Dokhan volcanics of Gabal Monqul area,North Eastern Desert,Egypt: geochemistry and petrogenesis

The Dokhan volcanics are represented by a thick stratified lava flows succession of basalt, andesite, imperial porphyry, dacite, rhyodacite, rhyolite, ignimbrites, and tuffs. These lavas are interbanded with their pyroclastics in some places including banded ash flow tuffs, lithic tuffs, crystal lapilli tuffs, and agglomerates. They are typical calc–alkaline and developed within volcanic arc environment. All rocks show moderate enrichment of most large ion lithophile elements relative to high field strength elements (HFSE). The incompatible trace elements increase from basalt through andesite to rhyolite. The felsic volcanics are characterized by moderate total rare earth elements (REE) contents (162 to 392 ppm), less fractionated patterns {(Ce/Yb)_N = (1.24 to 10.93)}, and large negative Eu anomaly {(Eu/Eu*) = (0.15 to 0.92)}. The mafic volcanics have the lowest REE contents (61 to 192 ppm) and are relatively steep {(Ce/Yb)_N = (3.2 to 8.5)}, with no negative Eu anomalies {(Eu/Eu*) = (0.88 to 1)}. The rhyolite displays larger negative Eu anomaly (Eu/Eu* = 0.28) than those of other varieties, indicating that the plagioclase was an early major fractionating phase. The mineralogical and chemical variations within volcanics are consistent with their evolution by fractional crystallization of plagioclase and clinopyroxene.     

Widespread transport of pyroclastic density currents from a large silicic tuff ring: the Glaramara tuff, Scafell caldera, English Lake District, UK

The Glaramara tuff presents extensive exposures of the medial and distal deposits of a large tuff ring (original area >800 km²) that grew within an alluvial to lacustrine caldera basin. Detailed analysis and correlation of 21 sections through the tuff show that the eruption involved phreatomagmatic to magmatic explosions resulting from the interaction of dacitic magma and shallow-aquifer water. As the eruption developed to peak intensity, numerous, powerful single-surge pyroclastic density currents reached beyond 8 km from the vent, probably >12 km. The currents were strongly depletive and deposited coarse lapilli (>5 cm in diameter) up to 5 km from source, with only fine ash and accretionary lapilli deposited beyond this. As the eruption intensity waned, currents deposited fine ash and accretionary lapilli across both distal and medial regions. The simple wax–wane cycle of the eruption produced an overall upward coarsening to fining sequence of the vertical lithofacies succession together with a corresponding progradational to retrogradational succession of lithofacies relative to the vent. Various downcurrent facies transitions record transformations of the depositional flow-boundary zones as the depletive currents evolved with distance, in some cases transforming from granular fluid-based to fully dilute currents primarily as a result of loss of granular fluid by deposition. The tuff-ring deposits share several characteristics with (larger) ignimbrite sheets formed during Plinian eruptions and this underscores some overall similarities between pyroclastic density currents that form tuff rings and those that deposit large-volume ignimbrites. Tuff-ring explosive activity with such a wide area of impact is not commonly recognized, but it records the possibility of such currents and this should be factored into hazard assessments.     

High-temperature emplacement and liquefaction of shallow-water caldera-forming eruption products: Early Miocene Tateyamazaki Dacite in the Oga Peninsula, NE Japan

The Early Miocene Tateyamazaki Dacite infills a 3.2 km diameter caldera. It comprises poorly sorted, massive, biotite-bearing dacite pumice lapilli tuff, in which huge blocks of densely welded dacite lapilli tuff, basaltic andesite lava, and other lithologies are commonly set. Dense blocks are variably cracked and intruded by the host lapilli tuff. Sparse blocks of bedded lapilli tuff and tuff are variably disaggregated to intermingle with the host rocks or are plastically deformed into irregular shapes. Rootless tuff veins millimeters to 30 cm thick are developed within the host rocks, mainly dipping at 10–30°, and are locally branched and mutually cut to form a network. Where thicker, they are stratified and locally carry accidental fragments. Accidental lapilli up to 2 or 3 cm wide and 30 cm long are locally set in near-vertical and variably sinuous arrays. Although poorly defined they are reminiscent of fluid escape structures. The host pumice lapilli tuff, however, retains in part a thermal remnant magnetization (TRM) vector stable at temperatures above 280 °C. Blocks in the caldera fill also retain TRM but the vectors are rotated significantly from those of the host pumice lapilli tuff and the adjacent volcanic rocks. Tateyamazaki Dacite is thus likely to have been emplaced at high temperatures, and intermingled with shattered basement rocks and ambient water to be partly liquefied within the caldera immediately after or during the caldera-forming eruption.     

Geochemistry of Mesoproterozoic Deonar Pyroclastics from Vindhyan Supergroup of Central India: Evidences of felsic magmatism in the Son valley

Deonar Pyroclastics of Semri Group in the Vindhyan Supergroup originated as a result of violent and explosive intrabasinal submarine volcanism during the Mesoproterozoic period. These pyroclastics are rhyolitic to rhyodacitic in composition, comprised of banded, massive, pumiceous flow, breccia, vitric tuff, lapilli and volcanic bomb. The pyroclastic deposits represent welded and non-welded ignimbrites, exhibit typical eutaxitic texture. Mantle normalized multi-element patterns show enrichment in LILs and depletion in HFSFs. Ti, Nb and REE contents show close correlation with Zr, indicating their immobile character. HFSEs and Th/Nb, La/Nb and Zr/Nb values indicate contamination and these signatures represent mixing between mantle-derived rocks and the average continental crust. Deonar Pyroclastics reflect continental rift environment. Felsic magma plausibly generated by underplating of the mature Proterozoic crust of the Indian craton (which acted like a ‘heating lens’) resulted in extensive melting of metabasalt in the lower crustal levels. The high heat flow beneath the Indian shield accentuated heat generation which led to extensive partial melting of metabasalts. Thus, generation of rhyolitic magma occurred along the reactivated deep seated fractures and rifting of the craton, resulting in the explosive intra-basinal felsic vulcanicity in the Vindhyan basin.     

Volcaniclastic stratigraphy of the Tiscapa maar crater walls (Managua, Nicaragua): implications for volcanic and seismic hazards and Holocene climate changes

The Tiscapa maar in the center of Managua city formed by a phreatomagmatic eruption <3 ka ago. The eruption excavated a crater deep into the basement exposing a coherent Pleistocene to Holocene volcaniclastic succession that we have divided into four formations. The lowermost, >60 ka old basaltic–andesitic formation F1 comprises mafic ignimbrites and phreatomagmatic tephras derived from the Las Sierras volcanic complex south of Managua. Formation F2 contains the ~60 ka basaltic–andesitic Fontana tephra erupted from the Las Nubes Caldera of the Las Sierras complex 15 km to the S, the 25 ka Upper Apoyo tephra from the Apoyo Caldera 35 km to the SE, and the Lower (~17 ka) and Upper (12.4 ka) Apoyeque tephras from the Chiltepe volcanic complex 15 km to the NW. These tephras are separated by weathering horizons and paleosols indicating dry climatic conditions. Fluvial deposits of a SSW-NNE running paleo-river system build formation F3. The fluvial sediments contain, from bottom to top, scoriae from the ~6 ka basaltic San Antonio tephra, pumice lapilli from the Apoyo and Apoyeque tephras and the 6.1 ka Xiloà tephra, and scoriae derived from the Fontana tephra. The fluvial sediment succession thus reflects progressively deeper carving erosion in the southern highlands (where a large-amplitude regional erosional unconformity exists at the appropriate stratigraphic level) that began after ~6 ka. This suggests that the mid-Holocene tropical high-precipitation climatic phase affected western Nicaragua about a thousand years later than other circum-Caribbean regions. The end of the wet climate phase ~3 ka ago is recorded by a deep weathering zone and paleosol atop formation F3 prior to the Tiscapa eruption. Formation F4 is the Tiscapa tuffring composed of pyroclastic surge and fallout deposits that cover a minimum area of 1.2 km². The 4 × 10⁹ kg of erupted basaltic magma is compositionally and genetically related to the low-Ti basalts of the N–S striking Nejapa-Miraflores volcanic–tectonic alignment 5 km to the West of Tiscapa. Ascent and eruption mode of the Tiscapa magma were controlled by the Tiscapa fault that has a very active seismic history as it achieved 12 m displacement in about 3000 years. Managua city is thus exposed to continued seismic and volcanic risks.     

Volcanic hazards zonation of the Nevado de Toluca Volcano,Central Mexico

Nevado de Toluca Volcano (NTV), located in central Mexico, is a large stratovolcano, with an explosive history. The area is one of the most important developing centers (>2 millions) in Mexico and in the last 30 yrs large population growth and expansion have increased the potential risk in case of a reactivation of the volcano. As part of a study to assess volcanic risk, this paper presents the results of the volcanic hazard analysis for the NTV. A total of 150 stratigraphic sections were made in the field and three new ages were obtained. Eruptions from NTV produced a complex sequence of pyroclastic deposits that have affected the area at least 18 times during the last 100,000 yrs. Eight vulcanian, four plinian and one-ultraplinian eruptions as well as the destruction of at least three domes occurred in the last 42,000 yr BP as well as two sector collapses in the last 100,000 yrs. Isopach and isopleth maps for the main ulraplinian eruption were also made. The original cone height (5,080 m.a.s.l) was reconstructed through geomorphologic methods. The maximum distance calculated with the energy line for the block and ash flows was 41 km, 35 km for pumice flows and 45 km for debris avalanches. The dominant wind direction at altitudes of 20–30 km is to the east-northeast from November to March, west-northwest in April and west from May to October. Five hazards maps (block and ash flows, pumice flows, ash fall, debris avalanches, and lahars) were made for the NTV. The pyroclastic flows and lahars represent very high to medium hazard for Toluca, Villa Guerrero, Coatepec, Tianguistengo, Metepec, Tenango, Lerma and Zinacantepec. A new debris avalanche would probably affect the south and northeast because of active faulting (E–W and NW–SE) and existing topographic differences in height.     

Grain size variations in ignimbrites and implications for the transport of pyroclastic flows

Ignimbrite flow units commonly show reverse grading of large pumice clasts and normal grading of large lithic clasts. Ignimbrites show coarse-tail grading, in which particles beneath a critical diameter, ranging from 64 to 2 mm, are ungraded. Above this size the larger the clast diameter the more pronounced the segregation. The grading is consistent with the theoretical settling rates of particles in a dispersion with a high particle concentration. Ignimbrite flow units show a reversely graded, fine grained basal layer which is attributed to the action of boundary forces during flow. Ignimbrites are commonly associated with cross-stratified pyroclastic surge deposits and fine ash fall deposits formed in the same eruption. The fine ash fall deposit is depleted in crystals and is thought to be the deposit of the fine turbulent cloud observed making up the upper parts of nuées ardentes. Pyroclastic flows are postulated to be dense, poorly expanded partly fluidized debris flows. Only its fine grained components can be fluidized by gas. Pyroclastic flows are believed to behave as a dispersion of larger clasts in a medium of fluidized fines, which acts as a lubricant similar to water in mud-flows. Poor sorting in ignimbrites is attributed to high particle concentrations not turbulence. Many pyroclastic flows may be laminar in their movement with apparent viscosities, deduced from the lateral grading of large lithic clasts, in the range 10¹?10³ poise.     

Two mantle sources, two plumbing systems: tholeiitic and alkaline magmatism of the Maymecha River basin, Siberian flood volcanic province

Rocks of two distinctly different magma series are found in a ∼4000-m-thick sequence of lavas and tuffs in the Maymecha River basin which is part of the Siberian flood-volcanic province. The tholeiites are typical low-Ti continental flood basalts with remarkably restricted, petrologically evolved compositions. They have basaltic MgO contents, moderate concentrations of incompatible trace elements, moderate fractionation of incompatible from compatible elements, distinct negative Ta(Nb) anomalies, and _Nd values of 0 to +2. The primary magmas were derived from a relatively shallow mantle source, and evolved in large crustal magma chambers where they acquired their relatively uniform compositions and became contaminated with continental crust. An alkaline series, in contrast, contains a wide range of rock types, from meymechite and picrite to trachytes, with a wide range of compositions (MgO from 0.7 to 38 wt%, SiO₂ from 40 to 69 wt%, Ce from 14 to 320 ppm), high concentrations of incompatible elements and extreme fractionation of incompatible from compatible elements (Al₂O₃/TiO₂∼1; Sm/Yb up to 11). These rocks lack Ta(Nb) anomalies and have a broad range of _Nd values, from −2 to +5. The parental magmas are believed to have formed by low-degree melting at extreme mantle depths (>200 km). They bypassed the large crustal magma chambers and ascended rapidly to the surface, a consequence, perhaps, of high volatile contents in the primary magmas. The tholeiitic series dominates the lower part of the sequence and the alkaline series the upper part; at the interface, the two types are interlayered. The succession thus provides evidence of a radical change in the site of mantle melting, and the simultaneous operation of two very different crustal plumbing systems, during the evolution of this flood-volcanic province. Received: 6 January 1998 / Accepted: 29 June 1998     

Nature of a flood-basalt-magma reservoir based on the compositional variation in a single flood-basalt flow and its feeder dike in the Mesozoic Hartford Basin, Connecticut

The erosional remains of the Mesozoic Holyoke basalt in the Hartford, Pomperaug, and Deerfield basins of Connecticut and Massachusetts indicate an original flow volume of >1200 km³. Its feeder dike, which is about 50 m wide and 160 km long, can be traced down through 2 km of Mesozoic sediments and, as a result of faulting associated with basin formation, through an additional 6 km of Paleozoic metamorphic rocks. Chemical profiles through the distal and proximal parts of the flow and through the dike at depths of 2, 4, and 8 km provide sequential samples of the magma that rose during this one eruptive event. The flow and dike have restricted compositions that indicate saturation with olivine, augite, and plagioclase at depth. The flow consisted largely of a liquid at the pigeonite reaction point. Dike compositions can be modeled as mixtures of this liquid with up to 24% crystals of plagioclase, augite, and olivine. The dike compositions indicate equilibration with these minerals at 3.8 kbar. This pressure corresponds to a depth of 12.2 km, which is believed to have been the depth of the brittle/ductile transition in the crust at the time. This transition appears to be the only reasonable barrier that could have caused ponding of the magma at the mid-crustal level. The Holyoke liquid is interpreted to have segregated from a compacting crystal mush following 30% crystallization of the magma in this mid-crustal reservoir. Eruption of the basalt exhausted the supply of segregated liquid, and when the remaining crystal mush began to rise in the dike, the average density of the magma column increased until it matched the average density of the intruded crust, and the eruption ended. By analogy with the differentiation that took place in the solidifying Holyoke flow on the surface, the mid-crustal magma reservoir is estimated to have had a volume of at least 12,000 km³. The magma in this chamber must have come from a still deeper chamber, because it was too fractionated to have come directly from a mantle source. Received: 3 October 1997 / Accepted: 5 May 1998     

Geology and utilization of underground space in metropolitan Kansas City area, USA

 Kansas City is the international leader in the commercial development of mined-out areas for warehousing, manufacturing, offices, business, and service-related activities. Over 3000 people work underground at about two dozen sites with a combined floor space of almost 2 sq km. Most of the space was created after mining a 6-m-thick bed of limestone that underlies the city and its environs. The most common usage of the space is for storage. The mined-out area can be converted to warehouse storage at a fraction of the cost of a comparable surface facility. Received: 2 October 1995 · Accepted: 6 November 1995     

Intra-crater deposits of the Toga tuff ring, Oga Peninsula, NE Japan

The Toga tuff ring is a large, dissected tuff ring located on the modern shoreline of the Oga Peninsula, NE Japan. The crater measures 2 km by 2.4 km and the inner crater walls are inclined inward at 40–50° to form a funnel shape. Intra-crater beds are mainly composed of platy or blocky, non- to variably vesicular glass shards and pumice lapilli of K-rich rhyolite composition and dip inward at 10°–30° or less. A gravity model suggests they fill the downward-tapering conduit to a depth of 548 m below sea level. Fission-track dates from the intra-crater deposits indicate the age of the Toga tuff ring is ca. 420 ka, likely corresponding to a stage of global sea-level fall, MIS 12. Subsequent sea-level rise and marine transgression is inferred to have resulted in erosion of almost the entire outer tuff ring by post-eruptive wave action.The intra-crater deposit`s are exposed over a thickness of 50 m in the deeply incised crater floor. They comprise mainly monomictic tephra of phreatomagmatic origin and are similar in grain-size distribution and sedimentary structures to relatively high and low density turbidites, although the constituents, sparse block-sag structures, and multiple fluid-escape dikes suggest that they are the subaqueous equivalents of high- and low-density pyroclastic currents with similar grain-sizes and degree of grain-size sorting. Marine diatom frustules sparsely contained in the deposits suggest that the crater was likely open to the sea, enabling rapid access of seawater to the vent. Pyroclasts ejected through the water flowed back into the crater to form eruption-fed oscillatory or circular turbidity currents and were repeatedly recycled and variably abraded by subsequent explosions, while many juvenile pumice lapilli and ash grains were carried beyond the crater rim to form relatively dilute pyroclastic currents. The Toga example suggests that primary deposits emplaced in crater lakes are well sorted, graded and stratified with polymodal flow directions, sparse block-sags, and vesicular and fragile fragments that are more or less abraded by repeated explosions and recycling.     

Characterization and correlation of the Hegawa-Kasamori 5 tephra,a widespread tephra aged c. 450 ka associated with large-scale pyroclastic flows from southern Kyushu,SW Japan

A Middle Pleistocene widespread tephra, defined here as Hegawa-Kasamori 5 tephra (Hgw-Ks5), has been newly recognized over a broad area of Japan. Large-scale pyroclastic flow deposits associated with co-ignimbrite ash fall deposits (CAFDs) of Hgw-Ks5 have been identified in the proximal southern Kyushu area, south-west Japan. Hgw-Ks5 possibly originated from the Aira caldera in southern Kyushu, and it is widely spread and intercalated with deposits of the Kasamori Formation, Honshu Island, more than 1000 km away from the source. In the north-west area of the Aira Caldera, the tephra is sparsely distributed in the form of non-welded ignimbrites, and is exposed stratigraphically above the well-known Kobayashi-Kasamori tephra. Hgw-Ks5 is characterized through petrographic features, major element geochemistry of glass shards, and refractive indices of orthopyroxene. The results of previous stratigraphic isotope studies indicate that the eruptive age of Hgw-Ks5 is 434–458 ka (Marine Isotope Stage 12). Assuming that the CAFDs originating from the Aira Caldera are distributed concentrically, the apparent volume of Hgw-Ks5, estimated from the area of distribution and CAFD thickness, is ~100 km³. Therefore, a volcanic explosivity index of 7 is assigned to the Hgw-Ks5 eruption.     

Petrology of the Great Abitibi Dyke, Superior Province, Canada

The Great Abitibi Dyke (GAD) which can be traced northeast,for >> 700 km, across the Abitibi Belt in the southeasternSuperior Province of the Canadian Shield, is composed of olivinegabbro to monzodiorite, weakly saturated to undersaturated insilica. All rocks of the GAD can be derived by mainly plagioclase andolivine fractionation from a parental magma corresponding incomposition to chilled margin samples. Two units can be distinguished,a marginal unit (Unit 1) representing 0–50% crystallizationand a central unit (Unit 2), found over about half of the dykelength, representing 50–70% crystallization. Modelling,using Pearce elemental ratio analysis, quantifies the fractionationhistory and allows mass–balance calculations over thepresent exposure level of the dyke. The approximate balancebetween the amounts of cumulate and fractionated rocks suggeststhat Unit 1 differentiated essentially in situ as a closed system.In contrast, Unit 2 rocks were formed by loss of substantialplagioclase and olivine from the parental magma. This fractionatemust have either been lost to depth or left behind in an externalchamber. Unit 2 rocks show depletion in plagioclase and enrichmentin mafic minerals along strike towards the southwest (deeperexposure level?), a trend explained by density stratification.Regional variation in Unit 1 chemistry is interpreted in termsof lateral magma injection towards the northeast from the locusof Keweenawan rift volcanism. Feldspar, olivine, and augitecompositions are linearly correlated with equilibrium temperatureand extent of magma evolution. The parent magma had a trace element chemistry correspondingto a ‘within–plate’ setting and was probablyderived from an incompatible–element enriched mantle similarto the source for ocean island basalts (OIBs).     

Metal-residence sites in lavas and tuffs from Volcán Popocatépetl, Mexico: implications for metal mobility in the environment

 Volcan Popocatépetl is a Quaternary stratovolcano located 60 km southeast of Mexico City. The summit crater is the site of recent ash eruptions, excess degassing, and dacite dome growth. The modern cone comprises mainly pyroclastic flow deposits, airfall tephras, debris flows, and reworked deposits of andesitic composition; it is flanked by more mafic monogenetic vents. In least-degassed fallout tuffs and mafic scoria, transition metals are concentrated in phases formed before eruption, during eruption, and after eruption. Preeruptive minerals occur in both lavas and tephra, and include oxides and sulfides in glass and phenocrysts. The magmatic oxides consist of magnetite, ilmenite, and chromite; the sulfides consist of both (Fe,Ni)_1-xS (MSS) and Cu–Fe sulfide (ISS). Syn- and posteruptive phases occur in vesicles in both lavas and tephra, and on surfaces of ash and along fractures. The mineral assemblages in lavas include Cu–Fe sulfide and Fe–Ti oxide in vesicles, and Fe sulfide and Cu–Fe sulfide in segregation vesicles. Assemblages in vesicles in scoria include Fe–Ti oxide and rare Fe–Cu–Sn sulfide. Vesicle fillings of Fe–Ti oxide, Ni-rich chromite, Fe sulfide, Cu sulfide, and barite are common to two pumice samples. The most coarse-grained of the vesicle fillings are Cu–Fe sulfide and Cu sulfide, which are as large as 50 μ in diameter. The youngest Plinian pumice also contains Zn(Fe) sulfide, as well as rare Ag–Cu sulfide, Ag–Fe sulfide, Ag bromide, Ag chloride, and Au–Cu telluride. The assemblage is similar to those typically observed in high-sulfidation epithermal mineralization. The fine-grained nature and abundance of syn- and/or posteruptive phases in porous rocks makes metals susceptible to mobilization by percolating fluids. The abundance of metal compounds in vesicles indicates that volatile exsolution prior to and/or during eruption played an important role in releasing metals to the atmosphere. Received: March 1997 · Accepted: 27 May 1997     

Recycling of orogenic arc crust triggers porphyry Cu mineralization in Kerman Cenozoic arc rocks,southeastern Iran

Pre-collisional Eocene–Oligocene arc diorites, quartzdiorites, granodiorites, and volcanic equivalents in the Kerman arc segment in central Iran lack porphyry Cu mineralization and ore deposits, whereas collisional middle-late Miocene adakite-like porphyritic granodiorites without volcanic equivalents host some of the world’s largest Cu ore deposits. Petrological and structural constraints suggest a direct link between orogenic arc crust evolution and the presence of a fertile metallogenic environment. Ore-hosting Kuh Panj porphyry intrusions exhibit high Sr (>400 ppm), low Y (<12 ppm) contents, significant REE fractionation (La/Yb > 20), no negative Eu anomalies (Eu/Eu^* ≥ 1), and relatively non-radiogenic Sr isotope signatures (⁸⁷Sr/⁸⁶Sr = 0.7042–0.7047), relative to Eocene–Oligocene granitoids (mainly Sr < 400 ppm; Y > 12; La/Yb < 15; Eu/Eu^* < 1; ⁸⁷Sr/⁸⁶Sr = 0.7053–0.7068). Trace element modeling indicates peridotite melting for the barren Eocene–Oligocene intrusions and a hydrous garnet-bearing amphibolite source for middle-late Miocene ore-hosting intrusions. The presence of garnet implies collisional arc crustal thickening by shortening and basaltic underplating from about 30–35 to 40–45 km or 12 kbar. The changes in residual mineralogy in the source of Eocene to Miocene rocks in the Kerman arc segment reflect probing of a thickening arc crust by recycling melting of the arc crustal keel. Underplating of Cu and sulfur-rich melts from fertile peridotite generated a fertile metallogenic reservoir at or near the crust–mantle boundary, and dehydration melting under oxidizing conditions produced syn- and post-collisional ore-hosting intrusions, while the lack of post-collisional volcanism prevented the venting of volatiles to the atmosphere from sulfur-rich and oxidized adakitic magmas. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Origin and interaction of mafic and felsic magmas in an evolving late orogenic setting: the Early Paleozoic Terra Nova Intrusive Complex, Antarctica

The Abbott Unit (∼508 Ma) and the Vegetation Unit (∼475 Ma) of the Terra Nova Intrusive Complex (northern Victoria Land, Antarctica) represent the latest magmatic events related to the Early Paleozoic Ross Orogeny. They show different emplacement styles and depths, ranging from forcible at 0.4–0.5 GPa for the Abbott Unit to passive at ∼0.2 GPa for the Vegetation Unit. Both units consist of mafic, felsic and intermediate facies which collectively define continuous chemical trends. The most mafic rocks from both units show different enrichment in trace element and Sr-Nd isotopic signatures. Once the possible effects of upper crustal assimilation-fractional crystallisation (AFC) and lower crustal coupled AFC and magma refilling processes have been taken into account the following features are recognised: (1) the modelled primary Abbott Unit magma shows a slightly enriched incompatible element distribution, similar to common continental arc basalts and (2) the modelled primary Vegetation Unit magma displays highly enriched isotope ratios and incompatible element patterns. We interpreted these major changes in magmatic affinity and emplacement style as linked to a major change in the tectonic setting affecting melt generation, rise and emplacement of the magmas. The Abbott Unit mafic melts were derived from a mantle wedge above a subduction zone, with subcontinental lithospheric mantle marginally involved in the melting column. The Vegetation Unit mafic melts are regarded as products of a different source involving an old layer of subcontinental lithospheric mantle. The crustal evolution of both types of mafic melts is marked by significant compositional contrasts in Sr and Nd isotopes between mafic and associated felsic rocks. The crustal isotope signature showed an increase with felsic character. Geochemical variations for both units can be accounted for by a similar two-stage hybridisation process. In the first stage, the most mafic magma evolved mainly by fractional crystallisation coupled with assimilation of metasedimentary rocks having crustal time-integrated Sr and Nd compositions similar to those of locally exposed metamorphic basement. The second stage involves contaminated products mixing with independently generated crustal melts. Petrographic, geochemical and isotope data also provide evidence of significant compositional differences in the felsic end-members, pointing to the involvement of metaigneous and metasedimentary source rocks for the Abbott granite and Vegetation leucogranite, respectively. Received: 31 March 1998 / Accepted: 3 May 1999     

Stability of dense hydrous magnesium silicate phases in the systems Mg2SiO4-H2O and MgSiO3-H2O at pressures up to 27 GPa

We conducted high-pressure phase equilibrium experiments in the systems MgSiO₃ with 15 wt% H₂O and Mg₂SiO₄ with 5 wt% and 11 wt% H₂O at 20 ∼ 27 GPa. Based on the phase relations in these systems, together with the previous works on the related systems, we have clarified the stability relations of dense hydrous magnesium silicates in the system MgO-SiO₂-H₂O in the pressure range from 10 to 27 GPa. The results show that the stability field of phase G, which is identical to phase D and phase F, expands with increasing water contents. Water stored in serpentine in the descending cold slabs is transported into depths greater than 200 km, where serpentine decomposes to a mixture of phase A, enstatite, and fluid. Reaction sequences of the hydrous phases which appear at higher pressures vary with water content. In the slabs with a water content less than about 2 wt%, phase A carries water to a depth of 450 km. Hydrous wadsleyite, hydrous ringwoodite, and ilmenite are the main water reservoirs in the transition zone from 450 to 660 km. Superhydrous phase B is the water reservoir in the uppermost part of the lower mantle from 670 to 800 km, whereas phase G appears in the lower mantle only at depths greater than 800 km. In cold slabs with local water enrichment greater than 2 wt%, the following hydrous phases appear with increasing depths; phase A to 450 km, phase A and phase G from 450 km to 550 km, brucite, superhydrous phase B, and phase G from 550 km to 800 km, and phase G at depths greater than 800 km. Received: 4 August 1999 / Accepted: 1 March 2000     

Detrital transport of metals by glaciers, an example from the Pinchi Mine, central British Columbia

 Abundant cinnabar (HgS) mineralization is associated with the Pinchi Fault in central British Columbia. Two formerly producing mercury mines have been developed on this fault: Pinchi and Bralorne Takla. The mercury content of till (a sediment type directly deposited by glaciers) in the area of this fault is primarily controlled by the occurrence of cinnabar mineralization in bedrock and the direction of ice flow. Cinnabar-bearing bedrock was eroded by glaciers, transported in the direction of ice flow, and deposited "down-ice" from its source. An example of such a dispersal train is documented for the Pinchi Mine area where mercury ore was transported over a distance of 12 km, as measured in the clay-sized fraction (< 0.002 mm) of till, and could have been transported over 24 km according to heavy mineral concentrates (specific gravity >3.3) of this same sediment. Antimony, chromium, and nickel dispersal trains were also detected in the region. These data indicate that natural glacial processes can result in the "mobilization" of metals in the surficial environment, a factor which has to be considered at mine sites in glaciated terrain, where mine reclamation and remediation measures are now required. Received: 31 October 1996 · Accepted: 27 May 1997     

Tephra glass chemistry provides storage and discharge details of five magma reservoirs which fed the 75 ka Youngest Toba Tuff eruption,northern Sumatra

The Youngest Toba Tuff contains five distinct glass populations, identified from Ba, Sr and Y compositions, termed PI (lowest Ba) – PV (highest Ba), representing five compositionally distinct pre-eruptive magma batches that fed the eruption. The PI–PV compositions display systematic changes, with higher FeO, CaO, MgO, TiO₂ and lower incompatible element concentrations in the low-SiO₂ PIV/PV, than the high-SiO₂ PI–PIII compositions. Glass shard abundances indicate PIV and PV were the least voluminous magma batches, and PI and PIII the most voluminous. Pressure estimates using rhyolite-MELTS indicate PV magma equilibrated at ~6 km, and PI magma at ~3.8 km. Glass population proportions in distal tephra and proximal (caldera-wall) material describe an eruption which commenced by emptying the deepest PIV and PV reservoirs, this being preferentially deposited in a narrow band across southern India (possibly due to jet-stream and/or plinian eruption transport), and as abundant pumice clasts in the lowermost proximal ignimbrites. Later, shallower magma reservoirs erupted, with PI being the most abundant as the eruption ended, sourcing the majority of distal ash from co-ignimbrite clouds (PI- and PIII-dominant), where associated ignimbrites isolated earlier (PIV- and PV-rich) deposits. This study shows how analysis of tephra glass compositional data can yield pre-eruption magma volume estimates, and enable aspects of magma storage conditions and eruption dynamics to be described.     

Anatexis at 700 to 1000 MPa in the aureole of the Marcy anorthosite, Adirondack Highlands, New York

High temperature (>900 °C) metamorphism affected the New Russia gneiss complex in the aureole of the Marcy anorthosite, Adirondack Highlands, New York. Dehydration melting of pargasitic hornblende and plagioclase in metagabbro during contact metamorphism produced garnet among other phases, an indicator that pressure exceeded 700 MPa during anatexis. Partial melting also occurred in mangerite and charnockite. Minerals that equilibrated during melting yield barometric estimates of 970 ± 100 MPa (garnet–orthopyroxene–plagioclase–quartz in metagabbro and mangerite) and 735 ± 100 and 985 ± 100 MPa (garnet–hornblende–plagioclase–quartz, metagabbro and mangerite, respectively). From these results we infer that the Marcy anorthosite was emplaced at a depth of at least 23 km and probably near 32 km. Received: 9 February 2000 / Accepted: 4 April 2000