Petrogenesis of the gabbros from Mt. St. Hilaire,Quebec, Canada

Mt. St. Hilaire occurs as a small funnel-shaped intrusion in the Monteregian petrographic province of Quebec and consists of alkali gabbros and later nepheline syenites. Based on field relations, petrography, and geochemistry, five types of gabbro are recognized. In order of intrusion these are: leucogabbro, foliated gabbro, kaersutite-biotite gabbro, kaersutite gabbro, and a gabbro-melagabbro series. Based on analyses of the early-forming ilmenite-titanomagnetite, the gabbros crystallized under high fO₂ conditions which lead to subsequent crystallization of olivines with high MnO contents. Fractionation of ilmenite and titanomagnetite was a major control on the Ti and A[TV]concentrations in the clinopyroxenes. Plagioclase compositions in the gabbros became richer in Ab contents in the sequence gabbro-melagabbro to leucogabbro. Whole-rock analyses suggest that the parental magma of alkali basaltic composition was fairly evolved prior to emplacement. Lack of olivine in the cumulate gabbro-melagabbros and low Ni and Cr in all gabbros may reflect either extreme olivine fractionation and/or a very low olivine content in the source material for these basalts. Differentiation of the gabbros occurred both pre- and post-emplacement, probably by a process of crystal-liquid fractionation at depths between 3-5 and 8 km. This is in accordance with geophysical measurements for other Monteregian intrusions. A model is presented for the mechanism of emplacement.     

Phase Equilibria Constraints on Relations of Ore‐bearing Intrusions with Flood Basalts in the Panxi Region,Southwestern China

There are two types of temporally and spatially associated intrusions within the Emeishan large igneous province (LIP); namely, small ultramafic subvolcanic sills that host magmatic Cu-Ni-Platinum Group Element (PGE)-bearing sulfide deposits and large mafic layered intrusions that host giant Ti-V magnetite deposits in the Panxi region. However, except for their coeval ages, the genetic relations between the ore-bearing intrusions and extrusive rocks are poorly understood. Phase equilibria analysis (Q-Pl-Ol-Opx-Cpx system) has been carried out to elucidate whether ore-bearing Panzhihua, Xinjie and Limahe intrusions are co-magmatic with the picrites and flood basalts (including high-Ti, low-Ti and alkali basalts), respectively. In this system, the parental magma can be classified as silica-undersaturated olivine basalt and silica-saturated tholeiite. The equivalents of the parental magma of the Xinjie and Limahe peridotites and picrites and low-Ti basalts are silica-undersaturated, whereas the Limahe gabbro-diorites and high-Ti basalts are silica-saturated. In contrast, the Panzhihua intrusion appears to be alkali character. Phase equilibria relations clearly show that the magmas that formed the Panzhihua intrusion and high-Ti basalts cannot be co-magmatic as there is no way to derive one liquid from another by fractional crystallization. On the other hand, the Panzhihua intrusion appears to be related to Permian alkali intrusions in the region, but does not appear to be related to the alkali basalts recognized in the Longzhoushan lava stratigraphy. Comparably, the Limahe intrusion appears to be a genetic relation to the picrites, whereas the Xinjie intrusion may be genetically related to be low-Ti basalts. Additionally, the gabbro-diorites and peridotites of the Limahe intrusion are not co-magmatic, and the former appears to be derived liquid from high-Ti basalts.     

Monteregian Hills II. Petrography, Major and Trace Element Geochemistry, and Strontium Isotopic Chemistry of the Eastern Intrusions: Mounts Shefford, Brome, and Megantic

The Monteregian Hills consist of a series of alkaline intrusionsand associated dikes and sills emplaced along a linear west-easttrend extending from 35 km west of Montreal, Quebec, to 190km east of Montreal. The igneous activity occurred between 117Ma and 141 Ma and the age data show a distinct bimodality, withstrongly undersaturated magmas emplaced at c. 118 Ma and slightlyundersaturated magmas emplaced at c. 136 Ma. The eastern intrusionslargely consist of gabbros, diorites, and a variety of felsicrocks as compared to the more mafic character of the westernintrusions which largely consist of pyroxenites, gabbros, anddiorites. The eastern intrusions were emplaced through a thicksequence of folded and faulted geosynclinal sediments whichseems to have played a role in determining their felsic character. Mounts Brome and Shefford are located approximately 80 km eastof Montreal. The dominant rock type at Mount Shefford is a dioritewhich has been intruded by arcuate bodies of pulaskite and nordmarkite.Mount Brome consists of an outer crescent-shaped gabbro body,composed of several cyclical units, and a large syenite bodydivided into a slightly quartz-undersaturated unit (pulaskite)and a slightly oversaturated unit (nordmarkite). The two unitsare chemically similar, but the nordmarkite has elevated initialSr ratios suggesting that crustal contamination is responsiblefor its oversaturation. Both complexes have been intruded bynepheline-bearing diorites, and at Mount Brome, foyaites, tinguaites,and laurdalites. Mount Megantic is located approximately 190km east of Montreal and consists of an outer normarkite annulus,separated by a gabbro-dioritic body from an inner granite plug. Five different magma types have been identified in the easternMonteregian Hills. Type 1, precursor to the gabbros and syenitesat Mounts Shefford and Brome, is an alkali picrite generatedby limited partial melting of a garnet lherzolite, with subsequentevolution controlled by the removal of olivine, pyroxene, andplagioclase. Type 2, precursor to the gabbros and nordmarkitesat Mount Megantic, is apparently produced by moderate partialmelting of a spinel lherzolite, and this magma subsequentlyevolved to a quartz-saturated residuum. Type 3, which producedthe rocks of the strongly undersaturated series, is equivalentto the basanite magmas generated by small degrees of meltingof a spinel lherzolite, which formed similar rocks in the westernMonteregian Hills. The high concentration of incompatible elementsand high-charge-density cations in these magmas indicates thatthe mantle source regions were enriched in these elements. Type4, Mount Shefford nordmarkite, and Type 5, Mount Megantic granite,magmas apparently originated in the crust by partial meltingof, respectively, amphibolite or granulite facies metadioriteand metagraywacke sources.     

Undersaturated lavas from Ambittle Island,Papua New Guinea

Ambittle is part of a gently arcuate chain of Quaternary volcanic islands east of the Bismarck Archipelago. Lavas include basanities, tephrites, ankaramitic lavas and trachyte. Phenocrysts of olivine, clinopyroxene and plagioclase are found in bauonite along with hauyne. Olivine and less commonly hauyne are absent from tephrite while amphibole and biotite are common. Nodules in the lavas are composed of phenocryst phases and are thought to represent crystal clots or cumulates. Low Ti, high Al, low Ni and Cr and high Sr, Rb mark these lavas as being very different from basanites of intra-continental regions. Suggested parallels are shoshonitic lavas from the Aeolian Islands and feldspathoidal lavas from Indonesia. Although the structure in the region of Ambittle is not well known it seems likely that the magmas were linked to tectonic processes along a subducting plate margin. The presence of basalts of transitional affinity in other islands of the chain suggests a petrochemical link with the undersaturated types. Derivation of undersaturated lavas form partial melts formed in the mantle only seems possible if ol + cpx are fractionated at moderate pressures in order to enrich residual liquids in Al. If the undersaturated lavas are related to a parental transitional tholeiite magma then fractionation processes alone are insufficient to explain their composition and processes of volatile differentiation may have occurred.     

Upper mantle source for some hawaiites,mugearites and benmoreites

Chemical analyses of over seventy lavas or dykes containing spinel lherzolite inclusions of high pressure mineralogy, show that most host magmas are of alkali olivine basalt or basanite composition with relatively rare olivine nephelinites, and olivine melilitites. The 100 Mg/Mg+Fe⁺⁺ ratios of host magmas display a strong maximum at about Mg₇₀ consistent with partial melting of source peridotite with olivine of Fo_88–90. In contrast to these primary magmas, there occur some host magmas with 100 Mg/Mg+Fe⁺⁺<60 and with chemical compositions resembling those of classical hawaiite, mugearite, and nepheline benmoreite magmas. It is inferred that these magmas have been produced by crystal fractionation, within the upper mantle, of parental basanites or alkali olivine basalts. The presence of kaersutitic hornblende xenocrysts accompanying the lherzolite inclusions, and the nature of the chemical variation between associated basanites and nepheline benmoreites suggests that crystal fractionation has been dominated by kaersutitic hornblende, together with olivine and, in some cases, probably clinopyroxene. The mantle-derived nepheline benmoreite magmas also show similarities to some plutonic nepheline syenites.     

Petrologic variations in quaternary volcanic rocks,British Columbia,and the nature of the underlying upper mantle

Volcanic activity has produced Late Tertiary and Quaternary cinder cones and flows between the Snake River Plain, U.S.A. and the Yukon Territory, Canada. The rock types include basanites, alkali olivine basalts, high-iron basalts, hawaiites, ankaramites, nephelinites, and olivine tholeiites. The alkali olivine basalts, basanites and hawaiites sampled are chemically similar to rocks from the mid-Atlantic islands. Associated with the volcanic rocks are xenoliths of ultramafic rocks, gabbros, granites and granulites.Seismic data indicate that the Moho throughout the region dips eastward at a very shallow angle. The low velocity zone has been located beneath southern British Columbia and displays a topographic high trending northwest-southeast. The nephelinite was erupted from near the crest of this high with less undersaturated lavas erupted from along its flanks.The suite of ultramafic xenoliths spans a greater variety of rock types than can be generated by maximum amounts of partial melting of a uniform source material to produce the lavas in the region. Calculated residual olivine compositions in equilibrium with the magmas at low velocity zone depths and liquidii temperatures are more iron-rich than the typical lherzolite xenolith olivine. This suggests that the residua from the partial melting episodes which produced the volcanic rocks are different from the upper mantle lid above the low velocity zone as represented by the ultramafic xenoliths.     

The Serra da Graciosa A-type Granites and Syenites, southern Brazil: Part 2: Petrographic and mineralogical evolution of the alkaline and aluminous associations

The Serra da Graciosa Granites and Syenites comprise five distinct plutons in the Brasiliano/Pan-African Graciosa A-type Province, southern Brazil. Six petrographic series can be identified in these plutons: (1) Alkaline series 1, composed of amphibole-bearing alkali feldspar syenites with varied mafic mineralogy and quartz contents, from alkali feldspar syenites with calcic amphibole, clinopyroxene, olivine and allanite to alkali feldspar quartz syenites with sodic–calcic amphibole and chevkinite–perrierite and to alkali feldspar granites with sodic amphibole; (2) Alkaline series 2, characterized by amphibole-bearing alkali feldspar granites, with limited modal variations but amphibole compositions also varying from calcic to sodic; (3) Alkaline series 3, of limited occurrence, which includes alkali feldspar syenites with olivine and clinopyroxene and no amphibole; (4) Aluminous series 1, of widespread occurrence, with various petrographic facies of biotite granites with amphibole; (5) Aluminous series 2, characterized by alkali feldspar granites with biotite and only minor amphibole; (6) Monzodiorites, typically with biotite, calcic amphibole and augitic clinopyroxene, partially mingled with granitic magmas. The mafic minerals present are, in general, Fe-rich with correspondingly low Mg and Al contents. In Alkaline series 1, amphiboles crystallized in progressively more oxidizing and alkaline conditions, while in Alkaline series 2, the initial conditions were somewhat more oxidizing and shifted to reducing in the final stages. In Aluminous series 1 and Aluminous series 2, amphiboles are calcic and comparatively homogeneous. The amphiboles in the monzodioritic rocks, while also homogeneous, are more Mg-rich and show compositions quite distinct from the calcic varieties in the other associations, and this is also the case for clinopyroxene. Mg# in biotite decreases from the monzodioritic rocks to Aluminous series 1 and further to Aluminous series 2. Contrasting evolution of the various associations suggests that several coeval magmatic series are present in the Serra da Graciosa granites.     

Nd isotopes demonstrate the role of contamination in the formation of coexisting quartz and nepheline syenites at the Abu Khruq Complex,Egypt

The petrogenesis of Abu Khruq, an 89 Ma alkaline ring complex of eastern Egypt which is composed of alkali gabbros and both silica over- and undersaturated syenites, has been investigated. Major and trace element relationships and Nd and Sr isotope data are consistent with formation of the gabbros from an alkaline mafic magma that experienced extensive fractionation, and all syenites from a felsic derivative of this melt. The parental magma had an ⁸⁷Sr/⁸⁶Sr of 0.7030 and an ¹⁴³Nd/¹⁴⁴Nd of 0.512750 (_Nd = +4.4) indicating derivation from a depeleted mantle source. The initial ¹⁴³Nd/¹⁴⁴Nd ratios are: 0.512721 to 0.512748 for the gabbros, 0.512739 to 0.512750 for the alkali syenites and trachytes, 0.512717 to 0.512755 for the nepheline syenites, and, 0.512706 to 0.512732 for the quartz syenites. In contrast, analyzed Precambrian granites from eastern Egypt have generally lower ¹⁴³Nd/¹⁴⁴Nd ratios (ranging from 0.51247 to 0.51261 or _Nd = -0.8 to 1.7, for 89Ma); their Nd model ages range from 775 to 935 Ma and suggest there was no significant input of pre-Pan-African crust in their formation. Among Abu Khruq rocks, ¹⁴³Nd/¹⁴⁴Nd ratios indicate that the quartz syenites formed by open-system, crustal contamination processes whereas the nepheline syenites experienced little or no contamination. Modeling shows that contamination occurred at various stages, affecting both mafic and more evolved compositions with input of about 20% crustal Nd for the most contaminated samples. The degree of contamination is related to the silica saturation of the quartz syenites. Simplified modeling of magma evolution within Petrogeny's Residua System demonstrates the ability of AFC processes to cause a critically undersaturated magma to evolve across the feldspar join and produce oversaturated rocks. The oversaturated syenites at Abu Khruq were produced in this manner whereas the nepheline syenites formed by fractionation without similarly large degrees of contamination. The results have broad implications for the formation of subvolcanic complexes in continental settings beyond the important production of silica oversaturated compositions from crustal interaction. They underscore the importance of crustal interactions in the formation of the various lithologies. Such interactions occur at various stages in the evolution of the magmas and, as such, are not strictly coupled with fractional crystallization. While previous study of Abu Khruq has demonstrated extensive hydrothermal alteration of O and Sr isotopes, the present work shows that the Nd isotope ratios were not significantly affected and thus reflect magmatic signatures. This feature combined with relatively small corrections for initial ratios emphasizes the utility of Nd isotopes for petrogenetic studies.     

Minor-element and Sr-isotope geochemistry of tertiary stocks,Colorado mineral belt

Rocks of the northeast portion of the Colorado mineral belt form two petrographically, chemically and geographically distinct rock suites: (1) a silica oversaturated granodiorite suite; and (2) a silica saturated, high alkali monzonite suite. Rocks of the granodiorite suite generally have Sr contents less than 1000 ppm, subparallel REE patterns and initial ⁸⁷Sr/ ⁸⁶Sr ratios greater than 0.707. Rocks of the monzonite suite are restricted to the northeast part of the mineral belt, where few rocks of the granodiorite suite occur, and generally have Sr contents greater than 1000 ppm, highly variable REE patterns and ⁸⁷Sr/⁸⁶Sr initial ratios less than 0.706.Despite forming simple, smooth trends on major element variation diagrams, trace element data for rocks of the granodiorite suite indicate that they were not derived from a single magma. These rocks were derived from magmas having similar REE patterns, but variable Rb and Sr contents, and Rb/Sr ratios. The preferred explanation for these rocks is that they were derived by partial melting of a mixed source, which yielded pyroxene granulite or pyroxenite residues.The monzonite suite is chemically and petrographically more complex than the granodiorite suite. It is subdivided here into alkalic and mafic monzonites, and quartz syenites, based on the textural relations of their ferromagnesian phases and quartz. The geochemistry of these three rock types require derivation from separate and chemically distinct magma types. The preferred explanation for the alkalic monzonites is derivation from a heterogeneous mafic source, leaving a residue dominated by garnet and clinopyroxene. Early crystallization of sphene from these magmas was responsible for the severe depletion of the REE observed in the residual magmas. The lower Sr content and higher Rb/Sr ratios of the mafic monzonites requires a plagioclase-bearing source.The Sr-isotope systematics of the majority of these rocks are interpreted to be largely primary, and not the result of crustal contamination. The positive correlation of Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios for the least fractionated samples indicate that the sources from which parent magmas of both the granodiorite and monzonite suites were derived are Precambrian in age.     

Geochemistry of phonolites and trachytes from the summit region of Mt. Kenya

Two suites of felsic eruptives and intrusives are represented in a set of samples from the summit region of the Plio-Pleistocene volcano, Mt. Kenya. Most of the samples are moderately or strongly undersaturated and have ⁸⁷Sr/⁸⁶Sr initial ratios in the range 0.70360–0.70368 (mean=0.70362). Members of this phonolitic suite are phonolites, nepheline syenites or kenytes and as a group they show a wide variation in TiO₂, FeO, P₂O₅, Sr, Ba, Zr and Nb. The minor and trace element geochemistry reflect variation in the nature of the parental basaltic magmas from which the phonolitic rocks evolved and variation in the crystal fractionation process in individual cases. Crystal fractionation involving plagioclase, alkali feldspar, clinopyroxene, olivine and magnetite is the process by which most of the phonolitic rocks evolved and variation in the relative proportions of these phases in individual cases has led to a broad spectrum of trace and minor element behaviour. The second suite of felsic samples is critically saturated and consists of trachytes showing either slight oversaturation or slight undersaturation with respect to SiO₂. This trachyte suite has lower initial ⁸⁷Sr/⁸⁶Sr ratios (mean=0.70355) and is derived from transitional alkalic basalts by low pressure (crustal) crystal fractionation involving feldspar, clinopyroxene, magnetite and olivine. The range in minor and trace element chemistry observed among the felsic rocks is a consequence of variation in the parental basalts which is related to mantle source variation and to the specific nature of the crystal fractionation process.     

Phase Equilibria Constraints on Relations of Ore-bearing Intrusionswith Flood Basalts in the Panxi Region, Southwestern China

There are two types of temporally and spatially associated intrusions within the Emeishan large igneous province (LIP); namely, small uitramafic subvolcanic sills that host magmatic Cu-Ni-Platinum Group Element (PGE)-bearing sulfide deposits and large mafic layered intrusions that host giant Ti-V magnetite deposits in the Panxi region. However, except for their coeval ages, the genetic relations between the ore-bearing intrusions and extrusive rocks are poorly understood. Phase equilibria analysis (Q-PI-OI-Opx-Cpx system) has been carried out to elucidate whether ore-bearing Panzhihua, Xinjie and Limahe intrusions are co-magmatic with the picrites and flood basalts (including high-Ti, low-Ti and alkali basalts), respectively. In this system, the parental magma can be classified as silica-undersaturated olivine basalt and silica-saturated tholeiite. The equivalents of the parental magma of the Xinjie and Limahe peridotites and picrites and iow-Ti basalts are silica-undersaturated, whereas the Limahe gabbro-diorites and high-Ti basalts are silica-saturated. In contrast, the Panzhihua intrusion appears to be alkali character. Phase equilibria relations clearly show that the magmas that formed the Panzhihua intrusion and high-Ti basalts cannot be co-magmatic as there is no way to derive one liquid from another by fractional crystallization. On the other hand, the Panzhihua intrusion appears to be related to Permian alkali intrusions in the region, but does not appear to be related to the alkali basalts recognized in the Longzhoushan lava stratigraphy. Comparably, the Limabe intrusion appears to be a genetic relation to the picrites, whereas the Xinjie intrusion may be genetically related to be low-Ti basaits. Additionally, the gabbro-diorites and peridotites of the Limahe intrusion are not co-magmatic, and the former appears to be derived liquid from high-Ti basalts.     

The M?gantic Unstrusive Complex, Qu?bec: a Study of the Derivation of Silica-Oversaturated Anorogenic Magmas of Alkaline Affinity

The Cretaceous M?gantic intrusive complex of southern Qu?beccontains early noritic gabbrodiorites which represent cumulatesfrom crustally contaminated hawaiite to syenite magmas. Wholerock and mineral chemistry, as well as textural evidence, indicatethat post-cumulus recrystallization and reaction were important,and most of the amphibole and biotite are thought to have formedin this way. A younger plutonic quartz-syenite ringdyke maynot be cogenetic with the gabbro-diorites sice it lacks orthopyroxene.It may, however, be cogenetic with basaltic to riebeckite granitedykes. Fractionation of olivine, plagioclase, aluminous clinopyroxene,and minor Ti-magnetite from critically undersaturated alkalibasaltic magmas generated hawaiitic magmas. The developmentof quartzbearing mugearitic and syenitic residua from the hawaiitescan best be modelled by fractionation of amphibole, plagioclase,olivine, oxides, and apatite. Attempts to model fractionationusing observed phenocrysts (including clinopyroxene) were unsuccessful.Amphibole fractionation is interpreted to have taken place througha reaction with still-porous, higher-temperature cumulates onthe walls of the magma chamber. The plutonic syenites probablyrepresent alkali feldspar cumulates from the residual syeniticmelts. Magnesian calc-alkaline lamprophyres exhibit olivineto phlogopite reaction textures, are enriched in Cr, Ni, K,Rb, Nb, Y, Zr, and Si relative to the basaltic dykes, yet havesimilar incompatible element ratios. Their relation to the basaltsis problematical. The late biotite-granite core to the complexis identical to typical White Mountain granites and may haveformed as an anatectic cap on rising, fractionating, mantlederivedmagmas.     

Petrology of middle proterozoic diabases and picrites from Lake Nipigon,Canada

Mafic rocks at Lake Nipigon provide a record of rift-related continental basaltic magmatism during the Keweenawan event at 1109 Ma. The mafic rocks consist of an early, volumetrically minor suite of picritic intrusions varying in composition from olivine gabbro to peridotite and a later suite of tholeiitic diabase dikes, sheets and sills. The diabase occurs primarily as two 150 to 200 m thick sills with a textural stratigraphy indicating that the sills represent single cooling units. Compositional variation in the sills indicates that they crystallized from several magma pulses.The diabases are similar in chemistry to olivine tholeiite flood basalts of the adjacent Keweenawan rift, particularly with respect to low TiO₂, K₂O and P₂O₅. The picrites have higher TiO₂, K₂O and P₂O₅ than the diabases and are similar to, but more primitive than, high Fe-Ti basalts which erupted early in the Keweenawan volcanic sequence.All of the rocks crystallized from fractionated liquids. The picrites are cumulate rocks derived at shallow crustal depths from a magma controlled predominantly by olivine fractionation. Picritic chills are in equilibrium with olivine phenocrysts of composition Fo₈₀ and are interpreted to represent the least evolved liquids observed. The parental magma of the picrites was probably Fe rich relative to the parental magma of the diabase. The diabase sills crystallized from an evolved basaltic liquid controlled by cotectic crystallization of plagioclase and lesser olivine and pyroxene.The emplacement of dense olivine phyric picritic magmas early in the sequence, followed by later voluminous compositionally evolved magmas of lower density suggests the development of a crustal density filter effect as the igneous event reached a peak. Delamination of the crust-mantle interface may have resulted in the transition from olivine controlled primitive magma to fractionated magma through the development of crustal underplating.     

The volcanic-subvolcanic rocks of the fernando de noronha archipelago,southern atlantic ocean: Mineral chemistry

Fernando de Noronha archipelago presents an older Remédios Formation with subvolcanic intrusions, belonging to two different alkaline series, the sodic (undersaturated: basanites, tephrites, essexites, tephriphonolites, phonolites), and potassic ones (mildly undersaturated to silicic, with alkali basalts, basaltic trachyandesites, trachyandesites, trachytes), and lamprophyres. The upper Quixaba Formation presents nephelinite flows and basanites. A third minor unit, São José, is constituted by basanites carrying mantle xenoliths. Magnesian olivines occur in the Remédios basanites and alkali basalts, and in nephelinites. Melilites are present as groundmass grains in melilite melanephelinites (MEM). Clinopyroxenes (cpx) are mostly salites to titaniferous salites (Remédios sodic series), grading into aegirines in the differentiated aphyric phonolites. Cpx in the lamprophyres show disequilibrium textures. In the Quixaba flows, cpx are salites, enriched in Mg (especially in MEM). Amphiboles, remarkably, are common in tephriphonolites and phonolites and in basaltic trachyandesites, sometimes with disequilibrum zoning textures, and a conspicuous phase in lamprophyres. Dark micas are present as groundmass plates in MEM, OLM and PYM (olivine and pyroxene melanephelinites), with compositional variety (enriched in Ti, Ba, Sr) depending on the composition of the parent rock; BaO can be as high as 16–19%. Feldspars crystallize as calcic plagioclases, sanidines and anorthoclases, depending on the rock types, as phenocrysts and in groundmass, both in Quixaba and Remédios rocks; they are absent in nephelinites. Nephelines are found in Remédios sodic series types and Quixaba rocks. Haüyne and noseane are rarely observed in Remédios rocks.     

Petrology of the Nandewar Volcano,N.S.W., Australia

The Nandewar Mountains, N.S.W., Australia, are the remains of a Miocene continental alkaline volcano whose products range from olivine basalts to comendites and alkali rhyolites. Intermediate hawaiites, mugearites and benmoreites predominate in the shield, in which olivine basalts are rare, and the trachytic rocks form many intrusions into the shield. The Nandewar alkaline series shows extreme fractionation of a relatively differentiated alkali olivine basalt magma, saturated with silica, to yield extremely oversaturated peralkaline comendites and peraluminous alkali rhyolites. The nature of the ferromagnesian phases forming was controlled by low oxygen fugacities. Throughout the series clinopyroxenes range from diopsidic augite, through sodic ferrohedenbergites to hedenbergite-acmite solid solutions. Riebeckite-arfvedsonite solid solutions appear in the trachytes and comendites, and aenigmatite appears in some of the peralkaline rocks. The feldspars in the series fractionate from calcic labradorite through potash oligoclase and calcic anothoclase towards the minimum melting alkali feldspar composition, Ab₆₅Or₃₅. The compositions of the alkali rhyolites approach the minimum in the system SiO₂-KAlSi₃O₈-NaAlSi₃O₈. All the mineralogical and chemical evidence points to the development of the Nandewar series by the processes of extreme crystallization differentiation of an alkali olivine basalt parent magma. No significant contamination occurred, xenoliths and xenocrysts are absent, and volatile transfer and metasomatism played a minor role.     

The mineral chemistry and origin of xenoliths from the lavas of Anjouan,Comores Archipelago,Western Indian Ocean

Mineralogical data for xenoliths occurring as inclusions in the fissure erupted alkali basalts and the basanitic tuffs of Anjouan reveal three xenolith suites: 1) the lherzolites, 2) the dunites and wehrlites, 3) the gabbros and syenites. The dunite-wehrlite suite and the gabbro suite are shown to represent high-level cumulate sequences resulting from ankaramitic fractionation of the hy-normative shield-building lavas and cotecictic fractionation of the alkali basalt lavas respectively, whilst the syenitic xenoliths represent evolved high-level intrusions. Mineralogical and rare earth element (REE) data indicate that the most likely origin for the spinel lherzolite xenoliths is by extraction of a basaltic phase from spinel peridotite, leaving a light REE-poor spinel lherzolite residuum. REE models, constructed using model peridotite assemblages, imply that the hy-normative basalt lavas may be derived by partial melting of spinel peridotite at pressures of <20–25 kb leaving a residual lherzolite, and that the alkali basalt and basanite melts are formed by small degrees of melting of a garnet-peridotite source at pressures of >20–25 kb. The spinel lherzolite source for the hy-normative basalts has been accidentally sampled during explosive eruption of the alkali basalt and basanite magmas.     

云南个旧西区贾沙辉长-二长岩体SIMS锆石U-Pb定年及地球化学研究

为确定云南省个旧地区晚中生代大规模岩浆活动过程中基性端元的时限、地幔源区特点及大地构造环境,选取贾沙辉长．二长岩体为对象进行年代学和地球化学研究。贾沙辉长．二长岩体位于个旧西区,岩性主要为辉长岩和二长岩。锆石U．Pb同位素测年结果表明,岩石侵位时代为（84．0＋0．6）Ma,属于晚白垩世,与个旧地区花岗岩、碱性岩和煌斑岩形成年代范围一致（76～85Ma）。贾沙岩体的辉长岩和二长岩Si02为47．3％～60．O％,K20＋Na20为7．31％～10．1％。稀土含量较高,轻稀土富集重稀土亏损,Eu异常不明显。相对于原始地幔,贾沙岩体富集轻稀土和大离子亲石元素Rb、K、Pb,亏损高场强元素Nb、Ta、Ti和P。地球化学研究显示贾沙岩体母岩浆起源于与俯冲有关的交代地幔,由石榴子石二辉橄榄岩经历了较低程度的（〈5％）部分熔融形成。原始岩浆在就位过程中经历了广泛的地壳混染和橄榄石、辉石的分离结晶作用。二长岩由辉长岩浆结晶分异作用形成。研究显示,贾沙辉长．二长岩体是晚白垩世滇东南．桂西地区大规模岩浆活动的产物,表明这些岩浆岩形成于统一的岩石圈伸展的动力学背景下。     

An Anorthosite Suite in a Ring-Complex: Crystallization and Emplacement of an Anorogenic Type from Abontorok, A?r, Niger

Laminated anorthosite grading outwards into leucogabbro, gabbro,and monzogabbro occurs in a 2.6-km-diameter funnel-shaped intrusion,cut by a quartz alkali syenite plug and concentric syenite andgranite ring-dykes. The anorthosite-gabbro series is laminatedbut not modally or otherwise texturally layered. The lamination,defined by large tabular plagioclase crystals, forms a set ofinwarddipping cones, the dips of which decrease from 60–45?in the central anorthosite to < 25? in the outer gabbros.Rocks close to the outer contact are medium-grained isotropicgabbros. Plagioclase, forming >80% of the series, generallyhas homogeneous labradorite cores (An_62–58 in the wholeseries) and thin strongly zoned rims, which follow progressivelylonger solidus paths from the anorthosites to the gabbros. Allrocks contain a late-magmatic alkali feldspar. Plagioclase isthe main or only cumulus phase, the anorthosites being ad- tomesocumulates and the gabbros orthocumulates. Olivine (FO_49–41)is more abundant than clinopyroxene in most of the series. Dependingon quartz content, the syenites and granites are hypersolvusor subsolvus and the depth of crystallization was calculatedto be 5 ? 2 km. A Rb/Sr isochron for the syenites and granites gave an age of399 ? 10 Ma with an initial strontium isotopic ratio of 0.7084? 0.0005. Ten samples from the anorthosite-gabbro scries havean average calculated initial ratio of 0.70582 ? 0-00004 at– 400 Ma, showing that the two series are not comagmatic.The anorthosite-gabbro series has parallel REE trends (La_N/Yb_N{small tilde} 7–10) with decreasing positive Eu anomaliesand increasing total REE contents from anorthosite to gabbro;two monzogabbros have almost no Eu anomaly. The liquid calculatedto be in equilibrium with the lowest anorthosite has almostno Eu anomaly and its normalized REE pattern lies just abovethose for the monzogabbros. The syenites and granites have complementaryREE patterns with negative Eu anomalies. The inferred parental magma was alkalic and leucotroctoliticwith high TiO₂ P₂O₅, Sr and K/Rb and with low MgO, very similarto parental magmas in the Gardar province, South Greenland.It was probably produced at depth by settling of olivine andclinopyroxene but not of plagioclase, which accumulated by flotation.It is suggested that plagioclase crystals from this lower chamberwere progressively entrained (from 0% in the gabbros to 30–40%in the anorthosites), giving rise to the flow lamination inthe upper chamber. The magma in the lower chamber may have beenlayered, because the plagioclase cores in the anorthosite areconsiderably richer in Or than those in the leucogabbros orgabbros. Overall convection did not occur in the upper chamber,whereas compositional convection occurred in the more slowlycooled central anorthositic adcumulates.     

Strongly undersaturated Tertiary volcanic rocks from the Kangerdlugssuaq area, east Greenland

Erratic blocks from the interior of Kangerdlugssuaq fjord are described with chemical analyses and microprobe data for their constituent minerals. A strongly peralkaline phonolite, unlike any previously known from the province, shows that some magmas which precipitated alkali feldspar, nepheline and amphibole (magnesio-katophorite) differentiated along a trend towards increasing peralkalinity, although this trend is unknown in the nearby plutons. Feldspar-free nephelinite and less undersaturated limburgite are also encountered, which apparently provided a petrographic link between the strongly undersatrated ultramafic rocks recently discovered in the area and the alkali basalts of Prinsen af Wales Bjerge.

Recognition of these rock types provides further important analogies with the plume-generated triple junctions of the African Rift and affords samples approximating closely to the compositions of the more unusual magmas of the area.     

Geology of Pulikonda and Dancherla alkaline complexes,Andhra Pradesh

Geological studies on saturated to oversaturated and subsolvus aegirine-riebeckite syenite bodies of the Pulikonda alkaline complex and Dancherla alkaline complex were carried out. The REE distribution of the Dancherla syenite shows a high fractionation between LREE and HREE. The absence of Eu anomaly suggests source from garnet peridotite. The Pulikonda syenite shows moderate fractionation between LREE and HREE as reflected by enrichment of HREE and moderate enrichment of LREE. The negative Eu anomaly indicates role of plagioclase fractionation.Three distinct co-eval primary magmas i.e. mafic syenite-, felsic syenite- and alkali basalt magmas — all derived from low-degrees of partial melting of mantle differentiates and enriched metasomatised lower crust played a major role in the genesis and emplacement of the syenites into overlying crust along deep seated regional scale trans-lithospheric strike-slip faults and shear zones following immediately after late-Archaean calc-alkaline arc magmatism at different time-space episodes i.e. initially at craton margin and later on into the thickened interior of the Eastern Dharwar craton. The ductile sheared and folded Pulikonda alkaline complex was evolved dominantly from the magmas derived from partial melting of lower crust and minor juvenile magmas from mantle. Differentiation and fractionation by liquid immiscibility of mafic magma and commingling-mixing of intermediate and felsic magmas followed by fractionational crystallisation under extensional tectonics during waning stages of calc-alkaline arc magmatism nearer to the craton margin were attributed as the main processes for the genesis of Pulikonda syenite complex. Commingling and limited mixing of independent mantle derived mafic and felsic syenitic magmas and accompanying fractionation resulting into soda rich and potash rich syenite variants was tentatively deduced mechanism for the origin of Dancherla, Danduvaripalle, Reddypalle syenites and other bodies belonging to Dancherla alkaline complex at the craton interior. The Peddavaduguru syenite was formed by differentiation of alkali mafic magma (gabbro to diorite) and it’s simultaneous mingling with fractionated felsic syenitic magma under incipient rift. Vannedoddi and Yeguvapalli syenites were derived due to desilicification and accompanying alkali feldspar mestasomatism of younger potash rich granites along Guntakal-Gooty fault and along Singanamala shear zone respectively.