Hybrid Magma Research Articles

Feldspar crystallization under magma-mixing conditions shown by cathodoluminescence and geochemical modelling – a case study from the Karkonosze pluton (SW Poland)

AbstractFeldspars from the Karkonosze pluton (SW Poland) display many features compatible with magma mixing. The mixing hypothesis has been tested using a geochemical mass balance law resulting in two possible paths of magma hybridization. Based on the results of the geochemical calculation, feldspar samples have been chosen along both mixing lines for cathodoluminescence (CL) investigation which was used as the main tool for the reconstruction of their crystallization path. Changes in the conditions of nucleation and crystallization of the feldspars as well as their movement within the magma chamber have been recognized due to different luminescence characteristics. These changes in the conditions of crystallization obtained by CL allow a precise determination of the genetic affinity of the samples to more mafic or more felsic environments.The results of the present study proved CL to be a valuable tool for the study of crystal-growth morphologies in a dynamic, turbulent environment and also as a geochemical tool for the reconstruction of various petrogenetic mechanisms (e.g. magma hybridization). Accordingly, the combination of CL with geochemical modelling provides corresponding information about magma evolution in an open system.

Experiments on magma mixing

The arrival of a new hot batch of magma changes the thermal and the chemical state of a volcanic plumbing system. Formation of hybrid magma by chemical dilution (liquid miscibility model) of different magma batches is generally accepted. Observations of drop‐like domains of less evolved (newer) magma in more evolved (host) magma point to the contribution of a mechanical mixing process (liquid immiscibility). The interplay of both mechanisms seems to control the time scale from the dormant to the active state of a volcano. We carried out laboratory experiments using melts from natural volcanic rock compositions with the aim to validate model calculations for shear rate dependent domain sizes. The results confirmed the model and also the importance of hydro‐mechanically mixing for chemical dilution. The key process, however, that is able to change the state of a magma reservoir on a critical time scale is hydrodynamic magma mixing.

Field Evidence of Magma Mixing from Microgranular Enclaves Hosted in Palaeoproterozoic Malanjkhand Granitoids, Central India

The Malanjkhand granitoids (MG) pluton (about 1500 sq km) occurs in the Balaghat district of Madhya Pradesh. The MG (~2400 Ma) represent an episode of Palaeoproterozoic felsic magmatism in Central India and hosts potential Cu (±Mo±Au) deposits. The enclaves hosted in MG can be broadly classified into two categories: microgranular enclaves (dark-coloured, fine-grained magmatic) and xenoliths of country rocks. The microgranular enclaves (ME) may be rounded, ellipsoidal, discoid, elongated, lenticular or tabular, and their size commonly reaches up to 2 metres across. The ME have sharp and in places, diffuse contacts with their host granitoids. The shape and size of ME indicate contemporaneous flow and mingling of partly crystalline felsic-mafic magmas. Some ME exhibit dark crenulated margins giving them a pillow-like form that has been attributed to undercooling of a ME magma as globules intruded into a granitoid magma. The presence of corroded felsic and mafic minerals (xenocrysts) in ME is interpreted as the result of mechanical transfer during the mafic-felsic magma interaction and mixing event. Mafic minerals (biotite) rim the quartz xenocrysts giving rise to ocellar texture, which exhibit signatures of resorption under hybrid (enclave) magma conditions. All these features suggest an origin for the calc-alkaline intermediate granitoid magma in Malanjkhand involving a magma mixing process.

Late Magmatism of the Galway Granite Batholith: II. Composite Dolerite–Rhyolite Dikes

Composite dolerite-rhyolite dikes traverse the Galway Granite batholith and its adjacent envelope. The dikes pertain to the Teach D6ite suite and were previously considered to be of Carboniferous age. New and extended examination of field relationships supports recent radiometric dating for an intrusive period that overlapped with the final consolidation of the Galway batholith. Regional crustal extension produced a complex pattern of fissuring, controlled by various preexisting structures, which permitted ascent of mantle-derived melts into and around the Galway batholith. Ponding of mafic magma at an intermediate level facilitated crustal partial melting and the generation of high-silica, high-alumina rhyolitic melts. The two contrasting magmas then rose into common or proximate dike fissures, rhyolitic injection immediately following that of dolerite. Magma storage in stratified chambers occasionally resulted in the development of a hybrid magma layer, but in all cases minor mingling and mixing beween dolerite and rhyolite magma continued up into the dikes. Rhyolite geochemistry precludes a genetic relationship with the Galway granitoids, despite a few instances where granitic material was entrained into rhyolitic magma. Introduction and setting The 400Ma Galway Granite batholith was emplaced into 470Ma island-arc orthogneisses in the Connemara sector of the Caledonides. This emplacement was followed by the intrusion of two hypabyssal suites: earlier microphyric ('porphyry') dacite dikes (Kinahan 1869; Mohr 2003) and a later complex nexus of dolerite dikes, the Teach D6ite (TD) suite (Mitchell and Mohr 1987; Fig. 1). The numerous and widespread dacite dikes have consistently been considered the youngest igneous rocks pertaining to the Galway batholith (Wager 1932; Wright 1961; Harvey 1967; Coats and Wilson 1971; Senior 1973; Leake 1974). However, new work summarised here suggests that the subsequent TD dikes were a final manifestation of the magmatic episode responsible for the batholith. The regional pattern of TD dikes comprises three major linear trends (Fig. 1). In the central part of the Galway batholith and its northern envelope, the NNE-trending Seanabhain system of dikes is intimately associated with the Shanawon Fault that separates the central and western blocks of the batholith (Feely and Madden 1988; Mohr 1993; Callaghan 1999). The na hUillinni dike system, 3.5km west of the Seanabhan system and parallel to it, projects much farther NNE into the orthogneiss envelope (Fig. 1). Secondly, a grid of ENE-trending dikes Irish Journal of Earth Sciences 22 (2004), 15-32. © Royal Irish Academy 15 This content downloaded from 207.46.13.174 on Sun, 10 Jul 2016 05:04:50 UTC All use subject to http://about.jstor.org/terms 16 Irish Journal of Earth Sciences (2004)

A 550‐year‐old Plinian eruption at El Chichón Volcano, Chiapas, Mexico: Explosive volcanism linked to reheating of the magma reservoir

Some 550 years ago (1320–1433 A.D.), a powerful Plinian eruption at El Chichón Volcano in southern Mexico produced a widespread pumice fall deposit. We subdivided the deposit into three parts on the basis of structural and textural characteristics, pumice lithology and density, granulometry, and petrologic‐geochemical attributes. The deposit covers an area of 1500 km2 within the 1‐cm isopach and has a minimum estimated bulk volume of 2.8 km3 (1.1 km3 dense rock equivalent (DRE)); its eruptive column reached an altitude of ∼31 km. Consideration of field evidence, the presence and nature of mafic enclaves, and chemical data strongly suggest that the 550 year B.P. eruption is linked with the intrusion of a high‐temperature basaltic magma into preexisting but stagnated trachyandesitic magma beneath El Chichón. Thorough mixing of the two magmas produced a compositionally uniform hybrid trachyandesite magma (average SiO2 55.3 wt %), which subsequently underwent crystal growth and gas exsolution, ultimately overpressurizing the zoned magmatic system to erupt explosively. On the basis of El Chichón's known eruptive history, the intrusion‐mixing event occurred sometime after the 900 year B.P. eruption. The hybrid magma had a preeruption temperature of 820–830°C and was water undersaturated (5–6 wt % H2O) at pressures of ∼2–2.5 kbar.

Tectono-Magmatic Precursors for Porphyry Cu-(Mo-Au) Deposit Formation

Porphyry Cu-(Mo-Au) deposits are relatively rare but reproducible products of subduction-related magmatism. No unique processes appear to be required for their formation, although additive combinations of common tectono-magmatic processes, or optimization of these processes, can affect the grade and size as well as the location of the resulting deposits. These various contributing processes are reviewed, from partial melting in the mantle wedge overlying the subducting plate, through processes of magma interaction with the lithosphere, to mechanisms for magma emplacement and volatile exsolution in the upper crust. Specific ore-forming processes, such as magmatic-hydrothermal fluid evolution, are not discussed. Hot, hydrous, relatively oxidized, sulfur-rich mafic magmas (predominantly basalts) generated in the metasomatized mantle wedge above a subducting oceanic slab rise buoyantly to the base of the overlying crust where they stall due to density contrasts. Because these magmas are oxidized, sulfur is dominantly present as sulfate, and chalcophile elements such as Cu and Au are incompatible (i.e., they are retained in the melt). As these magmas begin to crystallize they release heat which causes partial melting of crustal rocks. Mixing between crustal- and mantle-derived magmas yields evolved (andesitic to dacitic), volatile-rich, metalliferous, hybrid magmas, which are of sufficiently low density to rise through the crust. Magma ascent is driven primarily by buoyancy forces and is dominantly a fracture-controlled phenomenon. As such, crustal stress and strain patterns play an important role in guiding the ascent of magma from the lower crust. In particular, translithospheric, orogen-parallel, strike-slip structures serve as a primary control on magma emplacement in many volcanic arcs worldwide. A feedback mechanism operates, whereby preexisting faults facilitate magma ascent, the heat from which further weakens the crust and focuses strain. Certain structural geometries, such as fault jogs, step-overs, and fault intersections, offer low-stress extensional volumes during transpressional strain. Such sites represent vertical conduits of relatively high permeability, up which magmas will preferentially ascend. Large upper crustal plutonic complexes may therefore be localized within these structural settings. Having delivered a sufficient volume of evolved, fertile arc magma to a focused position in the upper crust, magmatic fractionation, recharge, and volatile exsolution lead to the development of ore-forming magmatic-hydrothermal systems. To a first approximation, the size of the resulting deposit will be limited by the magma volume delivered to the upper crustal magma chamber. System-specific details such as magmatic-hydrothermal evolution, the nature of the country rocks, and subsequent erosional and weathering history will ultimately control the value of the deposit, but these factors fall outside the scope of this paper.

A Petrologic Perspective of Kilauea Volcano's Summit Magma Reservoir

Two hundred years of magmatic history are documented by the lavas and tephra sampled from Kīlauea's historical summit eruptions. This paper presents detailed petrographic and geochemical data for a comprehensive suite of samples erupted within or near Kīlauea's summit caldera since the 17th century. Our results elucidate the range of magmatic processes that operate within the volcano's summit magma reservoir and document two compositional trends that span nearly the entire known range for the volcano. Prior to the 1924 summit crater collapse, a trend of increasing incompatible element and CaO and decreasing SiO2 abundances (at a constant MgO) prevailed. Thereafter, the trend reversed direction and has persisted for the rest of the 20th century, including during the current Pu`u `Ō`ō eruption. The rapid and systematic nature of these temporal geochemical variations indicates that the summit reservoir is a single, relatively small body rather than a plexus of dikes and sills. Olivine fractionation is the dominant petrologic process within this reservoir. Petrographic observations and olivine and whole-rock geochemical data suggest that the summit reservoir has a crown of aphyric, more evolved, low-density magma. Differentiation within this crown involving clinopyroxene and plagioclase is more extensive than previously recognized in Kīlauea summit lavas. The effects of crystal fractionation are superimposed upon an evolving hybrid magma composition produced by mixing new, mantle-derived magmas with more fractionated reservoir magma. Frequent eruptions of these hybrid reservoir magmas document the rapid variation in parental magma composition. These compositional variations correlate with magma supply rate; both are thought to be influenced by the degree of melting of small-scale source heterogeneities within the Hawaiian plume. However, Kīlauea's source compositions and partial-melting processes have varied only within a narrow range over the past 350 kyr.

Magma emplacement and mafic–felsic magma hybridization: structural evidence from the Pan-African Negash pluton, Northern Ethiopia

The Negash pluton (50 km 2) consists of late Pan-African, high-K, calc-alkaline granitoids intruded into low-grade metavolcanics–metasediments. This almost circular massif consists of monzogranites, granodiorites, diorites–gabbrodiorites, and hybrid diorites. The anisotropy of magnetic susceptibility (AMS) method was used to determine internal structures of the pluton. The foliation trajectories are concentric and inward dipping. The lineation pattern displays an external zone characterised by horizontal concentrically oriented lineations and an internal zone with NW–SE oriented lineations. These petro-structural data clearly locate the feeder zone at the north-western tip of the pluton and indicate the subsequent expansion of the magmas towards the SE. The pluton is a result of in-situ assembly of four magma batches, which were forcefully injected into pre-existing foliated country rocks in relation to transpressional tectonic regime. Two types of mafic–felsic magma interactions are recognised: homogeneous and heterogeneous hybrid diorites at the north-western part, and mingled interfaces at the diorite–granodiorite contact zones mainly visible in the eastern and south-eastern parts. The in-situ mingling along diorite–granodiorite contacts was achieved at the level of emplacement during the injection of dioritic magma into the felsic magmas while the hybrid diorites are assumed to result from two-way conduit mixing and mingling during simultaneous rising of mafic and felsic magmas.

A sulphide-bearing orthopyroxenite layer in the Bjerkreim–Sokndal Intrusion, Norway: implications for processes during magma-chamber replenishment

The Bjerkreim–Sokndal Layered Intrusion contains an up to 3-m-thick layer of sulphide-bearing orthopyroxenite or melanorite that can be followed for about 30 km along the boundary between megacyclic units II and III. The layer of orthopyroxenite is developed within a sequence of ilmenite leuconorites and defines the base of zone IIIa. The leuconorites are generally succeeded by ilmenite–magnetite leucotroctolite, which defines the base of zone IIIb. The orthopyroxenite layer has an initial Sr-isotopic ratio similar to the leucotroctolites (ca. 0.7051) and lower than the enveloping ilmenite leuconorites of zones IIc and IIIa (ca. 0.7054). Variations of tetrahedrally coordinated Al in orthopyroxene and elevated Cr/TiO 2 in the orthopyroxenite layer and the leucotroctolite imply that the orthopyroxenite is genetically more related to the leucotroctolite than the enveloping leuconorites. On the basis of field observations and analytical results, we conclude that the orthopyroxenite crystallised as a result of mixing between relatively primitive magma and differentiated resident magma, both of which were sulphide-saturated, during chamber replenishment. The injected magma formed a buoyant plume that spread out laterally at its level of neutral buoyancy within the compositionally zoned resident magma, some distance above the magma-chamber floor. Mixing in the plume resulted in a hybrid saturated in orthopyroxene, ilmenite and sulphide melt. Batches of relatively dense magma containing crystals of orthopyroxene and ilmenite and droplets of sulphide melt sank to the floor of the chamber from the hybrid magma layer and formed a layer of orthopyroxenite on the magma-chamber floor. Removal of orthopyroxene from the hybrid melt resulted in the sporadic crystallisation of plagioclase at the orthopyroxene–plagioclase cotectic. Plagioclase joined orthopyroxene and sulphide droplets in the dense melt batches which sank to the floor, resulting in the local development of melanorite instead of orthopyroxenite. Crystallisation of ilmenite leuconorite from the resident magma below the hybrid magma layer resumed after formation of the orthopyroxenite layer. This continued as the hybrid layer thickened and eventually came into contact with the chamber floor when leucotroctolites of zone IIIb started to crystallise on the deeper parts of the floor. Elsewhere, lower-temperature cumulates formed on elevated portions of the floor from magma higher in the stratified column. The sulphide-bearing orthopyroxenite layer is not associated with PGE mineralisation because both the replenishing and differentiated resident magmas had lost PGE through earlier sulphide saturation. Zone IIIb is followed by a sequence of lithologies that reflect progressive fractional crystallisation with the appearance of cumulus orthopyroxene at the expense of olivine (zone IIIc), and the successive entries of magnetite (zone IIId) and clinopyroxene plus apatite (zone IIIe). A new major influx of magma took place at this time to produce megacyclic unit IV.

The magmatic evolution of the late Miocene laccolith–pluton–dyke granitic complex of Elba Island, Italy

Since late Miocene time, post-collisional extension of the internal parts of the Apennine orogenic belt has led to the opening of the Tyrrhenian basin. Extensive, mainly acidic peraluminous magmatism affected the Tuscan Archipelago and the Italian mainland during this time, building up the Tuscan Magmatic Province as the fold belt was progressively thinned, heated and intruded by mafic magmas. An intrusive complex was progressively built on western Elba Island by emplacement, within a stack of nappes, of multiple, shallow-level porphyritic laccoliths, a major pluton, and a final dyke swarm, all within the span from about 8 to 6.8 Ma. New geochemical and Sr–Nd isotopic investigations constrain the compositions of materials involved in the genesis of the magmas of Elba Island compared to the whole Tuscan Magmatic Province. Several distinct magma sources, in both the crust and mantle, have been identified as contributing to the Elba magmatism as it evolved from crust-, to hybrid-, to mantle-dominated. However, a restricted number of components, geochemically similar to mafic K-andesites of the Island of Capraia and crustal melts like the Cotoncello dyke at Elba, are sufficient to account for the generation by melt hybridization of the most voluminous magmas (c. εNd(t) −8.5, 87Sr/86Sr 0.715). Unusual magmas were emplaced at the beginning and end of the igneous activity, without contributing to the generation of these hybrid magmas. These are represented by early peraluminous melts of a different crustal origin (εNd(t) between −9.5 and −10.0, 87Sr/86Sr variable between 0.7115 and 0.7146), and late mantle-derived magma strongly enriched in incompatible elements (εNd(t) = −7.0, 87Sr/86Sr = 0.7114) with geochemical–isotopic characteristics intermediate between contemporaneous Capraia K-andesites and later lamproites from the Tuscan Magmatic Province. Magmas not involved in the generation of the main hybrid products are not volumetrically significant, but their occurrence emphasizes the highly variable nature of crust and mantle sources that can be activated in a short time span during post-collisional magmatism.

Petrogenesis and Implications of Calc-Alkaline Cryptic Hybrid Magmas from Washburn Volcano, Absaroka Volcanic Province, USA

AbstractThe petrogenesis of calc-alkaline magmatism in the Eocene Absaroka Volcanic Province (AVP) is investigated at Washburn volcano, a major eruptive center in the low-K western belt of the AVP. New 40Ar/39Ar age determinations indicate that magmatism at the volcano commenced as early as 55 Ma and continued until at least 52 Ma. Although mineral and whole-rock compositional data reflect near equilibrium crystallization of modal phenocrysts, petrogenetic modeling demonstrates that intermediate composition magmas are hybrids formed by mixing variably fractionated and contaminated mantle-derived melts and heterogeneous silicic crustal melts. Nd and Sr isotopic compositions along with trace element data indicate that silicic melts in the Washburn system are derived from deep-crustal rocks broadly similar in composition to granulite-facies xenoliths in the Wyoming Province. Our preferred explanation for these features is that mantle-derived basaltic magma intruded repeatedly in the deep continental crust leading to fractional crystallization, silicic melt production, and homogenization of magmas, followed by ascent to shallow reservoirs and crystallization of new plagioclase-rich mineral assemblages in equilibrium with the intermediate hybrid liquids. The implications of this process are that (1) some calc-alkaline magmas may only be recognized as hybrids on purely chemical grounds, particularly in systems where mixing precedes and is widely separated from crystallization in space and time, and (2) given the role ascribed to crustal processes at Washburn volcano, the variation between rocks that follow calc-alkaline trends in the western AVP and those that follow shoshonitic trends in the east cannot simply reflect higher pressures of fractionation to the east in Moho-level magma chambers in the absence of crustal interaction.

Mantle and Crustal Sources in the Genesis of Late-Hercynian Granitoids (NW Portugal): Geochemical and Sr-Nd Isotopic Constraints

Large volumes of granitoids were emplaced in the Hercynian Central Iberian Zone during the last ductile deformation phase (D3, 300-320 Ma). The biotite-rich granitoids are the most abundant: (1) syn-D3 granodiorites-monzogranites (313-319 Ma) with calc-alkaline and aluminopotassic affinities; (2) late-D3 granodiorites-monzogranites (306-311 Ma), related to subalkaline and aluminopotassic series. These granitoids are associated with coeval gabbro-norite to granodiorite bodies and/or mafic microgranular enclaves. Both granitoids and basic-intermediate rocks show petrological, geochemical and isotopic evidence of interaction between felsic and mafic magmas. The mantle-derived melts, represented by shoshonitic gabbro-norites, were probably derived from an enriched and isotopically homogeneous source (Sr i = 0.7049 to 0.7053, e Nd = -2.1 to -2.5). In some syn- and late-D3 plutons there are evidences of essentially crustal granites, represented by moderately peraluminous monzogranites of aluminopotassic affinity. They have similar Nd model ages (1.4 Ga) but different isotopic compositions (Sr i = 0.7089 to 0.7106, e Nd = -5.6 to -6.8), revealing a heterogeneous crust. Potential protoliths are metasedimentary (immature sediments) and/or felsic meta-igneous lower crust materials. Large amounts of hybrid magmas were generated by the interaction of these coeval mantle- and crust-derived liquids, giving rise to slightly peraluminous monzogranites/granodiorites of calc-alkaline and subalkaline affinities, which display more depleted isotopic compositions than the crustal end-members (Sr i = 0.7064 to 0.7085, e Nd = -4.4 to -6.2). Petrogenetic processes involving mingling and/or mixing and fractional crystallization (at variable degrees) in multiple reservoirs are suggested. A major crustal growth event occurred in late-Hercynian times (∼305-320 Ma) related to the input of juvenile mantle magmas and leading to the genesis of composite calc-alkaline and subalkaline plutons, largely represented in the Central Iberian Zone.

Timing of magma extraction during the Campanian Ignimbrite eruption (Campi Flegrei Caldera)

A core drilled within the northern part of the city of Napoli has offered the unique opportunity to observe in one single sequence the superposition of the four pyroclastic flow units emplaced during the Campanian Ignimbrite (CI) eruption. Such a stratigraphic succession has never been encountered before in natural or in man made exposures. Therefore the CI sequence was reconstructed only on the basis of stratigraphic correlations and compositional data (in literature). The occurrence of four superposed CI flows, together with all the data available (in literature) allowed us to better constrain the chemical stratigraphy of the deposit and the compositional structure of the CI magma chamber. The CI magma chamber includes two cogenetic magma layers, separated by a compositional gap. The upper magma layer was contaminated by interaction with radiogenic fluids. The two magma layers were extruded either individually or simultaneously during the course of the eruption. In the latter case they produced a hybrid magma. But no evidence of input of new geochemically and isotopically distinct magma batches just prior or during the eruption has been found. Comparison with the exposed CI deposits has permitted reconstruction of variable eruption phases and related magma withdrawal and caldera collapse episodes. The eruption was likely to have began with phreatomagmatic explosions followed by the formation of a sustained plinian eruption column fed by the simultaneous extraction from both magma layers. Towards the end of this phase the upward migration of the fragmentation surface and the decrease in magma eruption rate and/or activation of fractures formed an unstable pulsating column that was fed only by the most-evolved magma layer. This plinian phase was followed by the collapse of the eruption column and the beginning of caldera formation. At this stage expanded pyroclastic flows fed by the upper magma layer in the chamber generated. During the following major caldera collapse episode, the maximum mass discharge rate was reached and both magma layers were tapped, generating expanded pyroclastic flows. Towards the end of the eruption, only the deeper and less differentiated magma layer was tapped producing more concentrated pyroclastic flows that traveled short distances.

Sr,Nd isotope evidence for an enriched mantle component in the origins of the Hercynian gabbro-granite series of the ''Serie dei Laghi'' (Southern Alps, NW Italy)

In the “Serie dei Laghi” of the Southwestern Alps, large Permian granite plutons and mafic-intermediate stocks and dykes (the “Appinite suite”) were emplaced in a post-collision environment. The magmatic rocks of the Serie dei Laghi are metaluminous and exhibit petrographic and geochemical characters typical of medium-K, calc-alkaline series. Granites and leucocratic Appinites show LREE and LILE enriched patterns and negative Nb, P and Ti spikes typical of calcalkaline series. In the mafic Appinite samples, the above characteristics become progressively less pronounced, as their acidity decreases. The gabbro-noritic Appinites (Mg# = 67–75, Ni and Cr contents up to 163 ppm and 882 ppm, respectively) are the most primitive of the Permian magmatic rocks reported to date in the Southern Alps east of the Ivrea Verbano Zone. On the basis of their geochemical and isotope patterns, they can, with a reasonable degree of confidence, be considered mantle derivatives that underwent very little, if any, crustal contamination. Their overall geochemical and isotope characteristics (ϵNd = −0.06/-2.45, (87Sr/86Sr)280 = 0.7044–0.7072) suggest an enriched mantle as the source of the Serie dei Laghi magmatic series. The strong overall correlation of the whole rock series in the Nd and Sr covariation diagrams indicates the prominence of mantlecrust interactions in the evolution of the Permian plutonic rocks from the Serie dei Laghi. Moreover, the correlation between isotope ratios and SiO2 suggests that the magmas evolved in crustal chambers. The entire Permian intrusive series was generated through complex crust-mantle interaction mechanisms which began with a mantle-derived magma isotopically similar to gabbro-noritic Appinites. The following picture can thus be envisioned: a basaltic magma rising from the mantle resided in a magma chamber at the base of the crust and underwent combined assimilation and crystal fractionation (AFC); the amount of assimilated crust at this stage was about 10%. A late-orogenic transtensive regime allowed small magma batches to rise from the chamber directly to the surface without further crustal exchange (Appinites). Continued magma input, crustal assimilation, crystallization and tapping extended the duration of the magma chamber up until the post-orogenic stage. Crustal extension enabled the formation of shallower magma chambers in which large volumes of hybrid magmas (the Alzo-Roccapietra, Montorfano and Baveno-Mottarone plutons) resided and further assimilated crustal material (20–25%).

PGE AND Ag MINERALIZATION IN A BRECCIA ZONE OF THE PRECAMBRIAN NUASAHI ULTRAMAFIC-MAFIC COMPLEX, ORISSA, INDIA

The Nuasahi ultramafic–mafic complex, in Orissa, India, contains high concentrations of platinum-group elements (Pt up to 9.6 ppm, Pd up to 17.9 ppm) and Ag (up to 6.6 ppm) in a chromiferous sulfide-rich breccia zone. The plutonic complex occurs within an Archean belt of supracrustal rocks. The complex consists of an ultramafic unit in fault contact with and overlain by gabbros whose stratigraphic top is characterized by a zone of brecciation. Lode chromite deposits occur in the ultramafic suite and are currently being mined. Magnesiochromite compositions in the lodes are refractory and show little variation within the ultramafic suite (Cr 2 O 3 ~60%, Al 2 O 3 ~12%, MgO ~15%). Sudburyite, (Pd,Bi)Sb, members of the michenerite–testibiopalladite series, and irarsite–hollingworthite (IrAsS–RhAsS) are the common platinum-group minerals (PGM) in the chromite–sulfide assemblage of the breccia zone. Sudburyite is preferentially included within coarse-grained ferrian chromite, along with numerous inclusions of chalcopyrite, pyrrhotite, rutile, ilmenite and oriented lamellae of clinochlore. The distinctive noble-metal composition (high Pd, Pt and Ag with high Cu) and dominant Pd–Bi–Te–Sb-bearing PGM in the mineralized portion indicate its evolved nature. Mineralization developed by magmatic processes; a hybrid magma resulting from partial incorporation of serpentinized chromite-bearing ultramafic rocks and chromitite in the younger gabbroic magma was emplaced within the pre-existing breccia zone. The breccia zone developed during brittle deformation along a NNW-trending shear zone.

Age and origin of the Böllsteiner Odenwald

The granitoid protoliths of the gneisses of the Bollsteiner Odenwald, a part of the Mid German Crystalline Rise, intruded at 405 ± 3 Ma. This age was obtained by single zircon 207Pb/206Pb evaporation measurements. It is supported by an upper discordia intercept age of 410 ± 11 Ma from single zircon U/Pb data. These granitoids were derived from hybrid magmas according to their geochemical characteristics. Inherited zircon grains with apparent ages of 882 and 1138 Ma further constrain the source heterogeneity. The isotope data with Nd model ages for the granitoid gneisses between 1.3 and 1.7 Ga also underline the involvement of Proterozoic material into the magma sources. Since the geochemical data indicate a subduction zone environment we suggest that the Bollsteiner Odenwald was formed as part of a Silurian/Early Devonian magmatic arc in an active continental margin. The age data imply that the Bollsteiner Odenwald is closely related to the Spessart and Ruhla Crystalline Complexes but not to the Bergstrasser Odenwald. Therefore the Otzberg zone is considered to be a Variscan suture zone.

THE PLIO-QUATERNARY MAGMATISM OF SOUTHERN TUSCANY AND NORTHERN LATIUM: COMPOSITIONAL CHARACTERISTICS, GENESIS AND GEODYNAMIC SIGNIFICANCE

Southern Tuscany has been traditionally considered as a typical crustal anatectic magmatic province. However, extensive petrological and geochemical investigations carried out in the last years revealed a much more complex magmatic setting given by the occurrence of a large variety of rock types. The main rock associations are represented by: 1) acid volcanics and intrusives of crustal anatectic origin; 2) Ultrapotassic rocks with lamproitic (LMP) affinity; 3) Hybrid rocks between LMP and Roman-type (ROM) potassic and highly potassic magmas; 4) High-potassium calcalkaline (HKCA) and shoshonitic (SHO) magmas. Acid anatectic rocks are generally associated and mixed with mafic lavas and enclaves of variable composition. LMP, HKCA, SHO, and ROM-LMP hybrid magmas are of mantle origin. Their high enrichment in incompatible elements and the crustal-like isotopic signatures are consistent with a genesis in an anomalous upper mantle that was metasomatized by addition of subduction-related upper crustal material. However, variations of some key petrological and geochemical parameters reveal significant compositional heterogeneity of mantle beneath Tuscany. Petrological and geochemical data for mafic magmas suggest a complex evolutionary history of mantle sources, with at least two metasomatic events. These affected lithospheric and asthenospheric mantle, generating strong vertical and lateral compositional heterogeneity. The close similarity of Tuscany LMP rocks as those occurring in the Western Alps suggests that the earliest metasomatic modification was likely related to Alpine subduction processes. Younger mantle modifications occurred during subduction of the Adria Plate; this was responsible for the widespread Roman magmatism, but also produced some effects in the Tuscany province, as indicated by the occurrence of hybrids between Roman-type and lamproitic magmas.

OLIVINE LIQUID RELATIONS OF LAVA ERUPTED BY KILAUEA VOLCANO FROM 1994 TO 1998: IMPLICATIONS FOR SHALLOW MAGMATIC PROCESSES ASSOCIATED WITH THE ONGOING EAST-RIFT-ZONE ERUPTION

From 1994 through 1998, the eruption of K¯ilauea, in Hawai'i, was dominated by steady-state effusion at Pu'u ' O'¯ o that was briefly disrupted by an eruption 4 km uprift at Napau Crater on January 30, 1997. In this paper, I describe the systematic relations of whole-rock, glass, olivine, and olivine-inclusion compositions of lava samples collected throughout this interval. This suit e comprises vent samples and tube-contained flows collected at variable distances from the vent. The glass composition of tube lava varies systematically with distance and allows for the vent-correction of glass thermometry and olivine-liquid KD as a function of tube-transport distance. Combined olivine-liquid data for vent samples and vent-corrected lava-tube samples are used to document pre-eruptive magmatic conditions. KD values determined for matrix glasses and forsterite cores define three types of olivine phenocrysts: type A (in equilibrium with host glass), type B (Mg-rich relative to host glass) and type C (Mg-p oor relative to host glass). All three types of olivine have a cognate association with melts that are present within the shallow m ag- matic plumbing system during this interval. During steady-state eruptive activity, the compositions of whole-rock, glass and mo st olivine phenocrysts (type A) all vary sympathetically over time and as influenced by changes of magmatic pressure within the summit-rift-zone plumbing system. Type-A olivine is interpreted as having grown during passage from the summit magma- chamber along the east-rift-zone conduit. Type-B olivine (high Fo) is consistent with equilibrium crystallization from bulk-roc k compositions and is likely to have grown within the summit magma-chamber. Lower-temperature, fractionated lava was erupted during non-steady-state activity of the Napau Crater eruption. Type-A and type-B olivine-liquid relations indicate that this lava is a mixture of rift-stored and summit-derived magmas. Post-Napau lava (at Pu'u '¯ O'¯ o) gradually increases in temperature and MgO content, and contains type-C olivine with complex zoning, indicating magma hybridization associated with the flushing of rift-stored components through the eruption conduit.

A petrogenetic study of anorogenic felsic magmatism in the Cretaceous Paresis ring complex, Namibia: evidence for mixing of crust and mantle-derived components

Paresis is one of a group of Cretaceous ring complexes extending from the coast some 350 km NE across the Damara Belt. It consists of over 90% rhyolites and comendites, with subordinate intrusions of quartz syenite and alkali-feldspar syenite. These felsic units are accompanied by very minor amounts of silica-undersaturated basalt, phonolite and lamprophyre. Located near the edge of the Congo Craton, Paresis is the farthest inland of any felsic complex in the Cretaceous ring complex group. The other ring complexes in the vicinity consist of carbonatites and undersaturated alkaline rocks (e.g., Okorusu, Kalkfeld, Etaneno, Ondurakorume). Geochemical and isotopic data provide evidence for both mantle and crustal components in the sources of the Paresis magmas. The alkaline basalts, phonolite and lamprophyre have overlapping Sr and Nd isotopic initial ratios which plot within the mantle array close to bulk-earth values ( εNd=−0.9 to −2.8 and 87Sr/ 86Sr(i)=0.7042–0.7054) and may indicate a mantle plume component. The rhyolite units comprise metaluminous feldspar rhyolites and peraluminous, more differentiated quartz-feldspar rhyolites. Both units show prominent negative Nb and Ta anomalies on mantle-normalized multielement diagrams and have extremely nonradiogenic Nd ratios ( εNd=−21) and Sr initial ratios of 0.7117–0.7138. These isotopic values suggest a crustal origin from pre-Damara (Early Precambrian) gneisses and granitoids, which are exposed in the Congo craton and related inliers in northern Namibia. The comendites are peralkaline, highly differentiated rocks. In contrast to the rhyolites, comendites lack mantle-normalized Nb and Ta anomalies, have higher HFSE and LREE contents, extreme negative Eu anomalies and εNd values of −11. Like the comendites, alkali-feldspar syenites are peralkaline, lack Nb and Ta anomalies, have negative Eu anomalies and high HFSE concentrations. Their εNd values are −6.5 to −8. The quartz syenites overlap with the peralkaline units in isotopic composition but they are chemically very different. They are metaluminous, have a low degree of differentiation and have prominent negative mantle-normalized Nb and Ta anomalies like the rhyolites. The Sr–Nd isotopic composition and incompatible element ratios of the comendites, quartz syenites and alkali-feldspar syenites are intermediate between those of the rhyolites and basalts, precluding a simple origin by basalt fractionation or crustal melting. Instead, the data indicate that the comendite and syenites formed from hybrid magmas with both crustal and mantle-derived components. The metaluminous quartz syenites can be modelled by assimilation of pre-Damara crust by basaltic magma. The peralkaline nature and trace element characteristics of comendite and alkali-feldspar syenites, on the other hand, require an alkaline and HFSE-enriched basic endmember, whose fractionated equivalent may be the Paresis phonolite.

POST-ACCRETION MAGMATISM WITHIN THE KUIU-ETOLIN IGNEOUS BELT, SOUTHEASTERN ALASKA

Petrological and geochronological studies of Tertiary granite and gabbro plutons in southeastern Alaska provide information about the Mid-Miocene post-accretionary magmatic history of the region. The Burnett Inlet Igneous Complex is a 20 Ma bimodal granite–gabbro complex located on Etolin Island and adjacent areas in central southeastern Alaska. It consists of three main members: granite, alkali granite, and a gabbro–diorite unit consisting of intermingled mafic, hybrid and granitic rocks. Mafic magmatic enclaves are ubiquitous in the complex. They occur as isolated inclusions in the silicic plutons and as packed masses in the intermingled zones of the gabbro–diorite unit. The mafic enclaves typically display round and pillow-like shapes, igneous textures, and chilled margins, and thus denote the mingling of mafic and felsic magmas. Petrographic and geochemical features indicate that the complex developed via a combination of fractional crystallization, crystal accumulation and magma mixing. Systematic changes in mineralogy and coherent, curvilinear trends on geochemical diagrams indicate that the mafic and felsic magmas each evolved separately, mainly by crystal accumulation and fractionation. Magma-mixing textures, such as micro-inclusions and quartz xenocrysts in some mafic rocks, suggest mafic–felsic magma mixing and hybridization. However, the compositional and viscosity differences precluded bulk mixing between end-member granite and basalt magmas. Mixing probably involved intermediate magmas, whose smaller differences in composition and viscosity would have permitted hybridization. Both the mafic and felsic units have within-plate geochemical characteristics, indicating that post-accretionary magmatism occurred within an overall extensional tectonic setting. Distributions of apatite fission-track lengths and mineral-cooling curves imply that the magmatic rocks cooled to less than 100°C within 5 million years of emplacement. The complex is interpreted to have initially evolved as a shallow-level silicic magma chamber heated by underplated basaltic magma derived from partial melting of enriched upper mantle. Subsequent invasion of the silicic magma chamber by basaltic intrusions induced an explosive eruption phase, followed by rapid crystallization and cooling of the entire complex.