Salic Magmas Research Articles

Alkaline–Salicic Kurung Complex of the Ket-Kap–Yuna Igneous Province (Aldan Shield, Russia): Petrogenesis and Geodynamic Conditions of Formation

Abstract —Integrated geological data, petrogeochemical characteristics of magmatites of the Kurung complex (the Ket-Kap–Yuna igneous province of the Aldan Shield), and previously published isotopic data along with the results of geochronology and geochemistry research provide evidence for this complex evolving in the Late Cretaceous in a setting of riftogenesis (final phase) of continental margins. Similar to the Late Jurassic–Early Cretaceous polyformational volcano-plutonism of the Aldan Shield, the Late Cretaceous alkaline magmatism in the Ket-Kap–Yuna province is associated with the tectonomagmatic activation which manifested itself mainly as continental margin rifting associated with deep reorganization of the southwestern, southern and southeastern framing of the Siberian craton triggered by gravitational sliding of the lithospheric plates. Results of the petrogeochemical analysis of the studied magmatites revealed that the heterogeneous rock groups identified within this rock complex are not related to each other by fractional or other type of differentiation, thus suggesting their possible derivation as a result of fluid syntexis-type interaction between mantle-derived alkali–basite melt and felsic crustal melts. Alkali–basite parent melts are shown to have formed during partial melting of variably enriched mantle which is widely spread beneath the Aldan Shield and is varied in composition (from BSE enriched to nearly EM-I). Origination of such deep-seated melts (magma sources) was associated with the rifting processes (strike-slip tectonics) triggered by the setting of gravitational sliding of lithospheric plates. The formation of alkaline–salic magmas of the Kurung complex is associated with large-scale selective assimilation of crustal material by high-temperature fluidized mantle magmas during their ascent to the surface, possibly as a result of fluid syntexis of alkaline–basite mantle magmas and crustal smeltings formed under their influence.

Petrogenesis of composite dikes in granitoids of western Transbaikalia

The performed studies have revealed two varieties of composite dikes differing in morphology and internal structure depending on (1) the proportions of salic and basic components and (2) the rheologic state of the host environment. The latter can be both a solid substrate with open fractures and a melt at different stages of crystallization. The evaluated isotopic age of dikes in the Shaluta massif, 290.8 ± 2.7 Ma and 283.4 ± 3.4 Ma, is correlated with the time of the pluton formation. The age of the composite dike breaking through the metamorphic deposits on the eastern shore of Lake Baikal is 284.10 ± 0.96 Ma. The mass formation of composite dikes in western Transbaikalia is correlated with the Late Paleozoic magmatism, which resulted in one of the Earth’s largest granitoid provinces. The intrusion of dikes was not a single-stage event; it lasted at least 10-12 Myr and was apparently related to the repeated intrusion of large volumes of salic magmas and the formation of granitoid plutons. Early Mesozoic composite dikes associated with alkali-granitoid plutons of the Late Kunalei igneous complex (230-210 Ma) are much scarcer. Basic magmas of the composite dikes were generated at depths greater than 75 km as a result of the melting of a modified (enriched in crustal components) mantle source. Salic components of the dikes, independently of their geologic position, are generally similar in composition to the granitoids of the plutonic facies, and the differences are apparently due to hybridization proceeding at great depths.© 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Numerical modeling of the mechanisms of magma mingling and mixing: A case study of the formation of complex intrusions

This paper describes a thermomechanical mathematical model of magma mingling and mixing during the formation of complex intrusions and presents the first results of numerical modeling. The model considers one-pulse intrusion of mafic or intermediate melts into a granitoid magma chamber. The model is based on literature data on the composition and structure of two polychronous intrusions: the Burgas quartz syenite massif and the Magadan granitoid batholith. The modeling shows that the main parameter controlling the convection regime is the density difference. The density and viscosity contrasts of interacting magmas during mingling and mixing are estimated. Depending on the density difference, one of the possible processes dominates: In the case of a small difference (less than 30-40 kg/m ), magma mixing and hybridization in a small contact zone takes place; in the case of a large difference (100 kg/m3 or more), magma mingling predominates. The viscosity contrast, in turn, determines whether interpenetration of melts or fragmentation of melts in the form of drops, spheres, etc. occurs. There is a limiting viscosity of salic magma (10 Pas) at which the flows freeze in the chamber and further cooling occurs with a slowly moving fluid. The time of formation of mingling structures is estimated to be several days to several hundred years, depending on the initial melt viscosity.

MINGLING PROCESSES IN THE EARTH'S CRUST: GEOLOGICAL OBSERVATIONS AND MATHEMATICAL SIMULATION

This issue of Geodynamics & Tectonophysics presents the state-of-the-art studies of mingling processes and investigations of the interactions between the mafic and acidic melts of contrasting compositions at various depths of the Earth's crust. The diagnostic features and the genesis of magmatic and synmetamorphic mingling are discussed with respect to the reference objects of Altaides and Uralides. Several papers report on the results of mathematical simulation used to discover the mechanism for lifting the high-density basite inclusions in a chamber or a dyke filled with salic magma. This special issue is intended for researchers and specialists in geology, petrology and geodynamics of magmatic processes in the crust, as well as for lecturers, post-graduate students and university students.

NUMERICAL SIMULATION OF MAGMA MINGLING (CASE OF BAYANKOL GABBRO‐GRANITE SERIES, SANGILEN, TUVA)

A new numerical model has been developed that makes it possible to describe the process of formation of a dyke of a combined composition on the basis of the dynamics of a viscous compressible fluid. The numerical thermo‐mechanical model shows the processes of magma mingling and taking into account multiphase interaction of melts which are different in composition and properties. The models suggest a mechanism for uplifting of high‐density mafic enclaves in a chamber/dyke filled with salic magma by gravitational floating in the enclosing gran‐ ite magma that has been cooled and lost volatile components. The performed simulation shows that the main parame‐ ter controlling the shape and size of the ascending bodies is the difference in densities. The viscosity contrast determines whether interpenetration and hybridization of magmas occur. The limiting ratio of felsic material in the mix‐ ture, which is capable of uplifting denser mafic enclaves, is estimated. The duration of melt uplifting in combined dykes is estimated with respect to the viscosity parameters. At a typical rate of 2–3 km per year, it amounts to almost 12 months.

Petrogenesis of cogenetic silica-oversaturated and -undersaturated syenites of Abu Khruq ring complex, South Eastern Desert, Egypt

The upper Cretaceous Abu Khruq ring complex (ARC) is located in the South Eastern Desert of Egypt displays concentric zonation of syenitic rocks from quartz-rich syenite at the margin, through alkali feldspar syenite to nepheline syenite in the centre. The syenitic rocks occur with nepheline monzogabbro, volcanic rocks (phonolite and trachyte) and the quartz- and nepheline-bearing pegmatites. Rocks of contrasting composition (mafic and salic) exhibit sophisticate geometric relationships. The nepheline monzogabbroic rocks have pillowy xenoliths forms within the salic (nepheline syenite and quartz alkali feldspar syenite) rocks, suggesting synchronous emplacement of the mafic and salic magmas. Clinopyroxene analysis of mafic and salic plutonic rocks of the ARC revealed that the overall pyroxene trend suggesting that fractionation involved a late, progressive increase in Na, in a reaction of the type Ca Mg Fe2+↔Na Fe3+. The chemistry of the analysed amphiboles are compositionally similar to those from typical differentiated peralkaline suites. Geochemically, the complex is enriched in the LILE, HFSE and REE. The concentrations of the compatible elements (V, Sr and Ba) generally decrease with increasing silica, consistent with fractional crystallization. A generalised increase in the Nb/Ta from the nepheline monzogabbro to nepheline syenite compositions is attributed to titanite fractionation. All the rock samples show relative increment of the LREE content than the HREE indicating weak to steep fractionated REE patterns (La/Yb) from 9.43 to 10.86, and thus retaining the geochemical characteristics of anorogenic suites. The magma sources of ARC are not derived from normal primitive mantle. The early stages of differentiation involved extensive olivine and pyroxene fractionation, the fractionation of amphibole, titanite, magnetite, apatite and feldspar may have been involved in the genesis of the salic differentiated compositions. The deviation towards silica-oversaturated members attributed to crustal assimilation and associated with fractional crystallization of the basic magma at a relatively early stage during the development of the complex. The whole complex is deduced to have originated from alkali basaltic to basanitic magma of OIB-like character. The general increasing of the amphibole and biotite among the different rocks of the complex reflects the hydrous nature of ARC magmas.

Composition, sources, and mechanisms of origin of rare-metal granitoids in the Late Paleozoic Eastern Sayan zone of alkaline magmatism: A case study of the Ulaan Tolgoi massif

The Ulaan Tolgoi massif of rare-metal (Ta, Nb, and Zr) granites was formed at approximately 300Ma in the Eastern Sayan zone of rare-metal alkaline magmatism. The massif consists of alkaline salic rocks of various composition (listed in chronologic order of their emplacement): alkaline syenite → alkaline syenite pegmatite → pantellerite → alkaline granite, including ore-bearing alkaline granite, whose Ta and Nb concentrations reach significant values. The evolution of the massif ended with the emplacement of trachybasaltic andesite. The rocks of the massif show systematic enrichment in incompatible elements in the final differentiation products of the alkaline salic magmas. The differentiation processes during the early evolution of the massif occurred in an open system, with influx of melts that contained various proportions of incompatible elements. The magma system was closed during the origin of the ore-bearing granites. Rare-metal granitoids in the Eastern Sayan zone were produced by magmas formed by interaction between mantle melts (which formed the mafic dikes) with crustal material. The mantle melts likely affected the lower parts of the crust and either induced its melting, with later mixing the anatectic and mantle magmas, or assimilated crustal material and generated melts with crustal–mantle characteristics. The origin of the Eastern Sayan zone of rare-metal alkaline magmatism was related to rifting, which was triggered by interaction between the Tarim and Barguzin mantle plumes. The Eastern Sayan zone was formed in the marginal part of the Barguzin magmatic province, and rare-metal magmas in it were likely generated in relation with the activity of the Barguzin plume.

Complex subvolcanic magma plumbing system of an alkali basaltic maar-diatreme volcano (Elie Ness, Fife, Scotland)

Alkali basaltic diatremes such as Elie Ness (Fife, Scotland) expose a range of volcanic lithofacies that points to a complex, multi-stage emplacement history. Here, basanites contain phenocrysts including pyrope garnet and sub-calcic augites from depths of ~60km. Volcanic rocks from all units, pyroclastic and hypabyssal, are characterised by rare earth element (REE) patterns that show continuous enrichment from heavy REE (HREE) to light REE (LREE), and high Zr/Y that are consistent with retention of garnet in the mantle source during melting of peridotite in a garnet lherzolite facies. Erupted garnets are euhedral and unresorbed, signifying rapid ascent through the lithosphere. The magmas also transported abundant pyroxenitic clasts, cognate with the basanite host, from shallower depths (~35–40km). These clasts exhibit wide variation in texture, mode and mineralogy, consistent with growth from a range of compositionally diverse melts. Further, clinopyroxene phenocrysts from both the hypabyssal and pyroclastic units exhibit a very wide compositional range, indicative of polybaric fractionation and magma mixing. This is attributed to stalling of earlier magmas in the lower crust — principally from ~22 to 28km — as indicated by pyroxene thermobarometry. Many clinopyroxenes display chemical zoning profiles, occasionally with mantles and rims of higher magnesium number (Mg#) suggesting the magmas were mobilised by juvenile basanite magma. The tuffs also contain alkali feldspar megacrysts together with Fe-clinopyroxene, zircon and related salic xenoliths, of the ‘anorthoclasite suite’ — inferred to have crystallised at upper mantle to lower crustal depths from salic magma in advance of the mafic host magmas. Despite evidence for entrainment of heterogeneous crystal mushes, the rapidly ascending melts experienced negligible crustal contamination. The complex association of phenocrysts, megacrysts and autoliths at Elie Ness indicates thorough mixing in a dynamic system immediately prior to explosive diatreme-forming eruptions.

Late Paleozoic gabbroids of western Transbaikalia: U-Pb and Ar-Ar isotopic ages, composition, and petrogenesis

We provide new isotope-geochronological evidence for the synchronous occurrence of Late Paleozoic basic and granitoid magmatism in western Transbaikalia; this is a strong argument for the contribution of mantle magmas to granitoid petrogenesis. The Late Paleozoic basic rocks originated from the phlogopite-garnet-bearing lherzolitic mantle, which melted under “hydration conditions.” The specific features of Late Paleozoic magmatism in western Transbaikalia were determined by the combination of the activity of a low-energy mantle plume with the final stage of the Hercynian orogeny in space and time. At the early stage of magmatism, during the formation of the Barguzin granites,the plume had only a thermal influence on the crustal rocks heated as a result of Hercynian fold-thrust deformations. The mixing of mantle basic and crustal salic magmas at different levels marked the transition from crustal to mixed (mantle-crustal) granites, which include all post-Barguzin complexes (probably, except for alkali granites). In the geologic evolution of Transbaikalia, the Late Paleozoic magmatism was postorogenic, but it was initiated and influenced by the mantle plume.

The Mull Paleocene dykes: some insights into the nature of major dyke swarms

At about 1000 km long and with a magmatic volume exceeding 850 km 3 , the Paleocene Mull Dyke Swarm qualifies as a giant dyke swarm. As such, it offers important insights into the nature of such swarms. Magma flowage laterally from the Mull Central Complex is supported by evidence of recent magma flowage in rift-related volcanic areas. Conduits were frequently used by more than one magma pulse at different times. Pre-heating of conduits by basaltic magma enabled salic magmas to propagate for >400 km from Mull. The Southern Upland Fault had a major influence on dyke propagation in the southern sector of the swarm, variably arresting magma migration or diverting magma vertically or laterally, in one case for 16 km laterally. A significant role for crustal heterogeneity in controlling dyke dynamics is advocated. Recent models point to important interactions between the dykes and adjacent Paleocene central complexes. The dyke swarm can potentially contribute significant information on the evolution of the Mull plumbing system, such as the formation of compositional zoning and the role of magma input in chamber rupture.

Geochemistry and petrology of Mesozoic-Cenozoic magmatic complexes of the southern framing of the Aldan shield: Geodynamic problems

This paper summarizes the results of long-term geological, petrological, and geochemical investigations of the Mesozoic-Cenozoic complexes of the Stanovoy Range in order to determine the main reasons for their generation and evolution. The analysis of this material showed that the compositionally variable Late Mesozoic igneous complexes of the Stanovoy Range were formed in various depth facies, from abyssal to surficial. The majority of their salic complexes show minor compositional variations, whereas the mafic complexes are more variable, especially in the southeast of the region. The southeastern Stanovoy Range comprises comparable amounts of both subalkaline and low-alkali igneous rocks, whereas the central part is dominated by subalkaline rocks, and the northwestern part contains rocks only of the shoshonite-latite series. This zoning is fundamentally different from that of typical island arcs, which are characterized by the occurrence of volcanic rocks of similar alkalinity in each zone. Extrusive and intrusive rocks with similar alkali and silica contents (and schlieren-like inclusions in the granitoids of the region) were formed from common magmas of corresponding chemical compositions. In addition, the mafic and most of the salic magmas were formed as independent melting products, whereas the magmas of intermediate composition were formed mainly by mixing of chemically contrasting liquids (i.e., salic and basic). It was shown that the available information on the magmatism of the region is best interpreted in terms of the model of mantle diapirism. In particular, mantle diapirs ascended rather slowly during the Mesozoic and occurred over the whole territory of the Stanovoy Range during the Jurassic-Cretaceous stage (J3-K1), when alkaline and subalkaline basalts were formed. During the Early-Late Cretaceous stage, mantle diapirs produced alkali-poor basalts in the central and eastern parts. During the Cenozoic, the diapir ascended rather rapidly but only in a small area in the eastern part of the region forming alkali basalts. In contrast to the Cenozoic, the Earth’s crust was strongly affected by mantle diapirs and related mafic magmas in the Mesozoic. As a result, crustal sequences were reworked by fluids and subsequently yielded tremendous volumes of compositionally corresponding salic magmas, which interacted and mixed with mafic magmas producing the corresponding chemical zoning. The maximum generation of crustal magmas was confined to the axial zones of ascending diapirs, where the highest energy effects took place, whereas the role of autochthonous gneissic granites increased away from the axis at the expense of typical intrusive complexes.

The central Kenya peralkaline province: Insights into the evolution of peralkaline salic magmas

The central Kenya peralkaline province comprises five young (< 1 Ma) volcanic complexes dominated by peralkaline trachytes and rhyolites. The geological and geochemical evolution of each complex is described and issues related to the development of peralkalinity in salic magmas are highlighted. The peralkaline trachytes may have formed by fractionation of basaltic magma via metaluminous trachyte and in turn generated pantellerite by the same mechanism. Comenditic rhyolites are thought to have formed by volatile-induced crustal anatexis and may themselves have been parental to pantelleritic melts by crystal fractionation. The rhyolites record very low temperatures of equilibration (≤ 700 °C) at low fO 2 (≤ FMQ). The development of compositional zonation within the magma reservoirs has been ubiquitous, involving up to tens of cubic km of magma at timescales of 10 3–10 4 years. Magma mixing has also been commonplace, sometimes between adjacent centres. Isotopic evidence relating to rates and timescales of pre-eruptive residence times and crystal fractionation processes is summarized.

Initial gold contents of silicic volcanic rocks: Bearing on the behavior of gold in magmatic systems

Many fresh silicic volcanic rocks have markedly lower initial gold contents than previously recognized. Of 129 carefully selected glassy silicic volcanic rocks analyzed, 113 contain f (O 2 ) more readily accommodate gold. The mean of 23 relatively silicic intermediate rocks is 0.54 ppb Au, with tholeiitic andesites (icelandites) generally higher in gold than calc-alkalic types. There is little evidence that particular geologic regions are intrinsically richer in gold than others. Bulk composition, melt structure, and the amount and timing of separation of vapor, mineral, and sulfide and/or metal melt phases would appear to largely determine the gold contents of silicic magmas. The various lines of evidence for the removal of gold from magma by the separation of vapor, crystalline, and immiscible melt phases indicate that fresh volcanic rocks provide only minimum limits on magmatic gold concentrations. In most cases, magmatic values will be much higher. Elevated gold contents (0.3 to >1.0 ppm) of some porphyry deposits suggest the existence of salic magmas containing gold in concentrations appreciably greater than the 1-2+ ppb concentrations of basalts.

Evolution of the Tugtutôq Central Complex, South Greenland: A high-level, rift-axial, late-Gardar centre

Abstract The Tugtutoq Central Complex (TCC) comprises at least six intrusions emplaced by ring-faulting and/or stoping. These intrusions are of syenites, quartz-syenites and granites, with peralkaline affinities, crystallized at relatively shallow depths, probably as a subvolcanic complex. The intrusions transect a major dyke-swarm marking the axial zone of a narrow mid-Proterozoic continental rift. The TCC appears to have formed from a succession of increasingly fractionated magma batches which are inferred to have arisen from a differentiating parent magma body near the base of the crust. A culminating perthosite intrusion (Bla Mane So Syenite) joined the two separate early intrusive centres to form a composite body elongate along the rift axis. A Rb-Sr whole-rock isochron indicates an age of 1124 ± 20 Ma, with a (87Sr/86Sr)i of 0.70345. The magmas were relatively anhydrous (although probably halogen-rich) producing hypersolvus rocks. Olivines range from Fo9Fa88Tp3 to FoOFa95Tp5 and pyroxenes from Di35Hd62Ac3 to virtually pure aegirine. Relative to members of the preceding dyke swarm (regarded as fast-quenched magmas derived from the same deep crustal site of differentiation) the TCC units are enriched in alkali-feldspar. The TCC rocks are interpreted in part as alkali-feldspar-rich roof-facies cumulates and in part as products of salic magmas selectively depleted in Fe-rich ferromagnesian minerals by gravitative separation. As with the salic members of the dyke swarm, the TCC rocks exhibit marked negative Eu anomalies. The TCC is inferred to be underlain by layered meso- to melanocratic syenite cumulates comparable to those seen in some of the other South Greenland alkaline intrusions.

History of Tertiary igneous activity in the N Atlantic borderlands

Summary Stretching and thinning of the Laurasian continental lithosphere, which had proceeded intermittently from late Palaeozoic through the Mesozoic, reached a climax in early Tertiary times with copious generation of basalt magma, invasion of the attenuated crust as dyke swarms and surface eruption, principally by fissure volcanism. Continued basin subsidence allowed basaltic lavas, erupted at near sea-level, to accumulate to thicknesses which in places (e.g. W Greenland, E Greenland, Faeroes) attained several kilometres. In some zones dyke-swarm injection and crustal attenuation led to sea floor spreading and generation of ocean crust. Fissure swarms not infrequently changed position so that there could be one or more ‘unsuccessful’ rifting events prior to establishment of a ‘successful’ spreading axis. While some uncertainty still exists regarding the precise timing of the onset of magmatism in the various zones of failure, the bulk occurred between 60 and 50 Ma. Across the British Isles basalt magmatism occurred early (ca. 60-59 Ma) whereas the great volumes of the E Greenland basalts appear to have erupted rapidly over the interval 54-52 Ma. The onset of magmatism in the Faeroes may have preceded that in E Greenland and possibly that of W Greenland-Baffin Island. Major sill swarms developed, commonly within the Mesozoic sedimentary strata. At favoured channels for magma ascent, commonly controlled by major faults, longer-lived central-vent volcanoes developed. Slow cooling of large magma bodies at such foci produced mafic and ultramafic cumulates; production of salic magmas was mainly confined to these central complexes, the majority of which are located close to the E Greenland coast between ca. 66° and 74°N and a N-S zone through the British Isles from ca. 58°-51°N. Although the salic magmas were predominantly silicic (rhyolitic), feldspathoidal (phonolitic) magmas were important in some of the E Greenland centres. Generation of salic magmas was generally late with respect to the main phases of basalt eruption in the various sectors of activity. However, notable sequences of peraluminous silicic lavas may have preceded basaltic eruption in the Rockall Trough and Vøring Plateau areas. It is probable that magmatic activity in some of the sectors that had experienced the most intense early-phase volcanism persisted, on a small-scale, long after the sea floor spreading had been initiated and activity on the passive margins of both W and E Greenland may not have finally terminated until ca. 30 Ma. The total compositional range of early Tertiary magmas associated with the N Atlantic marginal regions was extensive. Whereas tholeiites, varying from FeTi-rich to N-type MORB, were erupted in greatest volume, compositions varied from picrites, through tholeiites to ferro-basalts, icelandites, dacites and rhyolites. More alkalic mafic magmas included alkali olivine-basalt (and its differentiated products), nephelinites, lamprophyres and melilitites, with some associated carbonatites. Salic magmas ranged widely from silicic to feldspathoidal and from peraluminous to peralkaline.

The Tugtutôq older giant dyke complex: mineralogy and geochemistry of an alkali gabbro-augite-syenite-foyaite association in the Gardar Province of South Greenland

AbstractThe Older Giant Dyke Complex is a differentiated alkaline intrusion of Proterozoic age (1154±16 Ma) and is the earliest of the late Gardar intrusions in the Tugtutôq-Ilímaussaq region. The dyke is approximately 20 km long by 0·5 km broad and comprises (i) marginal ‘border group’ rocks of alkali olivine gabbro, grading inwards to ferro-syenogabbro and (ii) an axial ‘central group’ of salic rocks ranging from augite syenite in the WSW to sodalite foyaite in the ENE.Chilled margins contain plagioclase (An53), olivine (Fo53), magnetite, ilmenite, and apatite as liquidus phases and later-crystallized augite (Di69Hd27Ac4) and biotite (Annite32). The coexisting Fe-Ti oxides indicate fO2 and T values just below the synthetic QFM buffer curve. In the border group, plagioclase cores zone into anorthoclase and soda-sanidine rims, olivines reach Fo16, pyroxenes Di32Hd59Ac9, and biotites Annite86. Interstitial pargasitic amphibole appears close to the innermost margins. In the central group, feldspars are all perthitic alkali feldspars and nepheline becomes a major, early crystallizing phase. Olivines range from Fo10-Fo4, in the augite-syenites where they coexist with ferro-salites Di50Hd47Ac3, but olivine is absent from foyaitic assemblages in which the pyroxenes range through aegirine-augite to pure aegirine. Interstitial amphiboles range from ferro-pargasite or hastingsite to katophorite and thence towards arfvedsonite, but are absent from the most differentiated rocks, whereas biotite occurs throughout the entire group in the range Annite71-Annite100.The parental magma, represented by the chilled margins, was a relatively anhydrous alkali olivine-basalt with an initial 87Sr/86Sr ratio of 0.70326. Its high Ti, P, Ba, and F contents are inferred to be features inherited from a primary magma, derived from the mantle as a small partial melt fraction which involved significant amounts of fluor-apatite and phlogopite. While all lithologies are considered as differentiates from this parental magma, there is both a well-defined field junction and a compositional hiatus between the border group and the central group rocks. Mineralogical considerations and REE patterns suggest that the later, more salic (?benmoreitic) magma from which the central group crystallized, related to the parental magma by ol-fsp-ap-FeTi oxide fractionation. Congelation in both border group and central group occurred by side-wall crystallization, but the salic magma became compositionally stratified, with upward concentration of alkalis and volatiles to produce a phonolitic upper facies which is preserved at the ENE end of the intrusion owing to subsequent axial tilting.

Estudio vulcanológico y qeoquímico del maar de la Caldera del Rey. Tenerife (Canarias)

In this study a cartographic, morphological, geochemical and petrographic study is made of the ''Caldera del Rey. The Caldera del Rey is a volcanic structure formed by two overlapping maars, The second one (maar) that was formed is of greaten dimensions and destroyed part of the first one, Both maars erupted throught a possible fracture N 35 E, which is one of the directions with regional importance in the Archipelago. The eruption, which was very explosive, has been thought to be due in part to the great importance of the gaseous phase of the salic magma and also to the steam produced during the interaction of the magma with underground water. This explosivity can be clearly seen in the cleaf resalte cul out in the Serie Basaltica Antigua to some extent penetrated and fragmented by the eruption, The phreatomagmatic character of the eruption is evident because of the existence of accretionary lapilli. The materials emitted are exc1usively of aerial projection: agglomerates, tuffs cinerites. There was no flow of lava. ' Some of the fragments of tuffs as cinerites and pumice are comagmatic. These correspond to salic trachytic phonolitic rocks, which represent one of the last stages of differentiation of the alkaline oceanic magmas. The geochemical character of the materials of the Caldera del Rey is characteristic and can be easily distinguished from other nearly salic deposits formed in different cycles.

The Topsails igneous terrane, Western Newfoundland: evidence for magma mixing

The Topsails igneous terrane of Western Newfoundland contains a diverse suite of igneous rocks, but consists mainly of Silurian alkaline to peralkaline granites and rhyolites. The terrane exhibits evidence for the coexistence of mafic and salic magmas in the form of composite dykes and flows, sinuous, boudined mafic dykes cutting granites and net vein complexes. Field data and major and trace element chemical data suggest that these magmas mixed to produce limited volumes of more or less homogeneous hydrids.

Crystal fractionation of the basalt comendite series of the mount Edziza volcanic complex, British Columbia: Major and trace elements

Abstract The Mount Edziza Volcanic Complex in north-central British Columbia includes a group of overlapping basaltic shields, salic composite volcanoes, domes and small calderas that range in age from 7.5 Ma to less than 2000 years B.P. The volcanic assemblage is chemically bimodal, comprising voluminous alkali olivine basalt and hawaiite, a salic suite of mainly peralkaline trachyte and comendite, plus a relatively small volume of intermediate rocks (trachybasalt, tristanite, mugearite, benmoreite). The complex is the product of five cycles of magmatic activity, each of which began with alkali olivine basalt and culminated with the eruption of salic magma. The regular chemical variation shown by almost 100 major- and trace-element analyses suggests a genetic lineage between the basic and salic members of each cycle. Least-squares mathematical modelling, indicates that the salic rocks (trachyte and comendite) have formed by fractionation of observed phenocryst and cumulate nodule mineral phases from a common alkali olivine basalt parent magma. Hawaiite is thought to be a cumulate rock, formed by partial fractionation and feldspar accumulation within rising columns of primary alkali olivine basalt. Fractionation leading from alkali olivine basalt through trachybasalt and trachyte to comendite is believed to have taken place where primary basalt became trapped in large crustal reservoirs. The early removal of olivine, clinopyroxene and plagioclase, leading to a trachytic residuum, and subsequent fractionation of mainly alkali feldspar, leading to the peralkaline end members, is consistent with major- and trace-element variation and with isotopic and REE data. The chemical diversity of the complex is attributed to its location over a zone of crustal extension where mantle-derived basalt, trapped in large high-level reservoirs, underwent prolonged fractionation.

The volatile component of some pumice-forming alkaline magmas from the Azores and Canary Islands

The abundances of pre-eruptive magmatic volatile species in the system H-O-S may be determined by application of thermodynamic methods to phenocryst assemblages commonly found in volcanic rocks, as demonstrated by Rutherford and Heming (1978). These methods are applied to alkaline pumice deposits, of airfall and ignimbrite type, from Tenerife (Canary Islands), Sao Miguel and Faial (Azores). It is argued that reliable temperature and fO2 buffering mechanisms found in rhyolitic magmas appear not to operate in more alkaline liquids. fH2O is estimated using biotite; the high values found are shown to be compatible with the violently explosive nature of the magmas concerned. fS2 is estimated from pyrrhotite composition. fH2, fH2S, fSO2, fSO3 are calculated from gas equilibria. Water fugacity may be very roughly estimated for non-biotite bearing samples from data on the sulphur species. Abundances of these species are similar in alkaline and calc-alkaline salic magmas. Volcanological implications, relating to the release of volatiles during explosive eruptions, are considered.