Compositional relations of pyroxenes in a ferrogabbro from Beaver Bay intrusion,Minnesota

Four different types of pyroxene found in a Beaver Bay ferrogabbro were analysed by microprobe. The crystallization sequence of pyroxene is augite-ferroaugite with exsolution lamellae of Ca-poor clinopyroxene → ferropigeonite with exsolution lamellae of augite → ferrohypersthene without exsolution lamellae+augite and ferroaugite without exsolution lamellae. The core of augite-ferroaugite with exsolution lamellae is cumulus pyroxene, whereas others, including the margin of augite-ferroaugite with lamellae, are interpreted to have crystallized from the intercumulus liquid. The sequence of crystallization of minerals from intercumulus liquid is different from that of minerals which have accumulated successively to the bottom and which were related to the fractionation of the whole layered series. The difference may be attributed to the different oxidation state of crystallization.     

Nomenclature of Pyroxenes

Summary This is the final report on the nomenclature of pyroxenes by the Subcommittee on Pyroxenes established by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. The recommendations of the Subcommittee as put forward in this report have been formally accepted by the Commission. Accepted and widely used names have been chemically defined, by combining new and conventional methods, to agree as far as possible with the consensus of present use. Twenty names are formally accepted, among which thirteen are used to represent the end-members of definite chemical compositions. In common binary solid-solution series, species names are given to the two end-members by the 50% rule. Adjectival modifiers for pyroxene mineral names are defined to indicate unusual amounts of chemical constituents. This report includes a list of 105 previously used pyroxene names that have been formally discarded by the Commission.

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Die Nomenklatur von Pyroxenen

Zusammenfassung Diese Arbeit über die Nomenklatur der Pyroxene ist der Endbericht eines Unterkomitees der Commission on New Minerals und Mineral Names der International Mineralogical Association. Die in dieser Arbeit vorgelegten Empfehlungen des Unterkomitees wurden von der Kommission angenommen. Akzeptierte und häufig verwendete Namen wurden unter Verwendung neuer und konventioneller Methoden und unter möglichst großer Übereinstimmung mit dem derzeitigen Gebrauch chemisch definiert. Zwanzig Namen wurden als bindend angenommen, von denen dreizehn Endglieder mit chemisch genau definierten Zusammensetzungen sind. Für gewöhnlich wurden die Namen von zwei Endgliedern einer binären Mischkristallreihe durch Anwendung der 50% Regel ermittelt. Zur Darstellung ungewöhnlicher chemischer Zusammensetzungen wurden den Pyroxennamen ergänzende Eigenschaftswörter beigefügt. 105 ursprünglich verwendete Namen von Pyroxenen wurden bereits früher von der Kommission ausgesondert und sind in diesem Bericht aufgelistet.

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With 7 figures     

The Pyroxenes and Olivines from some Acid Glasses

The coexisting pyroxenes and olivines from nine British andIcelandic Tertiary porphyritic acid glasses have been separatedand analysed chemically. Eight new analyses of augite and ferroaugites,one of orthopyroxene, and five of iron-rich olivines are presented,together with their optical properties. The trend of crystallizationof the ferroaugites is discussed. New optical determinativecurves for these ferroaugites and the iron-rich olivines arepresented. The relationship of the pyroxenes and olivines totheir analysed residual glasses is considered.     

Pyroxenes from lherzolite inclusions of Itinome-gata,Japan

The lherzolites have recrystallized to plagioclase lherzolites consisting of olvine, pyroxenes, chromian spinel, plagioclase and pargasite at a depth of 20 to 25 km in the uppermost part of the mantle. It is believed that the garnet lherzolites and spinel lherzolites were originally derived from depths of 50–75 km and 30–50 km respectively. The clinopyroxenes contained about 10 mol. % of jadeite and Tschermak's molecules, respectively and the orthopyroxenes also included about 5–10% of Tschermak's component. Transported upward, the garnet was transformed through pyroxene-spinel symplectite to olivine, plagioclase and spinel aggregates, and most of the jadeite amd some Tschermak's components in the pyroxenes formed secondary pyroxenes and pargasite, and finally plagioclase under isochemical conditions.     

The Pyroxenes and Olivines from some Tertiary Acid Glasses

The coexisting pyroxenes and olivines from nine British andIcelandic Tertiary porphyritic acid glasses have been separatedand analysed chemically. Eight new analyses of augite and ferroaugites,one of orthopyroxene, and five of iron-rich olivines are presented,together with their optical properties. The trend of crystallizationof the ferroaugites is discussed. New optical determinativecurves for these ferroaugites and the iron-rich olivines arepresented. The relationship of the pyroxenes and olivines totheir analysed residual glasses is considered.     

Pyroxenes of the Dufek Intrusion, Antarctica

The Dufek intrusion is a stratiform mafic body, 24,000 to 34,000km² in area and 8 to 9 km thick, in the Pensacola Mountainsof Antarctica. Textures, structures, magmatic stratigraphy,and chemical variation indicate that layered gabbros and relatedrocks of this body developed by accumulation of crystals thatsettled on the floor of a magma chamber. The major cumulus phasesin the exposed part of the intrusion are plagioclase, pyroxene,and iron-titanium oxides. The base of the Dufek intrusion is not exposed, and both Ca-richand Ca-poor pyroxene coexist as cumulus phases in the lowerexposed rocks. The Ca-rich pyroxenes belong to an augite-ferroaugiteseries (Ca_36.4Mg_48.7Fe_14.9-Ca_30.0Mg_23.5Fe_46.5) that extendsup through the 300 m thick capping granophyre. The Ca-poor pyroxenesbelong to a bronzite-inverted pigeonite series (Ca_3.5Mg_69.1Fe_27.4-Ca_11.4Mg_34.0Fe_54.6)that extends only to about 200 m below the granophyre layer.In addition to the cumulus pyroxenes some rocks contain post-cumulusgreen calcic augite and ferrohypersthene. The compositional change of the cumulus pyroxenes with stratigraphicheight is one of general iron enrichment. Superimposed on thistrend are (1) a 1 km thick section in the lower part of thebody that shows slight to no iron enrichment and (2) a markedreversal in the Fe/(Fe+Mg) ratio about 1 km below the top ofthe body. The variations from the general trend are associatedwith cyclic units and are best explained by convective overturnof the magma. In general, the pyroxene compositional trends are similar tothose of the Skaergaard and Bushveld intrusions. One significantdifference in the Dufek intrusion is the limited iron enrichmentof its Ca-rich pyroxenes, that may relate to a slower decreaseof P_O2 during crystallization of the Dufek magma.     

Magma chamber processes in central volcanic systems of Iceland: constraints from layered gabbro of the Austurhorn intrusive complex

New field work and petrological investigations of the largest gabbro outcrop in Iceland, the Hvalnesfjall gabbro of the 6–7 Ma Austurhorn intrusive complex, have established a stratigraphic sequence exceeding 800 m composed of at least 8 macrorhythmic units. The bases of the macrorhythmic units are composed of 2–10 m thick melanocratic layers rich in clinopyroxene and sometimes olivine, relative to the thicker overlying leucocratic oxide gabbros. While the overall compositional variation is limited (Mg# clinopyroxene 72–84; An% plagioclase 56–85), the melanocratic bases display spikes in Mg# and Cr₂O₃ of clinopyroxene and magnetite indicative of magma replenishment. Some macrorhythmic units show mineral trends indicative of up-section fractional crystallisation over up to 100 m, whereas others show little variation. Two populations of plagioclase crystals (large, An-rich and small, less An-rich) indicate that the recharge magma carried plagioclase xenocrysts (high An-type). The lack of evolved gabbros suggests formation in a dynamic magma chamber with frequent recharge, tapping and fractionation. Modelling of these compositional trends shows that the parent magma was similar to known transitional olivine basalts from Iceland that had undergone about 20% crystallisation of olivine, plagioclase and clinopyroxene and that the macrorhythmic units formed from thin magma layers not exceeding 200–300 m. Such a “mushy” magma chamber is akin to volcanic plumbing systems in settings of high magma supply rate including the mid-ocean ridges and present-day magma chambers over the Iceland mantle plume. The Austurhorn central volcano likely formed in an off-rift flank zone proximal to the Iceland mantle plume during a major rift relocation. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Pyroxenes of the Bushveld Intrusion, South Africa

New analyses are presented, for major, minor, and trace elements,of eleven Ca-rich pyroxenes, four bronzites, and two invertedpigeonites from the Bushveld layered basic intrusion. The twenty-threeanalyses now available are believed to represent the entireBushveld fractionation sequence. The Ca-rich pyroxene trendis from Ca_45.4Mg_49.6Fe_5.0 to Ca_42.7Mg_0.6Fe_56.8, the ferrohedenbergitesshowing no evidence of inversion from ferriferous ß-wollastonites.The Ca-poor pyroxene trend is from bronzite (Ca_2.8Mg_85.0Fe_12.2)through pigeonites to ferropigeonites (approximately Ca₃Mg₂₇Fe₅₅).All the pigeonitic pyroxenes have inverted to orthopyroxene. The compositional trends are remarkably similar to those ofthe Skaergaard pyroxene series, but the Bushveld sequence isthe most complete known for a single fractionated intrusion.The compositional and other variations of the pyroxenes, consideredtogether with those of the coexisting olivines and feldspars,leave little doubt that the Bushveld rocks originated by crystalaccumulation from a slowly cooled and fractionated intrusionof tholeiitic basalt magma. The slight but significant differences between the Bushveldand Skaergaard pyroxene trend characteristics can be explainedin terms of a displacement, in one intrusion as compared withthe other, of the liquidus and solidus surfaces relative tothe solvus and inversion surfaces in the system Wo—En—Fs.This may be due to minor differences in the initial magma compositionsof the two intrusions. Differences in the Mg/Fe ratios of Bushveldand Skaergaard coexisting pyroxene pairs are believed to bedue, at least in part, to the greater depth of the Bushveldmagma chamber. The Bushveld trends are briefly discussed in the light of recentexperimental studies on compositions within the Di-Hed-En-Fspyroxene quadrilate     

福建马坑铁矿辉石的成因矿物学研究

本文研究了福建马坑铁矿床中辉石族矿物的时空分布、产状、种属及其晶体形态标型、物理标型、化学成分标型。通过不同成因的辉石(308个不同成因辉石)化学成分标型对比、图解等综合判别方法确定马坑铁矿中辉石有火山岩变质、沉积变质、接触交代和岩浆四种成因。矿体底部成矿建造为富Fe、K、Mn火山岩建造,中下部为富F、Mn硅铁质沉积建造,上部为富Ca、Mg、Mn、Fe、Si碳酸盐沉积建造。马坑矿床主矿体为海底火山沉积变质型铁矿。提出利用辉石的种属、含铁系数、化学标型判断有无矿体及其贫富的矿物学标志。     

Pyroxenes from the Late Stages of Fractionation of the Skaergaard Intrusion, East Greenland

The pyroxenes of the upper zone of the Skaergaard layered seriesconsist of an iron-rich series of brown and green clinopyroxenes.Five new analyses are presented, together with a revised trendline which includes the full range of clinopyroxenes believedrepresentative of the Skaergaard fractionation sequence. Therange for the augite-ferroaugite-ferrohedenbergite series isfrom Ca_42?4 Mg_47?9 Fe_9?7 to Ca_42?5 Mg_0?4 Fe_57?1, the most ferriferousvariety coexisting with a pure fayalite. A re-study of the compositionsand textures of certain green ferrohedenbergites supports thebelief that they are the product of sub-solidus inversion offerriferous ß-wollaston?tes, which crystallized asa temporary phase between the periods when brown ferrohedenbergitescrystallized directly from the magma. A consideration of thecompositional and textural relationships between green and brownpyroxenes, and the significance of the mosaic inversion texture,have led to an interpretation of the crystallization and inversionsequence for these minerals. Four new analyses of ferrohedenbergitesfrom the downward-crystallized upper border group rocks provideevidence for a trend which differs slightly from that for thelayered series ferrohedenbergites.     

黄山东镁铁超镁铁杂岩中的辉石化学成分研究

通过对黄山东镁铁超镁铁杂岩的辉石化学成分研究，证明辉石的化学成分受寄主岩石类型的制约，从超镁铁岩相到角闪辉长岩相斜方辉石的化学成分由富ＭｇＯ、ＳｉＯ＿２、Ａｌ＿２Ｏ＿３、Ｃｒ＿２Ｏ＿３到富ＦｅＯ、ＴｉＯ＿２、ＣａＯ、ＭｎＯ；单斜辉石化学成分由富Ａｌ＿２Ｏ＿３、Ｃｒ＿２Ｏ＿３、Ｎａ＿２Ｏ、ＭｇＯ到富ＦｅＯ、ＭｎＯ、ＣｏＯ。在同一寄主岩石中，单斜辉石比斜方辉石富ＴｉＯ＿２、Ａｌ＿２Ｏ＿３、Ｃｒ＿２Ｏ＿３、ＣａＯ和Ｎａ＿２Ｏ，贫ＭｇＯ、ＳｉＯ＿２、ＭｎＯ和ＦｅＯ。根据辉石化学成分特征得出黄山东杂岩形成于岛弧环境，为上地幔石榴二辉檄榄岩部分熔融形成的拉斑玄武岩岩浆结晶分异作用的产物。     

Pyroxenes of the Khibiny alkaline pluton, Kola Peninsula

Seven pyroxene varieties were identified in nepheline syenites and foidolites of the Khibiny pluton: enstatite, ferrosilite, diopside, hedenbergite, augite, aegirine-augite, and aegirine. Enstatite and augite are typical of alkaline and ultramafic rocks of dike series. Ferrosilite was found in country quartzitic hornfels. Diopside is a rock-forming mineral in alkaline and ultramafic rocks, alkali gabbroids, hornfels in xenoliths of volcanic and sedimentary rocks and foyaite, melteigite-urtite that assimilate them, and certain hydrothermal pegmatite veins. Hedenbergite was noted in hornfels from xenoliths of volcanic and sedimentary rocks and in a hydrothermal pegmatite vein at Mount Eveslogchorr. Aegirine-augite is the predominant pyroxene in all types of nepheline syenites, phonolites and tinguaites, foidolites, alkaline and ultramafic rocks of dike series, fenitized wall rocks surrounding the pluton, and xenoliths of Devonian volcanic and sedimentary rocks. Aegirine is an abundant primary or, more often, secondary mineral in nepheline syenites, foidolites, and hydrothermal pegmatite veins. It occurs as separate crystals, outer zones of diopside and aegirine-augite crystals, and homoaxial pseudomorphs after Na-Ca amphiboles. Microprobe analyses of 265 pyroxenes samples allowed us to distinguish ten principal trends of isomorphic replacement and corresponding typomorphic features of pyroxenes. Compositional variations in clinopyroxenes along the sampled 35-km profile from the margin of the Khibiny pluton to its center confirm the symmetric zoning of the foyaite pluton relative to semicircular faults of the Minor Arc and the Main (Central) Ring marked by Devonian volcanic and sedimentary rocks, foidolites, and related metasomatic rocks (rischorrite, albitite, and aegirinite). Changes in the composition of pyroxenes are explained mainly by the redistribution of elements between coexisting minerals of foyaites in the process of their intense differentiation under the effect of foidolite melts that have intruded into the circular fault zones.     

Compositional Zoning in Pyroxenes from Lunar Rock 12021, Oceanus Procellarum

Electron probe study of the pyroxene phenocrysts in rock 12021of the Apollo 12 sample reveals an extraordinary variety ofzoning phenomena. These phenocrysts have cores of relativelyhomogeneous pigeonite which are mantled by Ca-rich pyroxenes;the mantles are zoned outward from about 20 mol per cent FeSiO₃at the core-mantle boundaries to 70 mol per cent FeSiO₃ at themargins of the phenocrysts. The character of the normal zoningdiffers in each growth sector. Four different trends characterizinggrowth on (110), (100), (010), and parallel to c are described.Pronounced but erratic oscillatory effects are impressed onthe normal zoning in all sectors. Normal zoning in lunar pyroxenes is generally more extensivethan in terrestrial pyroxenes, probably because the lunar lavaswere an order of magnitude more fluid than terrestrial basalts.Turbulent flow in lunar lavas may have caused some of the oscillatoryeffects. The cyclic nature of augite and pigeonite crystallizationin these pyroxenes is probably kinetic in origin; conceivablyit could reflect the presence of an augite-pigeonite peritectic.It is likely that further knowledge of the crystal structureof pigeonite at high temperatures will be required before astructural model can be formulated that will satisfactorilyexplain the details of the sector zoning. Coarse-grained pyroxferroite and subcalcic ferroaugite havecrystallized in apparent ‘equilibrium’ in the finalstages of formation of the groundmass in rock 12021. Nevertheless,the crystallization of the pyroxferroite was probably metastablerelative to the assemblage ferroaugite+ fayalite+ tridymite.Data on the effect of Mg substitution on the stability of pyroxferroitewill be needed to establish this point. We suggest that the pigeonite cores of the phenocrysts in rock12021 crystallized under quiescent conditions, probably in amagma chamber below the lunar surface, and that the stronglyzoned augite mantles crystallized during emplacement of thelava on the surface.     

Paragenetic Types and Compositions of Metamorphic Pyroxenes

Paragenetic types of pyroxenes, selected according to mineralassociations, differ in their average contents of the majorcations. By comparing with the average composition it is seenthat both the Fe/(Mg+Fe) ratio and the Fe⁺², Mn, Mg, Fe⁺³, andCr contents are determined in the main by the composition ofthe host rocks, but the AI_z, Al_y, Ca, and Na contents in pyroxenesare influenced by the conditions under which the rock was formed.The dependence of the Al_z and Al_y contents of orthopyroxenesand clinopyroxenes on temperature and pressure is shown withthe help of a new P-T diagram and by comparison with chemicalanalyses of natural pyroxenes. The correlation between the compound cations in pyroxenes isused to test the hypothesis of real isomorphous substitutionsin each paragenetic type of pyroxenes, and to determine rationalmethods of calculating pyroxene analyses into components. The calculated discriminant functions are useful for referring(with 5–10 per cent error) analyses of pyroxenes to oneof a number of paragenetic types, and in particular for distinguishingpyroxenes from magmatic and metamorphosed gabbros, and fromhigh- and low-temperature two-pyroxene granulites.     

Origin and timing of the post-Variscan gabbro–granite complex of Porto (Western Corsica)

The post-Variscan complex of Porto consists of metaluminous to slightly peraluminous A-type biotite granites mingled with gabbro-dioritic rocks, and late dykes with basaltic to trachyandesitic composition. U-Pb zircon dating by LA-ICP-MS on two mafic intrusive samples constrains the time of the gabbro–granite crystallisation at 281 ± 3 Ma and 283 ± 2 Ma. Hornblende ⁴⁰Ar-³⁹Ar ages from a late trachyandesite dyke date the dyking event at 280 ± 2 Ma, which is within error the U-Pb zircon ages of the intrusives. Biotite granites show variable major and trace element compositions and similar initial ε_Nd (−0.3 to +0.9). Whole rock chemistry variations and trace element compositions of plagioclase and allanite indicate that the granites are genetically linked, essentially through fractional crystallisation of feldspars and minor allanite. On the basis of whole-rock chemistry e.g. initial ε_Nd +4.9 to +1.7 and trace element clinopyroxene compositions, we have ascertained that the mafic intrusives and basic dykes formed from isotopically depleted mantle source-derived melts with similar trace element signature. These basic melts experienced slightly different evolutionary histories, controlled by fractional crystallisation and crustal contamination, mainly by the acid magma that gave rise to the associated biotite granites, but also by the enclosing older Variscan granitoids. U-Pb zircon data suggest that the Porto complex was affected by hydrothermal fluid circulation at 259 ± 9 Ma.     

班公湖-怒江洋的扩张脊俯冲: 宗白增生杂岩中侏罗世辉长岩脉地球化学和Sr-Nd同位素特征

丁青蛇绿岩位于班公湖-怒江缝合带东段,被宗白增生杂岩分为东、西2个蛇绿岩体,面积分别约为400 km2和150 km2。宗白增生杂岩由异地体亚宗混杂岩和上部原地体陆缘碎屑沉积岩组成。亚宗混杂岩由低变质岩带、砾岩带、玄武质凝灰岩夹薄层泥硅质岩带和作为主要基质的泥页岩夹薄层杂砂岩组成,其中泥页岩基质被中侏罗世辉长岩脉侵入。辉长岩脉发育双侧冷凝边,走向90°~110°不等,主量、稀土和微量元素成分均介于富集型洋中脊玄武岩和洋岛玄武岩之间,结合εNd（t）=1.88~2.41和（87Sr/86Sr）t=0.70912 ~ 0.70919,指示宗白辉长岩岩浆为大洋岩石圈地幔底部地震波低速带（LVZ）顶部富集挥发分和不相容元素的洋岛玄武岩型熔体,与源自LVZ下部亏损软流圈地幔的正常洋中脊玄武岩型熔体混合的产物,形成于弧前扩张脊与俯冲带相互作用的板片窗环境。     

Pyroxenes in the Shaw (L-7) chondrite

The Shaw L-group chondrite differs from orthodox type 6 ordinary chondrites in ways which suggest that it experienced unusually high metamorphic temperatures and anatexis. Electron microprobe and single crystal X-ray diffraction data indicate that Shaw contains three pyroxenes: the augite (Fs_11.3Wo_38.2) and calcic orthopyroxene (Fs_19.4WO_4·5) reported by other workers and a second, Ca- and Al-poor orthopyroxene (Fs_16·8Wo_1·2) which we interpret as inverted protobronzite. Comparison of the Shaw assemblage with experimental data suggests that a two-phase (augite-protobronzite) assemblage developed at peak metamorphic temperatures of ~1250–1300°C, that partial reaction of augite and protobronzite produced calcic orthopyroxene and by-product spinel at temperatures approximately 150°C lower and that protobronzite inverted to bronzite free of stacking disorder during subsequent slow cooling. The intracrystalline distribution of Fe and Mg in the Ca-poor bronzite (K_E + 0·07; determined by crystal structure analysis) indicates an equilibration temperature of ~500°C.Shaw differs sufficiently in texture and mineralogy from type 6 ordinary chondrites to justify its assignment to a separate petrologic type: L-7.     

Mylonitic deformation of gabbro in the lower crust: A case study from the Pankenushi gabbro in the Hidaka metamorphic belt of central Hokkaido, Japan

The mylonitization of the Pankenushi gabbro in the Hidaka metamorphic belt of central Hokkaido, Japan, occurred along its western margin at ≈600 MPa and 660–700 °C through dynamic recrystallization of plagioclase and a retrograde reaction from granulite facies to amphibolite facies (orthopyroxene + clinopyroxene + plagioclase + H₂O = hornblende + quartz). The reaction produced a fine-grained (≤100 μm) polymineralic aggregate composed of orthopyroxene, clinopyroxene, quartz, hornblende, biotite and ilmenite, into which strain is localized. The dynamic recrystallization of plagioclase occurred by grain boundary migration, and produced a monomineralic aggregate of grains whose crystallographic orientations are mostly unrelated to those of porphyroclasts. The monomineralic plagioclase aggregates and the fine-grained polymineralic aggregates are interlayered and define the mylonitic foliation, while the latter is also mixed into the former by grain boundary sliding to form a rather homogeneous polymineralic matrix in ultramylonites. However in both mylonite and ultramylonite, plagioclase aggregates form a stress-supporting framework, and therefore controlled the rock rheology. Crystal plastic deformation of pyroxenes and plagioclase with dominant (100)[001] and (001)1/2 slip systems, respectively, produced distinct shape- and crystallographic-preferred orientations of pyroxene porphyroclasts and dynamically recrystallized plagioclase grains in both mylonite and ultramylonite. Euhedral to subhedral growth of hornblende in pyroxene porphyroclast tails during the reaction and its subsequent rigid rotation in the fine-grained polymineralic aggregate or matrix produced clear shape- and crystallographic-preferred orientations of hornblende grains in both mylonite and ultramylonite. In contrast, the dominant grain boundary sliding of pyroxene and quartz grains in the fine-grained polymineralic aggregate of the mylonite resulted in their very weak shape- and crystallographic-preferred orientations. In the fine-grained polymineralic matrix of the ultramylonite, however, pyroxene and quartz grains became scattered and isolated in the plagioclase aggregate so that they were crystal-plastically deformed leading to stronger shape- and crystallographic-preferred orientations than those seen in the mylonite.     

Channel deposits of peridotite, gabbro and chromitite from turbidity currents in the stratiform Fiskenæsset anorthosite complex, southwest Greenland

Trough-shaped layers of peridotite, gabbro and chromitite occur sporadically in a 500 m thick intrusion of mainly anorthosite. The layers are interpreted as deposits in channels which were excavated and filled in by turbidity currents on the floor of the intrusion. They show that these currents were important concentrators of olivine and chromite.     

Petrogenesis of gabbro and orthopyroxene gabbro from the Phenai Mata Igneous Complex,Deccan volcanic province: Products of concurrent assimilation and fractional crystallization

We report the occurrence of orthopyroxene gabbro from the Phenai Mata Igneous Complex (containing thoeliitic and alkaline rocks) that occur within Deccan Traps. The P-T calculations based on two pyroxene thermometry vary from 8.5±1.0 kbar and 963±39 °C. These gabbroic rocks exhibit high Mg# (0.67 to 0.71). But their primary magma signature can be negated due to their high SiO₂ (> 50 wt %), low Ni (32–35 ppm) and Cr (105–182 ppm) contents. Further, simple fractional crystallization was not responsible for the modification of the magma. Modeling carried out using trace element concentrations revealed that concurrent assimilation and fractional crystallization (AFC) was responsible for the genesis of these rocks. Small pods of magma could have accumulated in the crustal portions and concurrent assimilation and fractional crystallization have taken place in the generation of gabbro and orthopyroxene gabbro in the present study area.