Post-eruptive processes in the Campanian Ignimbrite

Summary ¶This paper deals with the mineralogical features of the most important and widespread pyroclastic formation of Southern Italy: the Campanian Ignimbrite. This formation is characterized by four stratigraphic units, different in colour and mineralogical composition. The lowermost unit (USAF) is constituted by an incoherent, thin (few tens of cm) sequence of glass or feldspar-rich layers. The middle part of the succession is formed by two main units: a lower grey, welded unit (WGI) capped by a yellow and lithified unit (LYT) that differ in both volcanological features and mineralogical composition. Locally these units are found separately. An incoherent unit (max thickness about 2m) mainly represented by fresh glass, forms the upper part of the succession. A detailed analysis has been carried out on six sections representative of the entire succession. The WGI is feldspathized by authigenic mineralization processes, and the feldspar content ranges between 30 and 90%. The LYT has been deeply affected by zeolitization processes and the total zeolite content often reaches 60%. Based on a comparative examination of both volcanological and mineralogical data, it is hypothesized that these two units were involved in different secondary mineralization processes. The portion of the succession characterized by the highest emplacement temperatures suffered welding and feldspathization and the result of these processes gave rise to the WGI unit. In contrast, the overlying lower-temperature and incoherent portion of the deposit was affected by circulation of meteoric water, thus enhancing zeolitization and lithification. The product of this process gave rise to the LYT unit.Received July 4, 2002; revised version accepted April 23, 2003     

中国东部黄山的成山过程及其构造意义

黄山是否存在过冰川是长期争议的科学问题,反对者的证据之一是黄山的海拔高度不能满足终年积雪的要求。这种认识源于地理学家对黄山成山过程的推测,没有考虑黄山岩体和区域地质演化所反映的成因信息。黄山岩体为一个主要由4个岩性单元组成的早白垩世复式小岩基,从早到晚、由边缘到中心依次为温泉细粒黑云母二长花岗岩、云谷寺连续不等粒粗粒黑云母正长花岗岩、狮子岭斑状黑云母正长花岗岩和贡阳山细粒黑云母碱长花岗岩组成。这种结构变化表明黄山地区发生过同侵位剥蚀作用,暗示岩浆侵位深度越来越小,受挤压构造力的驱动。与此相应,狮子岭单元的钾长石斑晶定向排列和云谷寺单元中的原生节理垂直于相邻两单元的接触界面,暗示黄山花岗岩具有强力侵位的特点。因此,黄山岩体形成于挤压构造环境,而不是前人推测的伸展环境。这种认识与区域构造变形和盆地分析的证据一致,暗示黄山地区曾经处于挤压造山环境,具有造山带型厚陆壳。对比岩浆起源深度和现今地壳厚度,可以合理地推测,晚白垩世以来黄山地区地壳减薄了约14～29 km,其地貌响应约为194～418 km。据此,黄山在早白垩世的海拔高度可能达到2 693～4 776 m。因此,黄山形成于地壳沉陷过程中的差异侵蚀,而不是地壳的阶段式隆升。     

A Pb isotope investigation of the Lovozero Agpaitic Nepheline Syenite,Kola Peninsula,Russia

For the first time Pb isotope composition was established in Lovozero rocks and raremetal ores, which is important for identifying their sources. The world’s largest layered intrusion of agpaitic nepheline syenite-the Lovozero alkaline massif—is located near the center of the Kola Peninsula in Russia. This superlarge complex plutonic body hosts the economically important loparite and eudiallyte deposits [1]. These deposits contain immense resources of REE, Nb, Ta, Zr, and constitute a world class mineral district. The Lovozero massif belongs to the Kola ultramafic alkaline and carbonatitic province (KACP) of Devonian age. Previous bulk rock studies have shown that the initial Sr and Nd isotope ratios of Lovozero rocks plot in the depleted mantle quadrant of Sr-Nd diagrams [2]. More recently, Hf isotope data obtained by Kogarko et al. (3) confirm that the Lovozero and Khibina massifs with ?_Hf between 6 and 8 are derived predominantly from a depleted mantle source. It was shown that Sr, Nd, and Hf abundances are significantly elevated in the Kola alkaline rocks, and thus their isotopic compositions are relatively insensitive to minor contamination by the overlying crustal rocks. By contrast, Pb in the KACP rocks is a much more sensitive indicator of a crustal component. In this paper we investigate the lead isotopic signature of all resentative types of Lovozero rocks (Table 1) in order to further characterize their mantle sources. The Lovozero massif consists of four intrusive phases. Rocks of phase I (mostly nepheline syenites) comprise about 5% of the total volume, phase II (urtites, foyaite, lujavrites) forms the main portion of the massif comprising 77% in volume, and phase III (eudialyte lujavrites) contributes about 18%. Country rocks are represented by Devonian effusive rocks and Archean gneisses.     

Mafic xenoliths from Egyptian Tertiary basalts and their petrogenetic implications

Mineralogical data, coupled with whole-rock major and trace element data of mafic xenoliths from two occurrences of the Egyptian Tertiary basalts, namely Abu Zaabal (AZ) near Cairo and Gabal Mandisha (GM) in the Bahariya Oases, are presented for the first time. Chemically, AZ basalts are sodic transitional, while those of GM are alkaline. In spite of the different petrographic and geochemical features of the host rocks, mafic xenoliths from the two occurrences are broadly similar and composed essentially of clinopyroxene, plagioclase, alkali feldspar, and Fe–Ti oxides. The analytical results of host rocks, xenoliths and their minerals suggest that the xenoliths are cognate to their host magmas rather than basement material. The mafic xenoliths are olivine-free and contain alkali feldspar contrary to the phenocryst assemblage of the host rocks, confirming that they are not cumulates from the host magma. The geochemical and mineralogical characteristics show that the precursor magmas of these xenoliths are more fractionated and possibly contaminated compared to those of the host rocks. Estimated crystallization conditions are  1–3 kbar for xenoliths from both areas, and temperature of  950–1100 °C vs. 920–1050 °C for AZ and GM, respectively. These cognate xenoliths probably crystallized from early-formed, highly-fractionated anhydrous magma batches solidified in shallow crustal levels, possibly underwent some AFC during their ascent, and later ripped-up during fresh magma pulses. The xenoliths, although rare, provide an evidence for the importance of crystal fractionation at early evolution of the Egyptian Tertiary basalts.     

U-Pb SHRIMP-II ages of titanite and timing constraints on apatite-nepheline mineralization in the Khibiny and Lovozero alkaline massifs (Kola Peninsula)

Results of this study of titanite samples collected from silicate rocks and apatite-nepheline-(sphene) ores from Paleozoic polyphase alkaline nepheline syenite complexes of the Khibiny and Lovozero massifs revealed the possibility of their in-situ U-Pb dating using sensitive high-resolution ion microprobe SHRIMP-II with an accuracy of 1.0-1.5%, which is comparable with that of U-Pb zircon analysis. Employing different approaches to age determination of the formation of the U-Pb system of titanites, the combined isochrons and mixing lines were plotted from the data obtained from the differentiated complex samples (121 analyses of five Khibiny samples and 52 analyses of one Lovozero sample) and apatite-nepheline ores (120 analyses of five Khibiny samples and 88 analyses of three Lovozero samples). They indicate synchronous crystallization of titanite in silicate rocks throughout the complexes: 374.1 ± 3.7 Ma for the Khibiny massif and 380.9 ± 4.5 Ma for the Lovozero massif, and attest to the later formation of phosphate-rare-metal ores: 371.0 ± 4.2 and 361.4 ± 3.2 Ma, respectively. The relatively delayed ore mineralization specific to the Lovozero massif can be accounted for the significantly lower volumes of magmatic melt and ore fluid involved, different thermal conditions, and the pattern of the investigated mineralization. As such, the obtained U-Pb data from titanite make it possible to limit significantly the time interval (most likely, not exceeding 15-20 Ma) comprising the evolution and activity of the ore-magmatic system of major agpaitic complexes, which is probably associated with plume magmatism.     

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.     

Early to Middle Jurassic (ca. 182–164 Ma) fractionated granitoids in the Korean Peninsula: Implication for the tectonomagmatic history of East Asia

The Jurassic granitoids (200–164 Ma) are distributed in the Korean Peninsula due to the Paleo-Pacific plate subduction. Early Jurassic (200–182 Ma) granitoids are mainly distributed in the southern Korean Peninsula. By contrast, Early to Middle Jurassic (182–164 Ma) granitoids are distributed in the central Korean Peninsula. In this study, we report detailed petrology, zircon U–Pb ages, and whole-rock geochemistry from the Seoul–Uijeongbu and Pocheon–Gimhwa pluton units in the central Korean Peninsula. The Seoul–Uijeongbu unit is dominated by biotite granite, with minor porphyritic biotite and garnet-biotite granite while the Pocheon–Gimhwa unit consists of biotite granite and porphyritic biotite granite, garnet-biotite granite, and two-mica granite. Zircon U–Pb age from those granites gives 180–167 Ma. The granitoids in the Pocheon-Gimhwa unit formed through fractional crystallization from biotite granite and porphyritic biotite granite to garnet-biotite granite, and two-mica granite based on gradually decreasing their Nb/Ta, Zr/Hf, and Eu/Eu* ratios. The strongly fractionated granitoids are garnet-biotite granite and two-mica granite. The LILE enrichment, Ta–Nb, Sr–P, and Eu–Ti troughs, and Ba depletion in most granitoids are similar to those of granitoids due to the subduction in the arc environment. Thus, these Jurassic granitoids (180–167 Ma) are mainly peraluminous granites with moderate crystal fractionation corresponding to I-type granite. Alkali feldspar granite associated with ore mineralization occurs in the Gwanaksan pluton from the southwestern Seoul–Uijeongbu unit. The alkali feldspar granite displays distinct negative Eu anomaly with high contents of Rb, Hf, Cs, and Nb compared with other granites. These characteristics imply that alkali feldspar granite experienced strong hydrothermal activity leading to feldspar ore mineralization compared to the other granites. The formation of a wide range of moderately evolved peraluminous granitoids is presumed to be related to rapid flat-subduction during 182–164 Ma, and the mineralization-related alkali feldspar granite indicates the termination of Jurassic granitoid magmatism in the central Korean Peninsula.     

Rate control in dissolution of alkali feldspars—I. Study of residual feldspar grains by X-ray photoelectron spectroscopy

Sanidine grains (100–600 μm in diameter) were subjected to dissolution at 82°C in aqueous electrolyte solutions of pH ranging from 4 to 8 for 293 or 377 hr. Dissolution equivalent to the removal of silica from the outer 300–900 A of these grains was accomplished. The shallow subsurfaces of feldspar grains were then analyzed for K, Al, and Si by X-ray photoelectron spectroscopy. The results rule out any continuous precipitate layer; if an alkali-depleted subsurface zone (leached layer) was present in the feldspar, the thickness of such a zone approximated by linear increase of alkali concentration with depth was not more than about 17 Å.It is concluded that in the absence of a compact precipitate layer, dissolution of feldspars in the temperature range corresponding to deep diagenesis is controlled by the processes at the feldspar-solution interface and a leached layer more than one feldspar unit cell thick does not form. Whether the same applies at the temperatures of shallow diagenesis and weathering cannot be judged with certainty, but parallels with leached layers on alkali silicate glasses suggest that it does.     

The Koshrabad granite massif in Uzbekistan: petrogenesis, metallogeny, and geodynamic setting

The Koshrabad massif, referred to as the Hercynian postcollisional intrusions of the Tien Shan, is composed of two rock series: (1) mafic and quartz monzonites and (2) granites of the main phase. Porphyritic granitoids of the main phase contain ovoids of alkali feldspar, often rimmed with plagioclase. Mafic rocks developed locally in the massif core resulted from the injections of mafic magma into the still unconsolidated rocks of the main phase, which produced hybrid rocks and various dike series. All rocks of the massif are characterized by high f (Fe/(Fe + Mg)) values and contain fayalite, which points to the reducing conditions of their formation. Mafic rocks are the product of fractional crystallization of alkali-basaltic mantle melt, and granitoids of the main phase show signs of crustal-substance contamination. In high f values and HFSE contents the massif rocks are similar to A-type granites. Data on the geochemical evolution of the massif rocks confirm the genetic relationship of the massif gold deposits with magmatic processes and suggest the accumulation of gold in residual acid melts and the rapid formation of ore quartz veins in the same structures that controlled the intrusion of late dikes. The simultaneous intrusion of compositionally different postcollisional granitoids of the North Nuratau Ridge, including the Koshrabad granitoids, is due to the synchronous melting of different crustal protoliths in the zone of transcrustal shear, which was caused by the ascent of the hot asthenospheric matter in the dilatation setting. The resulting circulation of fluids led to the mobilization of ore elements from the crustal rocks and their accumulation in commercial concentrations.     

Geochemical discrimination of metabasalt rocks of the Fan–Karategin transitional blueschist/greenschist belt, South Tianshan, Tajikistan: seamount volcanism and accretionary tectonics

The Fan–Karategin metamorphic belt, South Tianshan, Tajikistan, is regarded to be an ancient subduction–accretionary complex and is composed of three tectonostratigraphic units which display lithologies consistent with different tectonic settings. The mafic schists, which make up the major part of the older unit of the belt, contain both alkali and tholeiitic metabasalts. On the basis of rare-earth and other immobile element characteristics, the alkali metabasalts are akin to within-plate ocean island basalts, whereas the tholeiitic metabasalts resemble E-type MORB. The association is interpreted to have been formed on seamount-like structures under a within-plate plume. Bedded cherts and marbles in the unit are regarded as ancient pelagic sediments and carbonate caps developed upon basaltic seamounts, respectively. Dismemberment of the seamount-related basalts and pelagic sediments and the high-P/low-T prograde metamorphism of the unit rocks up to transitional blueschist/greenschist facies was the result of paleoseamount submergence into a subduction zone. This unit is tectonically overlain by arc-derived metavolcanic unit and a disrupted, mainly clastic unit of Upper Ordovician–Lower Silurian age. Metavolcanic and metasedimentary rocks of the two upper units have geochemical characteristics compatible with subduction-related origin. The lithological assemblages of the individual units and their juxtaposition suggest an origin involving collision–accretionary processes. The Fan–Karategin belt is a subduction–accretionary complex which formed during subduction of oceanic crust under a volcanic arc and was subjected to tectonic juxtaposition and imbrication of seamount, deep-sea, trench and volcanic arc sequences.     

Mineralogical,geochemical, and age characteristics of the rocks of the Inagli dunite-clinopyroxenite-shonkinite massif with platinum-chromite and Cr-diopside mineralization (Aldan Shield)

We consider the mineralogical and geochemical features of the rocks of the Inagli dunite-clinopyroxenite-shonkinite massif with platinum-chromite and unique jewelry Cr-diopside mineralization, which is a reference object of concentric zonal complexes. The massif rocks, from dunites to pulaskites, including peridotites, clinopyroxenites, shonkinites, and melanocratic alkali syenites, form a single continuous comagmatic series. This is confirmed by a clear dependence of the compositions of olivine, pyroxene, phlogopites, and Cr-spinels on the MgO content of the rocks and on the behavior of trace elements in them. The similar compositions of pyroxenes and trace-element patterns of clinopyroxenite rocks and Cr-diopsidite veins indicate a genetic similarity of these rocks. The age and mineralogical and geochemical compositions of the rocks and the geologic and morphological features of the intrusion prove that the Inagli massif formed from high-K picritoid melts, which underwent gradual decompression solidification during the ascent and formed a cylindrical diapir-like body at the subsurface level in the Early Cretaceous. The new portions of differentiates supplied from the lower horizons of the magma column determined the complex composition of the massif: It has a concentric zonal structure cut by numerous radial-circular vein bodies of pegmatites and pure anchimonomineral rocks (Cr-diopsidites), in places, of jewelry quality.     

Crystallization of loparite in alkaline fluid-magmatic systems (from experimental and mineralogical data)

We studied loparite-containing rocks (lujaurites, juvites, foyaite-juvites, etc.) sampled from a complex of differentiated rocks and, partly, from a complex of eudialytic lujaurites of the Lovozero alkaline massif. Zoned crystals of loparite (the zoning is due to variations in Ti, Nb, REE, Sr, and Th contents) were examined by microprobing. We also carried out experimental studies of loparite formation in complex silicate–salt systems including sodium carbonate, chloride, fluoride, or sulfate at 400–1200 °C and 1–2 kbar. They show that the composition of loparites depends on the physicochemical conditions of their formation (fluid composition) and that natural loparite can crystallize in a wide range of temperatures. The produced loparite crystals are zoned as a result of variations in Ti, Nb, La, Ce, Y, Ca, and Sr contents, which is probably related to the kinetic specifics of crystallization. Their zoning is similar to that of loparites of the Lovozero massif.     

Alkali-calcic and alkaline post-orogenic (PO) granite magmatism: petrologic constraints and geodynamic settings

The end of an orogenic Wilson cycle corresponds to amalgamation of terranes into a Pangaea and is marked by widespread magmatism dominated by granitoids. The post-collision event starts with magmatic processes still influenced by subducted crustal materials. The dominantly calc-alkaline suites show a shift from normal to high-K to very high-K associations. Source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle, affected by dehydration melting and generating more and more K- and LILE-rich magmas. In the vicinity of intra-crustal magma chambers, anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves. The post-collision event ends with emplacement of bimodal post-orogenic (PO) suites along transcurrent fault zones. Two suites are defined, (i) the alkali-calcic monzonite–monzogranite–syenogranite–alkali feldspar granite association characterised by [biotite+plagioclase] fractionation and moderate [LILE+HFSE] enrichments and (ii) the alkaline monzonite–syenite–alkali feldspar granite association characterised by [amphibole+alkali feldspar] fractionation and displaying two evolutionary trends, one peralkaline with sodic mafic mineralogy and higher enrichments in HFSE than in LILE, and the other aluminous biotite-bearing marked by HFSE depletion relative to LILE due to accessory mineral precipitation. Alkali-calcic and alkaline suites differ essentially in the amounts of water present within intra-crustal magma chambers, promoting crystallisation of various mineral assemblages. The ultimate enriched and not depleted mantle source is identical for the two PO suites. The more primitive LILE and HFSE-rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere. Present rock compositions are a mixture of major mantle contribution and various crustal components carried by F-rich aqueous fluids circulating within convective cells created around magma chambers. In favourable areas, PO suites pre-date a new orogenic Wilson cycle.     

Mid-ocean ridge and supra-subduction affinities in the Pindos ophiolites (Greece): implications for magma genesis in a forearc setting

The Pindos ophiolitic massif is considered an important key area within the Albanide–Hellenide ophiolitic belt and is represented by two tectonically distinct ophiolitic units: (1) a lower unit, including an intrusive and a volcanic section; and (2) an Upper Ophiolitic Unit, mainly including mantle harzburgites. Both units share similar metamorphic soles and tectono-sedimentary mélanges at their bases.

The intrusive section of the lower unit is composed by an alternation of troctolites with various ultramafic rock-types, including dunites, lherzolites, olivine-websterites, olivine-gabbros, anorthositic gabbros, gabbros and rare gabbronorites.

The volcanic and subvolcanic sequence of the lower unit can geochemically be subdivided into three groups of rocks: (1) basalts and basaltic andesites of the lower pillow section showing a clear high-Ti affinity; (2) basaltic andesites of the upper pillow section with high-Ti affinity, but showing many geochemical differences with respect to the first group; (3) very low-Ti (boninitic) basaltic and basaltic andesitic lava flows separating the lower and upper pillow sections, and dykes widespread throughout the Pindos ophiolites.

These different magmatic groups originated from fractional crystallization from different primary magmas, which were generated, in turn, from partial melting of mantle sources progressively depleted by previous melt extractions. Group 1 volcanics may have derived from partial melting (ca. 20%) of an undepleted lherzolitic source, while group 2 basaltic rocks may have derived from partial melting (ca. 10%) of a mantle that had previously experienced mid-ocean ridge basalt (MORB) extraction. Finally, the Group 3 boninites may have derived from partial melting (ca. 12–17%) of a mantle peridotite previously depleted by primary melt extraction of Groups 1 and 2 primary melts.

In order to explain the coexistence of these geochemically different magma groups, two petrogenetic models formerly proposed for the Albanian ophiolites are discussed.     

Potassium Chloride Treatment to Control Alkali Induced Heave in Black Cotton Soil

The results presented in this paper shows that high concentrations of sodium hydroxide causes abnormal changes on the volume change behaviour of illite–smectite (interstratified mineral) soil due to mineralogical changes. The higher swell that occurs is shown in the form of a new second stage of swelling. Increase in negative charges on soil particles and mineralogical changes after interaction with soil, respectively, are responsible for the swelling in these two stages. However, potassium hydroxide does not induce such high swelling in soils. This is mainly due to the fixation of potassium ions. Hence an attempt has been made to control the swelling induced by sodium hydroxide by making used of potassium chloride as an additive. Potassium fixation which is not substantial at neutral pH is favoured at higher pH Addition of potassium chloride salt solution (as 2 and 5% solution) can reduce only the first stage of swelling by linking the unit layers of mineral by reducing development diffuse double layer near clay surface. Potassium chloride is unable to prevent the formation of mineralogical alteration due to soil alkali interaction and hence the swelling associated with mineralogical changes. X-ray diffraction studies have revealed that mineralogical changes leading to formation of zeolite by soil alkali interaction is not inhibited by potassium ions. Morphological changes studied by scanning electron microscope corroborate these observations. Also the compressibility of soil which is increased in alkali solution is reduced in the presence of potassium salts. This reduction is due to reduction in the first stage of swelling.     

U-Pb zircon age of gabbroids of the Ust’-Belaya mafic-ultramafic massif (Chukotka) and its interpretation

The Ust’-Belaya mafic-ultramafic massif is assigned to the Western Koryak fold belt and largely composed of residual spinel peridotites, layered spinel and plagioclase peridotites, and gabbros. These rocks are crosscut by occasional plagiogranite and diorite veins and exhibit locally a close spatial association with basalts and carbonate-sedimentary deposits of Late Devonian and Early Carboniferous age. Based on this evidence, the massif was ascribed to the pre-Late Devonian ophiolite association. Our study presents new U-Pb SHPIMP II zircon ages and petrographic and mineralogical data on samples of the layered amphibole gabbro and vein diorite from the Ust’-Belaya massif. The approximate concordant U-Pb age corresponding to a timing of of amphibole gabbro crystallization is 799 ± 15 Ma, and the concordant U-Pb age reflecting a timing of of vein diorite crystallization is 575 ± 10 Ma. These ages coupled with geological studies of the massif, petrological and mineralogical investigations of the dated samples, as well as literature data on the petrology of peridotites and the age of formed plagiogranites suggest that the peridotites and layered gabbros of the Ust’-Belaya massif were formed by the Late Riphean, whereas the vein diorite and plagiogranite were resulted from a later (Vendian-Cambrian) magmatic stage. The peridotites and gabbros of the massif display no genetic relationship with spatially associated basalts and sedimentary rocks and, thus, they cannot be considered as members the pre-Late Devonian ophiolitic association. The results of this study will inevitably lead to a significant revision of geological and geodynamic interpretations of the Ust’-Belaya mafic-ultramafic massif. However, uneven study of the Precambrian complexes of the Koryak and Chukchi areas, their evolution in different structures of the region cannot yet be described by a single geodynamic scenario.     

Geochemical constraints on magma processes in a peralkaline system: The Paisano volcano,west Texas

Major and trace element data for a sequence of peralkaline silicic lavas and pyroclastic flows, exposed in the caldera wall of the Paisano volcano, west Texas, document systematic fractional crystallization during magmatic evolution and an open system, magma mixing event in the upper parts of the sequence. Stratigraphically lowest flows are comendite and comenditic quartz trachyte lavas and ash flow tufts. Overlying these units is a trachyte with compositional, textural and mineralogical features indicating that it is the product of magma-mixing; similar flows occur in other parts of the volcano at the same stratigraphic level. This composite trachyte is considered to be a mixture of mugearitic or mafic trachytic magma, derived from a similar source region which yielded the earlier caldera wall flows. Trace element concentrations of the post-trachyte comenditic quartz trachyte lavas suggest they were erupted from a chamber whose magma was diluted by an influx of mugearitic or mafic trachytic magma during a magma mixing event.Rayleigh fractionation calculations show that the comendites and comenditic quartz trachytes can be derived from a parental mugearite magma by 88% to 93% fractionation of dominantly plagioclase and alkali feldspar, with lesser amounts of clinopyroxene, magnetite and apatite. Zircon was not a significant fractionating phase. The composition, mineralogy and depth of the source region(s) which generated these magmas cannot be constrained from the present data set.     

Mineralogical,micromorphological and geochemical transformations in the initial steps of the weathering process of charnockite from the Caparaó Range,southeastern Brazil

X-ray diffraction (XRD), X-ray Fluorescence (XRF), optical microscopy, Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry (SEM-EDS) and Electron Probe micro-analyser (EPMA) and Wavelength-Dispersive Spectroscopy (WDS) were conducted on charnockite from the Caparaó Suite and its alteration cortex to determine the mineralogical, micromorphological and geochemical transformations resulting from the weathering process. The hydrolysis of the charnockite occurred in different stages, in accordance with the order of stability of the minerals with respect to weathering: andesine/orthopyroxene, pargasite and alkali feldspar. The rock modifications had begun with the formation of a layer of incipient alteration due to the percolation of weathering solutions first in the pressure relief fractures and then in cleavage and mineral edges. The iron exuded from ferromagnesian minerals precipitated in the intermineral and intramineral discontinuities. The layer of incipient alteration evolves into an inner cortex where the plagioclase changes into gibbsite by direct alitisation, the ferromagnesian minerals initiate the formation of goethitic boxworks with kaolinitic cores, and the alkali feldspar initiates indirect transformation into gibbsite, forming an intermediate phase of illite and kaolinite. In the outer cortex, mostly traces of alkali feldspar remain, and they are surrounded by goethite and gibbsite as alteromorphics, characterising the formation of the isalteritic horizon that occurs along the slope and explains the bauxitization process at the Caparaó Range, SE Brazil.     

Experimental determination of partition coefficients for Rb,Sr and Ba between alkali feldspar and silicate liquid

Partitioning of Rb, Sr and Ba between alkali feldspar and a synthetic granitic melt has been determined at 8 kb and 720 to 780°C for a single quaternary granite composition. The results suggest that Henry's law is obeyed by Rb up to ~0.8 wt.% Rb₂O in both the liquid and in the alkali feldspar. The measured D values for Rb range from 0.77 to 1.1. For Ba, Henry's Law is obeyed up to ~0.6 wt.% BaO in the liquid and ~5 wt.% BaO in the alkali feldspar. D values for Ba range from 6.4 to 14. For Sr there is only a crude relationship between concentration in the liquid and concentration in the alkali feldspar at concentrations greater than ~0.6 wt.% SrO in the liquid and ~0.4 wt.% SrO in the alkali feldspar. D values for Sr range from 1.2 to 5.0. Partitioning of Sr is apparently sensitive to the concentration of Ba in the system and this partly explains the failure of Sr to obey Henry's Law.Linear least-squares fits to the partitioning data as a function of temperature suggest inverse correlation between temperature and D values. Rb shows only a slight temperature effect whereas Ba and Sr appear to be rather strongly affected by temperature, but the temperature range examined here is small compared to the scatter in the data making these trends relatively uncertain. Other factors that appear to affect partitioning, especially of Sr, are growth rate, development of sector zoning and Or content of the alkali feldspar. These factors severely limit the use of partitioning of these elements in alkali feldspar as geothermometers.The technique for measuring growth rates utilized here combined with measurement of trace element depletion in diffusion boundary layers adjacent to the alkali feldspar crystals makes it possible to estimate diffusivities for Ba and Sr. These estimates suggest a difference of 2 orders of magnitude between diffusivities for Ba and Sr in a vapor-saturated melt and those measured by HOFMANN and MAGARITZ (1976) for a dry obsidian glass.     

Geochemistry of fractionation of coherent elements (Zr and Hf) during the profound differentiation of peralkaline magmatic systems: A case study of the Lovozero Complex

The paper presents pioneering data on Hf distribution in peralkaline rocks, ores, and rock-forming and accessory minerals of the Lovozero Complex. Variations in the Zr/Hf ratio are determined in all rocks of the Lovozero alkaline massif. This ratio is proved to increase in the course of evolution of alkaline magma because of fractionation of alkaline pyroxene. The Hf distribution coefficient is evaluated for alkali-rich pyroxene, whose crystallization controls Zr and Hf fractionation during the differentiation of alkaline magma. These data and the equation of equilibrium and fractional crystallization are utilized in a model suggested for Zr and Hf fractionation in the course of evolution of the Lovozero intrusion.