fo_70–5, clinopyroxene Mg# = 75–5,plagioclase An_75–5) indicating almost perfect fractionalcrystallization, but whole-rock compositions do not show suchextreme variations. Thus, while residual liquids became highlyevolved in situ, they mainly became trapped within the crystalnetwork and did not undergo wholesale inward migration. Someinward (mainly upward) concentration of residual liquids didoccur to form a ‘sandwich horizon’, but the morevolatile-rich, late-stage liquids that did not crystallize insitu appear to have migrated to higher levels in the sill toform pegmatitic horizons. Parental liquid compositions are modelledfor the intrusive units and it is concluded that the originalparent magma formed by partial melting of upper mantle thatwas more depleted in LREE than the sources of most ScottishTertiary basaltic rocks. Incompatible trace elements in thepicrodolerite–crinanite intrusive unit support isotopeevidence that its parent magma was contaminated by crustal material.Attempts to reconcile the chemical characteristics of the sillwith a recently proposed petrogenetic model based on a singleintrusion of magma differentiated by novel, but controversial,processes fail comprehensively. It is predicted that the complexpetrogenetic history of the Shiant Isles sill is not unusualand could become the model for other large (>50 m thick)sills. KEY WORDS: alkali basalt; differentiation; geochemistry; multiple intrusion; Shiant Isles; sill     

The Shape and Volume of the Skaergaard Intrusion, Greenland: Implications for Mass Balance and Bulk Composition

Re-examination of the Skaergaard intrusion in the context ofits regional setting, combined with new data from explorationdrilling, has resulted in a revised structural model for theintrusion. It is modelled as an irregular box, c. 11 km fromnorth to south, up to 8 km from east to west, and 3·4–4km from the lower to the upper contact. The walls of the intrusionare inferred to follow pre-existing and penecontemporaneoussteep faults, and the floor and roof seem largely controlledby bedding planes in the host sediments and lavas, similar toregional sills. The suggested shape and volume are in agreementwith published gravimetric modelling. Crystallization alongall margins of the intrusion concentrated the evolving meltin the upper, central part of the intrusion, best visualizedas an ‘onion-skin’ structure inside the box. Thetotal volume is estimated to c. 280 ± 23 km³, of which13·7% are referred to the Upper Border Series (UBS),16·4% to the Marginal Border Series (MBS) and 69·9%to the Layered Series (LS). In the LS, the Lower Zone (LZ) isestimated to constitute 66·8%, the Middle Zone (MZ) 13·5%and the Upper Zone (UZ) 19·7%. The new volume relationshipsprovide a mass balance estimate of the major and trace elementbulk composition of the intrusion. The parental magma to theSkaergaard intrusion is similar to high-Ti East Greenland tholeiiticplateau basalts with Mg number c. 0.45. The intrusion representsthe solidification of contemporary plateau basalt magma trappedand crystallized under closed-system conditions in a crustalreservoir at the developing East Greenland continental margin. KEY WORDS: bulk composition; emplacement; mass proportions; Skaergaard intrusion; structure     

The Origin of HIMU in the SW Pacific: Evidence from Intraplate Volcanism in Southern New Zealand and Subantarctic Islands

This paper presents field, geochemical and isotopic (Sr, Nd,Pb) results on basalts from the Antipodes, Campbell and ChathamIslands, New Zealand. New ⁴⁰Ar/³⁹Ar age determinations alongwith previous K–Ar dates reveal three major episodes ofvolcanic activity on Chatham Island (85–82, 41–35,5 Ma). Chatham and Antipodes samples comprise basanite, alkaliand transitional basalts that have HIMU-like isotopic (²⁰⁶Pb/²⁰⁴Pb>20·3–20·8, ⁸⁷Sr/⁸⁶Sr <0·7033,¹⁴³Nd/¹⁴⁴Nd >0·5128) and trace element affinities(Ce/Pb 28–36, Nb/U 34–66, Ba/Nb 4–7). Thegeochemistry of transitional to Q-normative samples from CampbellIsland is explained by interaction with continental crust. Thevolcanism is part of a long-lived (100 Myr), low-volume, diffusealkaline magmatic province that includes deposits on the Northand South Islands of New Zealand as well as portions of WestAntarctica and SE Australia. All of these continental areaswere juxtaposed on the eastern margin of Gondwanaland at >83Ma. A ubiquitous feature of mafic alkaline rocks from this regionis their depletion in K and Pb relative to other highly incompatibleelements when normalized to primitive mantle values. The inversionof trace element data indicates enriched mantle sources thatcontain variable proportions of hydrous minerals. We proposethat the mantle sources represent continental lithosphere thathost amphibole/phlogopite-rich veins formed by plume- and/orsubduction-related metasomatism between 500 and 100 Ma. Thestrong HIMU signature (²⁰⁶Pb/²⁰⁴Pb >20·5) is consideredto be an in-grown feature generated by partial dehydration andloss of hydrophile elements (Pb, Rb, K) relative to more magmaphileelements (Th, U, Sr) during short-term storage at the base ofthe lithosphere. KEY WORDS: continental alkaline basalts; lithospheric mantle, mantle metasomatism; New Zealand; OIB, HIMU; Sr, Nd and Pb isotopes; West Antarctica     

Possible Non-melted Remnants of Subducted Lithosphere: Experimental and Geochemical Evidence from Corundum-Bearing Mafic Rocks in the Horoman Peridotite Complex, Japan

We report experimental results and whole-rock trace-elementcharacteristics of a corundum-bearing mafic rock from the Horomanperidotite complex, Japan. Coronitic textures around corundumin the sample suggest that corundum was not stable in maficrock compositions during the late-stage P–T conditionsrecorded in the complex (P < 1 GPa, T < 800°C). Basedon the experimental results, corundum is stable in aluminousmafic compositions at pressures of 2–3 GPa under dry conditions,suggesting that the corundum-bearing mineral assemblages developedunder upper-mantle conditions, probably within the surroundingperidotite. Variations in the trace-element compositions ofthe corundum-bearing mafic rock and related rocks can be controlledby modal variations of plagioclase, clinopyroxene and olivine,suggesting that they formed as gabbroic rocks at low-pressureconditions, and that the corundum-bearing mafic rock was derivedfrom a plagioclase-rich protolith. A complex P–T trajectory,involving metamorphism of the plagioclase-rich protolith ata pressure higher than that at which it was first formed, isneeded to explain the origin of the corundum-bearing mafic rocks.They show no evidence for partial melting after their formationas low-pressure cumulates. The Horoman complex is an exampleof a large peridotite body containing possible remnants of subductedoceanic lithosphere still retaining their original geochemicalsignatures without chemical modification during subduction andexhumation. KEY WORDS: Horoman; mafic rock; corundum; experiment; P–T history; recycling