Crystallization Conditions and Mineral Chemistry in the East of Tafresh,Central Iran,with Insights into Magmatic Processes

The Tafresh granitoids are located at the central part of the Urumieh-Dokhtar Magmatic Arc(UDMA) in Iran. These rocks, mainly consisting of diorite and granodiorite, were emplaced during the Early Miocene. They are composed of varying proportions of plagioclase + K-feldspar + hornblende ± quartz ± biotite. Discrimination diagrams and chemical indices of amphibole phases reveal a calc-alkaline affinity and fall clearly in the crust-mantle mixed source field. The estimated pressure, derived from Al in amphibole barometry, is approximately 3 Kb. The granitoids are I-type, metaluminous and belong to the calc-alkaline series. They are all enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements and display geochemical features typical of subduction-related calc-alkaline arc magmas. Most crystal size distribution(CSD) line patterns from the granitoids show a non-straight trend which points to the effect of physical processes during petrogenesis.The presence of numerous mafic enclaves, sieve texture and oscillatory zoning along with the CSD results show that magma mixing in the magma chamber had an important role in the petrogenesis of Tafresh granitoids. Moreover, the CSD analysis suggests that the plagioclase crystals were crystallized in a time span of less than 1000 years, which is indicative of shallow depth magma crystallization.     

Crystallization Pressure and Cooling History of the Giles Layered Igneous Complex, Central Australia

The Giles Complex, central Australia, consists of a series oflarge layered gabbroic/ultramafic intrusions emplaced in acidicand intermediate granulites of the Middle Proterozoic Musgraveblock. Lithologies range from well-layered dunite, wehrlite,and pyroxenite in the lower primitive series, to massive olivinegabbro, gabbronorite, and anorthosite in the main units, andferrodiorites, vanadife-rous magnetite layers, and granophyresin the upper, most fractionated parts. Unlike many layered intrusions,the Giles Complex is tectonically dismembered to an extent thata reconstruction of the original morphology is difficult. The Complex is believed to be a type example for medium- tohigh-pressure differentiation. (1) Chilled margin samples (wherepreserved) are orthopyroxene-phyric, and liquidus olivine isreplaced by liquidus orthopyroxene at an mg-number of 0.77,suggesting a pressure-related expansion of the orthopyroxenestability field (Goode & Moore, 1975). (2) Tschermaks substitutioninto pyroxene and plagioclase-orthoclase solid solution areextensive, indicating unusually high crystallization temperaturerelated to high pressure; antiperthites in the Giles Complexare amongst the most calcic reported for terrestrial rocks.(3) The lower primitive cumulate units of the Complex are coroniticand feature a variety of subsolidus high-pressure reaction textures;olivine and cumulus chromite have reacted with calcic plagioclaseto orthopyroxene-clinopyroxene-spinel, olivine-spinel, and clinopyroxene-spinelsymplectites. The principal reaction mechanism for the symplectites was continuousmass transfer of alumina from plagioclase toward spinel, asthe Complex passed from the olivine-plagioclase stability fieldinto the pyroxene-spinel field during cooling. Geothermometersapplicable to the cumulates record a wide range of equilibrationtemperatures from late-magmatic to granulite-metamorphic conditions.FeMg₁ exchange gives closure temperatures around 600–700?C,whereas Al₂Mg₁Si₁ net-transfer equilibria have preserved highertemperatures around 750–900 ?C. Defocused beam bulk analysesof exsolved cumulus clinopyroxenes and intercumulus plagioclasesrecover magmatic compositions; i. e., two-pyroxene solvus CaMg_-1temperatures plot around 1120?50?C, whereas two-feldspar thermometersgive 1200?C. Pressures are calculated from thermochemical data with the heterogeneousequilibria 2 fo + an = en + di + sp, fo + an = di + Mg-Ts, andfo + an = en + Ca-Ts, after correcting spinel activities forselective retrograde FeMg_-1 exchange during cooling. These equilibria,combined with orthopyroxene-spinel Al₂Mg_-1Si_-1 temperaturesfor metamorphic assemblages and two-pyroxene temperatures forcumulus phases define a medium-pressure cooling path extendingfrom 1150 ?C (at 6?5 kb) to 750 ?C (at 6?2 kb). The resultssuggest an isobaric cooling path for the Giles Complex, withno evidence for a post-intrusive metamorphic overprint. Themagmas intruded at lower to middle crustal levels after thepervasive deformation in the Musgrave block, and probably afterthe peak metamorphic event.     

The Ballachulish Igneous Complex, Scotland: Petrography, Mineral Chemistry, and Order of Crystallization in the Monzodiorite-Quartz Diorite Suite and in the Granite

The igneous complex of Ballachulish is a composite calc-alkalinepluton of Caledonian age (412 ? 28 Ma), emplaced in Dalradianmetasediments at a pressure of 3 ? 0–5 kb (c. 10 km depth).The 4 by 7 km intrusion is composed of a zoned monzodiorite-quartzdiorite envelope with a distinct flowand deformation-foliation,surrounding a younger core of porphyritic granite. Two-pyroxene thermometry, Fe-Ti oxide thermobarometry, and stabilityrelationships of ternary feldspars, biotite, and amphibolesare used to calibrate the 3 kb isobaric crystallization sequencewith respect to the following parameters: the fractionationstage of the host rocks, the water content of the magmas, phasecompositions, and oxygen fugacity. Plagioclase, augite, andoxides generally yielded submagmatic temperatures due to theextensive recrystallization and re-equilibration of these phasesin the 900–l550?C subsolidus range. The ‘dry’monzodiorites apparently contained less than 1 wt. % initialmagmatic water, and remained H₂O-deficient and vapor-absentthroughout their entire crystallization range. In contrast,2.5–3 wt.% initial H₂O is estimated for the more fractionatedquartz diorites and the younger granites. The main crystallizationinterval for Opx–Cpx–Plg primocrysts in the dioritescovers c. 1100–950?C. Late-magmatic biotite and alkalifeldspar join the paragenetic sequence below 980?860?C, at fO₂near NNO. A solidus temperature of c. 900?C is inferred forthis ‘dry’ system, in which amphiboles are entirelysubsolidus. At the present level of emplacement, crystallizationintervals of {small tilde} 1050–690?C and{small tilde}900–680?C are suggested for the quartz diorites and thegranites, which probably terminated crystallization in the presenceof a hydrous fluid.     

Crystallization Conditions of the Sybille Monzosyenite, Laramie Anorthosite Complex, Wyoming

The Sybille Monzosyenite, associated with the Laramie AnorthositeComplex, consists of rocks ranging in composition from monzogabbroto monzosyenite. There is a continuous range of mineral compositionswith plagioclase varying from An₄₅ to An₂₅ and olivine fromFa₇₅ to Fa₉₈ Strongly ternary (Or, Ab, An all > 10 mol%)feldspars–presently mesoperthites–are found in allrock types and define a continuous trend also in the feldsparternary system. The mineral compositions suggest that the rocktypes of the Sybille Monzosyenite could be part of a singledifferentiation sequence; contamination of the later units byassimilation of, or admixture of partial melts from, countryrock is also likely. Original magmatic temperatures of approximately950–1050?C are indicated by estimated original compositionsfor pyroxenes and feldspars; pressure was near 3 kb, as indicatedby the most magnesian olivine that coexists with quartz. Oxygenfugacity of crystallization is estimated as 1.5 to 2.0 log unitsbelow FMQ by using the displaced equilibrium: SiO₂ + 2Fe₂TiO₃= 2FeTiO₃ + Fe₂SiO₄. Such oxygen fugacities are consistent withthe occurrence of graphite and CO₂-rich fluid inclusions, whichsuggest that crystallization took place in the presence of aCO₂ vapor phase. Temperatures indicated by the present mineralassemblages show that all geothermometers used were reset duringcooling, first by intergrain and then by intragrain processes.     

云南鸡街碱性超基性岩杂岩体中橄榄石的矿物化学

分析了云南鸡街碱性超基性岩杂岩体主体岩石及包体中橄榄石矿物的化学成分。结果显示,不同类型岩石中橄榄石的矿物化学成分之间具有较好的线性关系;橄榄辉石岩包体中橄榄石的Fo值最高,其亚种属靠近镁橄榄石端元的贵橄榄石;从霞霓钠辉岩→霓霞岩→磷霞岩,橄榄石的Fo值逐渐降低,其亚种从靠近镁橄榄石端元的贵橄榄石→靠近透铁橄榄石端元的贵橄榄石→低铁镁铁橄榄石。这些特征表明,本区三类主体岩石为岩浆结晶分异作用的产物;橄榄辉石岩包体为Ⅱ类包体,可能为杂岩体原始岩浆早期结晶分异作用所形成的堆积岩碎块。     

The Klokken Gabbro-Syenite Complex, South Greenland: Quantitative Interpretation of Mineral Chemistry

The layered syenite series in the Klokken stock formed by continuousin situ fractionation of a trachytic magma in a chamber linedby gabbro with 3000 m of cover rocks. The following mineralsand reactions are assessed as geothermometers and barometers:two feldspars; hypersolvus ternary feldspars; ferrohedenbergite-ß-wollastonite;clinopyroxene-olivine Fe-Mg exchange; Fe-Ti oxides; sanidine-magnetite-annite;ferroedenite stability. Estimates of silica activity are obtainedfrom the silica-magnetite-fayalite assemblage. The gabbros ended magmatic crystallization at > 1000–1050°C.The less fractionated members of the syenite range probablycrystallized with P_H2O < P_total, at T > 870°C and,P_H2O 800 bars. In the more fractionated syenites P_H2O = P_totalduring intercumulus feldspar growth, and all magmatic phasescrystallized within the interval 940–830°C and P_H2O< 1100 bars. Magmatic f_O2 (bars) was 1 log unit below theQFM buffer. a_SIO2 in gabbros was slightly above the albite-nephelinebuffer, but rose suddenly to just <1 in the syenites, a jumpmirrored by minor elements in pyroxenes and opaque oxides. Biotitegrew subsolidus in most rocks, at f_O2 < QFM, except in intermediaterocks when f_O2 > QFM and was defined by the sanidine-magnetite-biotiteassemblage. In these rocks P_H2O of 450 bars at 760°C isobtained using existing experimental data, but application ofthis data to Fe-rich biotites in the layered series (where biotiteis an intercumulus phase) requires P > 10 kb at magmatictemperatures. High TiO₂ or F: OH probably accounts for increasedT stability of natural annites at low P. The syenitic liquid fractionated down a low temperature zonein a multicomponent system precipitating alk fsp + ol + cpx+ mt and the more fractionated members of the layered serieshad a negligible crystallization interval, a prerequisite forthe development of the unique Klokken-type inversely gradedmineral layering.     

黑龙江杂岩蓝片岩—云母片岩矿物成分及变质作用P-T条件

黑龙江构造混杂岩中保留着一套曾遭受过中高压变质作用的岩石,其代表性岩石为:蓝片岩-云母片岩;蓝闪石、铁蓝闪石和多硅白云母(单位结构中Si=3.37～3.46>3.30)等具有中高压性质的典型变质矿物在岩石中平衡共生.蓝片岩-云母片岩组合至少经历了早期和晚期两阶段变质,早期为绿帘蓝片岩相,变质温压条件为T=320～460℃,P=7.8×108～11×108Pa.晚期为高绿片岩相,出现富镁绿泥石(XMg=0.565～0.646)交代早期钠质角闪石和多硅白云母等高压变质矿物现象,变质温度升高而压力有所降低,由石榴石云母片岩所限定的高绿片岩相变质条件为T=536～598℃,P=6.8×108～9.5×108Pa,属中温和中高压变质.     

Evaluation of the Crystallization Conditions for the Calcalkaline Russian Peak Intrusive Complex, Klamath Mountains, Northern California

The mid-Jurassic calcalkaline Russian Peak intrusive complex,located in the Klamath Mountains of northern California, consistsof an elliptical peridotite-to-quartz diorite suite intrudedby two plutons of granodiorite. Several techniques were usedto decipher the crystallization conditions for ultramafic rocks,quartz diorite, and granodiorite, including comparison of parageneseswith crystallization experiments, application of geothermometersand barometers, and evaluation of phase equilibria. Contactmetamorphic assemblages, hornblende barometry, and amphibolesubstitution schemes indicate that pressures of intrusion were{small tilde}3 kbar. Plagioclase and pyroxene thermometry indicateintrusion temperatures of {small tilde}1000C for quartz dioriteand 900C for granodiorite. Phase equilibrium analysis for thereaction phlogopite+quartz=K-feldspar+enstatite+H₂O, coupledwith an estimate of the water-saturated quartz diorite solidus,suggests that the solidus of two-pyroxene quartz diorite wasat {small tilde}780C with a mole fraction of water of {smalltilde}0•55. The composition of granodiorite is very similarto that used in several crystallization experiments and indicatesa solidus of 70025C. Estimates of oxygen fugacity, obtainedfrom equilibrium relations of olivine, orthopyroxene, and spinelin ultramafic rocks, magnetite and ilmenite in quartz diorite,and magnetite, K-feldspar, and biotite in quartz diorite andgranodiorite are 2•1–2•5 and 1•0–1•3log units above the quartz-fayalite-magnetite (QFM) buffer forgranodiorite and quartz diorite at their respective solidustemperatures; and 1•0–4•0 log units above QFMfor ultramafic rocks and quartz diorite at subsolidus temperatures.Thus, the quartz diorite magma was hotter, drier, and slightlyreduced relative to the grandiorite magma, differences thatset important constraints on the genesis of the Russian Peakmagmas. These results also indicate that quartz diorite wasundersaturated with respect to H₂O as it reached its solidus,a condition that is consistent with the absence of deutericalteration in this unit. In contrast, granodiorite shows extensivedeuteric alteration and features pegmatites, quartz pods, andradial dikes as might be expected for H₂O-saturated conditions. Although calcalkaline plutonic complexes present serious difficultiesin estimating the intensive parameters of crystallization, judiciousapplication of appropriate methods may result in the successfulevaluation of the conditions of crystallization of such complexes.     

河北矾山超镁铁岩 正长岩杂岩体中黑云母的特征及其成岩指示意义

河北矾山超镁铁岩一正长岩杂岩体侵位于华北克拉通北缘,为硅不饱和过钾质碱性岩,起源于受古亚洲洋俯冲物质改造过的富集岩石圈地幔。矾山杂岩体侵位于晚三叠世,是华北克拉通北缘三叠纪碱性岩带的代表性岩体之一。本文利用电子探针(EMPA)和激光等离子质谱仪(LA-ICPMS)获得了该特殊岩浆体系下黑云母的主量和微量元素组成,并讨论了其成岩指示意义。矾山杂岩体各类型岩石中的云母均为黑云母,Fe/(Fe+Mg)为0.32~0.57,∑Al为1.179~1.375。黑云母成分记录石榴石辉石正长岩结晶时体系氧逸度LogfO_2=-12.5~-15,温度为680~780℃。矾山杂岩体的黑云母具有极低的稀土元素含量(小于0.100×10~(-6)~1.077×10~(-6)),对全岩稀土元素配分型式基本上没有影响。在微量元素组成上,黑云母是元素Rb、Ba、Nb、Ta、V、Cr、co和Ni的主要载体,其Sr、Zr、Hf、Y、Sc、Th和U的含量明显低于全岩。黑云母成分反映矾山杂岩体岩浆体系以富Fe、高氧逸度为特征,这可能是该杂岩体发育具有经济价值铁矿床的原因之一。     

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     

Geochemistry of the J-M Reef of the Stillwater Complex, Minneapolis Adit Area II. Silicate Mineral Chemistry and Petrogenesis

The platinum group element (PGE) rich J-M Reef of the StillwaterComplex is associated with the reappearance of olivine and chromiteas cumulus phases within a sequence of gabbroic and noriticcumulates. The Olivine-Bearing Subzone (OBZ I) which hosts theReef is interpreted as the result of fresh influxes of magmainto the Stillwater chamber. Cumulus pyroxenes from the Norite Subzone (No II) overlyingOBZ I are enriched in Fe over Mg, depleted in Cr and have similarNi contents relative to pyroxenes from the underlying GNo I.The most primitive pyroxene compositions are found in PBc andBc layers within OBZ I, and the most evolved as oikocrysts inOBZ I anorthositic layers. Plagioclase An content correlatesclosely with pyroxene MgO/FeO, except within OBZ I where a verticalAn/En trend is observed. These observations may be synthesized into a model involvingreplenishment of the chamber by high pressure differentiatesof a liquid P, representing the parental magma to the UltramaficSeries of the Stillwater. Liquid P crystallizes olivine at thepressure of the Stillwater chamber, but owing to migration ofthe olivine-orthopyroxene phase boundary is saturated with bronziteat higher pressures. The spectrum of cumulus mineralogy andmineral compositions within and above OBZ I may be generatedby a series of magma influxes having compositions derived byvarious degrees of high pressure fractionation from liquid P. The characteristic textures of the heterogenous plagioclase-olivinecumulates of OBZ I may be explained by mixing of P-type replenishingliquid with a fractionated resident liquid containing suspendedplagioclase crystals. Anorthositic layers within OBZ I formedfrom influxes of liquid derived by approximately 30 per centfractionation of P. Model density calculations indicate that the liquids shouldhave entered the chamber as a serious of buoyant plumes. Lateralvariations in OBZ I stratigraphy arise from pulses of differentsize spreading to varying distances from the feeder axis. TheReef formed from a single layer transgressing earlier faciestransitions.     

Fluorides and Fluorcarbonates in Rocks of the Katugin Complex,Eastern Siberia: Indicators of Geochemical Mineral Formation Conditions

The paper discusses the chemical composition and parageneses of fluorides and fluorcarbonates in rocks of the Katugin Complex, with which a unique deposit of REE–Nb–Ta ore with cryolite is associated. In mineralogy and chemical composition, the rocks correspond to biotite, biotite–amphibole, arfvedsonite, and aegirine–arfvedsonite granites, which were regarded in earlier publications as granite-like metasomatic rocks. Aegirine–arfvedsonite granite contains a cryolite–gagarinite assemblage, which reflects depletion of Ca in the mineral-forming medium and enrichment in Na and F. Arfvedsonite granite is characterized by intergrowth of yttrofluorite with fluocerite and gagarinite, which indicates a relative enrichment in Ca and low CO₂ content. Biotite granite is characterized by an assemblage of fluorite with titanite, apatite, and monazite as evidence for an elevated Ca concentration along with moderate F and P contents in the system. Neighborite, coulsellite, gagarinite, fluocerite, and tveitite-(Y) appear in biotite–amphibole granite along with replacement of annite with riebeckite and development of albite after microcline. All this indicates that a moderately alkaline Na-fluoride solution with a low Ca concentration affects biotite granite.     

Geological and Geochemical Characterization of Iron-titanium Mineral Indices in the Tertiary Anorogenic Complex of Bana (Cameroon Volcanic Line)

Please?refer?to?the?attachment(s)?for?more?details.     

Crystallization History of the Upper Layered Series, Kap Edvard Holm, East Greenland

Field studies of the Upper Layered Series of the Kap EdvardHolm Complex indicate that consolidation occurred by accumulationof primocrysts near the floor of the magma chamber; nucleationoccurred continuously near the roof of the chamber and intermittentlynear the base. Over an exposed thickness of 4000 m the plagioclaseshows no detectable cryptic variation, and a hypothesis of combinedliquid and crystal fractionation processes is invoked to explainthis feature. The crystallization history of mafic intrusivesis considered on the basis of available liquidus and soliduscurves for anhydrous and water–saturated magmas.     

On the black cotton soils of North Cameroon

During detailed geotechnical prospecting in North Cameroon, extensive black cotton soil deposits were encountered. Based on classification tests, three samples were selected for mineralogical analysis. The results indicated that the predominant clay mineral was the halloysite governing the geotechnical properties and behaviour of the cotton soils. New correlations were established between clay mineral content and Atterberg limits of the investigated materials.     

Mineral Chemistry of Mildly Alkalic Basalts from the 25 Ma Mont Crozier Section, Kerguelen Archipelago: Constraints on Phenocryst Crystallization Environments

Phenocryst compositions and mineral–melt equilibria inthe mildly alkalic basalts from the 25 Ma Mont Crozier sectionon the Kerguelen Archipelago are used to estimate the depthsat which magmas stalled and crystallized and to constrain therole of crustal structure in the evolution of magmas producedby the Kerguelen mantle plume. The Crozier section, of nearly1000 m height, consists of variably porphyritic flows (up to21 vol. % phenocrysts), dominated by plagioclase ± clinopyroxene± olivine ± Fe–Ti oxides. Feldspars showan extreme range of compositions from high-Ca plagioclase (An₈₈)to sanidine and variable textures that are related to extensivefractionation, degassing, and mixing in relatively low-pressure(sub-volcanic) magma chambers. Although clinopyroxene is a minorphenocryst type (0–3 vol. %), its non-quadrilateral components,principally Al (1·9–8·6 wt % Al₂O₃), varywidely. The results of clinopyroxene–liquid thermobarometryand clinopyroxene structural barometry indicate that the Croziermagmas crystallized at pressures ranging from     

金刚石包裹体矿物化学特征与华北东部克拉通岩石圈地幔属性

华北地台金刚石的包裹体中含有丰富的地幔信息.对山东蒙阴、辽宁复县两地金刚石中包裹体矿物包括橄榄石、石榴石、单斜辉石和铬铁矿的化学成分进行了深入分析,金刚石包裹体矿物的主量元素显示华北地台至少在古生代金伯利岩侵位时具高度亏损玄武质的难熔克拉通岩石圈地幔特征.应用适用于石榴石橄榄岩相的单斜辉石温压计对金刚石包裹体中透辉石进行了计算,得出华北地台金刚石形成于1 083～1 194℃、5.3-6.1 GPa的地质环境下.蒙阴和复县两地金刚石包裹体所反映的古老岩石圈地幔虽然都表现为克拉通岩石圈地幔特征,但也存在一定的不均一性.同时,通过对华北东部古生代与新生代地幔矿物的主量元素及地幔热状态的对比,结合现有的地幔岩石学和地幔地球化学的研究成果,认为软流圈物质对古老岩石圈地幔的侵蚀作用能较好地解释华北岩石圈减薄.     

Geology and Mineral Chemistry of the Deosai Volcanics, Baltistan, N. Pakistan

GEOLOGY AND MINERAL CHEMISTRY OF THE DEOSAI VOLCANICS, BALTISTAN, N. PAKISTAN     

The Petrogenesis of the Niquelandia Layered Basic-Ultrabasic Complex, Central Goias, Brazil

The Niquelandia igneous layered complex is made up of a lowersequence (LS), consisting of a basal layered zone (gabbros,peridotites, and pyroxenites), followed by an ultramafic zone(peridotites and pyroxenites), followed in turn by layered gabbros,and an upper sequence (US), composed of coronitic gabbros andanorthosites and upper amphibolites. Its stratigraphy and theproportion between ultramafic and gabbroic units resemble thoseof other layered complexes of the world, e.g. Stillwater andBushveld. The LS and US have different fractionation trendsrelated to the different liquidus phases, viz. ol (?sp), cpxand opx in the LS and ol, pl, and cpx in the US. Differentiatesappear at the top of both the LS and US. The possibility thatthe Niquelandia complex is formed by two unrelated intrusionsis rejected and a model accounting for the fractionation patternis presented. This model implies that, during crystallization,water caused a decrease in the temperature of the liquidus ofthe magma in the upper portion of the chamber. Taking into account the shape of the geotherm and its intersectionswith the liquidus temperature gradient, a point was reachedat which crystallization may have occurred simultaneously inthe lower and upper portions of the magma chamber, leaving differentiatedresidua both at the roof and in the middle of the complex. The inferred composition of the parent magma is that of a low-Tipicritic basalt of possibly transitional character, poor inincompatible elements and rich in Cr and Ni. These characteristicsimply a derivation by extensive partial melting of a relativelydepleted source in a tensional environment. The compositionof the liquid and the occurrence of metamorphic xenoliths ofthe country-rock in the uppermost gabbros suggest that the complexintruded the continental crust during a rifting episode. Theabnormal thickness of the complex (14 500 m) implies a continentalcrust comparable in thickness to the present one.